There are three fundamental limits to which all field compressors are subject:
• Compressor rod loading
• Speed
• Engine horsepower
In addition to calculating the actual engine horsepower by the above equation and comparing it to the name plate rating, the engine exhaust gas temperature should be checked. The engine manufacturer specifies a maximum exhaust temperature for the engine when running at maximum load. If this design temperature is 750°F, while the observed engine exhaust is 600°F, it is quite apparent that the engine is not running at its maximum load. On the other hand, if the cylinder clearance valve is closed a few turns, and the machine slows down (or even stalls) the engine is positively working as hard as it can. Of course, as with a car engine, adjustments to the carburetor and ignition systems can correct horsepower limits.
Do not forget that for a field compressor to develop its rated horsepower, -it must be operating at its maximum design speed. Slowing an engine down without reducing its horsepower load will raise the temperature of the exhaust gas. To economize on the available engine horsepower one can:
• Minimize pressure drop between the wellhead and the compressor suction. If the pressure difference between these two points exceeds 10 PSIG, there is an unnecessary restriction to flow. Perhaps the positive choke in the wellhead has not been removed. Oft-times the surface piping diameter has not been sized for low pressure gas. Gas heaters, necessary to prevent hydrate formation on high-pressure wells, should be by-passed when field compressors are installed.
• Withdraw gas from the suction of the compressor, rather than the discharge, for engine fuel. A 100 horsepower compressor will require 30 MSCFD of fuel or several percent of the unit’s capacity.
• Do not simply disable compressor valves when either the headend or crank-end is to be taken out of service. Remove the valve assembly completely from the cylinder. Even though the valve plate may have been removed from the suction valve, the remaining portions of the valve will still offer a substantial resistance to flow and hence absorb horsepower.
• By-pass the inter-cooler when on “crank-end” operation; alternately by-pass the after-cooler when on “head-end operation. • Wash the inter-cooler fin tubes to remove bugs and dust. Compressor horsepower required is proportional to gas inlet temperature.
