Suction recycle valve, discharge

The operators closed the discharge to the suction recycle valve, which reduced the flow and raised the discharge pressure. 

At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure. 

The suction pressure control system shown in Fig. 11.24 uses a fail-open recycle valve to return compressed chlorine from discharge to suction. In the common situation with multiple compressors, there is one control system, and the chlorine is piped from the common discharge header to the suction header. The control valve should have an equal percentage characteristic and should be sized to handle the total capacity of the compressors when 90% open. It should have excellent turndown in order to control small recycle flows and should be aided by a valve positioner. 

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

If the molar mass increases by 20 % and we control the suction drum pressure by recycling exit gas to the inlet (spill back control) the motor amps increase by 20% if control is by throttle of the suction line, the motor amps increase by 10%. For every 10% decrease in the total number of moles compressed, the amps load on the motor drive decreases by 5 %. For control valves on the suction or discharge side, allow a Ap of 5 % of the absolute suction pressure or 50 % of the dynamic loss, whichever is greater. 

A second valve then controls compressor suction pressure by allowing gas to recycle from the discharge of the compressor. Good control maintains a constant differential... 

Figure 11.26 shows a system in which compressed gas recycles fr om the discharge, upstream of the shutoff valve, to the inlet of the suction chiller. Passing the recycled gas through the suction chiller (or other precooler) cools the gas so that the control valve can handle the full compressor capacity. The control valve is a fail-open globe valve with an equal percentage characteristic and a positioner. It should be sized to handle full compressor output when 90% open. The valve seal can be a Monel bellows or a Hastelloy C stem with double PTFE packing. 

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