8 replies to this topic

[Papps]

Gas gets separated in Separators and feeds to the Engine Driven Reciprocating Compressors which converts 10 bar to 75 bar, which in turn partially feeds to another Engine Driven Reciprocating Booster Compressor which converts 75 bar to 120 bar and discharge of the Compressor goes as Lift Gas for the Wells. Rest of the 75 bar Gas goes to Gas Treatment Plant for further treatment.

 

Suction Pressure of the Booster Compressor is controlled by the Recycle Valve(1) (Spill back from discharge to suction) whereas Discharge pressure of the Compressor is controlled by the another Recycle Valve(2) (Spill back from Compressor discharge header to Compressor Suction header).

 

In case of any drop in discharge pressure, pressure can be increased by Engine speed (RPM) and in case of any increase in discharge pressure Recycle Valve (2) will open and maintain the discharge pressure as per the discharge pressure controller set point.

 

Any increase in Gas consumption in Lift Gas leads to Compressor discharge header pressure dropping below the required pressure. To maintain the required discharge pressure operator increases the Engine Speed (RPM) and vice versa for the other case (decrease in Compressor discharge header pressure).

 

As Gas flow rate to Lift Gas header is not constant to meet the discharge header pressure operator either increases/decreases the Engine RPM MANUALLY. Any opening of the Recycle Valve(2) causing flow disturbances which is causing upsets in Gas Treatment Plant.

 

Based on the above operation it is clear that primary control to maintain discharge header pressure is ENGINE SPEED.

 

Can we introduce a control system to maintain the Compressor discharge header pressure by modulating engine speed (Ramp up/down based on the set point) and also retaining the existing control system 

 

Will it work better than the current control system we have ???

 

What impact it will have on engines if the speed varies continuously by adopting to the proposed control system ????

 

It may not be the best control philosophy (proposed) so requesting to suggest any better adopted control philosophy currently used in these type of systems.

[Art Montemayor]

Why can't you simply draw a detailed sketch showing the control valves and the piping?

 

That would be so much simpler and easier to understand what you are operating.

 

A process can never be accurately described by words like a detailed drawing.

 

Additionally, it is not the engine speed (or the volumetric capacity that determines any reciprocating compressor discharge pressure.  It is the discharge resistance or existing process pressure that the discharge flow encounters.  Compressors, like pumps, are not pressure deliverers.  They are flow deliverers.  The resistance that is built up or introduced in the discharge piping or at the end of the piping is what determines the discharge pressure.  Your understanding of how a reciprocating machine works is not correct.   Discharge pressure is a result of resistance to flow.  You need to show in a sketch how the resistance exists or is applied.

[Papps]

Sir, Thanks for your reply.

 

I do understand that only the resistance caused in the pipeline determines the pressure of the reciprocating compressor and not the speed.

 

In our case, Drop in discharge pressure is due to the sudden consumption of gas and Increase in discharge pressure is due to reduction of gas consumption.

 

Increase in discharge pressure is taken by the Global Recycle PCV (2) whereas decrease in discharge pressure making the operator to increase the RPM requiring manual intervention.

 

What sort of control philosophy works best to reduce reduce the manual intervention caused due to the reducing in discharge pressure?

 

What are the general control philosophy used for controlling the discharge pressure of the engine driven reciprocating compressors?

 

Speed control system has been considered only to reduce the Global recycle PCV (2) opening as it causing upsets in Gas Treatment Plant.

 

Any other better control  philosophy shall be suggested.

 

Please go through the attached schematic for your reference.

 

I hope now the text can be correlated with the diagram for better understanding of my query.

 

 DIAGRAM.pdf   16.45KB   194 downloads

[Bobby Strain]

 You can ask the compressor manufacturer if it is permissible to vary the speed, by how much, and at what rate. Once you determine this, you can implement compressor speed control to minimize power (fuel) requirements. What control you implement depends on the answer to these questions.

 

And you are controlling the suction to the second compressor at the outlet of the gas plant, too. This is probably causing some problems, too. Unless you are not showing all the controls at the gas plant. With what you show, there is only one flow to the gas plant that will satisfy both pressure controllers. You need to do a lot more work.

 

Bobby

Edited by Bobby Strain, 15 July 2015 - 06:45 PM.

[kiansafaie]

Dear Paps
Considering that reciprocating compressor incresase pressure unlimittedly with blockage or decrease down stream consumption, the present control philosophy is not true(Use of 2 pcv on discharge of compressor lead to conflicting)
for each compressor discharge pressure shall be controlled by pcv due to risk of high pressure on discharge and recycle it to related suction.
Best Regards

[Graduate process ENG]

To maintain discharge pressure of compressors through speed not a good practice and also very challenging.

Insure why sale gas taken from first discharge compressor .This process will always result in fluctuations in the pressure to whole process.I am assumed because of different pressure between frist and second comps but again same process could be achieved through second comp.

You may need to consider install PCV downstream of second compressor instead taken from first comps . This PCV will take any excess gas from gas lift to Sale gas and maintain discharge comp pressure constant.You can set this control at required pressure for gas lift and comp discharge pressure .

[Papps]

Thanks all for your replies.

[Mahdi1980]

Dear Friends

It is a good discussion. So, please let me know more.

I have a process in which recycle gas should be handled to a reactor via compressor and a reciprocating compressor is available elsewhere which technical specifications is 2900 kg/h flow rate and suction pressure is 15~20 barg and discharge pressure is 35 barg.

However,the available condition for recycle gas is about 3000 kg/h at 28 barg which should be transferred to a 32 barg reactor.

If I correctly understand, I can reuse the available reciprocating compressor for this process due to the fact that the flow rate is the same and discharge pressure depends on the upstream resistance. Does it mean that the discharge pressure will be regulated automatically on for example 33 barg to go easily into reactor?  Is it wright ?

[Art Montemayor]

Mahdi1980:

 

As I have stated many times in the past: PLEASE FURNISH A P&ID OR AT LEAST A FLOW DIAGRAM when defining a chemical engineering process.  It is exceedingly difficult to interpret what you are trying to define with words.  For example, do you mean to state the following:

 

I have a proposed reaction process which requires 3,000 kg/h of recycle gas at 32 barg in the reactor.  In order to supply the required recycle gas I have an existing available reciprocating compressor that is specified as having a capacity of 2,900 kg/h of recycle gas at a suction pressure of 15~20 barg and discharge pressure is 35 barg.  The existing available recycle gas is 3,000 kg/h at 28 barg.

Can I employ the available reciprocating compressor for feeding recycle gas to the proposed reactor?

 

If the above is what you mean to communicate, then my response is:

• First, verify the mechanical and volumetric specifications and ratings for the existing compressor with the manufacturer;
• Verify the ability of the compressor to handle the recycle gas (unidentified as to composition) with the manufacturer at the required temperatures and pressures.
• If the manufacturer concurs with the application, then use the existing compressor - always with the correct and safe operational instrumentation controls.  This includes starting and stopping procedures.

​If I have misinterpreted your query, then specifically tell us what you mean.