Fundamentals of Gas Pressure Regulation

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Fundamentals of Gas

Pressure Regulation

Floyd D. Jury

Director of Education



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Fundamentals of

Gas Pressure Regulation

Introduction

Gas pressure regulators have become very familiaritems over the years, and nearly everyone has grownaccustomed to seeing them in factories, publicbuildings, by the roadside, and even in their ownhomes. As is frequently the case with many suchfamiliar items, we all have a tendency to take them forgranted. Even the gas man who handles regulatorsevery day as part of his job frequently tends to viewthe regulator simply as a piece of hardware which fitsin the line and regulates pressure. The fact that it willdo precisely that, for months on end without humanintervention, makes it easy to maintain such a view.It’s only when a problem develops or when we areselecting a regulator for a new application, that weneed to look more deeply into the fundamentals of theregulator’s operation.

Fundamental Essential Elements

The primary function of any gas regulator is to matchthe flow of gas through the regulator to thedemand for gas placed upon the system. At thesame time, the regulator must maintain the systempressure within certain acceptable limits.

Figure 1. Typical Regulator System A6002 / IL

A typical gas pressure system might be similar to thatshown in Figure 1, where the regulator is placedupstream of the valve or other device that is varying itsdemand for gas from the regulator.

If the load flow decreases, then the regulator flow mustdecrease also. Otherwise, the regulator would put toomuch gas into the system and the pressure P2 wouldtend to increase. On the other hand, if the load flowincreases, then the regulator flow must increase also

in order to keep P2 from decreasing due to a shortageof gas in the pressure system.

From this simple system it is easy to see that theprime job of the regulator is to put exactly as much gasinto the piping system as the load device takes out.

If the regulator were capable of instantaneouslymatching its flow to the load flow, then we would neverhave any major transient variation in the pressure P2as the load changed rapidly. From practical experiencewe all know that this is normally not the case, and inmost real-life applications we would expect somefluctuations in P2 whenever the load changes abruptly.How well the regulator is capable of performing underthese dynamic situations is one of the questions that

we would ask ourselves when selecting a regulator fora given application.

Since the regulator’s job is to modulate the flow of gasinto the system, we can see that one of the essentialelements of any regulator is a RESTRICTINGELEMENT that will fit into the flow stream and providea variable restriction that can modulate the flow of gasthrough the regulator.

Figure 2. Typical Restricting Element

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Figure 2 shows a schematic of a typical regulatorrestricting element. This restricting element is usuallysome type of valve arrangement. It can be asingle-port globe valve, a cage style valve, butterflyvalve, or any other type of valve that is capable ofoperating as a variable restriction to the flow.

In order to cause this restricting element to vary, sometype of loading force will have to be applied to it. Thuswe see that the second essential element of a gasregulator is a LOADING ELEMENT that can apply theneeded force to the restricting element. The loadingelement can be one of any number of things such as a



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weight, a handjack, a spring, a diaphragm actuator, ora piston actuator, to name a few of the more commonones.

A diaphragm actuator and a spring are frequentlycombined, as shown in Figure 3, to form the mostcommon type of loading element. A loading pressure

is applied to a diaphragm to produce a loading forcethat will act to close the restricting element. The springprovides a reverse loading force which acts toovercome the weight of the moving parts and toprovide a fail-safe operating action that is morepositive than a pressure force.

Figure 3. Typical Loading Element

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So far, we have a restricting element to modulate theflow through the regulator, and we have a loadingelement that can apply the necessary force to operatethe restricting element. But, how do we know when weare modulating the gas flow correctly? How do weknow when we have the regulator flow matched to theload flow? It is rather obvious that we need some typeof MEASURING ELEMENT which will tell us whenthese two flows have been perfectly matched. If wehad some economical method of directly measuringthese flows, we could use that approach; however, thisis not really a very feasible method.

We noticed earlier, in our discussion of Figure 1, that

the system pressure (P2) was directly related to thematching of the two flows. If the restricting elementallows too much gas into the system, P2 will increase.If the restricting element allows too little gas into thesystem, P2 will decrease. We can use this convenientfact to provide a simple means of measuring whetheror not the regulator is providing the proper flow.

Manometers, Bourdon tubes, bellows, pressuregauges, and diaphragms are some of the possiblemeasuring elements that we might use. Depending

Figure 4. Adding a Typical Measuring Element Completes the Regulator

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upon what we wish to accomplish, some of thesemeasuring elements would be more advantageousthan others. The diaphragm, for instance, will not onlyact as a measuring element which will respond tochanges in the measured pressure, but it will alsosimultaneously act as a loading element. As such, itwill produce a force to operate the restricting elementthat varies in response to changes in the measuredpressure.

