5 replies to this topic

[prashant.badgujar]

Dear Members,

I am working on reciprocating compressor blowdown calculations for compressed gas lift operations and one option client has requested is to reroute the blowdown to inlet low pressure gas gathering system. I need to validate that the settle out pressure post blowdown does not exceed the line design pressure. The automatic blowdown line has a restriction orifice after the blowdown valve. This is my first time doing these calcs so any inputs are appreciated.

 

Regards,

Prashant

[fallah]

 

I am working on reciprocating compressor blowdown calculations for compressed gas lift operations and one option client has requested is to reroute the blowdown to inlet low pressure gas gathering system. I need to validate that the settle out pressure post blowdown does not exceed the line design pressure. The automatic blowdown line has a restriction orifice after the blowdown valve. This is my first time doing these calcs so any inputs are appreciated.

 

 

Hi,

 

The term of  "Settle Out" pressure is actually related to a CENTIFUGAL compressor and there is no same thing for a reciprocating compressor. A reciprocating compressor is a positive displacement machine and unlike a centrifugal machine, discharge gas cannot freely flow from the discharge side back to the suction side through the compressor and the check valves on discharge and suction of any reciprocating compressor cylinder only allow the gas to flow downstream.

But in many reciprocating compressor systems there is a recycle valve around the whole system from the last stage discharge to the first stage suction which has to be opened after every compressor trip to be able to overcome the restarting torque. Obviously if the recycle valve doesn't open after a compressor trip, blowdown operation should be applied on the compressor train for subsequent compressor restarting.

Then, from "Settle Out" pressure if you mean such recycle valve/line that has to be opened after every trip to equalize the pressure of the first stage suction and the last stage discharge of a reciprocating compressor, yes the equalized pressure after such settle our shouldn't be exceeded the design pressure of the suction line.

[prashant.badgujar]

 

 

I am working on reciprocating compressor blowdown calculations for compressed gas lift operations and one option client has requested is to reroute the blowdown to inlet low pressure gas gathering system. I need to validate that the settle out pressure post blowdown does not exceed the line design pressure. The automatic blowdown line has a restriction orifice after the blowdown valve. This is my first time doing these calcs so any inputs are appreciated.

 

 

Hi,

 

The term of  "Settle Out" pressure is actually related to a CENTIFUGAL compressor and there is no same thing for a reciprocating compressor. A reciprocating compressor is a positive displacement machine and unlike a centrifugal machine, discharge gas cannot freely flow from the discharge side back to the suction side through the compressor and the check valves on discharge and suction of any reciprocating compressor cylinder only allow the gas to flow downstream.

But in many reciprocating compressor systems there is a recycle valve around the whole system from the last stage discharge to the first stage suction which has to be opened after every compressor trip to be able to overcome the restarting torque. Obviously if the recycle valve doesn't open after a compressor trip, blowdown operation should be applied on the compressor train for subsequent compressor restarting.

Then, from "Settle Out" pressure if you mean such recycle valve/line that has to be opened after every trip to equalize the pressure of the first stage suction and the last stage discharge of a reciprocating compressor, yes the equalized pressure after such settle our shouldn't be exceeded the design pressure of the suction line.

 

Hello Naser,

Thank you very much for your insights on this topic and I agree it should be equalization pressure more than settle out. I have attached the system sketch to simplify. I am trying to calculate the equalization pressure in the low pressure piping once the blowdown valve opens in case bypass valve does not open. One approach I see is to calculate flow of gas through RO for choked conditions and see at what pressure this volume of gas can fit into the volume of downstream piping, does this make sense? Let me know if you think otherwise, I am new to these calculations.

 

Regards,

Prashant

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Edited by prashant.badgujar, 04 January 2024 - 03:40 PM.

[shvet1]

@prashant.badgujar

I do not understand the core idea behind your logic. Please describe in details the overpressure scenario.

 

All reciprocating compressors in engineering I have been involved in had one of options below:

- design pressure (or max allowable working pressure - depends on jurisdiction) of suction side is the same as discharge side, or

- suction side is protected by PSV and compressor's manufacturer guarantees max credible backflow from discharge to suction

[fallah]

Hello Naser,

 

 

Thank you very much for your insights on this topic and I agree it should be equalization pressure more than settle out. I have attached the system sketch to simplify. I am trying to calculate the equalization pressure in the low pressure piping once the blowdown valve opens in case bypass valve does not open. One approach I see is to calculate flow of gas through RO for choked conditions and see at what pressure this volume of gas can fit into the volume of downstream piping, does this make sense? Let me know if you think otherwise, I am new to these calculations.

 

Regards,

Prashant

 

 

Hi,

 

You can calculate the equalized pressure (here it may called settle out pressure) between discharge side (the volume between the last stage discharge nozzle and the blowdown valve) and suction side (the volume between the first stage suction nozzle and the blowdown valve) after opening of blowdown valve while high discharge pressure at downstream propagate toward suction side through the mentioned valve.

Then you can check if the suction design pressure is adequate to cover such settle out pressure at blowdown valve opening after compressor trip.

[prashant.badgujar]

 

Hello Naser,

 

 

Thank you very much for your insights on this topic and I agree it should be equalization pressure more than settle out. I have attached the system sketch to simplify. I am trying to calculate the equalization pressure in the low pressure piping once the blowdown valve opens in case bypass valve does not open. One approach I see is to calculate flow of gas through RO for choked conditions and see at what pressure this volume of gas can fit into the volume of downstream piping, does this make sense? Let me know if you think otherwise, I am new to these calculations.

 

Regards,

Prashant

 

 

Hi,

 

You can calculate the equalized pressure (here it may called settle out pressure) between discharge side (the volume between the last stage discharge nozzle and the blowdown valve) and suction side (the volume between the first stage suction nozzle and the blowdown valve) after opening of blowdown valve while high discharge pressure at downstream propagate toward suction side through the mentioned valve.

Then you can check if the suction design pressure is adequate to cover such settle out pressure at blowdown valve opening after compressor trip.

 

Thank you, Naser for your inputs, really appreciate it. I was able to calculate the equalizing pressure.