Constant reciprocating discharge pressure 2

[mucour]

Hello,

Can someone please confirm to me that the discharge from a gas reciprocating compressor will be constant irrespective of the inlet volume flow. If the flow into a reciprocating compressor drops to 10 percent, will the discharge pressure remain the same as when the design inlet flow is 100%.
My believe is if the compressor discharge valve will open against a fixed pipeline discharge pressure, since it is like a spring check valve, I believe it does not matter whether the gas compressed is 10percent or 100percent. The discharge pressure will remain the same. Is this true?
Will the temperature increase if the inlet flow reduce or the gas compressed by the reciprocating compressor drops to 5 to 10percent?

 

[25362]

The discharge pressure is determined by the system.
As to the subject of unloading the compressor visit thread404-65749.

 

[pipehead]

The discharge pressure on a compressor is a function of the system 25362 states. The system may be 10 miles of pipe, or a control valve set to hold a pressure. It could be a switch that turns the compressor off at a given pressure. If the capacity of the compressor is decrease (lets just say we lowered the RPMs 10%, the discharge pressure may not drop at all, the control valve that is holding the pressure will close a little bit to compensate for the lower flow. OR a 10% drop in flow on a 10 mile line may only cause the inlet pressure in the line to drop 1%.
The discharge and suction valves are check valves that have spring pressures that require the pressure on 1 side to be anywhere from 1 oz/inch^2 to 20 psig to cause them to open.
Flow rate does NOT dictate discharge temperature of the gas. The pressure increase across the unit sets the discharge temperature. More exactly, it is the ratio of the discharge pressure divided by the suction the sets the discharge temperature. If the ratio is say 3, then going from 30 psia to 90 psia will have the same outlet tempeature as going from 300 psi to 900 psia (well there are some minor physical properties that will make a small change, not to worry about).
I n your question, a 10% lower inlet pressure will cause the compressor to move about 10% less volume and the discharge temperature will rise, if the discharge pressure we constant.

 

[25362]

If the large reduction in flow rate results in a greater loss of compression efficiency, the lost work transforms into heat and temperature may rise.

 

[dcasto]

by definition, a positive displacement compressor can have no reduced flow, unless you consider a lower suction pressure as a loss of flow and if that were the case, work is lost to entropy and to the machine because the unit will eventually fail due to excessive rod or pin load.

 

[25362]

Dcasto, flow reductions are a result of unloading. See, please, thread404-65749.

 

[mucour]

25362, loss of flow can also be as a result of loss in suction pressure into the compressor. And like you pointed out, unloaders (e.g. finger valve unloader, FVCP) can also result in reduction in flow. But I do not understand how work lost to the machine will cause a failure due to excessive rod load or pin load. Does a reduction in gas flow into the cylinder bore cause a higher compression load above what will be experienced at normal flow?

 

[25362]

mucour

[•] The wording "loss of flow" is to my grasping different from flowrate reduction as caused by unloaders, etc., although a reduction in volumetric efficiency is the common denominator.

[•] A pressure ratio increase by lowering the suction pressure could result in overstressing the piston rod; but this question should be addressed to dcasto.

 

[mucour]

25362, I do not think that lowering the suction pressure will increase the compression pressure ratio. The pressure ratio is constant unless there is blockage of the gas outlet line which should normally trigger a PSV to open to forestall pressure increase beyond the MAWP. This is to prevent equipment damage. The scenario of drop in suction pressure and thereby a reduction in process flow occur in process plant where the reservoir pressure has dropped. As a result, that should not lead to an increase in the road load I believe.

May be dcasto ay throw more light.

 

[dcasto]

mucour 25362, you added a constrant or variable to your responce, adding clearance with unloaders which changes everything, I stated that we assumed we changed the flow by slowing the speed of the compressor, but without these engineered interventions (unloaders or speed), the machine can only lower throughput by lowering the suction pressure, there is not a source on this planet that has a gauenteed flow of gas at a set pressure. Once you lower the suction pressure because the supply souurce has not performed (a lost well, a lost train, the outside temperature dropping), the differential pressure and ratios will increase naturally. Increased ratios, more work lost to TEMERATURE (delta s) and more energy PER pound processed through the compressor.

 

[25362]

Mucour,

[•] A reciprocating compressor is a volume displacing machine.
[•] The pressure ratio may indeed change. Both, suction and discharge pressures are fixed by the system as indicated by dcasto.
[•] A reduced suction pressure with a constant discharge pressure would result in additional heating of the gas.
[•] The adiabatic temperature change is given by: T₂/T₁ = (p₂/p₁)^(k-1)/k where k=Cp/Cv.
[•] Assuming k, the volumetric efficiency, and T₁ are constant the power consumed by compression is considered proportional to (p₂/p₁)^(k-1)/k - 1.

