Next, check the pressure rating of the existing unit:
During manufacture, the casing was hydrotested to 1½ times the maximum operating pressure (nameplate rating). If the pressures involved in the rerate exceed the nameplate rating, it will be necessary to re-hydrotest the casing for the new conditions.
Note the following items:
• It may be necessary to check with the manufacturer to confirm that the casing design pressure is adequate for rerating and rehydrotesting.
• Compressor operating characteristics, relief valve settings, or settle out pressures may set the maximum operating pressure.
• If set by compressor characteristics, use pressure rise to surge at maximum continuous speed.
• Side stream or compound compressors may have been hydrotested by sections with a different pressure for each. Check each section for compatibility with new conditions.
Check the compressor to determine its capability of producing the head required. Use Equation200-3 to calculate the head for the rerated condition based on the desired pressure ratio. An attempt may be made to re-use some or all of the existing impellers, based on an overall polytropic efficiency of 70% for the initial estimate. Initially estimate the speed from the affinity law (see later discussion regarding speed limitations):
N1 = original speed
N2 = rerated speed
Hp1 = head for rerated pressure
Hp2 = head for original pressure
This same procedure will work for applications involving side loads or intercooling between sections. The head for each section is determined based on the conditions for that section, and the total head is the sum of the individual section heads.