Process Description

The process gas to be compressed enters the suction of the first stage of the reciprocating compressor through a suction pressure control valve. The make-up process gas is mixed with the kickback flow before passing through the inlet cooler.

The process gas leaving the first stage of the compressor passes through an inter cooler before being compressed by the second stage of the compressor.

The compressed process gas is then drawn off by users from the discharge of the second stage. Excess gas may either by sent back to the suction of the first stage through the discharge pressure control valve, or may be vented.

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Approximately 2500 PPH of process gas is to be compressed from an upstream process gas pressure of 100 PSIG to a downstream process pressure of 800 PSIG. The suction pressure of the first stage of the reciprocating compressor is maintained at 62.5 PSIG and the discharge pressure of the second stage is maintained at 1000 PSIG. The pressure between the two compressor stages is approximately 259 PSIG, resulting in a compression ratio of approximately four to one for each stage.

The discharge temperature of each of the stages is maintained at less than 500 Deg F. This is accomplished with coolers that maintain the inlet of each stage at no more than 100 Deg F.

The compressor runs at 500 RPM and consumes energy at a rate of approximately 221 horsepower.

Under design conditions, the compressor is capable of displacing approximately 2500 PPH of gas. This capacity can be doubled by either loading all of the clearance pockets or by doubling the speed of the machine. Please note that the same number of clearance pockets in each stage should be loaded at all times. Individual controls of the clearance pockets for each stage have been provided for training and demonstrative purposes.

Both the inlet cooler and the inter cooler have been designed with sufficient capacity to maintain the design suction temperature for each stage, even when the machine is running on total kickback with all the clearance pockets loaded.

All the valves in the system have linear flow characteristics, and have been designed with sufficient capacity to allow the processing of approximately 5000 PPH of gas under design pressures.

Stage 1 suction pressure is controlled by PIC-101. Upstream process gas temperature and pressure are indicated by TI-100 and PI-100 respectively. Makeup gas flow is indicated by FI-100.

The mix temperature of the makeup gas and the kickback gas is indicated by TI-101. Suction temperature to the first stage is controlled by TIC-102.

The discharge temperature and pressure of stage 1 is indicated by TI-103 and PI-102 respectively. The flow through stage 1 is indicated by FI-101.

The suction temperature of stage 2 is controlled by TIC-104. The discharge temperature of stage 2 is indicated by TI-105 and the flow through stage 2 is indicated by FI-102.

The gas demand is controlled by FIC-105 and the pressure of the downstream process is indicated by PI-105.

Discharge pressure of the second stage is controlled by the kickback controller PIC-103 and the kickback flow is indicated by FI-103. Discharge pressure may also by controlled by the vent controller PIC-104. Vent flow is indicated by FI-104.

Compressor speed is controlled by SIC-100. The compressor may be started and stopped with switch HS-100. The horsepower consumed by the motor is indicated by HP-100.

Since reciprocating compressors displace a constant volume of gas regardless of operating conditions, the compressor must be configured to displace slightly more gas than is required. The excess gas may then be sent back to the suction of the compressor through a kickback line or the excess gas may be vented or flared.

Alternatively, the speed of the compressor may be adjusted to trim the amount of gas displaced, however, this is not a typical arrangement since most reciprocating compressors are designed to run at a constant speed.

Therefore, another mechanism is required to vary the amount of gas that the compressor displaces. This mechanism is through loading and unloading the compressor. There are three methods of loading and unloading a reciprocating compressor. The first is by holding the intake valves open, the second is by holding the intake valves closed. The third method is through small pockets or reservoirs which are opened when unloading the compressor. The gas is compressed into these pockets on the compression stroke and reexpanded on the return stroke, thereby preventing the compression of any additional gas. This compressor has been provided with four clearance pockets per each stage, yielding a five-step control (full load, ¾ load, ½ load, ¼ load, and no load). Both stages of this compressor should be equally loaded at all times to maintain a relatively consistent compression ratio for each stage. If the stages are unequally loaded, then the pressure between the two stages will be either to high or to low and this might result in damage to the machine.

The machine should always be configured to displace slightly more gas than is required either through varying the loading, or through speed control, or both. The excess gas should then either be sent back to the suction of the machine through a kickback line or the excess gas should be vented or flared. This allows the suction and discharged pressures to be controlled.

All faults can be failed high or low to any degree with any of 8 fault function generators (step change, square wave, staircase, stairs, ramp, sawtooth, slope, or sine wave). Faults can be programmed to start and/or stop at various times during a simulation exercise.

You may create a virtually unlimited number of scenarios and training exercises by programming the faults described in the previous section. You can then establish performance standards for each one of those exercises. Simtronics provides a number of exercises with established performance standards for each process simulation. The objective, time to complete the exercise, cause, effect, solution, and procedure for each exercise is documented. You may modify these procedures to more closely reflect your particular process plant operating procedures.

• Exercise 1: Design
• Exercise 2: Cold Start
• Exercise 3: Gas Demand Increases
• Exercise 4: Kickback Valve Fails Closed