Te c h n i c a l
The Technical Reference section includes articles covering regulator theory,
sizing, selection, overpressure protection, and other topics relating to
regulators. This section begins with the basic theory of regulators and ends
with conversion tables and other informative charts.
This section is for general reference only. For more detailed information
please visit www.emersonprocess.com/regulators or contact your
local Sales Ofce.
Table of Contents
572
Te c h n i c a l
Regulator Control Theory
Fundamentals of Gas Pressure Regulators ................................. 577
Pilot-Operated Regulators .......................................................... 578
Conclusion .................................................................................. 578
Regulator Components
Straight Stem Style Direct-Operated .......................................... 579
Lever Style Direct-Operated ....................................................... 580
Loading Style (Two-Path Control) Pilot-Operated ..................... 581
Unloading Style Pilot-Operated .................................................... 582
Introduction to Regulators
Specic Regulator Types ............................................................ 583
Pressure Reducing Regulators ............................................ 583
Backpressure Regulators and Relief Valves ....................... 584
Pressure Switching Valves .................................................. 584
Vaccum Regulators and Breakers .......................................... 584
Types of Regulators .................................................................... 583
Direct-Operated (Self-Operated) Regulators ...................... 583
Pilot-Operated Regulators .................................................. 583
Regulator Selection Criteria ....................................................... 584
Control Application ............................................................ 585
Pressure Reducing Regulator Selection .............................. 585
Outlet Pressure to be Maintained ....................................... 585
Inlet Pressure of the Regulator ........................................... 585
Capacity Required .............................................................. 585
Shutoff Capability .............................................................. 585
Process Fluid ...................................................................... 585
Process Fluid Temperature ................................................. 585
Accuracy Required ............................................................ 586
Pipe Size Required ......................................................... 586
End Connection Style .................................................... 586
Required Materials ......................................................... 586
Control Lines .................................................................. 586
Stroking Speeds .............................................................. 586
Overpressure Protection ................................................ 586
Regulator Replacement ................................................. 586
Regulator Price ................................................................ 587
Backpressure Regulator Selection ................................... 587
Relief Valve Selection ............................................................ 587
Theory
Principles of Direct-Operated Regulators
Introduction ................................................................................ 588
Regulator Basics ......................................................................... 588
Essential Elements ...................................................................... 588
Restricting Element ............................................................ 589
Measuring Element ............................................................ 589
Loading Element ................................................................ 589
Regulator Operation ................................................................... 589
Increasing Demand ............................................................ 589
Decreasing Demand ........................................................... 589
Weights versus Springs ...................................................... 589
Spring Rate ......................................................................... 590
Equilibrium with a Spring .................................................. 590
Spring as Loading Element ........................................................ 590
Throttling Example ............................................................ 590
Regulator Operation and P2 ............................................... 591
Regulator Performance ....................................................... 591
Performance Criteria .......................................................... 591
Setpoint ............................................................................... 591
Droop .................................................................................. 591
Capacity .............................................................................. 591
Accuracy ............................................................................. 591
Lockup ................................................................................ 591
Spring Rate and Regulator Accuracy ......................................... 592
Spring Rate and Droop ....................................................... 592
Effect on Plug Travel .......................................................... 592
Light Spring Rate ............................................................... 592
Practical Limits................................................................... 592
Diaphragm Area and Regulator Accuracy ................................. 592
Diaphragm Area ................................................................. 592
Increasing Diaphragm Area ................................................ 593
Diaphragm Size and Sensitivity ......................................... 593
Restricting Element and Regulator Performance ....................... 593
Critical Flow ....................................................................... 593
Orice Size and Capacity ................................................... 594
Orice Size and Stability .................................................... 594
Orice Size, Lockup, and Wear .......................................... 594
Orice Guideline ................................................................ 594
Increasing P1 ....................................................................... 594
Factors Affecting Regulator Accuracy ....................................... 594
Performance Limits .................................................................... 594
Cycling ............................................................................... 594
Design Variations ................................................................ 594
Improving Regulator Accuracy with a Pitot Tube .............. 595
Numerical Example ............................................................ 595
Decreased Droop (Boost) ................................................... 595
Improving Performance with a Lever ........................................ 595
Table of Contents
573
Te c h n i c a l
Overpressure Protection Methods
Methods of Overpressure Protection .................................. 604
Relief Valves ........................................................................... 604
Types of Relief Valves ................................................... 604
Advantages ...................................................................... 604
Disadvantages ................................................................ 604
Monitoring Regulators .......................................................... 605
Advantages ...................................................................... 605
Disadvantages ................................................................. 605
Working Monitor .................................................................... 605
Series Regulation .................................................................... 605
Advantages ...................................................................... 606
Disadvantages ................................................................. 606
Shutoff Devices ...................................................................... 606
Advantages ...................................................................... 606
Disadvantages ........................................................... 606
Relief Monitor ................................................................. 606
Summary .......................................................................... 607
Principles of Relief Valves
Overpressure Protection .................................................. 608
Maximum Pressure Considerations ................................. 608
Downstream Equipment ........................................... 608
Main Regulator ......................................................... 608
Piping ....................................................................... 608
Relief Valves ................................................................... 608
Relief Valve Popularity ............................................ 609
Relief Valve Types .................................................... 609
Selection Criteria ............................................................. 609
Pressure Build-up ...................................................... 609
Periodic Maintenance ............................................... 609
Cost versus Performance .......................................... 609
Installation and Maintenance Considerations .......... 609
Pop Type Relief Valve ..................................................... 609
Operation .................................................................. 609
Typical Applications ................................................. 610
Advantages ............................................................... 610
Disadvantage ............................................................ 610
Direct-Operated Relief Valves .................................. 610
Operation .................................................................. 610
Product Example ...................................................... 611
Typical Applications ................................................. 611
Selection Criteria ...................................................... 611
Pilot-Operated Relief Valves ........................................... 612
Operation .................................................................. 612
Product Example ...................................................... 612
Performance ............................................................. 613
Typical Applications ................................................. 613
Selection Criteria ...................................................... 613
Principles of Pilot-Operated Regulators
Pilot-Operated Regulator Basics ................................................ 596
Regulator Pilots .................................................................. 596
Gain .................................................................................... 596
Identifying Pilots ................................................................ 596
Setpoint ............................................................................... 596
Spring Action ...................................................................... 596
Pilot Advantage .................................................................. 596
Gain and Restrictions ................................................................. 596
Stability .............................................................................. 596
Restrictions, Response Time, and Gain ............................. 597
Loading and Unloading Designs ................................................ 597
Two-Path Control (Loading Design) .................................. 597
Two-Path Control Advantages ............................................ 598
Unloading Control .............................................................. 598
Unloading Control Advantages .......................................... 598
Performance Summary ............................................................... 598
Accuracy ............................................................................. 598
Capacity .............................................................................. 598
Lockup ................................................................................ 599
Applications ....................................................................... 599
Two-Path Control ....................................................................... 599
Type 1098-EGR .................................................................. 599
Type 99 ............................................................................... 600
Unloading Design ....................................................................... 600
Selecting and Sizing Pressure Reducing Regulators
Introduction ................................................................................ 601
Quick Selection Guides ................................................. 601
Product Pages .................................................................. 601
The Role of Experience ................................................. 601
Special Requirements .................................................... 601
Sizing Equations ..................................................................... 601
General Sizing Guidelines .................................................... 602
Body Size ......................................................................... 602
Construction .................................................................... 602
Pressure Ratings ............................................................. 602
Wide-Open Flow Rate ................................................... 602
Outlet Pressure Ranges and Springs............................ 602
Accuracy .......................................................................... 602
Inlet Pressure Losses ...................................................... 602
Orice Diameter ............................................................. 602
Speed of Response ......................................................... 602
Turn-Down Ratio ............................................................ 602
Sizing Exercise: Industrial Plant Gas Supply ................... 602
Quick Selection Guide ................................................ 603
Product Pages ................................................. 603
Final Selection ................................................................ 603
Table of Contents
574
Te c h n i c a l
Vacuum Control
Vacuum Applications ................................................................. 620
Vacuum Terminology ......................................................... 620
Vacuum Control Devices ................................................... 620
Vacuum Regulators ................................................................... 620
Vacuum Breakers (Relief Valves) .............................................. 620