If we add this typical measuring element to the loadingelement and the restricting element that we selectedearlier, we will have a complete gas pressure regulatoras shown in Figure 4.

Let’s review the action of this regulator. If therestricting element tries to put too much gas into thesystem, the pressure P2 will increase. The diaphragm,as a measuring element, responds to this increase inpressure and, as a loading element, produces a forcewhich compresses the spring and thereby restricts theamount of gas going into the system. On the otherhand, if the regulator doesn’t put enough gas into thesystem, the pressure falls and the diaphragmresponds by producing less force. The spring will thenovercome the reduced diaphragm force and open thevalve to allow more gas into the system. This type ofself-correcting action is known as negative feedback.

This example illustrates that there are three essentialelements needed to make any operating gas pressureregulator. They are a RESTRICTING ELEMENT, aLOADING ELEMENT, and a MEASURING ELEMENT.Regardless of how sophisticated the system maybecome, it still must contain these three essentialelements. Since these elements are so essential to the



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operation of any gas pressure regulator, perhaps weshould analyze each of them in more detail.

Restricting Element

The restricting element that we use will undoubtedlybe some type of valve. Regardless of the style of valvewhich we use, we must remember that its basicpurpose is to form a restriction to the flow. It forms abottleneck which allows us to convert from a highpressure system to a low pressure system.

The pressure differential that exists across the valverepresents a difference in potential energy that causesthe gas to flow, just as water flows downhill. Ourcommon sense tells us that if we increase thispressure differential across the valve, then we shouldthereby increase the flow of gas through the valve. Inactual practice, this is true only up to a certain critical

point.

If we look at a typical regulator installation, such as inFigure 5, we can analyze how the pressure variesalong the length of the valve under steady-state flowconditions.

Figure 5. Profile of Pressure Across a Valve

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The valve forms a major restriction in the line, andconsequently its flow area is considerably smaller thanthat of the surrounding pipe. If we are to maintain asteady-state flow through this system, we must have

just as much gas flow through the small area at therestriction as we have in the larger pipe area. It isobvious then that the velocity of the gas must bemuch greater at the restriction in order to maintain theflow of gas. The velocity will be the greatest at thepoint of maximum restriction of the flow stream. Thispoint is called the VENA CONTRACTA and is normallylocated a short distance downstream of the actualorifice.

This increase in velocity represents an increase inkinetic energy that must come at the expense of thepotential energy which is represented by pressure.Thus, in Figure 5, we see that the pressure decreasesto a minimum at the vena contracta where the velocityis the greatest. As the gas slows down again in thelarger downstream piping, we gain back some of the

pressure that we lost. This is called pressure recovery.

If we try to increase the flow through this valve, wemust, of course, increase the velocity of the gas at allpoints in the system. As we continue to increase theflow, we will eventually reach a point where the gasvelocity reaches the speed of sound at the venacontracta. Since we cannot normally make the gastravel faster than this limiting sonic velocity, we have ineffect reached the point where we can no longerincrease the volume rate of gas flow through the valvesimply by lowering the outlet pressure.

The point where we reach sonic velocity and the flow

becomes limited is known as CRITICAL FLOW, andthe pressure drop that exists across the valve at thatpoint is known as CRITICAL PRESSURE DROP. If wewere to increase the pressure drop beyond the criticalpoint by lowering the outlet pressure of the valve, wewould get no further increase in flow through the valve.The actual value of the critical pressure drop variesconsiderably depending upon the style and flowgeometry of the valve.

As the gas passes through the valve, a certain amountof turbulence occurs which results in an energy losswithin the valve. Part of the kinetic energy of the gas ischanged into heat energy and part of it is changed into

noise energy. A valve such as a ball valve or butterflyvalve, that has a fairly streamlined flow pattern has aminimum of energy loss in the valve. This results in arelatively high downstream pressure recovery. Thistype of valve would be called a high recovery valveand is shown in Figure 6 as the solid curve. On theother hand, a valve, such as a double ported globevalve, which has a relatively turbulent flow pattern, willhave a fairly high energy loss which means poordownstream pressure recovery. This type of valvewould be called a low recovery valve and is shown inFigure 6 as the dashed curve.

Those who specialize in the theory of fluid flow tell usthat the pressure differential between the inlet and thevena contracta (P1 — Pvc) is a direct measure of flowregardless of the style of valve; whereas, the pressuredrop observed across the valve (∆P = P1 – P2) is highlydependent upon valve style. Thus, if the two valvesplotted in Figure 6 have equal flow areas, then wesee that they would also have equal flow, since thepressure drop that determines the flow (P1 – Pvc) is thesame for both valves even though the observedpressure drops across the valves (P1 – P2) are quitedifferent. The observed pressure drop across the valve



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Figure 6. High and Low Recovery Valves

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(∆P = P1 – P2) is a direct measure of the pressure loss

in the valve. Since it is quite difficult for us to measurethe pressure at the vena contracta, we must, as apractical matter, resort to using the ∆P across thevalve as a measure of the flow instead of (P1 — Pvc).Since the ∆P across the valve is highly dependentupon valve style, it is obvious that experimental meansmust be used to find the relationship between thispressure drop and the flow for any given style of valve.This is the reason for the many tables of valve sizingdata published by the valve manufacturer.

