The number of stages is governed by the following factors:
1. Allowable discharge temperature.
2. Rod loading.
3. Existence of a fixed sidestream pressure level (where flow is added to or withdrawn from main flow of compressor).
4. Allowable working pressure of available cylinders.
Discharge temperature is the most important factor affecting the number of stages. Class A and B reciprocating compressors are generally limited to 300°F for most gases in upstream and downstream plants. API 618 further limits the discharge temperature of hydrogen-rich gases to 275°F. These limits restrict the stage pressure ratios. It is often necessary to increase the number of stages so that intercoolers can be added to keep the discharge temperature within limits, while achieving the required overall pressure ratio. Knowing the discharge temperature limit, Equation 300-21 can be rewritten to find the allowable pressure ratio as follows:
Figure 300-9 may also be used to find r (pressure ratio) corresponding to a given discharge temperature limit.
Adding intercoolers to a centrifugal compressor tends to save horsepower. With reciprocating compressors, however, there will seldom be any benefit in adding intercoolers beyond those needed to maintain discharge temperature limits. The reasons are: (1) reciprocating compressors are already highly efficient, and adding an intercooler adds pressure drop which offsets the power savings, and (2) addition of a stage requires additional cylinder(s), pulsation dampers, knockout drum and piping.
The rod-load limit can affect the number of stages since the combined rod loading is related to the differential pressure across the cylinder. Increasing the number of stages obviously reduces the differential pressure of each stage. Quite often a rod loading problem can be solved by using two cylinders for one compression stage. In this case, the differential pressure would remain the same, but piston area, upon which the differential pressure acts, would be reduced. Rod loading is covered in further detail in Section 350, “Selection Criteria.”
Sometimes a compressor application has more than one suction or discharge pressure level. For example, in an oil field gas system, the compressor may take different quantities of gas from the separator at two pressures, say 40 and 250 psig. This machine could also be required to deliver a portion of the gas at 1000 psig for gas lift, and the remainder at 2500 psig for injection back into the formation. In this case, these pressures would set the interstage pressures so that the sidestreams are accommodated. Note also that two stages might be required between the 40 and 250 psig levels (depending on suction temperature and k value) to stay below the discharge temperature limits.
Allowable Working Pressure. Occasionally a given pressure ratio might be achieved in one stage with satisfactory discharge temperature and rod loading, but an actual cylinder does not exist to handle both the capacity (ICFM) and pressure. In these situations, it is necessary to use two stages, or use two smaller single-stage cylinders depending on hardware and economics.