As the wellhead pressure falls, the differential pressure that the field compressor must deliver increases. This is because the collection header into which the compressor discharges remains relatively constant. As this differential pressure rises, the compressor may become limited by “rod loading”. A machine may be only utilizing a fraction of the available engine horsepower and trip-out due to low suction pressure or high discharge temperature. Both of these trip points are a function of the maximum compressor rod loading which, is, in turn, a function of the differential pressure across an individual stage and the cylinder geometry. Note that at a fixed discharge pressure, a falling suction pressure always results in an increase in discharge temperature.
Naturally, operating field personnel will try to avoid repeated compressor shut-downs due to low suction pressure or high discharge temperature. The proper response would be to convert the compressor from single-stage to tandum (i.e. two-stage) operation. However, for reasons enumerated below, field personnel may choose to remain on single-stage operation and:
• If on crank-end operation, reduce rpm.
• If on head-end operation, open the cylinder clearance valve.
Both of these methods will effectively eliminate trips caused by high discharge temperature or low suction pressure. Unfortunately, they also reduce natural gas production. Why is it then, that operating field personnel do not go immediately to tandum operation to eliminate trips caused by excessive rod loading? A few of the reasons are:
• Making the conversion requires tools, valve parts and time. Also, the machine must be shut-down and re-started.
• Often, the well will produce large quantities of water or condensate for several hours after the tandum operation is initiated. The vapor-liquid separator drum on the compressor suction line may not be able to keep up with the liquid flow. Manual draining of the drum is therefore appropriate. In practice, this means that an operator must remain at the well site for half a day to monitor and control the liquid level in the compressor suction drum.
• It is human nature to avoid step-changes. Converting from single stage to tandum operation entirely alters the wells characteristics; whereas small reductions in speed or suction volume may be made gradually over a period of time.
Converting to tandum operation reduces the rod loading by spreading the differential pressure out over two stages. For a given wellhead pressure, the two-stage operation also lowers the compressor discharge temperature.