Emerson Fisher Vee-Ball V150E Reference Manual

CONTROL VALVE

SOURCEBOOK

PULP & PAPER

Copyright © 2011 Fisher Controls International LLC
All Rights Reserved.

Fisher, ENVIRO-SEAL, Whisper Trim, Cavitrol, WhisperFlo, Vee‐Ball, Control‐Disk, NotchFlo, easy‐e and FIELDVUE are marks
owned by Fisher Controls International LLC, a business of Emerson Process Management. The Emerson logo is a trademark and
service mark of Emerson Electric Co. All other marks are the property of their respective owners.

This publication may not be reproduced, stored in a retrieval system, or transmitted in whole or in part, in any form or by any means,
electronic, mechanical, photocopying, recording or otherwise, without the written permission of Fisher Controls International LLC.

Printed in U.S.A., First Edition

Table of Contents

Introduction v

Chapter 1 Control Valve Selection 1-1

Chapter 2 Actuator Selection 2-1

Chapter 3 Liquid Valve Sizing 3-1

Chapter 4 Cavitation & Flashing 4-1

Chapter 5 Gas Valve Sizing 5-1

Chapter 6 Control Valve Noise 6-1

Chapter 7 Steam Conditioning 7-1

Chapter 8 Process Overview 8-1

Chapter 9 Pulping 9-1

Chapter 10A Batch Digesters 10A-1

Chapter 10B Continuous Digesters 10B-1

Chapter 11 Black Liquor Evaporators/Concentrators 11-1

Chapter 12 Kraft Recovery Boiler 12-1

Chapter 13 Recausticizing & Lime Recovery 13-1

Chapter 14 Bleaching & Brightening 14-1

Chapter 15 Stock Preparation 15-1

Chapter 16 Wet End Chemistry 16-1

Chapter 17 Paper Machine 17-1

Chapter 18 Power & Recovery Boiler 18-1

iii

iv

Pulp and Paper Control Valves

Introduction

This sourcebook’s intent is to introduce a pulp
and paper mill’s processes, as well as the use of
control valves in many of the processes found in
the mill. It is intended to help you:

D Understand pulp and paper processes
D Learn where control valves are typically

located within each process

D Identify valves commonly used for specific

applications

D Identify troublesome/problem valves within

the process
The information provided will follow a standard

format of:

D Description of the process
D Functional drawing of the process
D FisherR valves to be considered in each

process and their associated function

Control Valves

Valves described within a chapter are labeled
and numbered corresponding to the identification
used in the proces s flow chart for that chapter.
Their valve function is described, and a
specification section gives added information on
process conditions, names of Fisher valves that
may be considered, process impact of the valve,
and any special considerations for the process
and valve(s) of choice.

Process Drawings

The process drawings within each chapter show
major equipment items, their typical placement
within the processing system, and process flow
direction. Utilities and pumps are not shown
unless otherwise stated.

Many original equipment manufacturers (OEMs)
provide equipment to the pulp and paper
industry, each with their own processes and
proprietary information. Process drawings are
based on general equipment configurations
unless otherwise stated.

D Impacts and/or considerations for

troublesome/problem valves

Valve Selection

The information presented in this sourcebook is
intended to assist in understanding the control
valve requirements of general pulp and paper
mill’s processes.

Since every mill is different in technology and
layout, the control valve requirements and
recommendations presented by this sourcebook
should be considered as general guidelines.
Under no circumstances should this information
alone be used to select a control valve without
ensuring the proper valve construction is identified
for the application and process conditions.

All valve considerations should be reviewed by the
local business representative as part of any valve
selection or specification activity.

Problem Valves

Often there are references to valve-caused
problems or difficulties. The list of problems
include valve erosion from process media,
stickiness caused by excessive friction (stiction),
excessive play in valve to actuator linkages
(typically found in rotary valves) that causes
deadband, excessive valve stem packing
leakage, and valve materials that are
incompatible with the flowing medium. Any one,
or a combination of these difficulties, may affect
process quality and throughput with a resulting
negative impact on mill profitability.

Many of these problems can be avoided or
minimized through proper valve selection.
Consideration should be given to valve style and
size, actuator capabilities, analog versus digital
instrumentation, materials of construction, etc.
Although not being all-inclusive, the information
found in this sourcebook should facilitate the
valve selection process.

v

vi

Chapter 1

Control Valve Selection

In the past, a customer simply requested a control
valve and the manufacturer offered the product
best-suited for the job. The choices among the
manufacturers were always dependent upon
obvious matters such as cost, delivery, vendor
relationships, and user preference. However,
accurate control valve selection can be
considerably more complex, especially for
engineers with limited experience or those who
have not kept up with changes in the control valve
industry.

An assortment of sliding-stem and rotary valve
styles are available for many applications. Some
are touted as “universal” valves for almost any
size and service, while others are claimed to be
optimum solutions for narrowly defined needs.
Even the most knowledgeable user may wonder
whether they are really getting the most for their
money in the control valves they have specified.

Like most decisions, selection of a control valve
involves a great number of variables; the everyday
selection process tends to overlook a number of
these important variables. The following
discussion includes categorization of available
valve types and a set of criteria to be considered in
the selection process.

What Is A Control Valve?

Process plants consist of hundreds, or even
thousands, of control loops all networked together
to produce a product to be offered for sale. Each
of these control loops is designed to control a
critical process variable such as pressure, flow,
level, temperature, etc., within a required operating
range to ensure the quality of the end-product.

These loops receive, and internally create,
disturbances that detrimentally affect the process
variable. Interaction from other loops in the
network provides disturbances that influence the
process variable. To reduce the effect of these
load disturbances, sensors and transmitters collect
information regarding the process variable and its
relationship to a desired set point. A controller then
processes this information and decides what must
occur in order to get the process variable back to
where it should be after a load disturbance occurs.
When all measuring, comparing, and calculating
are complete, the strategy selected by the
controller is implemented via some type of final
control element. The most common final control
element in the process control industries is the
control valve.

A control valve manipulates a flowing fluid such as
gas, steam, water, or chemical compounds to
compensate for the load disturbance and keep the
regulated process variable as close as possible to
the desired set point.

Many people who speak of “control valves” are
actually referring to “control valve assemblies.”
The control valve assembly typically consists of
the valve body, the internal trim parts, an actuator
to provide the motive power to operate the valve,
and a variety of additional valve accessories,
which may include positioners, transducers, supply
pressure regulators, manual operators, snubbers,
or limit switches.

It is best to think of a control loop as an
instrumentation chain. Like any other chain, the
entire chain is only as good as its weakest link. It
is important to ensure that the control valve is not
the weakest link.

www.Fisher.com

Valve Types and Characteristics

The control valve regulates the rate of fluid flow as
the position of the valve plug or disk is changed by
force from the actuator. To do this, the valve must:

D Contain the fluid without external leakage.
D Have adequate capacity for the intended

service.

D Be capable of withstanding the erosive,
corrosive, and temperature influences of the
process.

D Incorporate appropriate end connections to
mate with adjacent pipelines and actuator
attachment means to permit transmission of
actuator thrust to the valve plug stem or rotary
shaft.