We said earlier that in general we could increase theflow by increasing the pressure drop across the valve.

We have just discussed the fact that when we increasethis pressure drop by lowering the downstreampressure, we eventually reach a critical conditionwhere the flow no longer increases. We have reachedcritical flow because we have achieved the limitingsonic velocity at the vena contracta.

As it turns out, we can continue to increase the flowthrough the valve even after we have reached thecritical flow condition. We do this by increasing theinlet pressure (P1) to the valve. We would still havesonic velocity at the vena contracta and we would stillhave critical flow through the same flow area. What we

have done, however, by increasing P1 is increase thedensity of the gas entering the valve and havethereby packed more standard cubic feet of gas intoeach actual cubic foot of volume passing through theregulator. In effect, we have not changed the cubicfeet per hour of flow, but we have increased theSCFH, or standard cubic feet per hour. As you willrecall, a standard cubic foot of gas is that amount ofgas that will occupy a volume of one cubic foot understandard conditions of 60oF and 14.73 psia.

Loading Element

Except for a few of the older weight loaded units,almost all gas regulators have springs. In fact, springsand diaphragms are by far the most universally usedloading elements in modern gas regulators.

From a design point of view, there are several springfactors that are important such as type of material,wire diameter, spring diameter, free length, and thenumber of coils. From the strictly operational point ofview of the gas man, however, there is only one factorof real significance. That factor is known as the springrate.

Spring rate (k) is defined as the number of pounds offorce (F) necessary to compress a spring by one inch.If a certain spring requires a force of 60 pounds tocompress it one inch, then we would say that thespring rate was 60 pounds per inch. Since the springrate is a linear relationship within its normal operatingrange, this same spring would require 90 pounds tocompress it 1.5 inches.

If we took another spring and found that it wouldcompress 2 inches under a force of 100 pounds, wecould determine the spring rate by dividing the 100pounds force by the 2 inch compression to obtain 50lbs./in. This example suggests that we could define asimple formula that would describe this relationship.

k+ F X

where:

F = force (lbs.)

X = compression (in.)

k = spring rate (lbs./in.)

We can easily manipulate this equation into anothercommon form (F = kX) that will allow us to find thespring force developed for any given springcompression. This form of this simple equation is veryuseful in the study of gas regulators.

As useful as a spring is, it can provide a loading forcein one direction only. Energy must be supplied tocompress the spring in the other direction. This energyusually comes from the force developed by pressure

acting upon a diaphragm.

A diaphragm is simply a piece of fabric coated withsome type of rubber-like material. The coatingprovides a seal to contain the pressure while the fabricgives it enough strength to withstand the pressure.Some diaphragms are molded into special shapes butmost are simply cut from flat sheets.

For economic reasons the coating on the diaphragmmay be much thinner on one side than the other. The



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side with the thinner coating is usually identified byhaving some type of design or pattern printed on it.When a diaphragm of this type is used, it should beinstalled so that the pressure is applied to thenon-patterned side or the side with the thicker coating.

When we speak of pressure, many of us are inclined

to refer to it incorrectly as so many “pounds” ofpressure. This is not only wrong, but it tends toobscure the real meaning of pressure. In reality,pressure is a force (lbs.) that is uniformly distributedover an area (in.2). Thus, we should more correctlyrefer to pressure as so many pounds per square inch.We can define a simple formula that will describe thisrelationship for us.

P + F A

where:

F = force (lbs.)

A = area (in.2)

P = pressure (lbs./in.2 or psi)

Again, we can easily manipulate this equation intoanother common form (F = PA) that will allow us to findthe force developed by a pressure acting upon an areasuch as a diaphragm. This equation is also very usefulto us in the study of gas regulators.

We don’t have to be high powered mathematicians tomake a rather interesting and informative study of gas

pressure regulation. In fact, just an understanding ofthe two simple equations that we have described sofar is enough to gain a rather comprehensiveknowledge of the fundamentals of gas pressureregulation.

Measuring Element

A variety of devices is available for us as gas pressuremeasuring elements. Two of these, the manometerand the pressure gauge, are widely used as gaspressure measuring elements; however, they do notreadily lend themselves to most control applications,so they are of little interest to us in our study of gaspressure regulation.

