The easiest, but least cost effective method, to operate a field compressor is the crank-end mode. When only the Crank-end (i.e. second stage) is in operation, capacity, compression ratio, as well as engine horsepower load and compressor rod loading are minimized. Left to their own devices, field personnel oft-times run compressors on the crank-end only. To increase the wellhead tube velocity, it is usually necessary to switch the compressor operation to the headend mode. This involves removing the crank-end cylinder valves and re-installing the head-end cylinder valves.
The head-end cylinder clearance valve should then be closed as far as possible so as to fully utilize the available engine horsepower. To calculate approximate horsepowerT the following equation may be used:
Maximizing engine horsepower and hence gas flow immediately after switching to head-end operation is helpful in achieving the tubing entrainment velocity. A gradual increase in gas flow will not be as effective in unloading the well. Therefore, the engine rpm should be set at maximum and the head-end cylinder clearance setting should be minimized as soon as the machine is put back on line.