Many styles of control valve bodies have been
developed. Some can be used effectively in a
number of applications while others meet specific
service demands or conditions and are used less
frequently. The subsequent text describes popular
control valve body styles utilized today.

Globe Valves

Single-Port Valve Bodies

Single-port is the most common valve body style
and is simple in construction. Single-port valves
are available in various forms, such as globe,
angle, bar stock, forged, and split constructions.
Generally, single-port valves are specified for
applications with stringent shutoff requirements.
They use metal-to-metal seating surfaces or
soft-seating with PTFE or other composition
materials forming the seal.

W7027-1

Figure 1-1. Single-Ported Globe-Style Valve

Body

characteristics. Retainer-style trim also offers ease
of maintenance with flow characteristics altered by
changing the plug. Cage or retainer-style
single-seated valve bodies can also be easily
modified by a change of trim parts to provide
reduced-capacity flow, noise attenuation, or
cavitation eliminating or reducing trim (see
chapter 4).

Figure 1-1 shows one of the more popular styles of
single-ported or single-seated globe valve bodies.
They are widely used in process control
applications, particularly in sizes NPS 1 through
NPS 4. Normal flow direction is most often flow-up
through the seat ring.

Angle valves are nearly always single ported, as
shown in figure 1-2. This valve has cage-style trim
construction. Others might have screwed-in seat
rings, expanded outlet connections, restricted trim,
and outlet liners for reduction of erosion damage.

Single-port valves can handle most service
requirements. Because high pressure fluid is
normally loading the entire area of the port, the
unbalance force created must be considered when
selecting actuators for single-port control valve
bodies. Although most popular in the smaller
sizes, single-port valves can often be used in NPS
4 to 8 with high thrust actuators.

Many modern single-seated valve bodies use cage
or retainer-style construction to retain the seat ring
cage, provide valve plug guiding, and provide a
means for establishing particular valve flow

1−2

Bar stock valve bodies are often specified for
corrosive applications in the chemical industry
(figure 1-3), but may also be requested in other
low flow corrosive applications. They can be
machined from any metallic bar-stock material and
from some plastics. When exotic metal alloys are
required for corrosion resistance, a bar-stock valve
body is normally less expensive than a valve body
produced from a casting.

High pressure single-ported globe valves are often
found in power plants due to high pressure steam
(figure 1-4). Variations available include

W0971

Figure 1-2. Flanged Angle-Style

Control Valve Body

W0540

Figure 1-4. High Pressure Globe-Style

Control Valve Body

W9756

Figure 1-3. Bar Stock Valve Body

cage-guided trim, bolted body-to-bonnet
connection, and others. Flanged versions are
available with ratings to Class 2500.

Balanced-Plug Cage-Style Valve
Bodies

This popular valve body style, single-ported in the
sense that only one seat ring is used, provides the
advantages of a balanced valve plug often

W0992-4

Figure 1-5. Valve Body with Cage-Style Trim,

Balanced Valve Plug, and Soft Seat

associated only with double-ported valve bodies
(figure 1-5). Cage-style trim provides valve plug
guiding, seat ring retention, and flow
characterization. In addition, a sliding piston
ring-type seal between the upper portion of the
valve plug and the wall of the cage cylinder
virtually eliminates leakage of the upstream high
pressure fluid into the lower pressure downstream
system.

1−3

W0997

Figure 1-6. High Capacity Valve Body with

Cage-Style Noise Abatement Trim

liquid service. The flow direction depends upon the
intended service and trim selection, with
unbalanced constructions normally flow-up and
balanced constructions normally flow-down.

Port-Guided Single-Port Valve Bodies

D Usually limited to 150 psi (10 bar) maximum

pressure drop.

D Susceptible to velocity-induced vibration.
D Typically provided with screwed in seat rings

which might be difficult to remove after use.

Three-Way Valve Bodies

D Provide general converging (flow-mixing) or

diverging (flow-splitting) service.

D Best designs use cage-style trim for positive

valve plug guiding and ease of maintenance.

Downstream pressure acts upon both the top and
bottom sides of the valve plug, thereby nullifying
most of the static unbalance force. Reduced
unbalance permits operation of the valve with
smaller actuators than those necessary for
conventional single-ported valve bodies.

Interchangeability of trim permits the choice of
several flow characteristics or of noise attenuation
or anticavitation components. For most available
trim designs, the standard direction of flow is in
through the cage openings and down through the
seat ring. These are available in various material
combinations, sizes through NPS 20, and pressure
ratings to Class 2500.

High Capacity, Cage-Guided Valve
Bodies

This adaptation of the cage-guided bodies
mentioned above was designed for noise
applications, such as high pressure power plants,
where sonic steam velocities are often
encountered at the outlet of conventional valve
bodies (figure 1-6).

The design incorporates oversized end
connections with a streamlined flow path and the
ease of trim maintenance inherent with cage-style
constructions. Use of noise abatement trim
reduces overall noise levels by as much as 35
decibels. The design is also available in cageless
versions with a bolted seat ring, end connection
sizes through NPS 20, Class 600, and versions for

D Variations include trim materials selected for
high temperature service. Standard end
connections (flanged, screwed, butt weld, etc.) can
be specified to mate with most any piping scheme.

D Actuator selection demands careful
consideration, particularly for constructions with
unbalanced valve plug.

A balanced valve plug style three-way valve body
is shown with the cylindrical valve plug in the down
position (figure 1-7). This position opens the
bottom common port to the right-hand port and
shuts off the left-hand port. The construction can
be used for throttling mid-travel position control of
either converging or diverging fluids.

Rotary Valves

Traditional Butterfly Valve

Standard butterfly valves are available in sizes
through NPS 72 for miscellaneous control valve
applications. Smaller sizes can use versions of
traditional diaphragm or piston pneumatic
actuators, including the modern rotary actuator
styles. Larger sizes might require high output
electric or long-stroke pneumatic cylinder
actuators.

Butterfly valves exhibit an approximately equal
percentage flow characteristic. They can be used

1−4

W8380

W9045-1

Figure 1-7. Three Way Valve with

Balanced Valve Plug

W4641

Figure 1-8. High-Performance Butterfly

Control Valve

for throttling service or for on-off control. Soft-seat
constructions can be obtained by utilizing a liner or
by including an adjustable soft ring in the body or
on the face of the disk.

Figure 1-9. Eccentric-Disk Rotary-Shaft

Control Valve

D Offer an economic advantage, particularly in
larger sizes and in terms of flow capacity per dollar
investment.

D Mate with standard raised-face pipeline
flanges.

D Depending on size, might require high output
or oversized actuators due to valve size valves or
large operating torques from large pressure drops.

D Standard liner can provide precise shutoff
and quality corrosion protection with nitrile or
PTFE liner.

Eccentric-Disk Control Valve

Eccentric disk rotary control valves are intended
for general service applications not requiring
precision throttling control. They are frequently
applied in applications requiring large sizes and
high temperatures due to their lower cost relative
to other styles of control valves. The control range
for this style of valve is approximately one third as
large as a ball or globe-style valves.
Consequently, additional care is required in sizing
and applying this style of valve to eliminate control
problems associated with process load changes.
They are well-suited for constant process load
applications.

