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# Minimum Pressure to Start Cycle Gas Compressor

## Minimum Pressure to Start Cycle Gas Compressor

(OP)
Hi everyone,

I got a case study about cycle gas compressor in polyolefin industry. The cycle gas compressors are used for the gas phase polymerization. Ethylene and other raw materials as well as catalyst are fed continuously to the reactors, the gas is cycled using cycle gas compressor type centrifugal. I was given compressor data sheet. Datas given: flow rate, suction condition (pressure, temperature, MW, compressiblity factor, cp/cv, inlet volume flow), discharge condition (pressure, temperature, cp/cv, compressibilty factor), polytropic efficiency, polytropic head,flow coefficient, total bhp-all loss.

Then I was asked to calculate minimum pressure to make the compressor online without surge. As I looked for in the journals, I found the inlet volume calculation and head calculation, but I haven't found the minimum pressure calculation.

Is there any other data needed to calculate minimum pressure? Then, may be if you know the equation to calculate minimum pressure to make the compressor online, please let me know.

Thank you.

### RE: Minimum Pressure to Start Cycle Gas Compressor

You need to find the compressor pressure map specific for that compressor.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Minimum Pressure to Start Cycle Gas Compressor

On an existing system, minimum pressure is the cracking pressure of the check valve normally located just downstream the anti-surge loop take off. When you reach that pressure the flow will start to deliver to the downstream system, the anti-surge control system will command the anti-surge valve to gradually close. Below that pressure (sometimes speed is specified as trigger); the anti-surge control system does not switch to auto mode - the anti-surge valve is fully open and compressor run in recycle at low head (more like a fan).

If you are investigating however that pressure (say in conceptual design phase) and assuming machine operates on single speed curve the closest point to the surge which still maintain a safety margin for the machine to not surge is located on the control line (which is typically set 10 percent right off the surge line). The corresponding pressure maybe the pressure you are looking for but it is NOT a minimum. The minimum pressure is on the far right side of the curve and correspond to a choked flow. That is by the way the most typical path when starting a compressor, that is to say to position the operating point on the right side of the curve and gradually bring the machine to its nominal operation (that is to say, following control sequence whereby anti-surge valve close and after check valve opens the compressor can be put online). Definitely you need the actual compressor performance curve / map. You also need to define the process control method (which type of driver is considered is also part of the picture).

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