Bourdon tubes and bellows are gas pressuremeasuring elements that can be used for automaticcontrol. They both can be made quite accurate andreliable; however, they also have the disadvantagethat they require some type of auxiliary equipment tosupplement their use.

By far the most universally used gas pressuremeasuring element is the diaphragm. There areseveral reasons that account for this. The diaphragmis simple, very economical, highly versatile, easy tomaintain, and has the definite advantage of notrequiring any additional equipment to supplement itsaction. In other words, the same diaphragm that

serves as a measuring element can also serve as aloading element with no intermediate hardware beingneeded.

Because of its extreme simplicity, the diaphragm isfrequently taken for granted and does not achieve thetheoretical attention that it deserves. One importantfactor that the gas man must be concerned withregarding diaphragms is a proper determination of thearea that the pressure will be acting upon.

Figure 7. Actuator Diaphragm Assembly

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If we take a look at Figure 7, we can see that theloading pressure (PL) acts over the entire exposedsurface of the diaphragm. The diameter of theexposed diaphragm area which the pressure actsupon is the same as the inside diameter of the uppercasing.

If we are not careful, we can lead ourselves into a trapat this point. Just because the pressure acts over theentire exposed surface doesn’t mean that all of thatarea is useful to us in terms of providing a loadingforce. In fact, once the question is raised, we mightanalyze the situation again and conclude that the onlyarea which is useful is the area of the diaphragm plate,

since that is the only place where the pressure is reallypushing down on the spring assembly.

If we allow ourselves to come to this conclusion, wewould be wrong again! The actual answer liessomewhere between these two extremes, but where?To find out, let’s take a close-up look at the smallportion of the diaphragm where it is unsupportedbetween the diaphragm plate and the case flange.Because the diaphragm is unsupported here, thepressure acting upon it takes up the slack in the



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Figure 8. Close-up of Diaphragm Convolution Section E0443 / IL

diaphragm and shapes it into what is known as aconvolution.

Figure 8 provides a close-up look at how the loadingpressure is uniformly distributed over the diaphragmconvolution. We have to remember that this pressure

is not only uniformly distributed, but also actsperpendicular to the diaphragm surface at every point.

As the pressure shapes the diaphragm convolution,we can visualize a line drawn tangent to thediaphragm at any point along this convolution. As wedo this, we will discover that there is one, and onlyone, point where the tangent line is horizontal. We canidentify this as the point of horizontal tangency. Avertical dashed line is drawn through this point inFigure 8 to divide the diaphragm convolution into twosections.

At this point we need to emphasize the fact that a

diaphragm is basically a rubber coated fabric. Theextreme flexibility of this type of material means that itcan support neither shear nor compressive forces. Theonly force that can be sustained by any flexiblematerial such as this would be a tension force that isalways acting directly parallel to the fabric at any point.If we follow this line of reasoning in Figure 8, we cansee that a horizontal tension force will exist in thediaphragm at the point of horizontal tangency.

The pressure acting to the left of the horizontal tangentpoint can only be transmitted to the diaphragm platethrough tension in the diaphragm material. At thehorizontal tangent point, this tensile force is horizontal,as shown in Figure 9, and therefore can contributenothing to the up and down motion or the thrust of theactuator. In other words, the area outside of the pointof horizontal tangency is not effective area. Thus,when we use the formula (F = PA) for the forcedeveloped by a pressure acting on an area, we mustuse the effective area in the calculation. As we have

just shown, this effective area is calculated using thediameter between the points of horizontal tangency onthe convolutions.

Figure 9. Tensile Force in a Diaphragm

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But let’s look out! There is another trap here that wemust avoid. It would be easy to assume that the pointof horizontal tangency falls in the center of theconvolution. This is only true for the one position in thestroke where the diaphragm plate is level with theflange. At other positions in the stroke, the effective

area changes as the point of horizontal tangencymoves inward or outward.

It is easy to visualize how this effective area changes ifyou hold a slack piece of string between youroutstretched hands to simulate the diaphragmconvolution. As you move one hand up and down tosimulate the motion of the diaphragm plate, you willdiscover the following relationship.

As the diaphragm moves to COMPRESS the spring,the effective area DECREASES. As the diaphragmmoves to RELAX the spring, the effective areaINCREASES. This relationship will remain at yourfingertips if you remember that the effective area

decreases to its least amount just when it is neededmost to help compress the spring.

Spring and Diaphragm Effect

Now let’s use some of the fundamentals that we havedeveloped about the three essential elements in orderto study the operational performance of gas regulators.

