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All gases flow from higher pressure to lower pressure. Choked flow can occur at the change of the cross section in a de Laval nozzle or through an orifice plate. The choked velocity is observed upstream of an orifice or nozzle. The upstream volumetric flow rate is lower than the downstream condition because of the higher upstream density.
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.
A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. [1] This enables the direct control of flow rate and the consequential control of process quantities such as pressure , temperature , and liquid level.
Process control instrumentation and designation (names, numbers, unique tag identifiers), including: Valves and their types and identifications (e.g. isolation, shutoff, relief and safety valves, valve interlocks) Control inputs and outputs (sensors and final elements, interlocks)
Gate valve, mainly for on–off control, with low pressure drop; Globe valve, good for regulating flow. Uses a cylinder movement over a seat; Knife valve, similar to a gate valve, but usually more compact. Often used for slurries or powders on–off control; Needle valve for accurate flow control; Pinch valve, for slurry flow regulation and control
More complex servo valves can control other parameters. For instance, some have internal feedback so that the input signal effectively control flow or output pressure, rather than spool position. Servo valves are often used in a feedback control where the position or force on a hydraulic cylinder is measured, and fed back into a controller that ...