The PLC makes process choices using logic that involves certain restrictions. Some of the more important constraints are as follows:
Limit operation of only a single unit to compression into only one pipeline.
Compress no more than two units into one pipeline.
Operate the three units to meet pipeline load demand while keeping the K3 unit's (the 14,000-horsepower unit) stack emissions within hourly, daily, and annual permit limits.
Allow designation of any unit as the base load to L400 or to L401, or as a standby unit, and provide for changing the designations while the units are online.
Do not allow start-up of a third unit unless the station is actually compressing into separate headers.
In order for the station to operate in common header, keep the actual line pressures within 25 psig of one another and the pressure set points for each line within 10 psig of one another.
Because the size of units K1 and K2 does not match the size of the unit K3, a start of new unit in parallel with the unit already online requires different logic set for each combination of the units and also for controlling the two station cold recycle valves.