D Provide effective throttling control.

D Require minimum space for installation

(figure 1-8).

D Provide high capacity with low pressure loss

through the valves.

D Linear flow characteristic through 90 degrees
of disk rotation (figure 1-9).

D Eccentric mounting of disk pulls it away from
the seal after it begins to open, minimizing seal
wear.

1−5

W9425 W9418

WAFER STYLE SINGLE FLANGE STYLE

Figure 1-10. Fisher Control-Disk Valve with 2052 Actuator and FIELDVUE DVC6200 Digital Valve Controller

D Bodies are available in sizes through NPS 24

compatible with standard ASME flanges.

D Utilize standard pneumatic diaphragm or

piston rotary actuators.

D Standard flow direction is dependent upon
seal design; reverse flow results in reduced
capacity.

Control-Disk Valve

The Control-Diskt valve (figure 1-10) offers
excellent throttling performance, while maintaining
the size (face-to-face) of a traditional butterfly
valve. The Control-Disk valve is first in class in
controllability, rangeability, and tight shutoff, and it
is designed to meet worldwide standards.

D Utilizes a contoured edge and unique
patented disk to provide an improved control range
of 15 - 70% of valve travel. Traditional butterfly

lever design to increase torque range within each
actuator size.

valves are typically limited to 25% - 50% control
range.

V-notch Ball Control Valve

D Includes a tested valve sealing design,
available in both metal and soft seats, to provide
an unmatched cycle life while still maintaining
excellent shutoff

D Spring loaded shaft positions disk against the
inboard bearing nearest the actuator allowing for
the disk to close in the same position in the seal,
and allows for either horizontal or vertical
mounting.

D Complimenting actuator comes in three,
compact sizes, has nested springs and a patented

This construction is similar to a conventional ball
valve, but with patented, contoured V-notch in the
ball (figure 1-11). The V-notch produces an
equal-percentage flow characteristic. These
control valves provide precise rangeability, control,
and tight shutoff.

pressure drop.

erosive or viscous fluids, paper stock, or other
slurries containing entrained solids or fibers.

W8172-2

Figure 1-11. Rotary-Shaft Control Valve

with V-Notch Ball

D Straight-through flow design produces little

D Bodies are suited to provide control of

1−6

on-off operation. The flanged or flangeless valves
feature streamlined flow passages and rugged
metal-trim components for dependable service in
slurry applications.

W4170-4

Figure 1-12. Sectional of Eccentric-Plug

Control Valve Body

D They utilize standard diaphragm or piston
rotary actuators.

D Ball remains in contact with seal during
rotation, which produces a shearing effect as the
ball closes and minimizes clogging.

D Bodies are available with either heavy-duty or
PTFE-filled composition ball seal ring to provide
excellent rangeability in excess of 300:1.

D Bodies are available in flangeless or
flanged-body end connections. Both flanged and
flangeless valves mate with Class 150, 300, or 600
flanges or DIN flanges.

D Valves are capable of energy absorbing
special attenuating trim to provide improved
performance for demanding applications.

Eccentric-Plug Control Valve

Control Valve End Connections

The three common methods of installing control
valves in pipelines are by means of:

D Screwed pipe threads
D Bolted gasketed flanges
D Welded end connections

Screwed Pipe Threads

Screwed end connections, popular in small control
valves, are typically more economical than flanged
ends. The threads usually specified are tapered
female National Pipe Thread (NPT) on the valve
body. They form a metal-to-metal seal by wedging
over the mating male threads on the pipeline ends.
This connection style, usually limited to valves not
larger than NPS 2, is not recommended for
elevated temperature service. Valve maintenance
might be complicated by screwed end connections
if it is necessary to take the body out of the
pipeline. This is because the valve cannot be
removed without breaking a flanged joint or union
connection to permit unscrewing the valve body
from the pipeline.

D Valve assembly combats erosion. The
rugged body and trim design handle temperatures
to 800°F (427°C) and shutoff pressure drops to
1500 psi (103 bar).

D Path of eccentric plug minimizes contact with
the seat ring when opening, thus reducing seat
wear and friction, prolonging seat life, and
improving throttling performance (figure 1-12).

D Self-centering seat ring and rugged plug
allow forward or reverse-flow with tight shutoff in
either direction. Plug, seat ring, and retainer are
available in hardened materials, including
ceramics, for selection of erosion resistance.

D Designs offering a segmented V-notch ball in
place of the plug for higher capacity requirements
are available.

This style of rotary control valve is well-suited for
control of erosive, coking, and other
hard-to-handle fluids, providing either throttling or

Bolted Gasketed Flanges

Flanged end valves are easily removed from the
piping and are suitable for use through the range
of working pressures for which most control valves
are manufactured (figure 1-13). Flanged end
connections can be used in a temperature range
from absolute zero to approximately 1500°F
(815°C). They are used on all valve sizes. The
most common flanged end connections include
flat-face, raised-face, and ring-type joint.

The flat face variety allows the matching flanges to
be in full-face contact with the gasket clamped
between them. This construction is commonly
used in low pressure, cast iron, and brass valves,
and minimizes flange stresses caused by initial
bolting-up force.

The raised-face flange features a circular
raised-face with the inside diameter the same as
the valve opening, and the outside diameter less
than the bolt circle diameter. The raised-face is

1−7

or Monelt, but is available in almost any metal.
This makes an excellent joint at high pressures
and is used up to 15,000 psig (1034 bar),
however, it is generally not used at high
temperatures. It is furnished only on steel and
alloy valve bodies when specified.

Welding End Connections

Welding ends on control valves (figure 1-14) are
leak-tight at all pressures and temperatures, and
are economical in first cost. Welding end valves
are more difficult to take from the line and are
limited to weldable materials. Welding ends come
in two styles:

D Socket welding

A7098

Figure 1-13. Popular Varieties of

Bolted Flange Connections

A7099

Figure 1-14. Common Welded End Connections

finished with concentric circular grooves for
precise sealing and resistance to gasket blowout.
This kind of flange is used with a variety of gasket
materials and flange materials for pressures
through the 6000 psig (414 bar) pressure range
and for temperatures through 1500°F (815°C).
This style of flanging is normally standard on Class
250 cast iron bodies and all steel and alloy steel
bodies.

The ring-type joint flange is similar in looks to the
raised-face flange except that a U-shaped groove
is cut in the raised-face concentric with the valve
opening. The gasket consists of a metal ring with
either an elliptical or octagonal cross-section.
When the flange bolts are tightened, the gasket is
wedged into the groove of the mating flange and a
tight seal is made. The gasket is generally soft iron

D Buttwelding

The socket welding ends are prepared by boring in
a socket at each end of the valve with an inside
diameter slightly larger than the pipe outside
diameter. The pipe slips into the socket where it
butts against a shoulder and then joins to the valve
with a fillet weld. Socket welding ends in a given
size are dimensionally the same regardless of pipe
schedule. They are usually furnished in sizes
through NPS 2.

