Wellhead Compression – Varying Speed

If a compressor has an excessively high second-stage (crankend) discharge temperature and a low first-stage (head-end) discharge temperature, one should proceed as follows:
• Reduce the adjustable clearance on the head-end.
• Slow the machine down.
• Balance the above two steps to restore the original wellhead pressure.

This technique switches load from the crank-end to the headend without changing gas flow. Note that to minimize horsepower the pressure ratio for both stages should be about equal. Operating with the “head-end” cylinder clearance valve wide open will tend to over-load the crank-end, under-load the head-end and waste net engine horsepower. Regardless of other circumstances, a compressor should never be run over its rated speed. However, if the machine will not come-up to its rated speed when it is runnning below its rated horsepower (as calculated above), then something is amiss with the engine.

To further complicate the adjustment of a field compressor, one needs to be aware of certain transient effects that the well imposed on the compressor.
• Many wells, immediately after unloading liquids exhibit an increase in wellhead pressure sufficient to overload and stall the engine.
• Opening the head-end cylinder clearance valve to reduce the first-stage discharge temperature will immediately increase this discharge temperature and can trip-off the compressor. However, once the wellhead pressure rises due to less gas being moved, the head-end discharge temperature will drop.
• After switching a compressor from single-stage to tandum operation, the second-stage discharge temperature will tend to increase for a few days as the wellhead pressure drops. This often leads to compressors tripping off unless corrective action is taken.
• The immediate effects of soap-sticking a well (i.e. unloading liquids by adding a foaming agent into the well’s tubing) may be to over-load the engine due to excessive suction pressure.
• A compressor which has operated properly in a tandum mode is shut-down for maintenance and thereafter repeatedly trips off on high discharge temperature. The problem is that the well has loaded-up with liquids and the resulting low wellhead pressure is causing too high a compression ratio.

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