Figure 10. Typical Self Operated Regulator

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Let’s look at a typical self operated regulator, such asthat in Figure 10, when it is operating understeady-state conditions. In this situation the valve plugis in equilibrium with the pressure force exactlybalancing the spring force. For simplicity, and toillustrate a point, the unbalance force on the valve plugis temporarily ignored. The pressure force is

developed by the sensed pressure (P2) acting againstthe diaphragm area. The spring force is developedfrom the compression that exists in the spring. We canexpress this relationship in equation form using theformulas that we developed earlier.

P 2 A+ kX

We can rearrange this equation into a moreconvenient form by solving for P2.

P 2+ kX A

Let’s use this simple formula now to make someinteresting observations about a regulator such as thatshown in Figure 10. An example should serve toillustrate the basic principles. Assume that theregulator has the following values for its parameters.

k = 160 lbs./in. (spring rate)A = 80 in.2 (effective diaphragm area)T = 2 in. (total valve travel)

Furthermore, we are going to assume that the spring isadjusted so that it has one inch of compression evenin the fully extended wide open valve position. Thismeans that when the valve plug travels its full twoinches to the closed position, the spring compressionwill be three inches.

Now apply our formula to the conditions at each end ofthe valve plug travel. Under very low load flowconditions the regulator will not be required to supplymuch gas to the system and the valve plug will beessentially in the closed position where the springcompression is three inches. The downstreamcontrolled pressure, which is acting upon thediaphragm, can be determined from our formula.

P 2+ kX A

+ (160Ă lbs.ńin.)Ă(3Ăin.)(80ăin.2)

+ 6Ă psig Ă( lowĂ loadĂ flow)

If the demand on the system were to change now sothat the regulator must supply a high flow, the valveplug would have to open up to its wide open position. Ifwe look carefully at our system in Figure 10, we seethat it is the controlled pressure (P2) that is actually

holding the valve plug closed. In order for the valveplug to open, P2 must decrease, thereby allowing thespring to push it open. When the valve plug gets open,there will again be a balance of forces, and we canfind the new value of P2. Remember, the spring stillhas one inch of compression in this open position.

P 2 + kX A

+(160Ă lbs.ńin.)Ă(1Ăin.)

(80ăin.2)

+ 2Ă psig Ă( fullĂ loadĂ flow)

Thus, we see that the pressure had to decrease from 6psig at low load to 2 psig in order to open the valveplug sufficiently to pass the full load flow. Furthermore,since this 2 psig is the pressure that will just hold thevalve plug in the open position, this value of thepressure will have to continue as long as the high loadflow condition exists. The pressure will only return tothe original 6 psig when the load flow demand returnsto its original low value.

If you wish to change our assumption regarding theamount of initial spring compression in the wide opencondition, you can quickly verify, by similarcalculations, that the actual values for P2 will changealso, but the decrease in P2 will always be the same 4psi. The actual magnitude of the decrease in controlledpressure required to open the valve is a function of thedesign parameters for the given regulator and iscaused by the required change in spring compression.This is why it is occasionally referred to as springeffect.

Adjusting the amount of initial compression will changethe value of the pressure at which we will operateunder any given load condition. As a matter of fact,this is exactly how we adjust the set point pressure onour regulator. As we have seen, however, adjustingthis spring compression does not change the amountof spring effect.

The decrease in controlled pressure that occurs as weincrease the load is called droop. In the gas industry,the amount of droop that occurs in the controlledpressure when we go from a low load flow condition toa full load flow condition is defined as

PROPORTIONAL BAND. This is frequently designatedas PB.

In the last example we saw that the proportional bandof the regulator was caused directly by the springeffect. In that example we assumed that the effectivearea of the diaphragm was constant. We know fromour work in a previous section that this assumption isnot always true. The effective area of the diaphragmmay be 80 in.2 when the valve plug is closed and thespring has its maximum compression, but when the



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spring is in its most relaxed state and the valve plug isfully open, the effective area of the diaphragm willhave to be something greater than 80 in2. A typicalvalue might be 100 in2.

When we previously assumed a constant effectivearea, we found that our example regulator had a 4 psi

proportional band. Let’s see how the change ineffective area would affect the proportional band.

PB+ ( P 2)Ă lowĂ load* ( P 2)Ă highĂ load

+ kX A

Ă lowĂ load* kX A

Ă highĂ load

+(160Ă lbs.ńin.)Ă(3Ăin.)

(80ăin.2)*

(160Ă lbs.ńin.)Ă(1Ăin.)

(100ăin.2)

+ 6Ă psig * 1.6Ă psig

+ 4.4Ă psig

We see that the proportional band of our regulatorincreased an additional 0.4 psi as a direct result of thechange in effective area of the diaphragm. Thus, droopand proportional band are caused by both spring effectand diaphragm effect, as we can see from the plot inFigure 11.

