Page 308 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 293

TABLE 6.4 Standard Instrumentation for Oil-Flooded Screw Compressors

Suction Pressure Alarm Low Trip Low
Suction pressure Alarm high Trip high
To prevent overcompression
or high torque
Discharge pressure Alarm high Trip high
Suction temperature Alarm high Trip high
Discharge temperature Alarm high Trip high
Thrust bearing temperature (if Alarm high Trip high
hydrodynamic)
Journal bearing temperature Alarm high Trip high
Casing vibration or shaft vibration Alarm high Trip high
monitoring a
Shaft axial thrust Alarm high Trip high
Pressure difference between lube oil and Alarm low Trip low
discharge gas
Alarm high
Lube oil temperature Alarm high Trip high
Start-up interlock low
Shaft seal oil flowrate Alarm low Trip low
Alarm high Trip high
Driveshaft seal oil pressure Alarm low Trip low
Alarm high
Temperature difference driveshaft seal Alarm high Trip high
oil outlet over inlet
a
Due to the stiff casing design, 1 accelerometer per compressor or 1 accelerometer at each compressor
end (max 2 in total) is sufficient for automatic monitoring with shutdown.

The pressure ratio is limited by the discharge temperature and the under-
compression. Extreme undercompression may cause severe gas pulsations in
the discharge line. If the pressure ratio is too high the discharge temperature
may be higher than allowable for the rotor clearances. The discharge tempera-
ture depends also on the gas molecular weight and the isentropic exponent. For
dry screw compressors without liquid injection the maximum pressure ratio
may range from approximately 2.5 for light gases up to approximately 6 for
hydrocarbons with molecular weights of 50 or higher. Again a case-by-case
decision is required.