I am reviewing one vendor offer for HP Gas compressor. It is gas compressor used in gas processsing train upstream oil and gas Industry to boost the operating pressure of the gas. There are total 4 operating cases. Compressor is requried to design to increse the pressure from 1.05 MPag to 4.7 MPag (Refer Attached HP Compressor Datasheet for more Data). It is two stage compressor. Further please note that For alternate cases it is envisaged to use the same designed compressor to operate at 7.0 Mpag suction pressure to 4.7 MPag discharge pressure. Low suction pressure(e.g. 7.0 MPag) is due to a fact that after prolonged production from wells, the operating pressure of incoming fluid will decrease. Capacity required for comoressor is 1.6 MMSCMD for normal case. We asked vendor to design machine for 1.6 MMSCMD at 1.05 MPag suction pressure. And also asked vendor to give flow rate when operate at less suction pressure as above.
We received the vendor offer (Refer attached Vendor Offer). As per vendor offer Compressor will deliver less flow rate during alternate operating case (low suction pressure case). Now here I have confusion. My limited knowledge about the reciprocating compressor is not makeing clerity on this point. As I know the compressors are constant volume machines. So here, when suction pressure is decresed means density is decreased. That means low mass will flow from the compressor resutling in low power consumption. But I am not able to understand how the capacity is reduced for low suction pressure case. But RPM of motor remains same. Does it mean the same energy from motor is delivered to the fluid ...then how come power is reduced.
Appriciate if you would throw some light in this case. I mean how compressor capacity will be reduced in this alternate case (low suction pressure case)