Vacuum Regulator Installation Examples .................................. 622
Vacuum Breaker Installation Examples .................................. 623
Gas Blanketing in Vacuum ......................................................... 625
Features of Fisher® Brand Vacuum Regulators and Breakers ... 625
Valve Sizing Calculations (Traditional Method)
Introduction ..................................................................... 626
Sizing for Liquid Service ................................................ 626
Viscosity Corrections ............................................... 626
Finding Valve Size ................................................... 626
Nomograph Instructions ................................................... 627
Nomograph Equations ................................................... 627
Nomograph Procedure ................................................... 627
Predicting Flow Rate ............................................... 628
Predicting Pressure Drop .......................................... 628
Flashing and Cavitation ............................................ 628
Choked Flow ............................................................ 629
Liquid Sizing Summary ........................................... 631
Liquid Sizing Nomenclature .................................... 631
Sizing for Gas or Steam Service ...................................... 632
Universal Gas Sizing Equation ................................ 632
General Adaptation for Steam and Vapors ............... 633
Special Equation for Steam Below 1000 psig .......... 633
Gas and Steam Sizing Summary .............................. 634
Valve Sizing (Standardized Method)
Introduction ....................................................... 635
Liquid Sizing ............................................................... 635
Sizing Valves for Liquids ............................................... 635
Liquid Sizing Sample Problem ...................................... 638
Gas and Steam Sizing ................................................... 641
Sizing Valves for Compressible Fluids ......................... 641
Compressible Fluid Sizing Sample Problem ................ 642
Temperature Considerations
Cold Temperature Considerations
Regulators Rated for Low Temperatures ......................... 647
Selection Criteria ............................................................. 647
Internal Relief .................................................................. 613
Operation .................................................................. 613
Product Example ...................................................... 613
Performance and Typical Applications ..................... 614
Selection Criteria ...................................................... 614
Selection and Sizing Criteria ........................................... 614
Maximum Allowable Pressure ................................. 614
Regulator Ratings ..................................................... 614
Piping ....................................................................... 614
Maximum Allowable System Pressure .................... 614
Determining Required Relief Valve Flow ................ 614
Determine Constant Demand ................................... 615
Selecting Relief Valves .................................................... 615
Required Information ............................................... 615
Regulator Lockup Pressure ...................................... 615
Identify Appropriate Relief Valves ........................... 615
Final Selection .......................................................... 615
Applicable Regulations ............................................ 615
Sizing and Selection Exercise ......................................... 615
Initial Parameters ..................................................... 615
Performance Considerations .................................... 615
Upstream Regulator .................................................. 615
Pressure Limits ......................................................... 615
Relief Valve Flow Capacity ...................................... 615
Relief Valve Selection .............................................. 616
Principles of Series Regulation and Monitor Regulators
Series Regulation ............................................................. 617
Failed System Response ........................................... 617
Regulator Considerations ......................................... 617
Applications and Limitations ................................... 617
Upstream Wide-Open Monitors ...................................... 617
System Values ........................................................... 617
Normal Operation ..................................................... 617
Worker Regulator B Fails ......................................... 618
Equipment Considerations ....................................... 618
Downstream Wide-Open Monitors .................................. 618
Normal Operation ..................................................... 618
Worker Regulator A Fails ......................................... 618
Upstream Versus Downstream Monitors ......................... 618
Working Monitors ............................................................ 618
Downstream Regulator ..................................................... 619
Upstream Regulator ......................................... 619
Normal Operation ..................................................... 619
Downstream Regulator Fails ......................................... 619
Upstream Regulator Fails ..................................................... 619
Sizing Monitor Regulators .............................................. 619
Estimating Flow when Pressure Drop is Critical ..... 619
Assuming P
intermediate
to Determine Flow .................... 619
Fisher Monitor Sizing Program ....................................... 619
Table of Contents
575
Te c h n i c a l
Freezing
Introduction ..................................................................... 648
Reducing Freezing Problems ........................................... 648
Heat the Gas ............................................................. 648
Antifreeze Solution .................................................. 648
Equipment Selection ................................................ 648
System Design .......................................................... 649
Water Removal ......................................................... 649
Summary .......................................................................... 649
Sulfide Stress Cracking—NACE MR0175-2002,
MR0175/ISO 15156
The Details ....................................................................... 650
New Sulfide Stress Cracking Standards For Refineries .... 651
Responsibility .................................................................. 651
Applicability of NACE MR0175/ISO 15156 .................. 651
Basics of Sulde Stress Cracking (SSC) and
Stress Corrosion Cracking (SCC) ......................................... 651
Carbon Steel .................................................................... 652
Carbon and Low-Alloy Steel
Welding Hardness Requirements ....................................... 653
Low-Alloy Steel Welding Hardness Requirements .. 653
Cast Iron .......................................................................... 653
Stainless Steel .................................................................. 653
400 Series Stainless Steel ......................................... 653
300 Series Stainless Steel ......................................... 653
S20910 ...................................................................... 654
CK3MCuN ............................................................... 654
S17400 ...................................................................... 654
Duplex Stainless Steel .............................................. 654
Highly Alloyed Austenitic Stainless Steels .............. 654
Nonferrous Alloys ........................................................... 655
Nickel-Base Alloys .................................................. 655
Monel® K500 and Inconel® X750 ........................ 655
Cobalt-Base Alloys ................................................... 655
Aluminum and Copper Alloys .................................. 656
Titanium ................................................................... 656
Zirconium ................................................................. 656
Springs ............................................................................ 656
Coatings ........................................................................... 656
Stress Relieving ............................................................... 656
Bolting ............................................................................. 656
Bolting Coatings .............................................................. 657
Composition Materials .................................................... 657
Tubulars ............................................................................ 657
Expanded Limits and Materials ....................................... 657
Codes and Standards ........................................................ 658
Certifications ................................................................... 658
Reference
Chemical Compatibility of Elastomers and Metals
Introduction ..................................................................... 659
Elastomers: Chemical Names and Uses ......................... 659
General Properties of Elastomers .................................... 660
Fluid Compatibility of Elastomers .................................. 661
Compatibility of Metals ................................................... 662
Regulator Tips
Regulator Tips ................................................................. 664
Conversions, Equivalents, and Physical Data
Pressure Equivalents ....................................................... 666
Pressure Conversion - Pounds per Square Inch
(PSI) to Bar ....................................................... 666
Volume Equivalents ......................................................... 666
Volume Rate Equivalents ................................................ 667
Mass Conversion—Pounds to Kilograms ....................... 667
Temperature Conversion Formulas ................................. 667
Area Equivalents ............................................................. 667
Kinematic-Viscosity Conversion Formulas ..................... 667
Conversion Units ............................................................. 668
Other Useful Conversions ............................................... 668
Converting Volumes of Gas ............................................ 668
Fractional Inches to Millimeters ...................................... 669
Length Equivalents .......................................................... 669
Whole Inch-Millimeter Equivalents ................................ 669
Metric Prefixes and Symbols ........................................... 669
Greek Alphabet ................................................................ 669
Length Equivalents - Fractional and Decimal
Inches to Millimeters ........................................ 670
Temperature Conversions ................................................ 671
A.P.I. and Baumé Gravity Tables and Weight Factors ..... 674
Characteristics of the Elements ....................................... 675
Recommended Standard Specications for Valve
Materials Pressure-Containing Castings ........... 676
Physical Constants of Hydrocarbons ............................... 679
Physical Constants of Various Fluids .............................. 680
Properties of Water .......................................................... 682
Properties of Saturated Steam ......................................... 682
Properties of Saturated Steam—Metric Units ................. 685
Properties of Superheated Steam ..................................... 686
Determine Velocity of Steam in Pipes ............................. 689
Recommended Steam Pipe Line Velocities ..................... 689
Typical Condensation Rates in Insulated Pipes ............... 689
Typical Condensation Rates without Insulation .............. 689
Table of Contents
576
Te c h n i c a l
Flow of Water Through Schedule 40 Steel Pipes ............ 690
Flow of Air Through Schedule 40 Steel Pipes ................ 692
Average Properties of Propane ........................................ 694
Orifice Capacities for Propane ......................................... 694
Standard Domestic Propane Tank Specifications ............ 694
Approximate Vaporization Capacities
of Propane Tanks ............................................... 694
Pipe and Tubing Sizing .................................................... 695
Vapor Pressures of Propane ............................................. 695
Converting Volumes of Gas ............................................. 695
BTU Comparisons ......................................................... 695
Capacities of Spuds and Orifices ..................................... 696
Kinematic Viscosity - Centistokes ................................... 699
Specic Gravity of Typical Fluids vs
Temperature ................................................................ 700
Effect of Inlet Swage On Critical Flow
Cg Requirements ......................................................... 701
Seat Leakage Classications ...................................................... 702
Nominal Port Diameter and Leak Rate ...................................... 702
Flange, Valve Size, and Pressure-Temperature Rating
Designations ............................................................... 703
Equivalency Table .......................................................... 704
Pressure-Temperature Ratings for Valve Bodies ............. 704
ASME Face-To-Face Dimensions for
Flanged Regulators ..................................................... 706
Diameter of Bolt Circles ................................................. 706
Wear and Galling Resistance Chart of Material
Combinations ............................................................. 707
Equivalent Lengths of Pipe Fittings and Valves ......................... 707
Pipe Data: Carbon and Alloy Steel —Stainless Steel ..... 708
American Pipe Flange Dimensions ................................ 710
EN 1092-1 Cast Steel Flange Standards .................................. 710
EN 1092-1 Pressure/Temperature Ratings for
Cast Steel Valve Ratings ............................................... 711
Drill Sizes for Pipe Taps ................................................. 712
Standard Twist Drill Sizes .............................................. 712
Glossary
Glossary of Terms ...................................................................... 713
Regulator Control Theory
577
Te c h n i c a l
Flow
P
2
P
1
Restricting Element
Load
Flow
Regulator
Flow
Load
Regulator
P
2
Diaphragm
Spring
= Loading Pressure
Flow
P
1
P
2
P
L
Diaphragm
Spring
= Loading Pressure
Flow
P
L
P
2
P
1
Fundamentals of Gas Pressure Regulators
The primary function of any gas regulator is to match the ow of
gas through the regulator to the demand for gas placed upon the
system. At the same time, the regulator must maintain the system
pressure within certain acceptable limits.
