Dear All,
Setting the number of compression stages is a strong function of the discharge temperature limitation needed to protect the compressor system as well as power consumption across each stage. For a multi-stage reciprocating HC injection compressor (currently being designed), vendor confirmed that four stages would be required for the inlet suction pressure of 1.5barg to be increased to a normal rated discharge pressure of 200 barg.
However, when it comes to setting the discharge design conditions of each stage (temperature and pressure), specific excursion margins are applied on top of the maximum operating discharge pressure and temperature in order to allow for potential increase in the operating conditions in case of increase in the backpressure seen by the last compression stage and such that sufficient margins are maintained below the alarm and trip set points.
Now, the question is:
I set the discharge discharge design pressure at the last stage to be 250 barg with discharge design temperature of 180 C. I was questioned by the rotating engineer that now the number of stages will have to be increased to five stages instead of 4! I explained to him that design conditions are set for the system's rating while we use the rated operating parameters in determining the number of compression stages. The 250 barg design pressure was specified to allow some margin for potential increase in backpressure due to wellhead injection pressure requirements knowing that with an increase in the CR from 200 to 220 for example, due to the large temperature term as X+273.15 K , the change in the temperature term would not be highly increased.
However, when it comes to power consumption, would really the discharge PSV set pressure at each stage have to be used when estimating the power and rod load?
Therefore, when determining the number of compression stages, we will be basing our selection based on rated conditions or we shall confirm the power requirements even when discharge PSV set point is actually reached?