Figure 11. Plot of Droop Due to Spring and Diaphragm Effect

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From a performance point of view, it is desirable tokeep the proportional band as narrow, or small, aspossible. One way that will help is to minimize oreliminate the diaphragm effect. In actual practice thisis done quite frequently by using a molded diaphragmwith deep convolutions rather than a simple flat-sheetdiaphragm that has quite shallow convolutions. Youcan show yourself quite easily why the deeperconvolution minimizes the effective area change byperforming the string trick again that was suggested inthe last section. As you move one end of the string upand down, you will notice that for a given amount ofvalve travel, the change in the position of thehorizontal tangent point of the convolution is much lesswith a deeper convolution. This means that there will

be much less change in effective area with valvetravel, and consequently, the proportional band of theregulator will be smaller. The major disadvantage ofthis approach is the higher cost of the moldeddiaphragm.

We can also narrow the proportional band by installinga spring with a lower spring rate, but we mightintroduce problems with valve plug chattering becauseof the lower stiffness. Another way might be to install alarger valve size so that the valve travel can bereduced. We could also install a different regulatorwhose effective diaphragm area is greater. This wouldtheoretically reduce the proportional band, althoughwe would almost certainly run into problems ofexceeding the spring’s capacity for travel or initialcompression. The last two methods also have thedisadvantage of requiring a major change in hardware.

Pilot Operated Regulators

So far we have only discussed self operatedregulators. This is the name given to that class ofregulators where the measured pressure is applieddirectly to the loading element with no intermediatehardware. There are really only two basicconfigurations of self operated regulators that arepractical. These two basic types are illustrated inFigures 4 and 10.

If the proportional band of a given self operatedregulator is too great for a particular application, thereare a number of things that we can do. From ourprevious examples we recall that spring rate, valvetravel, and effective diaphragm area were the threeparameters that affected the proportional band. In thelast section, we pointed out the way to change theseparameters in order to improve the proportional band.If these changes are either inadequate or impractical,the next most logical step is to install a pressureamplifier in the measuring or sensing line. Thispressure amplifier is frequently referred to as a pilot.

A typical pilot amplifier consists of a double diaphragmassembly that is rigidly fastened together as shown inFigure 12. This complete diaphragm assembly movesup and down under the action of the spring, theloading pressure (PL) and the controlled pressure (P2).As the diaphragm assembly moves, it modulates theflow through the supply pressure nozzle according tochanges in P2. The maximum opening of this variableorifice must be larger than the fixed orifice shown as ahole through the upper diaphragm.



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Figure 12. Pilot Amplifier

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When we introduce a source of supply pressure(usually P1 from upstream of the regulator) through thenozzle or variable orifice, the loading pressure (PL)builds up between the diaphragms because it can’tescape fast enough through the fixed orifice. If weincrease P2 to cap off the nozzle then PL, will decreasesince flow through the fixed orifice is now greater thanthrough the nozzle. This means that the loadingpressure is inversely proportional to P2 ; i.e., PLincreases when P2 decreases and vice versa.

A pilot amplifier such as this can be easily designed sothat it takes very little change in P2 to cause the

double diaphragm assembly to move far enough tocompletely open or close the variable nozzle orifice.This means that a very small change in P2 will result inrather large changes in PL. In other words, the pilotamplifier has high gain. A typical pilot amplifier gainmight be nominally about twenty.

Now, let’s see how this pilot amplifier would operatewhen installed in the regulator sensing line as shownin Figure 13. Because of the pilot’s inverse relationshipbetween P2 and PL, you will note that it has beennecessary to change the direction of the valve plugaction in Figure 13. This is important because we stillwant an increase in P2 to result in a closing action of

the valve plug.

The pilot amplifier that we saw in Figure 12 had threepressure connections. As shown in Figure 13, one ofthese connections is for the controlled pressure (P2).The second connection is for the supply pressurewhich we are obtaining from the upstream pressure(P1). Finally, the output connection of the pilot is for theloading pressure (PL) which is applied directly to theregulator diaphragm.

Figure 13. Pilot Operated Regulator E0447 / IL

The purpose of the pilot amplifier is to sense changesin the controlled pressure and amplify them into largerchanges in loading pressure on the diaphragm. Theamount of amplification that we get is called the gainof the pilot. If we have a pilot amplifier with a gain of20, then a one psi change in P2 will cause a 20 psichange in the loading pressure on the diaphragm.

When the system load flow increases, the valve mustcome wide open just as before in the case of the selfoperating regulator. This is accomplished bydecreasing the diaphragm pressure by the sameamount as before. Previously, this amount of

diaphragm pressure change showed up directly asdroop in the controlled pressure. With our gain of 20 inthe pilot amplifier, however, the controlled pressureonly needs to droop one twentieth as much as beforein order to get the same pressure change on thediaphragm. Thus, we have reduced the proportionalband of our regulator by twenty, which is the amount ofgain in the pilot.