A typical gas pressure system might be similar to that shown in
Figure 1, where the regulator is placed upstream of the valve or
other device that is varying its demand for gas from the regulator.
The loading element can be one of any number of things such as
a weight, a hand jack, a spring, a diaphragm actuator, or a piston
actuator, to name a few of the more common ones.
A diaphragm actuator and a spring are frequently combined, as
shown in Figure 3, to form the most common type of loading
element. A loading pressure is applied to a diaphragm to produce
a loading force that will act to close the restricting element. The
spring provides a reverse loading force which acts to overcome
the weight of the moving parts and to provide a fail-safe operating
action that is more positive than a pressure force.
If the load ow decreases, the regulator ow must decrease also.
Otherwise, the regulator would put too much gas into the system,
and the pressure (P2) would tend to increase. On the other hand, if
the load ow increases, then the regulator ow must increase also
in order to keep P2 from decreasing due to a shortage of gas in the
pressure system.
From this simple system it is easy to see that the prime job of the
regulator is to put exactly as much gas into the piping system as the
load device takes out.
If the regulator were capable of instantaneously matching its
ow to the load ow, then we would never have major transient
variation in the pressure (P2) as the load changes rapidly. From
practical experience we all know that this is normally not the
case, and in most real-life applications, we would expect some
uctuations in P2 whenever the load changes abruptly.
Because the regulator’s job is to modulate the ow of gas into
the system, we can see that one of the essential elements of any
regulator is a restricting element that will t into the ow stream
and provide a variable restriction that can modulate the ow of gas
through the regulator.
Figure 2 shows a schematic of a typical regulator restricting
element. This restricting element is usually some type of valve
arrangement. It can be a single-port globe valve, a cage style
valve, buttery valve, or any other type of valve that is capable of
operating as a variable restriction to the ow.
In order to cause this restricting element to vary, some type of
loading force will have to be applied to it. Thus we see that the
second essential element of a gas regulator is a Loading Element
that can apply the needed force to the restricting element.
Figure 1
So far, we have a restricting element to modulate the ow through
the regulator, and we have a loading element that can apply the
necessary force to operate the restricting element. But, how do
we know when we are modulating the gas ow correctly? How
do we know when we have the regulator ow matched to the load
ow? It is rather obvious that we need some type of Measuring
Element which will tell us when these two ows have been
perfectly matched. If we had some economical method of directly
measuring these ows, we could use that approach; however, this
is not a very feasible method.
We noted earlier in our discussion of Figure 1 that the system
pressure (P2) was directly related to the matching of the two ows.
If the restricting element allows too much gas into the system, P2
will increase. If the restricting element allows too little gas into
the system, P2 will decrease. We can use this convenient fact to
provide a simple means of measuring whether or not the regulator
is providing the proper ow.
Figure 2
Figure 3
Regulator Control Theory
578
Te c h n i c a l
Flow
P
1
P
2
P
2
P
1
Manometers, Bourdon tubes, bellows, pressure gauges, and
diaphragms are some of the possible measuring elements that
we might use. Depending upon what we wish to accomplish,
some of these measuring elements would be more advantageous
than others. The diaphragm, for instance, will not only act as a
measuring element which responds to changes in the measured
pressure, but it also acts simultaneously as a loading element. As
such, it produces a force to operate the restricting element that
varies in response to changes in the measured pressure. If we
add this typical measuring element to the loading element and the
restricting element that we selected earlier, we will have a complete
gas pressure regulator as shown in Figure 4.
Let’s review the action of this regulator. If the restricting element
tries to put too much gas into the system, the pressure (P2) will
increase. The diaphragm, as a measuring element, responds to this
increase in pressure and, as a loading element, produces a force
which compresses the spring and thereby restricts the amount
of gas going into the system. On the other hand, if the regulator
doesn’t put enough gas into the system, the pressure (P2) falls and
the diaphragm responds by producing less force. The spring will
then overcome the reduced diaphragm force and open the valve
to allow more gas into the system. This type of self-correcting
action is known as negative feedback. This example illustrates
that there are three essential elements needed to make any
operating gas pressure regulator. They are a restricting element,
a loading element, and a measuring element. Regardless of how
sophisticated the system may become, it still must contain these
three essential elements.
Pilot-Operated Regulators
So far we have only discussed direct-operated regulators.
This is the name given to that class of regulators where the
measured pressure is applied directly to the loading element
with no intermediate hardware. There are really only two basic
congurations of direct-operated regulators that are practical.
These two basic types are illustrated in Figures 4 and 5.
Figure 4
Figure 5
If the proportional band of a given direct-operated regulator is
too great for a particular application, there are a number of things
we can do. From our previous examples we recall that spring
rate, valve travel, and effective diaphragm area were the three
parameters that affect the proportional band. In the last section we
pointed out the way to change these parameters in order to improve
the proportional band. If these changes are either inadequate or
impractical, the next logical step is to install a pressure amplier in
the measuring or sensing line. This pressure amplier is frequently
referred to as a pilot.
Conclusion
It should be obvious at this point that there are fundamentals to
understand in order to properly select and apply a gas regulator
to do a specic job. Although these fundamentals are profuse
in number and have a sound theoretical base, they are relatively
straightforward and easy to understand.
As you are probably aware by now, we made a number of
simplifying assumptions as we progressed. This was done in the
interest of gaining a clearer understanding of these fundamentals
without getting bogged down in special details and exceptions. By
no means has the complete story of gas pressure regulation been
told. The subject of gas pressure regulation is much broader in
scope than can be presented in a single document such as this, but
it is sincerely hoped that this application guide will help to gain
a working knowledge of some fundamentals that will enable one
to do a better job of designing, selecting, applying, evaluating, or
troubleshooting any gas pressure regulation equipment.
Regulator Components
579
Te c h n i c a l
Straight Stem Style Direct-Operated Regulator Components
Type 133L
ORIFICE
INLET PRESSURE
BOOST PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
SPRING SEAT
CAGE
VALVE STEM
BALANCING
DIAPHRAGM
SPRING ADJUSTOR
SPRING CASE
DIAPHRAGM
HEAD
VALVE DISK
CLOSING CAP
CONTROL SPRING
VENT
EXTERNAL
CONTROL LINE
CONNECTION
REGISTRATION
DIAPHRAGM
BODY
DIAPHRAGM
CASE
NOTE:
THE INFORMATION PRESENTED IS FOR REFERENCE ONLY. FOR MORE SPECIFIC APPLICATION
INFORMATION, PLEASE LOG ON TO: www.emersonprocess.com/regulators
A6555
Regulator Components
580
Te c h n i c a l
Lever Style Direct-Operated Regulator Components
Type 627
BODY
VALVE DISK
LEVER
VENT
VALVE STEM
SPRING SEAT
SPRING
DIAPHRAGM
ORIFICE
DIAPHRAGM
HEAD
DIAPHRAGM
CASE
CLOSING CAP
SPRING CASE
INTERNAL CONTROL
REGISTRATION
ADJUSTING SCREW
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
NOTE:
THE INFORMATION PRESENTED IS FOR REFERENCE ONLY. FOR MORE SPECIFIC APPLICATION
INFORMATION, PLEASE LOG ON TO: www.emersonprocess.com/regulators
A6557
Regulator Components
581
Te c h n i c a l
Loading Style (Two-Path Control) Pilot-Operated Regulator Components
Type 1098-EGR
A6563
Type 1098-EGR
TRAVEL INDICATOR
CONTROL LINE
(EXTERNAL)
TYPE 1098 ACTUATOR
PILOT
INLET PRESSURE
INLINE FILTER
VENT
PILOT SUPPLY
PRESSURE
MAIN SPRING
BODY
BALANCED EGR TRIM
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
ATMOSPHERIC PRESSURE
NOTE:
THE INFORMATION PRESENTED IS FOR REFERENCE ONLY. FOR MORE SPECIFIC APPLICATION
INFORMATION, PLEASE LOG ON TO: www.emersonprocess.com/regulators
A6563
Regulator Components
582
Te c h n i c a l
Unloading Style Pilot-Operated Regulator Components
Type EZR
TYPE 161EB PILOT
TYPE 252 PILOT
SUPPLY FILTER
TYPE 112 RESTRICTOR
STRAINER
BODY
VENT
EXTERNAL
CONTROL
LINE
TRAVEL
INDICATOR
PLUG AND
DIAPHRAGM
TRIM
CAGE
EXTERNAL PILOT
SUPPLY LINE
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
ATMOSPHERIC PRESSURE
NOTE:
THE INFORMATION PRESENTED IS FOR REFERENCE ONLY. FOR MORE SPECIFIC APPLICATION
INFORMATION, PLEASE LOG ON TO: www.emersonprocess.com/regulators
W7438
Introduction to Regulators
583
Te c h n i c a l
Instrument engineers agree that the simpler a system is the
better it is, as long as it provides adequate control. In general,
regulators are simpler devices than control valves. Regulators are
self-contained, direct-operated control devices which use energy
from the controlled system to operate whereas control valves
require external power sources, transmitting instruments, and
control instruments.