The buttwelding ends are prepared by beveling
each end of the valve to match a similar bevel on
the pipe. The two ends are then butted to the
pipeline and joined with a full penetration weld.
This type of joint is used on all valve styles and the
end preparation must be different for each
schedule of pipe. These are generally furnished for
control valves in NPS 2-1/2 and larger. Care must
be exercised when welding valve bodies in the
pipeline to prevent excessive heat transmitted to
valve trim parts. Trims with low-temperature
composition materials must be removed before
welding.

Valve Body Bonnets

The bonnet of a control valve is the part of the
body assembly through which the valve plug stem
or rotary shaft moves. On globe or angle bodies, it
is the pressure retaining component for one end of
the valve body. The bonnet normally provides a
means of mounting the actuator to the body and
houses the packing box. Generally, rotary valves
do not have bonnets. (On some rotary-shaft
valves, the packing is housed within an extension
of the valve body itself, or the packing box is a
separate component bolted between the valve
body and bonnet.)

1−8

W0989

Figure 1-15. Typical Bonnet, Flange,

and Stud Bolts

guides the valve plug to ensure proper valve plug
stem alignment with the packing.

As mentioned previously, the conventional bonnet
on a globe-type control valve houses the packing.
The packing is most often retained by a packing
follower held in place by a flange on the yoke boss
area of the bonnet (figure 1-15). An alternate
packing retention means is where the packing
follower is held in place by a screwed gland (figure
1-3). This alternate is compact, thus, it is often
used on small control valves, however, the user
cannot always be sure of thread engagement.
Therefore, caution should be used if adjusting the
packing compression when the control valve is in
service.

Most bolted-flange bonnets have an area on the
side of the packing box which can be drilled and
tapped. This opening is closed with a standard
pipe plug unless one of the following conditions
exists:

D It is necessary to purge the valve body and
bonnet of process fluid, in which case the opening
can be used as a purge connection.

On a typical globe-style control valve body, the
bonnet is made of the same material as the valve
body or is an equivalent forged material because it
is a pressure-containing member subject to the
same temperature and corrosion effects as the
body. Several styles of valve body-to-bonnet
connections are illustrated. The most common is
the bolted flange type shown in figure 1-15. A
bonnet with an integral flange is also illustrated in
figure 1-15. Figure 1-3 illustrates a bonnet with a
separable, slip-on flange held in place with a split
ring. The bonnet used on the high pressure globe
valve body illustrated in figure 1-4, is screwed into
the valve body. Figure 1-8 illustrates a rotary-shaft
control valve in which the packing is housed within
the valve body and a bonnet is not used. The
actuator linkage housing is not a pressure­containing part and is intended to enclose the
linkage for safety and environmental protection.

On control valve bodies with cage- or retainer-style
trim, the bonnet furnishes loading force to prevent
leakage between the bonnet flange and the valve
body, and also between the seat ring and the
valve body. The tightening of the body-bonnet
bolting compresses a flat sheet gasket to seal the
body-bonnet joint, compresses a spiral-wound
gasket on top of the cage, and compresses an
additional flat sheet gasket below the seat ring to
provide the seat ring-body seal. The bonnet also
provides alignment for the cage, which, in turn,

D The bonnet opening is being used to detect
leakage from the first set of packing or from a
failed bellows seal.

Extension Bonnets

Extension bonnets are used for either high or low
temperature service to protect valve stem packing
from extreme process temperatures. Standard
PTFE valve stem packing is useful for most
applications up to 450°F (232°C). However, it is
susceptible to damage at low process
temperatures if frost forms on the valve stem. The
frost crystals can cut grooves in the PTFE, thus,
forming leakage paths for process fluid along the
stem. Extension bonnets remove the packing box
of the bonnet far enough from the extreme
temperature of the process that the packing
temperature remains within the recommended
range.

Extension bonnets are either cast (figure 1-16) or
fabricated (figure 1-17). Cast extensions offer
better high temperature service because of greater
heat emissivity, which provides better cooling
effect. Conversely, smooth surfaces that can be
fabricated from stainless steel tubing are preferred
for cold service because heat influx is usually the
major concern. In either case, extension wall
thickness should be minimized to cut down heat
transfer. Stainless steel is usually preferable to

1−9

W0667-2

Figure 1-16. Extension Bonnet

W1416

Figure 1-17. Valve Body with

Fabricated Extension Bonnet

W6434

Figure 1-18. ENVIRO-SEALt Bellows

Seal Bonnet

Bellows Seal Bonnets

Bellows seal bonnets (figure 1-18) are used when
no leakage (less than 1x10−6 cc/sec of helium)
along the stem can be tolerated. They are often
used when the process fluid is toxic, volatile,
radioactive, or highly expensive. This special
bonnet construction protects both the stem and the
valve packing from contact with the process fluid.
Standard or environmental packing box
constructions above the bellows seal unit will
prevent catastrophic failure in case of rupture or
failure of the bellows.

As with other control valve pressure/ temperature
limitations, these pressure ratings decrease with
increasing temperature. Selection of a bellows
seal design should be carefully considered, and
particular attention should be paid to proper
inspection and maintenance after installation. The
bellows material should be carefully considered to
ensure the maximum cycle life.

Two types of bellows seal designs are used for
control valves:

D Mechanically formed as shown in figure 1-19

carbon steel because of its lower coefficient of
thermal conductivity. On cold service applications,
insulation can be added around the extension to
protect further against heat influx.

1−10

D Welded leaf bellows as shown in figure 1-20
The welded-leaf design offers a shorter total

package height. Due to its method of manufacture
and inherent design, service life may be limited.

B2565

Figure 1-21. Comprehensive Packing Material Arrangements

for Globe-Style Valve Bodies

Control Valve Packing

Most control valves use packing boxes with the
packing retained and adjusted by a flange and
stud bolts (figure 1-27). Several packing materials
can be used depending upon the service
conditions expected and whether the application
requires compliance to environmental regulations.
Brief descriptions and service condition guidelines
for several popular materials and typical packing
material arrangements are shown in figure 1-21.

A5954

Figure 1-19. Mechanically Formed Bellows

A5955

Figure 1-20. Welded Leaf Bellows

The mechanically formed bellows is taller in
comparison and is produced with a more
repeatable manufacturing process.

PTFE V-Ring

D Plastic material with inherent ability to
minimize friction.

D Molded in V-shaped rings that are spring
loaded and self-adjusting in the packing box.
Packing lubrication not required.

D Resistant to most known chemicals except
molten alkali metals.

D Requires extremely smooth (2 to 4
micro-inches RMS) stem finish to seal properly.
Will leak if stem or packing surface is damaged.

D Recommended temperature limits: −40°F to
+450°F (−40°C to +232°C)

D Not suitable for nuclear service because
PTFE is easily destroyed by radiation.

1−11

B2566

Figure 1-22. Measurement Frequency for Valves

Controlling Volatile Organic Chemicals (VOC)

Laminated and Filament Graphite

D Suitable for high temperature nuclear service
or where low chloride content is desirable (Grade
GTN).

D Provides leak-free operation, high thermal
conductivity, and long service life, but produces
high stem friction and resultant hysteresis.

D Impervious to most hard-to-handle fluids and
high radiation.

