The first step in troubleshooting reciprocating compressors is to quantify the extent of the problem. How much compression work has actually been lost? An approximate rule of thumb is:

Inserting the data from the El Gringo operation in the above equation I found:
HP (current) = 90 (1100/850 – 1) .520/ 520 = 2JL9
HP (two weeks ago) = 100 (1100/800 – 1). 520/520 =37.5
(Note that the station inlet temperature had remained constant at 60°F)

No wonder my client had chased me out of that courtroom in New York: useful compression work had dropped by 28% in just two weeks

The next step in my investigation was to decide if the lost compression work was due to an engine deficiency or a compressor problem. To ascertain that a gas engine driver is not limiting compression work, the following questions should all be answered in the affirmative:

• Are all engine exhaust gas temperatures running below maxium?
• Is the compressor running at its rated speed?
• Is the fuel gas manifold pressure below maximum? (At a constant speed, the engines torque is linearly proportioned to the fuel gas manifold pressure.)
• Are all unloader pockets closed?

For the El Gringo station, the answer to the above questions was yes. Hence, it was not the gas engine’s fault that I was ruining my expensive Italian shoes.

Next, I checked the unloader pockets. An unloader is a mechanical devise used to reduce the capacity of a compression cylinder, without reducing the compressor’s efficiency. Figure 7—1 illustrates the function of an unloading pocket. By increasing the clearance between the piston and the cylinder head, the volume of the gas compressed per stroke is reduced. As the engine was not limiting, and we were trying to move maximum gas, all the unloader pockets were closed.