Specific Regulator Types
Within the broad categories of direct-operated and pilotoperated regulators fall virtually all of the general regulator
designs, including:
• Pressure reducing regulators
• Backpressure regulators
• Pressure relief valves
• Pressure switching valves
• Vacuum regulators and breakers
Pressure Reducing Regulators
A pressure reducing regulator maintains a desired reduced outlet
pressure while providing the required uid ow to satisfy a
downstream demand. The pressure which the regulator maintains
is the outlet pressure setting (setpoint) of the regulator.
Types of Pressure Reducing Regulators
This section describes the various types of regulators. All
regulators t into one of the following two categories:
1. Direct-Operated (also sometimes called Self-Operated)
2. Pilot-Operated
Direct-Operated (Self-Operated) Regulators
Direct-operated regulators are the simplest style of regulators. At
low set pressures, typically below 1 psig (0,07 bar), they can have
very accurate (±1%) control. At high control pressures, up to
500 psig (34,5 bar), 10 to 20% control is typical.
In operation, a direct-operated, pressure reducing regulator
senses the downstream pressure through either internal pressure
registration or an external control line. This downstream pressure
opposes a spring which moves the diaphragm and valve plug to
change the size of the ow path through the regulator.
Pilot-Operated Regulators
Pilot-operated regulators are preferred for high ow rates or where
precise pressure control is required. A popular type of pilotoperated system uses two-path control. In two-path control, the
main valve diaphragm responds quickly to downstream pressure
Figure 1. Type 627 Direct-Operated Regulator and Operational Schematic
Figure 2. Type 1098-EGR Pilot-Operated Regulator and Operational Schematic
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
ATMOSPHERIC PRESSURE
Type 1098-EGR
W6956
A6563
A6557
W4793
Introduction to Regulators
584
Te c h n i c a l
changes, causing an immediate correction in the main valve plug
position. At the same time, the pilot diaphragm diverts some
of the reduced inlet pressure to the other side of the main valve
diaphragm to control the nal positioning of the main valve plug.
Two-path control results in fast response and accurate control.
Backpressure Regulators and Pressure Relief Valves
A backpressure regulator maintains a desired upstream pressure
by varying the ow in response to changes in upstream pressure.
A pressure relief valve limits pressure build-up (prevents
overpressure) at its location in a pressure system. The relief valve
opens to prevent a rise of internal pressure in excess of a specied
value. The pressure at which the relief valve begins to open
pressure is the relief pressure setting.
Relief valves and backpressure regulators are the same devices.
The name is determined by the application. Fisher® relief valves
are not ASME safety relief valves.
Vacuum Regulators and Breakers
Vacuum regulators and vacuum breakers are devices used to
control vacuum. A vacuum regulator maintains a constant vacuum
at the regulator inlet with a higher vacuum connected to the outlet.
During operation, a vacuum regulator remains closed until a
vacuum decrease (a rise in absolute pressure) exceeds the spring
setting and opens the valve disk. A vacuum breaker prevents a
vacuum from exceeding a specied value. During operation, a
vacuum breaker remains closed until an increase in vacuum
(a decrease in absolute pressure) exceeds the spring setting and
opens the valve disk.
Regulator Selection Criteria
This section describes the procedure normally used to select
regulators for various applications. For most applications, there
is generally a wide choice of regulators that will accomplish the
Figure 3. Type 63EG Backpressure Regulator/Relief Valve
Operational Schematic
Figure 4. Type Y690VB Vacuum Breaker and Type V695VR Vacuum Regulator
Operational Schematics
Pressure Switching Valves
Pressure switching valves are used in pneumatic logic systems. These
valves are for either two-way or three-way switching. Two-way
switching valves are used for on/off service in pneumatic systems.
Three-way switching valves direct inlet pressure from one outlet
port to another whenever the sensed pressure exceeds or drops
below a preset limit.
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LOADING PRESSURE
INLET PRESSURE
CONTROL PRESSURE (VACUUM)
ATMOSPHERIC PRESSURE
VACUUM
BEING CONTROLLED
HIGHER
VACUUM SOURCE
VACUUM
PUMP
VACUUM
PUMP
VACUUM
BEING LIMITED
A6929
B2583
B2582
TYPE Y690VB
TYPE Y695VR
Introduction to Regulators
585
Te c h n i c a l
required function. The vendor and the customer, working together,
have the task of deciding which of the available regulators is best
suited for the job at hand. The selection procedure is essentially a
process of elimination wherein the answers to a series of questions
narrow the choice down to a specic regulator.
Control Application
To begin the selection procedure, it’s necessary to dene what
the regulator is going to do. In other words, what is the control
application? The answer to this question will determine the general
type of regulator required, such as:
• Pressure reducing regulators
• Backpressure regulators
• Pressure relief valves
• Vacuum regulators
• Vacuum breaker
The selection criteria used in selecting each of these general regulator
types is described in greater detail in the following subsections.
Pressure Reducing Regulator Selection
The majority of applications require a pressure reducing regulator.
Assuming the application calls for a pressure reducing regulator,
the following parameters must be determined:
• Outlet pressure to be controlled
• Inlet pressure to the regulator
• Capacity required
• Shutoff capability required
• Process uid
• Process uid temperature
• Accuracy required
• Pipe size required
• End connection style
• Material requirements
• Control line needed
• Overpressure protection
Outlet Pressure to be Controlled
For a pressure reducing regulator, the rst parameter to determine
is the required outlet pressure. When the outlet pressure is known,
it helps determine:
• Spring requirements
• Casing pressure rating
• Body outlet rating
• Orice rating and size
• Regulator size
Inlet Pressure of the Regulator
The next parameter is the inlet pressure. The inlet pressure
(minimum and maximum) determines the:
• Pressure rating for the body inlet
• Orice pressure rating and size
• Main spring (in a pilot-operated regulator)
• Regulator size
If the inlet pressure varies signicantly, it can have an effect on:
• Accuracy of the controlled pressure
• Capacity of the regulator
• Regulator style (two-stage or unloading)
Capacity Required
The required ow capacity inuences the following decisions:
• Size of the regulator
• Orice size
• Style of regulator (direct-operated or pilot-operated)
Shutoff Capability
The required shutoff capability determines the type of
disk material:
• Standard disk materials are Nitrile (NBR) and Neoprene (CR),
these materials provide the tightest shutoff.
• Other materials, such as Nylon (PA), Polytetrauoroethylene
(PTFE), Fluoroelastomer (FKM), and Ethylenepropylene
(EPDM), are used when standard material cannot be used.
• Metal disks are used in high temperatures and when
elastomers are not compatible with the process uid;
however, tight shutoff is typically not achieved.
Process Fluid
Each process uid has its own set of unique characteristics in
terms of its chemical composition, corrosive properties, impurities,
ammability, hazardous nature, toxic effect, explosive limits, and
molecular structure. In some cases special care must be taken
to select the proper materials that will come in contact with the
process uid.
Process Fluid Temperature
Fluid temperature might determine the materials used in the
regulator. Standard regulators use Steel and Nitrile (NBR) or
Neoprene (CR) elastomers that are good for a temperature range
of -40° to 180°F (-40° to 82°C). Temperatures above and below
this range may require other materials, such as Stainless steel,
Ethylenepropylene (EPDM), or Peruoroelastomer (FFKM).
Introduction to Regulators
586
Te c h n i c a l
Accuracy Required
The accuracy requirement of the process determines the acceptable
droop (also called proportional band or offset). Regulators fall into
the following groups as far as droop is concerned:
• Rough-cut Group— This group generally includes many
rst-stage, rough-cut direct-operated regulators. This group
usually has the highest amount of droop. However, some
designs are very accurate, especially the low-pressure gas or
air types, such as house service regulators, which incorporate
a relatively large diaphragm casing.
• Close-control Group— This group usually includes pilot-
operated regulators. They provide high accuracy over a
large range of ows. Applications that require close control
include these examples:
• Burner control where the fuel/air ratio is critical to
burner efciency and the gas pressure has a signicant
effect on the fuel/air ratio.
• Metering devices, such as gas meters, which require
constant input pressures to ensure accurate measurement.
Pipe Size Required
If the pipe size is known, it gives the specier of a new regulator
a more dened starting point. If, after making an initial selection
of a regulator, the regulator is larger than the pipe size, it usually
means that an error has been made either in selecting the pipe size
or the regulator, or in determining the original parameters (such
as pressure or ow) required for regulator selection. In many
cases, the outlet piping needs to be larger than the regulator for the
regulator to reach full capacity.
End Connection Style
In general, the following end connections are available for the
indicated regulator sizes:
• Pipe threads or socket weld: 2-inch (DN 50) and smaller
• Flanged: 1-inch (DN 25) and larger
• Butt weld: 1-inch (DN 25) and larger
Note: Not all end connections are available for all regulators.
Required Materials
The regulator construction materials are generally dictated by the
application. Standard materials are:
• Aluminum
• Cast iron or Ductile iron
• Steel
• Bronze and Brass
• Stainless steel
Special materials required by the process can have an effect on the
type of regulator that can be used. Oxygen service, for example,
requires special materials, requires special cleaning preparation,
and requires that no oil or grease be in the regulator.
Control Lines
For pressure registration, control lines are connected downstream
of a pressure reducing regulator, and upstream of a backpressure
regulator. Typically large direct-operated regulators have
external control lines, and small direct-operated regulators have
internal registration instead of a control line. Most pilot-operated
regulators have external control lines, but this should be conrmed
for each regulator type considered.