 

[mucour]

DCASTO, 25362, Sorry to stretch this issue too long. Any opportunity for engineers to rub minds is always exciting.

A reciprocating compressor is a constant volume machine (am I mistaken?). The stroke is constant and that means it can only displace the gas from a point 1 to a new point 2. From the P-V curve, you have to compress "adiabatically" from V1 to V2 to achieve pressure increase from P1 to P2. If we reduce the suction pressure to say P0, lower than P1, a new discharge pressure P3 will also be lower than P2. The information I read from your threads- Dcasto said "Once you lower the suction pressure because the supply souurce has not performed (a lost well, a lost train, the outside temperature dropping), the differential pressure and ratios will increase naturally". Are you saying i will still get the same P2 as before. If that should happen, there will be an increase in temperature and the pressure ratio will also increase significantly and this can damage the machine.

In our reciprocating gas compressor station, there is a recycle valve that opens the moment the suction pressure has decreased below a set point. It bypasses the gas from the discharge back to the inlet. The reasons I believe the recycle valve is installed are to ensure that the discharge pressure is maintained constant and to avoid pulling vacuum.

 

[25362]

Mucour

[&bull;] A reciprocating compressor doesn't operate on a line on a P-V diagram. It does so on a cycle of four steps: intake, isobaric, from P1,V1 to P1,V2, compression from P1,V2 to P2,V3(<V2), discharge, isobaric, from P2,V3 to P2,V4(<V1), expansion from P2,V4 to P1,V1(>V4).

[&bull;] The steps are not smooth lines but rather ragged (valve effects). Note that P1 is lower than the suction line pressure, P2 is higher than the system downstream pressure, and V4 is greater than the clearance volume.

[&bull;] Take, for example, the reciprocating compressor in a hydrotreater. The suction pressure depends on the pressure ex a catalytic reformer which can go down. The discharge pressure not only can, simultaneously, be kept constant but even increased, for example by throttling a control valve downstream the hydrotreating reactor.

[&bull;] Thus the case of P_suct[&darr;] while P_disch[&uarr;] at the same time is a real possibility.

 

[mucour]

25362, thanks I understand the full curve. My previous thread just focused on the compression side , which is the point of interest where you get the compression side of the cycle. You are right and I am also aware that the isobaric sides is not a straight constant pressure line but ragged to use your word due to valve effect.
The question is: If the old suction pressure was 15psia and the old discharge pressure was 45psia. And the second scenario is that the suction pressure reduced to 10psia, will the new discharge pressure be higher than 45psia using this same compressor? The cylinder volume and stroke are the same in these two scenarios.

 

[Montemayor]

Mucour:

There is a large disconnect between the yourself and the quality and quantity of compressor know-how both 25362 and dcasto are so generously granting you.

Your understanding of how the basic positive displacement, reciprocating compressor works is just as weak as your recent post on centrifugal compressors. Please do not take my comments as derrogatory or insulting. This being a serious engineering forum, my intent here is to accurately and efficiently help you out - as 25362 and dcasto are attempting to do. However, I seriously believe that you should do research and study on the internal and external workings of compressors - both dynamic and positive displacement. Believe me, the time and effort will be well worth your while and you will better understand the principles on which they are based and manufactured. If you are able to obtain hands-on operation and maintenance of these type of machines, then by all means do so.

Nothing can be better in understanding the fine and detailed principles of the design and operation than being able to virtually disassemble one and succeed in putting it together again. This, coupled with a firm and expert knowledge of Thermodynamics will resolve any concerns or problems you may have with these machines in the future. I offer this as professional advice from one who has labored with these type of machines for 47 years. That's how I started my engineering career. Good Luck.

 

[25362]

A new 4.5:1 press. ratio or higher could be harmful to the compressor parts as well as to the lubricant due to higher discharge temperatures. That is assuming same k and T₁. A pressure or temperature protection may cut the unit out. New pressure ratios should be approved by the vendor.

At first hand it appears that the 5 psia reduction of suction pressure wouldn't endanger the motor mover, even when keeping the discharge pressure constant at 45 psia.

However, notwithstanding the lower mass of gas now compressed, the answer to your query lies on the discharge system's pressure controller, it could be [&uarr;], no change, or [&darr;].

With no pressure controller, as for example, in refrigeration, since there are now 1/3 less moles of gas compressed and only say a 10% increase in absolute temperature, the system friction resistance may drop reducing the discharge pressure.

Again, it is the system, and not the compressor, that determines the head pressure.

 

[mucour]

Montemayor, thanks for your posting. 47yrs of experience can realy help one to put things in better perspective which is the reason why questions are being asked from people that know or have worked with this equipment. Learning is continuous. So Montemayor bear with me.

