Control valves compressible fluids

The flow coefficient Cv is determined by calibration with water, and it is not entirely satisfactory for predicting the flow rate of compressible fluids under choked flow conditions. This has to do with the fact that different valves exhibit different pressure recovery characteristics with gases and hence will choke at different pressure ratios, which does not apply to liquids. For this reason, another flow coefficient, Cg, is often used for gases. Cg is determined by calibration with air under critical flow conditions (Fisher Controls, 1977). The corresponding flow equation for gas flow is... 

The compressor is a device that increases the pressure of air and pumps the compressed air into a tank. The compressed air tank is called a reservoir. The compressed air is then routed to the desired location through transmission lines. Transmission lines are tubing through which the compressed air flows. Control valves regulate the flow of gases. The pneumatic system uses a cylinder with a movable piston to convert its fluid power into mechanical power in the form of linear motion. 

The common equation for the flow of a non-compressible fluid through a control valve is given in Equation 2.4 [14], which can be derived from Bernoulli s equation. 

Finally, safety systems related to the manipulation of high pressure compressible fluids are necessary all along the process loop. Strict parameter control, safety control and periodic valve actuation requires an automation system the operator controls the process through a computer. 

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