Stroking Speed
Stroking speed is often an important selection criteria. Directoperated regulators are very fast, and pilot-operated regulators
are slightly slower. Both types are faster than most control valves.
When speed is critical, techniques can be used to decrease
stroking time.
Overpressure Protection
The need for overpressure protection should always be considered.
Overpressure protection is generally provided by an external relief
valve, or in some regulators, by an internal relief valve. Internal
relief is an option that you must choose at the time of purchase.
The capacity of internal relief is usually limited in comparison with
a separate relief valve. Other methods such as shutoff valves or
monitor regulators can also be used.
Regulator Replacement
When a regulator is being selected to replace an existing regulator,
the existing regulator can provide the following information:
• Style of regulator
• Size of regulator
• Type number of the regulator
• Special requirements for the regulator, such as downstream
pressure sensing through a control line versus internal
pressure registration.
Introduction to Regulators
587
Te c h n i c a l
Figure 5. Backpressure Regulator/Relief Valve Applications
RELIEF PRESSURE CONTROL AT RELIEF VALVE INLET BACKPRESSURE CONTROL
Regulator Price
The price of a regulator is only a part of the cost of ownership.
Additional costs include installation and maintenance. In selecting
a regulator, you should consider all of the costs that will accrue over
the life of the regulator. The regulator with a low initial cost might
not be the most economical in the long run. For example, a directoperated regulator is generally less expensive, but a pilot-operated
regulator might provide more capacity for the initial investment.
To illustrate, a 2-inch (DN 50) pilot-operated regulator can have
the same capacity and a lower price than a 3-inch (DN 80), direct-
operated regulator.
Backpressure Regulator Selection
Backpressure regulators control the inlet pressure rather than the
outlet pressure. The selection criteria for a backpressure regulator
the same as for a pressure reducing regulator.
Relief Valve Selection
An external relief valve is a form of backpressure regulator. A
relief valve opens when the inlet pressure exceeds a set value.
Relief is generally to atmosphere. The selection criteria is the same
as for a pressure reducing regulator.
MAIN VALVE
VENT VALVE B
BLOCK VALVE A
MAIN PRESSURE LINE
NONRESTRICTIVE
VENTS AND PIPING
ALTERNATE PILOT
EXHAUST PIPING
PILOT
CONTROL LINE
30B8289_A
BLOCK VALVE A
VENT VALVE B
MAIN VALVE
VENT VALVE D
ALTERNATE PILOT
EXHAUST PIPING
VENT VALVE C
PILOT
CONTROL LINE
30B8288_A
Principles of Direct-Operated Regulators
588
Te c h n i c a l
Introduction
Pressure regulators have become very familiar items over the years,
and nearly everyone has grown accustomed to seeing them in
factories, public buildings, by the roadside, and even on the outside
of their own homes. As is frequently the case with such familiar
items, we have a tendency to take them for granted. It’s only
when a problem develops, or when we are selecting a regulator
for a new application, that we need to look more deeply into the
fundamentals of the regulator’s operation.
Regulators provide a means of controlling the ow of a gas or
other uid supply to downstream processes or customers. An
ideal regulator would supply downstream demand while keeping
downstream pressure constant; however, the mechanics of direct-
operated regulator construction are such that there will always be
some deviation (droop or offset) in downstream pressure.
The service regulator mounted on the meter outside virtually every
home serves as an example. As appliances such as a furnace or
stove call for the ow of more gas, the service regulator responds
by delivering the required ow. As this happens, the pressure
should be held constant. This is important because the gas meter,
which is the cash register of the system, is often calibrated for a
given pressure.
Direct-operated regulators have many commercial and residential
uses. Typical applications include industrial, commercial, and
domestic gas service, instrument air supply, and a broad range of
applications in industrial processes.
Regulators automatically adjust ow to meet downstream demand.
Before regulators were invented, someone had to watch a pressure
gauge for pressure drops which signaled an increase in downstream
demand. When the downstream pressure decreased, more ow was
required. The operator then opened the regulating valve until the
gauge pressure increased, showing that downstream demand was
being met.
Essential Elements
Direct-operated regulators have three essential elements:
• A restricting element — a valve, disk, or plug
• A measuring element— generally a diaphragm
• A loading element— generally a spring
Figure 1. Direct-Operated Regulators
TYPE 630
Regulator Basics
A pressure reducing regulator must satisfy a downstream demand
while maintaining the system pressure within certain acceptable
limits. When the ow rate is low, the regulator plug or disk
approaches its seat and restricts the ow. When demand increases,
the plug or disk moves away from its seat, creating a larger
opening and increased ow. Ideally, a regulator should provide a
constant downstream pressure while delivering the required ow.
Figure 2. Three Essential Elements
TYPE HSR
133 SERIES
MEASURING
ELEMENT
LOADING
ELEMENT
(WEIGHT)
RESTRICTING
ELEMENT
W1327
W1934
Principles of Direct-Operated Regulators
589
Te c h n i c a l
Restricting Element
The regulator’s restricting element is generally a disk or plug that
can be positioned fully open, fully closed, or somewhere in between
to control the amount of ow. When fully closed, the disk or plug
seats tightly against the valve orice or seat ring to shutoff ow.
Measuring Element
The measuring element is usually a exible diaphragm that
senses downstream pressure (P2). The diaphragm moves
as pressure beneath it changes. The restricting element is
often attached to the diaphragm with a stem so that when the
diaphragm moves, so does the restricting element.
Loading Element
A weight or spring acts as the loading element. The loading
element counterbalances downstream pressure (P2). The amount of
unbalance between the loading element and the measuring element
determines the position of the restricting element. Therefore, we can
adjust the desired amount of ow through the regulator, or setpoint,
by varying the load. Some of the rst direct-operated regulators used
weights as loading elements. Most modern regulators use springs.
Regulator Operation
To examine how the regulator works, let’s consider these values for
a direct-operated regulator installation:
• Upstream Pressure (P
1
) = 100 psig
• Downstream Pressure (P
2
) = 10 psig
• Pressure Drop Across the Regulator (P) = 90 psi
• Diaphragm Area (A
D
) = 10 square inches
• Loading Weight = 100 pounds
Let’s examine a regulator in equilibrium as shown in Figure 3. The
pressure acting against the diaphragm creates a force acting up to
100 pounds.
Diaphragm Force (FD) = Pressure (P2) x Area of Diaphragm (AD)
or
FD = 10 psig x 10 square inches = 100 pounds
The 100 pounds weight acts down with a force of 100 pounds,
so all the opposing forces are equal, and the regulator plug
remains stationary.
Increasing Demand
If the downstream demand increases, P2 will drop. The pressure on
the diaphragm drops, allowing the regulator to open further. Suppose
in our example P2 drops to 9 psig. The force acting up then equals
90 pounds (9 psig x 10 square inches = 90 pounds). Because of the
unbalance of the measuring element and the loading element, the
restricting element will move to allow passage of more ow.
Decreasing Demand
If the downstream demand for ow decreases, downstream
pressure increases. In our example, suppose P2 increases to 11
psig. The force acting up against the weight becomes 110 pounds
(11 psig x 10 square inches = 110 pounds). In this case, unbalance
causes the restricting element to move up to pass less ow or
lockup.
Weights versus Springs
One of the problems with weight-loaded systems is that they are
slow to respond. So if downstream pressure changes rapidly,
our weight-loaded regulator may not be able to keep up. Always
behind, it may become unstable and cycle—continuously going
from the fully open to the fully closed position. There are other
problems. Because the amount of weight controls regulator
setpoint, the regulator is not easy to adjust. The weight will always
have to be on top of the diaphragm. So, let’s consider using a
spring. By using a spring instead of a weight, regulator stability
increases because a spring has less stiffness.
AT EQUILIBRIUM
Figure 3. Elements
100 LB
AREA = 10 IN
2
P2 = 10 PSIGP1 = 100 PSIG
FW = 100 LB
FD = 100 LB
FD = (P2 x AD) = (10 PSIG)(10 IN2) = 100 LB
OPEN
100 LB
AREA = 10 IN
2
P2 = 9 PSIGP1 = 100 PSIG
FW = 100 LB
FD = 90 LB
FD = (P2 x AD) = (9 PSIG)(10 IN2) = 90 LB
Principles of Direct-Operated Regulators
590
Te c h n i c a l
Spring Rate
We choose a spring for a regulator by its spring rate (K). K
represents the amount of force necessary to compress the spring
one inch. For example, a spring with a rate of 100 pounds per inch
needs 100 pounds of force to compress it one inch, 200 pounds of
force to compress it two inches, and so on.
Equilibrium with a Spring
Instead of a weight, let’s substitute a spring with a rate of
100 pounds per inch. And, with the regulator’s spring adjustor, we’ll
wind in one inch of compression to provide a spring force (FS) of
100 pounds. This amount of compression of the regulator spring
determines setpoint, or the downstream pressure that we want to
hold constant. By adjusting the initial spring compression, we
change the spring loading force, so P2 will be at a different value in
order to balance the spring force.
Now the spring acts down with a force of 100 pounds, and the
downstream pressure acts up against the diaphragm producing
a force of 100 pounds (FD = P2 x AD). Under these conditions
the regulator has achieved equilibrium; that is, the plug or disk is
holding a xed position.