What I can say is that there is a misunderstanding either from me or from one answering the question. The example I gave is hypothetical as I am not sizing a reciprocating compressor which is what I see 25362 is doing by telling me of the impact of high temperature due to a compression ratio more than 4. I am quite aware of the danger of a discharge temperature above 140deg C on the lubricant.

Straight questions demands giving straight answers. I appreaciate all answers bcos one can pick one lesson from it. This will prompt a repeat of the question if I believe my question has not been addressed.

I believe finally, 25362 has admitted and answered the question by his statement thus" However, notwithstanding the lower mass of gas now compressed, the answer to your query lies on the discharge system's pressure controller, it could be ?, no change, or ?."

In my question, I am assuming that the system resistance is negligible. So if the discharge system pressure controller does not impose higher back pressure to warrant an increase, the compressor discharge pressure can remain the same or lower.

Montemayor, I will still appreciate your take on the centrifugal compressor posting if you can.

Thanks 25362 and all for your patience.

 

[CompressorExpert]

As you know, reciprocating compressors designed for constant capacity. It means that your compressor must be fed by design capacity.
Ok. What will happen when inlet capacity decreased? In that case we must increase inlet capacity virtually by using of unloading devices (suction valve unloader, clearance pocket, by pass line and variable speed drive (it is not recommended for reciprocating compressor due to the critical speed of the system))'I have never seen variable stroke system for gas compressor before'.
By using of these systems the inlet volume will remain at design point and it means that your discharge pressure will remain constant.
If your capacity decrease and you don’t have any of these systems on your machine, then what will happen? During the suction process (when suction valve open) compressor sucks lower gas into the cylinder. Your piston stroke and cylinder capacity are constant, it means that you have lower gas in this constant volume and during compression stage your discharge pressure will be lower than design conditions.
But I never seen any gas compressor without capacity control devices.

 

[mucour]

CompressorExpert, It is interesting that basic fundamentals get one confused. This is just like a thread posted on centrifugal compressor under "Chemical Process Engineering group" It was put into proper pespective by JJPellin.

If you follow my response on the recip, I did mentioned that a reciprocating compressor is a constant volume machine. The stroke is constant, i.e moving from the Bottom Dead Centre to the Top Dead Center (using gas engine terminologies). As a result, I asked in my thread that if the suction volume reduces due to a depleting gas well or a loss in gas production, which causes the inlet pressure into the reciprocating compressor to reduce, the question was will the recip. compressor discharge pressure reduce also? This thinking was based on the fact that the compression ratio is constant. So since the pressure has reduced below the original pressure, I mentioned that I expected the discharge pressure to also reduce. In my thread I mentioned that a recycle line is used in our plant to ensure constant discharge pressure. It diverts the gas from the discharge to the suction if the suction pressure drops. Then a response came that the reciprocating compressor suction and discharge conditions are based on the system resistances. Yes I agree. So asked whether this remove the fact that the compression ratio is constant for the particular recip. machine?

So since that time I have been reflecting over it. lets use common home device. Using a car air tyre pump or bicycle pump. Assuming the car tyre is flat when you start pumping. The pump has a valve that opens each time it discharges air into the tyre. So lets say the pressure upstream the valve is P1 and the pressure downstraem the valve is P2. So what this means is that as far as the pump is taking air from the atmosphere, the pressure P1 remains constant, while P2 is the "system" that continues to buildup in pressure until it equals to the pressure required. If a scenario is created where atmospheric pressure reduces by 10%, I expect that P1 will also reduce.

In the case of a reciprocating compressor, you can still maintain a constant pressue in a scenario where the suction pressure has reduced by increasing clearance volume pocket. Otherwise, P1 will reduce.

If there is no form of flow control to recycle the gas back to suction and no unloaders, then if in a scenario that the suction pressure drops and the pressure downstream the valve is at that time higher than upstream (in a system with back pressure control valve), the gas will be trapped inside the cylinder bore since the valve is a check valve and it cannot open against a developed back pressure. It can only overcome the back pressure if the pressure builds up again with several strokes of compression inside the cylinder. But this would have been damaging to the compressor because of the temperature built up due to the energy expended and possibly compressor damage.


I my missing anything?

 

[dcasto]

The discharge valve WILL always open on the compression stroke, no matter ahat the suction or discharge pressure is. Now what I think you are proposing is a senerio like this: Assume a suction pressure of 1 psia discharge 30 psia: What will happen is that the volumetric effiency will drop below 0% and the compressor will not move any gas, it may take a ratio of 30 to 1 or more, and in your scenerio, the gas may get to hot (how about N2 at -150F). This 30 psi differential may not break a rod, but it will see a temperature much higher.

Also, remember, when you compress the gas it gets hot, if the gas doesn't leave the cylinder it will expand and get cooler (not quite starting temp)