D Suitable temperature range: Cryogenic
temperatures to 1200°F (649°C).

D Lubrication not required, but an extension
bonnet or steel yoke should be used when packing
box temperature exceeds 800°F (427°C).

USA Regulatory Requirements for
Fugitive Emissions

Fugitive emissions are non-point source volatile
organic emissions that result from process
equipment leaks. Equipment leaks in the United
States have been estimated at over 400 million
pounds per year. Strict government regulations,
developed by the US, dictate Leak Detection and
Repair (LDAR) programs. Valves and pumps have
been identified as key sources of fugitive
emissions. In the case of valves, this is the

leakage to atmosphere due to packing seal or
gasket failures.

The LDAR programs require industry to monitor all
valves (control and noncontrol) at an interval that
is determined by the percentage of valves found to
be leaking above a threshold level of 500 ppmv
(some cities use a 100 ppmv criteria). This
leakage level is so slight you cannot see or hear it.
The use of sophisticated portable monitoring
equipment is required for detection. Detection
occurs by sniffing the valve packing area for
leakage using an Environmental Protection
Agency (EPA) protocol. This is a costly and
burdensome process for industry.

The regulations do allow for the extension of the
monitoring period for up to one year if the facility
can demonstrate an extremely low ongoing
percentage of leaking valves (less than 0.5% of
the total valve population). The opportunity to
extend the measurement frequency is shown in
figure 1-22.

Packing systems designed for extremely low
leakage requirements also extend packing seal life
and performance to support an annual monitoring
objective. The ENVIRO-SEALt packing system is
one example. Its enhanced seals incorporate four
key design principles including:

D Containment of the pliable seal material

through an anti-extrusion component.

1−12

D Proper alignment of the valve stem or shaft
within the bonnet bore.

D Applying a constant packing stress through
Belleville springs.

D Minimizing the number of seal rings to reduce
consolidation, friction, and thermal expansion.

The traditional valve selection process meant
choosing a valve design based upon its pressure
and temperature capabilities as well as its flow
characteristics and material compatibility. Valve
stem packing used in the valve was determined
primarily by the operating temperature in the
packing box area. The available material choices
included PTFE for temperatures below 93°C
(200°F) and graphite for higher temperature
applications.

Today, choosing a valve packing system has
become much more complex due to the number of
considerations one must take into account. For
example, emissions control requirements, such as
those imposed by the Clean Air Act within the
United States and by other regulatory bodies,
place tighter restrictions on sealing performance.
Constant demands for improved process output
mean that the valve packing system must not
hinder valve performance. Also, today’s trend
toward extended maintenance schedules dictates
that valve packing systems provide the required
sealing over longer periods.

In addition, end user specifications that have
become de facto standards, as well as standards
organizations specifications, are used by
customers to place stringent fugitive emissions
leakage requirements and testing guidelines on
process control equipment vendors. Emerson
Process Management and its observance of
limiting fugitive emissions is evident by its reliable
valve sealing (packing and gasket) technologies,
global emissions testing procedures, and
emissions compliance approvals.

A6161-1

Figure 1-23. Single PTFE V-Ring Packing

Single PTFE V-Ring Packing (Fig.
1-23)

The single PTFE V-ring arrangement uses a coil
spring between the packing and packing follower.
It meets the 100 ppmv criteria, assuming that the
pressure does not exceed 20.7 bar (300 psi) and
the temperature is between −18°C and 93°C (0°F
and 200°F). It offers excellent sealing performance
with the lowest operating friction.

ENVIRO-SEAL PTFE Packing
(Fig. 1-24)

The ENVIRO-SEAL PTFE packing system is an
advanced packing method that utilizes a compact,
live-load spring design suited to environmental
applications up to 51.7 bar and 232°C (750 psi
and 450°F). While it most typically is thought of as
an emission-reducing packing system,
ENVIRO-SEAL PTFE packing is, also, well-suited
for non-environmental applications involving high
temperatures and pressures, yielding the benefit of
longer, ongoing service life.

ENVIRO-SEAL Duplex Packing
(Fig. 1-25)

Given the wide variety of valve applications and
service conditions within industry, these variables
(sealing ability, operating friction levels, operating
life) are difficult to quantify and compare. A proper
understanding requires a clarification of trade
names.

This special packing system provides the
capabilities of both PTFE and graphite
components to yield a low friction, low emission,
fire-tested solution (API Standard 589) for
applications with process temperatures up to
232°C (450°F).

1−13

A6163

Figure 1-24. ENVIRO-SEAL PTFE Packing System

Figure 1-25. ENVIRO-SEAL Duplex (PTFE and

Graphite) Packing System

39B4612-A

Figure 1-26. ENVIRO-SEAL Graphite

ULF Packing System

carbon fiber reinforced TFE, is suited to 260°C
(500°F) service.

KALREZt Valve Stem Packing (KVSP)
systems

The KVSP pressure and temperature limits
referenced are for Fisher valve applications only.
KVSP with PTFE is suited to environmental use up
to 24.1 bar and 204°C (350 psi and 400°F) and, to
some non-environmental services up to 103 bar
(1500 psi). KVSP with ZYMAXXt, which is a

1−14

ENVIRO-SEAL Graphite Ultra Low
Friction (ULF) Packing (Fig. 1-26)

This packing system is designed primarily for
environmental applications at temperatures in
excess of 232°C (450°F). The patented ULF
packing system incorporates thin PTFE layers
inside the packing rings and thin PTFE washers on
each side of the packing rings. This strategic

W6125-1

Figure 1-27. ENVIRO-SEAL Graphite

Packing System for Rotary Valves

placement of PTFE minimizes control problems,
reduces friction, promotes sealing, and extends
the cycle life of the packing set.

Braided graphite filament and double PTFE are
not acceptable environmental sealing solutions.

The following applies to rotary valves. In the case
of rotary valves, single PTFE and graphite ribbon
packing arrangements do not perform well as
fugitive emission sealing solutions.

The control of valve fugitive emissions and a
reduction in industry’s cost of regulatory
compliance can be achieved through these stem
sealing technologies.

While ENVIRO-SEAL packing systems have been
designed specifically for fugitive emission
applications, these technologies should also be
considered for any application where seal
performance and seal life have been an ongoing
concern or maintenance cost issue.

Characterization of Cage-Guided
Valve Bodies

HIGH-SEAL Graphite ULF Packing

Identical to the ENVIRO-SEAL graphite ULF
packing system below the packing follower, the
HIGH-SEAL system utilizes heavy-duty, large
diameter Belleville springs. These springs provide
additional follower travel and can be calibrated
with a load scale for a visual indication of packing
load and wear.

ENVIRO-SEAL Graphite Packing for
Rotary Valves (Fig. 1-27)

ENVIRO-SEAL graphite packing is designed for
environmental applications from −6°C to 316°C
(20°F to 600°F) or for those applications where fire
safety is a concern. It can be used with pressures
to 103 bar (1500 psi) and still satisfy the 500 ppmv
EPA leakage criteria.

Graphite Ribbon Packing for Rotary
Valves

Graphite ribbon packing is designed for
non-environmental applications that span a wide
temperature range from −198°C to 538°C (−325°F
to 1000°F).