Spring as Loading Element
By using a spring instead of a xed weight, we gain better control
and stability in the regulator. The regulator will now be less likely to
go fully open or fully closed for any change in downstream pressure
(P2). In effect, the spring acts like a multitude of different weights.
Throttling Example
Assume we still want to maintain 10 psig downstream. Consider
what happens now when downstream demand increases and
pressure P2 drops to 9 psig. The diaphragm force (FD) acting up is
now 90 pounds.
FD = P2 x AD
FD = 9 psig x 10 in
2
FD = 90 pounds
We can also determine how much the spring will move (extend)
which will also tell us how much the disk will travel. To keep the
regulator in equilibrium, the spring must produce a force (FS) equal
to the force of the diaphragm. The formula for determining spring
force (FS) is:
FS = (K)(X)
where K = spring rate in pounds/inch and X = travel or
compression in inches.
AREA = 10 IN
2
SPRING AS ELEMENT
FS = F
D
FS = (K)(X)
FD = (P2)(AD)
F
S
F
D
(TO KEEP DIAPHRAGM
FROM MOVING)
FD = (10 PSIG)(102) = 100 LB
FS = (100 LB/IN)(X) = 100 LB
X = 1-INCH COMPRESSION
FS = 90 LB
FD = 90 LB
P1 = 100 PSIG P2 = 9 PSIG
Figure 4. Spring as Element
Figure 5. Plug Travel
0.1-INCH
AT EQUILIBRIUM
P1 = 100 PSIG P2 = 10 PSIG
Principles of Direct-Operated Regulators
591
Te c h n i c a l
We know FS is 90 pounds and K is 100 pounds/inch, so we can
solve for X with:
X = FS ÷ K
X = 90 pounds ÷ 100 pounds/inch
X = 0.9 inch
The spring, and therefore the disk, has moved down 1/10-inch,
allowing more ow to pass through the regulator body.
Regulator Operation and P
2
Now we see the irony in this regulator design. We recall that the
purpose of an ideal regulator is to match downstream demand
while keeping P2 constant. But for this regulator design to increase
ow, there must be a change in P2.
Regulator Performance
We can check the performance of any regulating system by
examining its characteristics. Most of these characteristics can
be best described using pressure versus ow curves as shown in
Figure 6.
Performance Criteria
We can plot the performance of an ideal regulator such that
no matter how the demand changes, our regulator will match
that demand (within its capacity limits) with no change in the
downstream pressure (P2). This straight line performance becomes
the standard against which we can measure the performance of a
real regulator.
Setpoint
The constant pressure desired is represented by the setpoint. But
no regulator is ideal. The downward sloping line on the diagram
represents pressure (P2) plotted as a function of ow for an actual
direct-operated regulator. The setpoint is determined by the initial
compression of the regulator spring. By adjusting the initial spring
compression you change the spring loading force, so P2 will be
at a different value in order to balance the spring force. This
establishes setpoint.
Droop
Droop, proportional band, and offset are terms used to describe
the phenomenon of P2 dropping below setpoint as ow increases.
Droop is the amount of deviation from setpoint at a given ow,
expressed as a percentage of setpoint. This “droop” curve is
important to a user because it indicates regulating (useful) capacity.
Capacity
Capacities published by regulator manufacturers are given for
different amounts of droop. Let’s see why this is important.
Let’s say that for our original problem, with the regulator set at
10 psig, our process requires 200 SCFH (standard cubic feet per
hour) with no more than a 1 psi drop in setpoint. We need to keep
the pressure at or above 9 psig because we have a low limit safety
switch set at 9 psig that will shut the system down if pressure falls
below this point.
Figure 6 illustrates the performance of a regulator that can do the job.
And, if we can allow the downstream pressure to drop below 9 psig,
the regulator can allow even more ow.
The capacities of a regulator published by manufacturers are
generally given for 10% droop and 20% droop. In our example,
this would relate to ow at 9 psig and at 8 psig.
Accuracy
The accuracy of a regulator is determined by the amount of ow it
can pass for a given amount of droop. The closer the regulator is to
the ideal regulator curve (setpoint), the more accurate it is.
Lockup
Lockup is the pressure above setpoint that is required to shut the
regulator off tight. In many regulators, the orice has a knife
edge while the disk is a soft material. Some extra pressure, P2, is
Figure 6. Typical Performance Curve
AS THE FLOW RATE APPROACHES ZERO, P2 INCREASES STEEPLY. LOCKUP IS THE TERM APPLIED TO THE
VALUE OF P2 AT ZERO FLOW.
LOCKUP
SETPOINT
DROOP
(OFFSET)
WIDE-OPEN
P
2
FLOW
Principles of Direct-Operated Regulators
592
Te c h n i c a l
required to force the soft disk into the knife edge to make a tight
seal. The amount of extra pressure required is lockup pressure.
Lockup pressure may be important for a number of reasons.
Consider the example above where a low pressure limit switch
would shut down the system if P2 fell below 9 psig. Now consider
the same system with a high pressure safety cut out switch set a
10.5 psig. Because our regulator has a lockup pressure of 11 psig,
the high limit switch will shut the system down before the regulator
can establish tight shutoff. Obviously, we’ll want to select a
regulator with a lower lockup pressure.
Spring Rate and Regulator Accuracy
Using our initial problem as an example, let’s say we now need the
regulator to ow 300 SCFH at a droop of 10% from our original
setpoint of 10 psig. Ten percent of 10 psig = 1 psig, so P2 cannot
drop below 10 to 1, or 9 psi. Our present regulator would not be
accurate enough. For our regulator to pass 300 SCFH, P2 will have
to drop to 8 psig, or 20% droop.
Spring Rate and Droop
One way to make our regulator more accurate is to change to
a lighter spring rate. To see how spring rate affects regulator
accuracy, let’s return to our original example. We rst tried a
spring with a rate of 100 pounds/inch. Let’s substitute one with a
rate of 50 pounds/inch. To keep the regulator in equilibrium, we’ll
have to initially adjust the spring to balance the 100 pound force
produced by P2 acting on the diaphragm. Recall how we calculate
spring force:
FS = K (spring rate) x X (compression)
Knowing that FS must equal 100 pounds and K = 50 pounds/inch,
we can solve for X, or spring compression, with:
X = FS ÷ K, or X = 2 inches
So, we must wind in 2-inches of initial spring compression to
balance diaphragm force, FD.
Effect on Plug Travel
We saw before that with a spring rate of 100 pounds/inch, when
P2 dropped from 10 to 9 psig, the spring relaxed (and the valve
disk traveled) 0.1 inch. Now let’s solve for the amount of disk
travel with the lighter spring rate of 50 pounds per inch. The force
produced by the diaphragm is still 90 pounds.
FD = P2 x AD
To maintain equilibrium, the spring must also produce a force of
90 pounds. Recall the formula that determines spring force:
FS = (K)(X)
Because we know FS must equal 90 pounds and our spring rate (K)
is 50 pounds/inch, we can solve for compression (X) with:
X = FS ÷ K
X = 90 pounds ÷ 50 pounds/inch
X = 1.8 inches
To establish setpoint, we originally compressed this spring 2 inches.
Now it has relaxed so that it is only compressed 1.8 inches, a change
of 0.2-inch. So with a spring rate of 50 pounds/inch, the regulator
responded to a 1 psig drop in P2 by opening twice as far as it did
with a spring rate of 100 pounds/inch. Therefore, our regulator
is now more accurate because it has greater capacity for the same
change in P2. In other words, it has less droop or offset. Using this
example, it is easy to see how capacity and accuracy are related and
how they are related to spring rate.
Light Spring Rate
Experience has shown that choosing the lightest available spring
rate will provide the most accuracy (least droop). For example, a
spring with a range of 35 to 100 psig is more accurate than a spring
with a range of 90 to 200 psig. If you want to set your regulator at
100 psig, the 35 to 100 psig spring will provide better accuracy.
Practical Limits
While a lighter spring can reduce droop and improve accuracy,
using too light a spring can cause instability problems. Fortunately,
most of the work in spring selection is done by regulator
manufacturers. They determine spring rates that will provide good
performance for a given regulator, and publish these rates along
with other sizing information.
Diaphragm Area and Regulator Accuracy
Diaphragm Area
Until this point, we have assumed the diaphragm area to be
constant. In practice, the diaphragm area changes with travel.
We’re interested in this changing area because it has a major
inuence on accuracy and droop.
Diaphragms have convolutions in them so that they are exible
enough to move over a rated travel range. As they change position,
Principles of Direct-Operated Regulators
593
Te c h n i c a l
they also change shape because of the pressure applied to them.
Consider the example shown in Figure 7. As downstream pressure
(P2) drops, the diaphragm moves down. As it moves down, it
changes shape and diaphragm area increases because the centers of
the convolutions become further apart. The larger diaphragm area
magnies the effect of P2 so even less P2 is required to hold the
diaphragm in place. This is called diaphragm effect. The result is
decreased accuracy because incremental changes in P2 do not result
in corresponding changes in spring compression or disk position.
Increasing Diaphragm Area
To better understand the effects of changing diaphragm area, let’s
calculate the forces in the exaggerated example given in Figure 7.
