Regulator Control Theory
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
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P
1
Te c h n i c a l
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.
Figure 1
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.
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.
Figure 2
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 3
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Te c h n i c a l
Flow
P
1
P
2
P
2
P
1
Regulator Control Theory
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.
Figure 4
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.
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.
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Figure 5
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