The following table provides a comparison of
various sliding-stem packing selections and a
relative ranking of seal performance, service life,
and packing friction for environmental applications.

In valve bodies with cage-guided trim, the shape of
the flow openings or windows in the wall of the
cylindrical cage determines flow characterization.
As the valve plug is moved away from the seat
ring, the cage windows are opened to permit flow
through the valve. Standard cages have been
designed to produce linear, equal-percentage, and
quick-opening inherent flow characteristics. Note
the differences in the shapes of the cage windows
shown in figure 1-28. The flow rate/travel
relationship provided by valves utilizing these
cages is equivalent to the linear, quick-opening,
and equal-percentage curves shown for contoured
valve plugs (figure 1-29).

Cage-guided trim in a control valve provides a
distinct advantage over conventional valve body
assemblies in that maintenance and replacement
of internal parts is simplified. The inherent flow
characteristic of the valve can easily be changed
by installing a different cage. Interchange of cages
to provide a different inherent flow characteristic
does not require changing the valve plug or seat
ring. The standard cages shown can be used with
either balanced or unbalanced trim constructions.
Soft seating, when required, is available as a
retained insert in the seat ring and is independent
of cage or valve plug selection.

Cage interchangeability can be extended to
specialized cage designs that provide noise
attenuation or combat cavitation. These cages
furnish a modified linear inherent flow
characteristic, but require flow to be in a specific

1−15

W0958 W0959 W0957

QUICK OPENING LINEAR EQUAL PERCENTAGE

Figure 1-28. Characterized Cages for Globe-Style Valve Bodies

Figure 1-29. Inherent Flow

Characteristics Curves

direction through the cage openings. Therefore, it
could be necessary to reverse the valve body in
the pipeline to obtain proper flow direction.

Characterized Valve Plugs

The valve plug, the movable part of a globe-style
control valve assembly, provides a variable
restriction to fluid flow. Valve plug styles are each
designed to:

D Provide a specific flow characteristic.

D Permit a specified manner of guiding or

alignment with the seat ring.

D Have a particular shutoff or
damage-resistance capability.

Valve plugs are designed for either two-position or
throttling control. In two-position applications, the
valve plug is positioned by the actuator at either of
two points within the travel range of the assembly.
In throttling control, the valve plug can be
positioned at any point within the travel range as
dictated by the process requirements.

The contour of the valve plug surface next to the
seat ring is instrumental in determining the
inherent flow characteristic of a conventional
globe-style control valve. As the actuator moves
the valve plug through its travel range, the
unobstructed flow area changes in size and shape
depending upon the contour of the valve plug.
When a constant pressure differential is
maintained across the valve, the changing
relationship between percentage of maximum flow
capacity and percentage of total travel range can
be portrayed (figure 1-29), and is designated as
the inherent flow characteristic of the valve.

Commonly specified inherent flow characteristics
include:

Linear Flow

D A valve with an ideal linear inherent flow
characteristic produces a flow rate directly
proportional to the amount of valve plug travel
throughout the travel range. For instance, at 50%
of rated travel, flow rate is 50% of maximum flow;
at 80% of rated travel, flow rate is 80% of
maximum; etc. Change of flow rate is constant
with respect to valve plug travel. Valves with a
linear characteristic are often specified for liquid
level control and for flow control applications
requiring constant gain.

Equal-Percentage Flow

D Ideally, for equal increments of valve plug
travel, the change in flow rate regarding travel may
be expressed as a constant percent of the flow

1−16

Valve Plug Guiding

Accurate guiding of the valve plug is necessary for
proper alignment with the seat ring and efficient
control of the process fluid. The common methods
used are listed below.

A7100

Figure 1-30. Typical Construction to Provide

Quick-Opening Flow Characteristic

rate at the time of the change. The change in flow
rate observed regarding travel will be relatively
small when the valve plug is near its seat, and
relatively high when the valve plug is nearly wide
open. Therefore, a valve with an inherent
equal-percentage flow characteristic provides
precise throttling control through the lower portion
of the travel range and rapidly increasing capacity
as the valve plug nears the wide-open position.
Valves with equal-percentage flow characteristics
are used on pressure control applications, on
applications where a large percentage of the
pressure drop is normally absorbed by the system
itself with only a relatively small percentage
available at the control valve, and on applications
where highly varying pressure drop conditions can
be expected. In most physical systems, the inlet
pressure decreases as the rate of flow increases,
and an equal percentage characteristic is
appropriate. For this reason, equal percentage
flow is the most common valve characteristic.

D Cage Guiding: The outside diameter of the
valve plug is close to the inside wall surface of the
cylindrical cage throughout the travel range. Since
the bonnet, cage, and seat ring are self-aligning
upon assembly, the correct valve plug and seat
ring alignment is assured when the valve closes
(figure 1-15).

D Top Guiding: The valve plug is aligned by a
single guide bushing in the bonnet, valve body
(figure 1-4), or by packing arrangement.

D Stem Guiding: The valve plug is aligned with
the seat ring by a guide bushing in the bonnet that
acts upon the valve plug stem (figure 1-3, left
view).

D Top-and-Bottom Guiding: The valve plug is
aligned by guide bushings in the bonnet and
bottom flange.

D Port Guiding: The valve plug is aligned by the
valve body port. This construction is typical for
control valves utilizing small-diameter valve plugs
with fluted skirt projections to control low flow rates
(figure 1-3, right view).

Quick-Opening Flow

D A valve with a quick opening flow
characteristic provides a maximum change in flow
rate at low travels. The curve is essentially linear
through the first 40 percent of valve plug travel,
then flattens out noticeably to indicate little
increase in flow rate as travel approaches the
wide-open position. Control valves with
quick-opening flow characteristics are often used
for on/off applications where significant flow rate
must be established quickly as the valve begins to
open. As a result, they are often utilized in relief
valve applications. Quick-opening valves can also
be selected for many of the same applications for
which linear flow characteristics are
recommended. This is because the quick-opening
characteristic is linear up to about 70 percent of
maximum flow rate. Linearity decreases
significantly after flow area generated by valve
plug travel equals the flow area of the port. For a
typical quick-opening valve (figure 1-30), this
occurs when valve plug travel equals one-fourth of
port diameter.

Restricted-Capacity Control Valve
Trim

Most control valve manufacturers can provide
valves with reduced- or restricted- capacity trim
parts. The reduced flow rate might be desirable for
any of the following reasons:

D Restricted capacity trim may make it possible
to select a valve body large enough for increased
future flow requirements, but with trim capacity
properly sized for present needs.

D Valves can be selected for adequate
structural strength, yet retain reasonable
travel/capacity relationship.

D Large bodies with restricted capacity trim can
be used to reduce inlet and outlet fluid velocities.

D Purchase of expensive pipeline reducers can
be avoided.

D Over-sizing errors can be corrected by use of
restricted capacity trim parts.