First, assume that the regulator is in equilibrium with a downstream
pressure P2 of 10 psig. Also assume that the area of the diaphragm
in this position is 10 square inches. The diaphragm force (FD) is:
FD = (P2)(AD)
FD = (10 psi) (10 square inches)
FD = 100 pounds
Now assume that downstream pressure drops to 9 psig signaling the
need for increased ow. As the diaphragm moves, its area increases
to 11 square inches. The diaphragm force now produced is:
FD = (9 psi) (11 square inches)
FD = 99 pounds
The change in diaphragm area increases the regulator’s droop.
While it’s important to note that diaphragm effect contributes to
Figure 8. Critical Flow
droop, diaphragm sizes are generally determined by manufacturers
for different regulator types, so there is rarely a user option.
Diaphragm Size and Sensitivity
Also of interest is the fact that increasing diaphragm size can result
in increased sensitivity. A larger diaphragm area will produce
more force for a given change in P2. Therefore, larger diaphragms
are often used when measuring small changes in low-pressure
applications. Service regulators used in domestic gas service are
an example.
Restricting Element and Regulator Performance
Critical Flow
Although changing the orice size can increase capacity, a
regulator can pass only so much ow for a given orice size and
inlet pressure, no matter how much we improve the unit’s accuracy.
Shown in Figure 8, after the regulator is wide-open, reducing P2
does not result in higher ow. This area of the ow curve identies
critical ow. To increase the amount of ow through the regulator,
the owing uid must pass at higher and higher velocities. But, the
uid can only go so fast. Holding P1 constant while decreasing P2,
ow approaches a maximum which is the speed of sound in that
particular gas, or its sonic velocity. Sonic velocity depends on the
inlet pressure and temperature for the owing uid. Critical ow is
generally anticipated when downstream pressure (P2) approaches a
value that is less than or equal to one-half of inlet pressure (P1).
Figure 7. Changing Diaphragm Area
DIAPHRAGM EFFECT ON DROOP
A = 10 IN
2
F
D1
A = 11 IN
2
F
D2
WHEN P2 DROPS TO 9
FD = P2 x A
FD1 = 10 x 10 = 100 LB
FD2 = 9 x 11 = 99 LB
SETPOINT
WIDE-OPEN
FLOW
P
2
CRITICAL FLOW
Principles of Direct-Operated Regulators
594
Te c h n i c a l
Orifice Size and Capacity
One way to increase capacity is to increase the size of the orice.
The variable ow area between disk and orice depends directly
on orice diameter. Therefore, the disk will not have to travel
as far with a larger orice to establish the required regulator
ow rate, and droop is reduced. Sonic velocity is still a limiting
factor, but the ow rate at sonic velocity is greater because more
gas is passing through the larger orice.
Stated another way, a given change in P2 will produce a larger
change in ow rate with a larger orice than it would with a smaller
orice. However, there are denite limits to the size of orice that
can be used. Too large an orice makes the regulator more sensitive
to uctuating inlet pressures. If the regulator is overly sensitive, it
will have a tendency to become unstable and cycle.
Orifice Size and Stability
One condition that results from an oversized orice is known as
the “bathtub stopper” effect. As the disk gets very close to the
orice, the forces of uid ow tend to slam the disk into the orice
and shutoff ow. Downstream pressure drops and the disk opens.
This causes the regulator to cycle—open, closed, open, closed. By
selecting a smaller orice, the disk will operate farther away from
the orice so the regulator will be more stable.
Orifice Size, Lockup, and Wear
A larger orice size also requires a higher shutoff pressure,
or lockup pressure. In addition, an oversized orice usually
produces faster wear on the valve disk and orice because it
controls ow with the disk near the seat. This wear is accelerated
with high ow rates and when there is dirt or other erosive
material in the ow stream.
Orifice Guideline
Experience indicates that using the smallest possible orice is
generally the best rule-of-thumb for proper control and stability.
Increasing P1
Regulator capacity can be increased by increasing inlet pressure (P1).
Factors Affecting Regulator Accuracy
As we have seen, the design elements of a regulator—the spring,
diaphragm, and orice size—can affect its accuracy. Some
of these inherent limits can be overcome with changes to the
regulator design.
Performance Limits
The three curves in Figure 9 summarize the effects of spring rate,
diaphragm area, and orice size on the shape of the controlled
pressure-ow rate curve. Curve A is a reference curve representing
Figure 9. Increased Sensitivity
a typical regulator. Curve B represents the improved performance
from either increasing diaphragm area or decreasing spring rate.
Curve C represents the effect of increasing orice size. Note that
increased orice size also offers higher ow capabilities. But
remember that too large an orice size can produce problems that
will negate any gains in capacity.
Cycling
The sine wave in Figure 10 might be what we see if we increase
regulator sensitivity beyond certain limits. The sine wave indicates
instability and cycling.
Design Variations
All direct-operated regulators have performance limits that result
from droop. Some regulators are available with features designed
to overcome or minimize these limits.
FLOW
SETPOINT
A
B
C
P
2
Figure 10. Cycling
SETPOINT
TIME
P
2
FLOW INCREASED
Principles of Direct-Operated Regulators
595
Te c h n i c a l
E0908
Type HSR
Improving Regulator Accuracy with a Pitot Tube
In addition to the changes we can make to diaphragm area, spring
rate, orice size, and inlet pressure, we can also improve regulator
accuracy by adding a pitot tube as shown in Figure 11. Internal to
the regulator, the pitot tube connects the diaphragm casing with a
low-pressure, high velocity region within the regulator body. The
pressure at this area will be lower than P2 further downstream.
By using a pitot tube to measure the lower pressure, the regulator
will make more dramatic changes in response to any change in
P2. In other words, the pitot tube tricks the regulator, causing it to
respond more than it would otherwise.
Improving Performance with a Lever
The lever style regulator is a variation of the simple direct-operated
regulator. It operates in the same manner, except that it uses a lever
to gain mechanical advantage and provide a high shutoff force.
In earlier discussions, we noted that the use of a larger diaphragm
can result in increased sensitivity. This is because any change in P2
will result in a larger change in diaphragm force. The same result
is obtained by using a lever to multiply the force produced by the
diaphragm as shown in Figure 13.
The main advantage of lever designs is that they provide increased
force for lockup without the extra cost, size, and weight associated
with larger diaphragms, diaphragm casings, and associated parts.
Figure 11. Pitot Tube
P2 = 10 PSIGP1 = 100 PSIG
SENSE PRESSURE HERE
Decreased Droop (Boost)
The pitot tube offers one chief advantage for regulator accuracy, it
decreases droop. Shown in Figure 12, the diaphragm pressure, PD,
must drop just as low with a pitot tube as without to move the disk
far enough to supply the required ow. But the solid curve shows
that P2 does not decrease as much as it did without a pitot tube. In
fact, P2 may increase. This is called boost instead of droop. So
the use of a pitot tube, or similar device, can dramatically improve
droop characteristics of a regulator.
Figure 13. Lever Style Regulator
Figure 12. Performance with Pitot Tube
PRESSURE
SETPOINT
P
2
P
D
FLOW
P2 = DOWNSTREAM PRESSURE
PD = PRESSURE UNDER DIAPHRAGM
Numerical Example
For example, we’ll establish setpoint by placing a gauge
downstream and adjusting spring compression until the gauge
reads 10 psig for P2. Because of the pitot tube, the regulator might
actually be sensing a lower pressure. When P2 drops from 10 psig
to 9 psig, the pressure sensed by the pitot tube may drop from
8 psig to 6 psig. Therefore, the regulator opens further than it
would if it were sensing actual downstream pressure.
PIVOT POINT
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
E0908
Principles of Pilot-Operated Regulators
596
Te c h n i c a l
Pilot-Operated Regulator Basics
In the evolution of pressure regulator designs, the shortcomings of
the direct-operated regulator naturally led to attempts to improve
accuracy and capacity. A logical next step in regulator design is to
use what we know about regulator operation to explore a method
of increasing sensitivity that will improve all of the performance
criteria discussed.
Identifying Pilots
Analysis of pilot-operated regulators can be simplied by viewing
them as two independent regulators connected together. The
smaller of the two is generally the pilot.
Setpoint
We may think of the pilot as the “brains” of the system. Setpoint
and many performance variables are determined by the pilot. It
senses P2 directly and will continue to make changes in PL on
the main regulator until the system is in equilibrium. The main
regulator is the “muscle” of the system, and may be used to control
large ows and pressures.
Spring Action
Notice that the pilot uses a spring-open action as found in directoperated regulators. The main regulator, shown in Figure 1, uses
a spring-close action. The spring, rather than loading pressure, is
used to achieve shutoff. Increasing PL from the pilot onto the main
diaphragm opens the main regulator.
Pilot Advantage
Because the pilot is the controlling device, many of the
performance criteria we have discussed apply to the pilot. For
example, droop is determined mainly by the pilot. By using very
small pilot orices and light springs, droop can be made small.
Because of reduced droop, we will have greater usable capacity.
Pilot lockup determines the lockup characteristics for the system.
The main regulator spring provides tight shutoff whenever the pilot
is locked up.
Gain and Restrictions
Stability
Although increased gain (sensitivity) is often considered an
advantage, it also increases the gain of the entire pressure
regulator system. If the system gain is too high, it may become
unstable. In other words, the regulator might tend to oscillate;
over-reacting by continuously opening and closing. Pilot gain
can be modied to tune the regulator to the system. To provide
a means for changing gain, every pilot-operated regulator system
contains both a xed and a variable restriction. The relative size
of one restriction compared to the other can be varied to change
gain and speed of response.
