Compressor Duty

The first step in specifying compressors is to define the required compression duty. This includes defining:
• flow rate
• gas composition
• suction pressure
• suction temperature
• discharge pressure

The Compressor Pressure Profile Worksheet, CMP-EF-876, in the “Standard Drawings” section of this manual may be used for determining and recording inlet and discharge conditions.

These conditions should be confirmed and the variability of the conditions determined.

In most applications, future, or alternate operating conditions can significantly affect the sizing and characteristics of the compressor system. Following is a list of typical changes affecting compressor duty:
1. Build-up in discharge pressure of gas injection compressors as the resistance of the oil field formation increases with time, or as more compressors are added to the system.
2. Increase in flow rate and change in molecular weight of gas gathering system as an oil-field’s gas-to-oil ratio increases with time.
3. Large change in molecular weight, flow, and pressures during periodic catalyst regeneration in a process plant.
4. Increase in system pressure drop due to fouling of equipment during a run.
5. Change in feedstock to a process unit.
6. Seasonal changes of ambient temperature and cooling water temperature.
7. Start-of-run (SOR) versus end-of-run (EOR) conditions in a refining unit.
8. Routine turndown of compressor capacity.

Initially, you must accurately determine the pressure/flow envelope where the compressor operates. If available, review the process flow diagram and a pressure profile in detail with the operating, or project representative to assure that all factors have been considered. If a process flow diagram is unavailable, make a sketch of the flow circuit. If you question the pressure drop allowances, review the calculations with the operating, or project representative. Refer to the Fluid Flow Manual for information on pressure drop calculations.

It is extremely important that you accurately determine the suction pressure. For example, if a four-stage reciprocating compressor is sized for compression from 20 to 4400 psia, and the actual suction pressure turns out to be 25 psia, the actual weight flow will be 25% higher, and the horsepower required 19% higher than predicted.

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