1−17

Conventional globe-style valve bodies can be fitted
with seat rings with smaller port size than normal
and valve plugs sized to fit those smaller ports.
Valves with cage-guided trim often achieve the
reduced capacity effect by utilizing valve plug,
cage, and seat ring parts from a smaller valve size
of similar construction and adapter pieces above
the cage and below the seat ring to mate those
smaller parts with the valve body (figure 1-28).
Because reduced capacity service is not unusual,
leading manufacturers provide readily available
trim part combinations to perform the required
function. Many restricted capacity trim
combinations are designed to furnish
approximately 40% of full-size trim capacity.

General Selection Criteria

Most of the considerations that guide the selection
of valve type and brand are rather basic. However,
there are some matters that may be overlooked by
users whose familiarity is mainly limited to just one
or a few valve types. Table 1-1 below provides a
checklist of important criteria; each is discussed at
length following the table.

Table 1-1. Suggested General Criteria for Selecting Type

and Brand of Control Valve

Body pressure rating
High and low temperature limits
Material compatibility and durability
Inherent flow characteristic and rangeability
Maximum pressure drop (shutoff and flowing)
Noise and cavitation
End connections
Shutoff leakage
Capacity versus cost
Nature of flowing media
Dynamic performance

Pressure Ratings

Body pressure ratings ordinarily are considered
according to ANSI pressure classes — the most
common ones for steel and stainless steel being
Classes 150, 300 and 600. (Source documents
are ASME/ANSI Standards B16.34, “Steel
Valves,” and ANSI B16.1, “Cast Iron Pipe
Flanges and Flanged Fittings.”) For a given body
material, each NSI Class corresponds to a
prescribed profile of maximum pressures that
decrease with temperature according to the
strength of the material. Each material also has a

minimum and maximum service temperature
based upon loss of ductility or loss of strength. For
most applications, the required pressure rating is
dictated by the application. However, because all
products are not available for all ANSI Classes, it
is an important consideration for selection.

Temperature Considerations

Required temperature capabilities are also a
foregone conclusion, but one that is likely to
narrow valve selection possibilities. The
considerations include the strength or ductility of
the body material, as well as relative thermal
expansion of various parts.

Temperature limits also may be imposed due to
disintegration of soft parts at high temperatures or
loss of resiliency at low temperatures. The soft
materials under consideration include various
elastomers, plastics, and PTFE. They may be
found in parts such as seat rings, seal or piston
rings, packing, rotary shaft bearings and butterfly
valve liners. Typical upper temperature limits for
elastomers are in the 200 - 350°F range, and the
general limit for PTFE is 450°F.

Temperature affects valve selection by excluding
certain valves that do not have high or low
temperature options. It also may have some affect
on the valve’s performance. For instance, going
from PTFE to metal seals for high temperatures
generally increases the shutoff leakage flow.
Similarly, high temperature metal bearing sleeves
in rotary valves impose more friction upon the
shaft than do PTFE bearings, so that the shaft
cannot withstand as high a pressure-drop load at
shutoff. Selection of the valve packing is also
based largely upon service temperature.

Material Selection

The third criterion in table 1-1, “material
compatibility and durability”, is a more complex
consideration. Variables may include corrosion by
the process fluid, erosion by abrasive material,
flashing, cavitation or pressure and temperature
requirements. The piping material usually indicates
the body material. However, because the velocity
is higher in valves, other factors must be
considered. When these variables are included,
often valve and piping materials will differ. The trim
materials, in turn, are usually a function of the
body material, temperature range and qualities of
the fluid. When a body material other than carbon,
alloy, or stainless steel is required, use of an
alternate valve type, such as lined or bar stock,
should be considered.

1−18

Flow Characteristic

The next selection criterion, “inherent flow
characteristic”, refers to the pattern in which the
flow at constant pressure drop changes according
to valve position. Typical characteristics are
quick-opening, linear, and equal-percentage. The
choice of characteristic may have a strong
influence upon the stability or controllability of the
process (see table 1-3), as it represents the
change of valve gain relative to travel.

Most control valves are carefully “characterized”
by means of contours on a plug, cage, or ball
element. Some valves are available in a variety of
characteristics to suit the application, while others
offer little or no choice. To quantitatively determine
the best flow characteristic for a given application,
a dynamic analysis of the control loop can be
performed. In most cases, however, this is
unnecessary; reference to established rules of
thumb will suffice.

The accompanying drawing illustrates typical flow
characteristic curves (figure 1-29). The quick
opening flow characteristic provides for maximum
change in flow rate at low valve travels with a fairly
linear relationship. Additional increases in valve
travel give sharply reduced changes in flow rate,
and when the valve plug nears the wide open
position, the change in flow rate approaches zero.
In a control valve, the quick opening valve plug is
used primarily for on-off service; but it is also
suitable for many applications where a linear valve
plug would normally be specified.

Rangeability

operating stability. To a certain extent, a valve with
one inherent flow characteristic can also be made
to perform as though it had a different
characteristic by utilizing a nonlinear (i.e.,
characterized) positioner-actuator combination.
The limitation of this approach lies in the
positioner’s frequency response and phase lag
compared to the characteristic frequency of the
process. Although it is common practice to utilize a
positioner on every valve application, each
application should be reviewed carefully. There
are certain examples of high gain processes
where a positioner can hinder valve performance.

Pressure Drop

The maximum pressure drop a valve can tolerate
at shutoff, or when partially or fully open, is an
important selection criteria. Sliding-stem valves
are generally superior in both regards because of
the rugged nature of their moving parts. Many
rotary valves are limited to pressure drops well
below the body pressure rating, especially under
flowing conditions, due to dynamic stresses that
high velocity flow imposes on the disk or ball
segment.

Noise and Cavitation

Noise and cavitation are two considerations that
often are grouped together because both result
from high pressure drops and large flow rates.
They are treated by special modifications to
standard valves. Chapter four discusses the
cavitation phenomenon and its impact and
treatment, while chapter six discusses noise
generation and abatement.

Another aspect of a valve’s flow characteristic is its
rangeability, which is the ratio of its maximum and
minimum controllable flow rates. Exceptionally
wide rangeability may be required for certain
applications to handle wide load swings or a
combination of start-up, normal and maximum
working conditions. Generally speaking, rotary
valves—especially partial ball valves—have
greater rangeability than sliding-stem varieties.

Use of Positioners

A positioner is an instrument that helps improve
control by accurately positioning a control valve
actuator in response to a control signal. They are
useful in many applications and are required with
certain actuator styles in order to match actuator
and instrument pressure signals, or to provide

End Connections

The three common methods of installing control
valves in pipelines are by means of screwed pipe
threads, bolted flanges, and welded end
connections. At some point in the selection
process, the valve’s end connections must be
considered with the question simply being whether
the desired connection style is available in the
valve being considered.

In some situations, this matter can limit the
selection rather narrowly. For instance, if a piping
specification calls for welded connections only, the
choice usually is limited to sliding-stem valves.

Screwed end connections, popular in small control
valves, offer more economy than flanged ends.

1−19

The threads usually specified are tapered female
NPT on the valve body. They form a
metal-to-metal seal by wedging over the mating
male threads on the pipeline ends. This
connection style is usually limited to valves not
larger than NPS 2, and is not recommended for
elevated temperature service.