Regulator Pilots
To improve the sensitivity of our regulator, we would like to be
able to sense P2 and then somehow make a change in loading
pressure (PL) that is greater than the change in P2. To accomplish
this, we can use a device called a pilot, or pressure amplier.
The major function of the pilot is to increase regulator sensitivity.
If we can sense a change in P2 and translate it into a larger change
in PL, our regulator will be more responsive (sensitive) to changes
in demand. In addition, we can signicantly reduce droop so its
effect on accuracy and capacity is minimized.
Gain
The amount of amplication supplied by the pilot is called
“gain”. To illustrate, a pilot with a gain of 20 will multiply
the effect of a 1 psi change on the main diaphragm by 20. For
example, a decrease in P2 opens the pilot to increase PL 20 times
as much.
Figure 1. Pilot-Operated Regulator
MAIN
REGULATOR
PILOT
REGULATOR
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
Principles of Pilot-Operated Regulators
597
Te c h n i c a l
Loading and Unloading Designs
A loading pilot-operated design (Figure 2), also called two-path
control, is so named because the action of the pilot loads PL onto
the main regulator measuring element. The variable restriction, or
pilot orice, opens to increase PL.
An unloading pilot-operated design (Figure 3) is so named because
the action of the pilot unloads PL from the main regulator.
PILOT
FLOW
LARGER FIXED
RESTRICTION
VARIABLE
RESTRICTION
P
2
P
1
P
L
P
2
TO MAIN
REGULATOR
Figure 2. Fixed Restrictions and Gain (Used on Two-Path Control Systems)
Restrictions, Response Time, and Gain
Consider the example shown in Figure 2 with a small xed
restriction. Decreasing P2 will result in pressure PL increasing.
Increasing P2 will result in a decrease in PL while PL bleeds out
through the small xed restriction.
If a larger xed restriction is used with a variable restriction, the
gain (sensitivity) is reduced. A larger decrease in P2 is required to
increase PL to the desired level because of the larger xed restriction.
Two-Path Control (Loading Design)
In two-path control systems (Figure 4), the pilot is piped so that
P2 is registered on the pilot diaphragm and on the main regulator
diaphragm at the same time. When downstream demand is
constant, P2 positions the pilot diaphragm so that ow through the
pilot will keep P2 and PL on the main regulator diaphragm. When
P2 changes, the force on top of the main regulator diaphragm
and on the bottom of the pilot diaphragm changes. As P2 acts on
the main diaphragm, it begins repositioning the main valve plug.
This immediate reaction to changes in P2 tends to make two-path
designs faster than other pilot-operated regulators. Simultaneously,
P2 acting on the pilot diaphragm repositions the pilot valve and
Figure 3. Unloading Systems
Figure 4. Two-Path Control
VARIABLE
RESTRICTION
FLOW
FIXED RESTRICTION
PILOT
FLOW
P
2
P
1
P
L
VARIABLE
RESTRICTION
SMALLER FIXED
RESTRICTION
P
2
TO MAIN
REGULATOR
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
PILOT
FLOW
FIXED
RESTRICTION
VARIABLE
RESTRICTION
P
2
P
1
P
L
P
2
TO MAIN
REGULATOR
Principles of Pilot-Operated Regulators
598
Te c h n i c a l
changes PL on the main regulator diaphragm. This adjustment to
PL accurately positions the main regulator valve plug. PL on the
main regulator diaphragm bleeds through a xed restriction until
the forces on both sides are in equilibrium. At that point, ow
through the regulator valve matches the downstream demand.
Two-Path Control Advantages
The primary advantages of two-path control are speed and
accuracy. These systems may limit droop to less than 1%. They
are well suited to systems with requirements for high accuracy,
large capacity, and a wide range of pressures.
Unloading Control
Unloading systems (Figure 5) locate the pilot so that P2 acts only
on the pilot diaphragm. P1 constantly loads under the regulator
diaphragm and has access to the top of the diaphragm through a
xed restriction.
When downstream demand is constant, the pilot valve is open
enough that PL holds the position of the main regulator diaphragm.
When downstream demand changes, P2 changes and the pilot
diaphragm reacts accordingly. The pilot valve adjusts PL to
reposition and hold the main regulator diaphragm.
Unloading Control Advantages
Unloading systems are not quite as fast as two-path systems,
and they can require higher differential pressures to operate.
However, they are simple and more economical, especially in
large regulators. Unloading control is used with popular elastomer
diaphragm style regulators. These regulators use a exible
membrane to throttle ow.
Performance Summary
Accuracy
Because of their high gain, pilot-operated regulators are extremely
accurate. Droop for a direct-operated regulator might be in the
range of 10 to 20 % whereas pilot-operated regulators are between
one and 3% with values under 1% possible.
Capacity
Pilot-operated designs provide high capacity for two reasons.
First, we have shown that capacity is related to droop. And
because droop can be made very small by using a pilot, capacity is
increased. In addition, the pilot becomes the “brains” of the system
and controls a larger, sometimes much larger, main regulator. This
also allows increased ow capabilities.
DOWNSTREAM PRESSURE (P
2
)
HIGH CAPACITY
FLOW RATE
MINIMAL
DROOP
Figure 6. Pilot-Operated Regulator Performance Figure 5. Unloading Control
MAIN REGULATOR
DIAPHRAGM
FIXED
RESTRICTION
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
Principles of Pilot-Operated Regulators
599
Te c h n i c a l
Figure 8. Type 99, Typical Two-Path Control
with Integrally Mounted Pilot
Lockup
The lockup characteristics for a pilot-operated regulator are the
lockup characteristics of the pilot. Therefore, with small orices,
lockup pressures can be small.
Applications
Pilot-operated regulators should be considered whenever accuracy,
capacity, and/or high pressure are important selection criteria.
They can often be applied to high capacity services with greater
economy than a control valve and actuator with controller.
Two-Path Control
In some designs (Figure 7), the pilot and main regulator are
separate components. In others (Figure 8), the system is integrated
into a single package. All, however, follow the basic design
concepts discussed earlier.
Type 1098-EGR
The schematic in Figure 7 illustrates the Type 1098-EGR
regulator’s operation. It can be viewed as a model for all twopath, pilot-operated regulators. The pilot is simply a sensitive
direct-operated regulator used to send loading pressure to the main
regulator diaphragm.
Identify the inlet pressure (P1). Find the downstream pressure (P2).
Follow it to where it opposes the loading pressure on the main
regulator diaphragm. Then, trace P2 back to where it opposes the
control spring in the pilot. Finally, locate the route of P2 between
the pilot and the regulator diaphragm.
Changes in P2 register on the pilot and main regulator diaphragms
at the same time. As P2 acts on the main diaphragm, it begins
repositioning the main valve plug. Simultaneously, P2 acting on
the pilot diaphragm repositions the pilot valve and changes PL on
the main regulator diaphragm. This adjustment in PL accurately
positions the main regulator valve plug.
Figure 7. Type 1098-EGR, Typical Two-Path Control
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
Type 1098-EGR
A6563
A6469
Principles of Pilot-Operated Regulators
600
Te c h n i c a l
As downstream demand is met, P2 rises. Because P2 acts directly
on both the pilot and main regulator diaphragms, this design
provides fast response.
Type 99
The schematic in Figure 8 illustrates another typical two-path
control design, the Type 99. The difference between the
Type 1098-EGR and the Type 99 is the integrally mounted pilot
of the Type 99.
The pilot diaphragm measures P2. When P2 falls below the pilot
setpoint, the diaphragm moves away from the pilot orice and
allows loading pressure to increase. This loads the top of the
main regulator diaphragm and strokes the main regulator valve
open further.
Unloading Design
Unloading designs incorporate a molded composition diaphragm
that serves as the combined loading and restricting component of
the main regulator. Full upstream pressure (P1) is used to load the
regulator diaphragm when it is seated. The regulator shown in
Figure 9 incorporates an elastomeric valve closure member.
Figure 9. Type EZR, Unloading Design
Unloading regulator designs are slower than two-path control
systems because the pilot must rst react to changes in P2 before
the main regulator valve moves. Recall that in two-path designs,
the pilot and main regulator diaphragms react simultaneously.
P1 passes through a xed restriction and lls the space above the
regulator diaphragm. This xed restriction can be adjusted to
increase or decrease regulator gain. P1 also lls the cavity below
the regulator diaphragm. Because the surface area on the top side
of the diaphragm is larger than the area exposed to P1 below, the
diaphragm is forced down against the cage to close the regulator.
When downstream demand increases, the pilot opens. When the
pilot opens, regulator loading pressure escapes downstream much
faster than P1 can bleed through the xed restriction. As pressure
above the regulator diaphragm decreases, P1 forces the diaphragm
away from its seat.
When downstream demand is reduced, P2 increases until it’s
high enough to compress the pilot spring and close the pilot
valve. As the pilot valve closes, P1 continues to pass through the
xed restriction and ows into the area above the main regulator
diaphragm. This loading pressure, PL, forces the diaphragm back
toward the cage, reducing ow through the regulator.
INLET PRESSURE, P
1
OUTLET PRESSURE, P
2
ATMOSPHERIC PRESSURE
LOADING PRESSURE, P
L
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