In our prior example of the self operated regulator, wehad a proportional band of 4.4 psi. If we were to installa pilot with a gain of 20 in that regulator, theproportional band would be only 0.22 psi. A ratherimpressive difference!

With the rather sensational improvement inproportional band that we can achieve with pilotoperated regulators, one might wonder why allregulators aren’t made this way. The answer istwofold, economics and stability. Pilot operatedregulators are more expensive than similar selfoperated regulators, and the improvement inproportional band may not be sufficiently necessary to

justify the increased cost. On the other hand, the gainof the pilot amplifier increases the gain or sensitivity of



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the entire pressure regulator loop. If this loop gain isincreased too much, the loop can become unstableand the regulator will oscillate or hunt.

When we discussed the supply pressure source for thepilot amplifier in Figure 12, we mentioned that thissupply pressure is usually obtained from the gas

upstream of the regulator. Since this is true, the pilot isnormally designed so that when we bleed PL down, thegas does not exhaust to the atmosphere but returnsinstead to the lower controlled pressure downstream ofthe regulator. Here its basic value as a source ofenergy can still be utilized.

In order to cause the loading gas to vent downstream,we have to make certain that the loading pressure (PL)is always greater than the downstream pressure (P2).One of the easiest ways to accomplish this is toprovide some way for the downstream pressure (P2) toact on the underside of the regulator diaphragm as wehave done in Figure 13. Thus, the loading pressure

(PL) will always be forced to operate at a level greaterthan P2 in order to be able to stroke the valve.

So far we have discussed spring and diaphragm effectand shown how this results in droop of the controlledpressure of a gas regulator. When we are dealing withstrictly proportional control, as is the case in nearly allof the common gas regulators, this droop is a fact oflife we must deal with. There is no way we caneliminate it. We can only minimize it with high gain inour regulator loop. We have also seen that the use ofa pilot amplifier is one way to reduce this droop.

Service Regulators

A pilot amplifier is very effective in reducing droop in aregulator, but it tends to be expensive. Anothermethod we can use to overcome droop is known asvelocity boosting. In general, this method is not asgood as the pilot amplifier method, but it has theadvantage of being less expensive and can be madeto work very well under the proper circumstances.Velocity boosting is most frequently used in houseservice types of regulators.

If we refer back to Figure 5, which shows the pressureprofile across a typical regulator, we recall that thevena contracta pressure occurs just a short distancedownstream of the actual restriction. Furtherdownstream this pressure recovers to the value of P2,which is the controlled pressure and the one which isnormally applied to the diaphragm.

A regulator which employs velocity boosting isdesigned so that the controlled pressure (P2) is nolonger applied to the diaphragm. Instead, a pitot tube,such as shown in Figure 14, or some similar design is

arranged so that the lower pressure near the venacontracta acts upon the diaphragm rather than usingthe higher P2.

Figure 14. Velocity Boost with a Pitot Tube

A6008 / IL

As the load flow starts to increase, the sensedpressure at the pitot tube begins to droop just as P2does. Since the sensed pressure is near the venacontracta, and the gas velocity is greater there, thispressure, which is applied to the diaphragm,decreases more than P2. Consequently the valve isallowed to open slightly wider than it would if P2 wereacting on the diaphragm. This has the effect ofkeeping P2 relatively more constant and thuspreventing a large droop with high load flow.

Figure 15. Performance Curves Showing Effect of Velocity Boosting

A6009 / IL

Spring and diaphragm effect still cause a droopingpressure at the sensing point similar to the basicregulator shown in Figure 15, but this is overcome bythe subsequent downstream pressure recovery of P2.

On any service regulator, such as that shown in Figure16, there is always the danger that the customer mayshut off all of his appliances including the pilot lights. Ifthat happens, the regulator must shut off for zero flow.This is called lock up.



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Figure 16. Basic Service Regulator Without Velocity Boost

E0448 / IL

To get tight shutoff the regulator uses soft seats whichmust be tightly compressed. This compression forcecan only come from the action of P2 acting on thediaphragm, which means that P2 must rise enough at

zero flow to hold the seal tight. This lock up pressurecan be clearly seen on the performance curves inFigure 15. The amount of pressure rise that we get inthis lock up region is directly dependent upon thestiffness of the elastomeric material on the valve plug.

For economic reasons the house service regulator isusually a single port valve as shown in Figure 16. If welook at the upstream or inlet pressure (P1) we can seethat it is producing a force that is trying to open thevalve. The controlled pressure (P2) is acting on theback side of the valve plug trying to close it, but in atypical application, P1 is much greater than P2. Thisleaves us with a rather large unbalanced force that is

acting to open the valve plug. The only way that wehave to balance this force is by P2 acting on thediaphragm. If we get any kind of variation in the inletpressure, then we can see that we will also get avariation in our controlled pressure.