Valve maintenance might be complicated by
screwed end connections if it is necessary to take
the body out of the pipeline. Screwed connections
require breaking a flanged joint or union
connection to permit unscrewing the valve body
from the pipeline.

Flanged end valves are easily removed from the
piping and are suitable for use through the range
of working pressures that most control valves are
manufactured (figure 1-13).

Flanged end connections can be utilized in a
temperature range from absolute zero (−273°F) to
approximately 1500°F (815°C). They are utilized
on all valve sizes. The most common flanged end
connections include flat face, raised face, and ring
type joint.

the trim. Special precautions in seat material
selection, seat preparation and seat load are
necessary to ensure success.

Flow Capacity

Finally, the criterion of capacity or size can be an
overriding constraint on selection. For extremely
large lines, sliding-stem valves are more
expensive than rotary types. On the other hand,
for extremely small flows, a suitable rotary valve
may not be available. If future plans call for
significantly larger flow, then a sliding-stem valve
with replaceable restricted trim may be the
answer. The trim can be changed to full size trim
to accommodate higher flow rates at less cost than
replacing the entire valve body assembly.

Rotary style products generally have much higher
maximum capacity than sliding-stem valves for a
given body size. This fact makes rotary products
attractive in applications where the pressure drop
available is rather small. However, it is of little or
no advantage in high pressure drop applications
such as pressure regulation or letdown.

Welded ends on control valves are leak-tight at all
pressures and temperatures and are economical
in initial cost (figure 1-14). Welded end valves are
more difficult to remove from the line and are
limited to weldable materials. Welded ends come
in two styles, socket weld and buttweld.

Shutoff Capability

Some consideration must be given to a valve’s
shutoff capability, which is usually rated in terms of
classes specified in ANSI/FCI70-2 (table 1-4). In
service, shutoff leakage depends upon many
factors, including but not limited to, pressure drop,
temperature, and the condition of the sealing
surfaces. Because shutoff ratings are based upon
standard test conditions that can be different from
service conditions, service leakage cannot be
predicted accurately. However, the shutoff class
provides a good basis for comparison among
valves of similar configuration. It is not uncommon
for valve users to overestimate the shutoff class
required.

Because tight shutoff valves generally cost more
both in initial cost, as well as in later maintenance
expense, serious consideration is warranted. Tight
shutoff is particularly critical in high pressure
valves, considering that leakage in these
applications can lead to the ultimate destruction of

Conclusion

For most general applications, it makes sense
both economically, as well as technically, to use
sliding-stem valves for lower flow ranges, ball
valves for intermediate capacities, and high
performance butterfly valves for the very largest
required flows. However, there are numerous
other factors in selecting control valves, and
general selection principles are not always the
best choice.

Selecting a control valve is more of and art than a
science. Process conditions, physical fluid
phenomena, customer preference, customer
experience, supplier experience, among numerous
other criteria must be considered in order to obtain
the best possible solution. Many applications are
beyond that of general service, and as chapter 4
will present, there are of number of selection
criteria that must be considered when dealing with
these sometimes severe flows.

Special considerations may require out-of-the­ordinary valve solutions; there are valve designs
and special trims available to handle high noise
applications, flashing, cavitation, high pressure,
high temperature and combinations of these
conditions.

1−20

After going through all the criteria for a given
application, the selection process may point to
several types of valves. From there on, selection
becomes a matter of price versus capability,

institutional preferences. As no single control valve
package is cost-effective over the full range of
applications, it is important to keep an open mind
to alternative choices.

coupled with the inevitable personal and

Table 1-2. Major Categories and Subcategories of Control Valves with Typical General Characteristics

Valve Style

Regular

Sliding-stem

Bar Stock

Economy

Sliding-stem

Thru-Bore

Ball

Partial Ball

Eccentric Plug Erosion Resistance 1 to 8

Swing-Thru

Butterfly

Lined Butterfly

High

Performance

Butterfly

Main

Characteristics

Heavy Duty

Versatile

Machined from Bar

Stock

Light Duty

Inexpensive

On-Of f Service 1 to 24

Characterized for

Throttling

No Seal 2 to 96

Elastomer or

TFE Liner

Offset Disk

General Service

Typical Size

Range,
inches

1 to 24

½ to 3

½ to 2

1 to 24

2 to 96

2 to 72

Typical

Standard Body

Materials

Carbon Steel

Cast Iron
Stainless

Variety of Alloys

Bronze

Cast Iron

Carbon Steel
Carbon Steel

Stainless

Carbon Steel

Stainless

Carbon Steel

Stainless

Carbon Steel

Cast Iron
Stainless

Carbon Steel

Cast Iron
Stainless

Carbon Steel

Stainless

Typical Standard

End Connection

ANSI Flanged

Welded

Screwed

Flangeless

Screwed

Screwed To ANSI 125 Moderate Good

Flangeless To ANSI 900 High Excellent
Flangeless

Flanged
Flanged To ANSI 600 Moderate Excellent

Flangeless

Lugged
Welded

Flangeless

Lugged

Flangeless

Lugged

Typical

Pressure

Ratings

To ANSI 2500 Moderate Excellent

To ANSI 600 Low Excellent

To ANSI 600 High Excellent

To ANSI 2500 High Poor

To ANSI 300 High Good

To ANSI 600 High Excellent

Relative Flow

Capacity

Relative

Shutoff

Capability

1−21

Table 1-3. Control Valve Characteristic Recommendations

Liquid Level Systems

Control Valve Pressure Drop

Constant ΔP Linear
Decreasing ΔP with increasing load, ΔP at maximum load > 20% of minimum load ΔP Linear
Decreasing ΔP with increasing load, ΔP at maximum load < 20% of minimum load ΔP Equal-percentage
Increasing ΔP with increasing load, ΔP at maximum load < 200% of minimum load ΔP Linear
Increasing ΔP with increasing load, ΔP at maximum load > 200% of minimum load ΔP Quick Opening

Best Inherent

Characteristic

Pressure Control Systems

Application

Liquid Process Equal-Percentage
Gas Process, Large Volume (Process has a receiver, Distribution System or Transmission Line Exceeding 100 ft. of

Nominal Pipe Volume), Decreasing ΔP with Increasing Load, ΔP at Maximum Load > 20% of Minimum Load ΔP Linear
Gas Process, Large Volume, Decreasing ΔP with Increasing Load, ΔP at Maximum Load < 20% of Minimum Load ΔP Equal-Percentage
Gas Process, Small Volume, Less than 10 ft. of Pipe between Control Valve and Load Valve Equal-Percentage

Best Inherent

Characteristic

Flow Control Processes

Application Best Inherent Characteristic

Flow Measurement Signal to

Controller

Proportional to Flow In Series Linear Equal-Percentage

Proportional to Flow Squared In Series Linear Equal-Percentage

*When control valve closes, flow rate increases in measuring element.

Location of Control Valve in Relation

to Measuring Element

In Bypass* Linear Equal-Percentage

In Bypass* Equal-Percentage Equal-Percentage

Wide Range of Flow Set Point

Small Range of Flow but

Large ΔP Change at Valve

with Increasing Load

1−22