Emerson VSSG, VSG manual RTDs and Pressure Transducers, Control System, Theory of Operation

Section 2 • Theory of Operation

suction. The discharge temperature of the compressor must be kept a minimum of 30°F (or 17°C) above the dis- charge gas dew point to prevent the condensing of liq- uids in the oil separator. The oil separator shell and legs must be insulated when the gas stream has a high prob- ability of having condensables.

Oil in the gas compressor unit serves three primary pur- poses. They are compressor lubrication, sealing clear- ances between moving parts, and heat removal result- ing from heat of compression and friction. Initially, oil flow is driven by a mechanical gear pump (7). Once the system reaches design conditions, the oil pump is shut off and oil flow is maintained by differential pressure in the gas system.

As the oil is separated from the gas in the oil separator, it is pumped or syphoned through an oil cooler (9), then filtered through a single (11) or dual oil filters (15) and back to the injection port of the compressor (14). The standard oil cooler is a shell and tube water cooled heat exchanger (9). The other option is to air cool oil remotely through a finned fan heat exchanger (8).

Furthermore, to collect oil from the coalescing side of the oil separator (12), a line is installed between the oil separator and the compressor. By opening the shut-off valve (6), this will allow oil dripping off the coalescing fil- ters to be fed back to the compressor. In addition, the oil cooler (8 or 9) is piped in parallel to the oil temperature control valve (oil mixing vlave) (10), which acts as a by- pass valve.

On units with a full-time oil pump, oil pressure is regu- lated by the oil pressure regulator (12). It controls up- stream pressure to the compressor bearings and should be adjusted to hold the oil pressure at 20 psig above suc- tion pressure. Excess oil not required for bearing lubrica- tion is passed through the regulator and back into the oil separator (13).

This is a continuous cycle.

RTDs and Pressure Transducers

Resistance Thermometers (RTDs) and pressure trans- ducers are instruments used to measure temperatures and pressures at specific locations on the gas compressor unit, see Figure 1-2. Gas Compressor Unit Components. RTDs are typically mounted on the suction pipe, dis- charge pipe, oil separator and oil filter outlet pipe. Pressure transducers are typically mounted on the block and bleed assembly and directly on the suction pipe. The pressure transducers measure suction pressure, inlet and outlet oil pressure, and oil separator pressure.

Control System

The gas compressor unit is controlled by a Programmable Logic Controller (PLC) panel. This PLC panel’s main func- tion is to control the gas compression system from the data that it receives from the sensors around the unit. Refer to Compact Logix PLC manual (35391CL) for addi- tional information.

2 – 2 VSG/VSSG • Installation, Operation and Maintenance Manual •Vilter/Emerson • 35391SSG