One easy way that we can decrease this variation inP2, due to fluctuations in P1, is to use a mechanicallever ratio connecting the diaphragm assembly to thevalve plug. In a typical service regulator, this lever ratiois about three to one. This means that P2 only has tovary one third as much as before in order to balancefluctuations in P1. The lever only reduces the effect ofvalve unbalance, it does not eliminate it.

Since the magnitude of the unbalanced force on thevalve is directly proportional to the valve area, it is toour advantage to select an orifice size which is nolarger than necessary to pass the required flow of gas.

The magnitude of the unbalanced force on the valvealso increases when P1 increases. Thus, even thoughthe lever system is helping to reduce the effect ofvalve unbalance, we still must have an increase in P2

in order to balance either an increase in orifice size oran increase in P1

A family of curves is shown in Figure 17 for a givenorifice size which clearly illustrates that there is adifferent outlet pressure curve for each inlet pressureon a typical service regulator.

Most house service regulators control pressures in thevicinity of a few inches of water column, yet since eachindividual gas company’s pressure setting may beslightly different, we need some method of adjustingthe setpoint pressure.

We know from our previous discussions that P2 actingon the diaphragm must provide a force that willbalance the spring compression force. We can use theadjusting screw on top of the spring, as shown inFigure 16, to increase the total compression force inthe spring. P2 must then also increase in order tobalance the larger spring force and hold the valve plugin the proper position. Thus, by changing the initial

compression in the spring, we can adjust P2 to operateat any setpoint value that we wish within a certainrange defined by the load limits on the spring.

Another factor that we frequently need to take intoconsideration when working with service regulators isthe weight of the moving parts in the regulator. Wheninstalled with the regulator spring on top, the weight ofthe spring and diaphragm assembly produces anadditional downward force that must be balanced byan additional increase in P2. If this same regulator wasturned upside down, the weight of the moving partswould then operate in the opposite direction and thesetpoint pressure would change. Since, in this upside

down position, the weight is opposed by the springinstead of the pressure, we can use the initialcompression in the spring to support the weight of theparts.

The upper casing of the regulator is designed so that itwill hold the spring properly, but it also protects theregulator parts from exposure to the weather, dirt, etc.Just as important, it also keeps a cushion of air abovethe diaphragm. As the diaphragm assembly moves upand down, the air in this upper casing must move inand out through the vent hole, otherwise, thecompression and rarefaction of air in the upper casingwould interfere with the diaphragm movement.

If we restrict the flow of air in and out of the uppercasing by just the right amount, we can damp thetendency of the diaphragm assembly to go into asustained oscillation. This type of oscillation is calledhunting or buzzing. This is why a regulator willsometimes buzz when we remove the closing cap toadjust the spring. When we close the cap again, theregulator stops buzzing. The same thing cansometimes happen if the vent hole is enlarged forsome reason.



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Figure 17. Variation of Controlled Pressure with Changes in Inlet Pressure on a Tyical Service Regulator

B2367 / IL

ConclusionIt should be obvious at this point that there are a great

many important fundamentals to understand in orderto properly select and apply a gas regulator to do aspecific job. Although these fundamentals are profusein number and have a sound theoretical base, they arerelatively straightforward and easy to understand.

As you are probably aware by now, we made anumber of simplifying assumptions as we progressed.This was done in the interest of gaining a clearer

understanding of these fundamentals without gettingbogged down in special details and exceptions. By no

means has the complete story of gas pressureregulation been told. The subject of gas pressureregulation is much broader in scope than can bepresented in a single document such as this, but it issincerely hoped that this paper will help the gas manto gain a working knowledge of some fundamentalsthat will enable him to do a better job of designing,selecting, applying, evaluating, or troubleshooting anygas pressure regulation equipment.



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Fisher Controls International, Inc.205 South Center StreetMarshalltown, Iowa 50158 USAPhone: (641) 754Ć3011Fax: (641) 754Ć2830Email: fcĆ[email protected]: www.fisher.com

The contents of this publication are presented for informational purposes only, and while every effort has

been made to ensure their accuracy, they are not to be

construed as warranties or guarantees, express or implied,

regarding the products or services described herein or

their use or applicability. We reserve the right to modify or

improve the designs or specifications of such products at

any time without notice.

E Fisher Controls International, Inc. 1972;

All Rights Reserved

Fisher and FisherĆRosemount are marks owned by

Fisher Controls International, Inc. or

FisherĆRosemount Systems, Inc.

All other marks are the property of their respective owners.

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Fundamentals of Gas Pressure Regulation

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