RMP Lecture Notes

September 6, 2013 – 11:09
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RMP Lecture Notes

Control Valves

In most processes, the final control element is an automatic valve. (See the figures in Riggs on pp. 45-48)

Most industrial control valves are globe valves, so called because of the shape of their body. Within the body, the valve stem is moved up and down (strokes) by the actuator. This opens and closes a gap between the valve plug and the valve seat.

The parts of the valve that come in contact with the process fluid including the valve seat, plug, etc. are collectively known as the valve trim. Trims are designed and machined to regulate and "shape" the flow according to a specific characteristic.

Valve actuators are usually pneumatic/spring devices, although piston (air on both sides), electric motor, and hydraulic actuators are made. The signal from the controller passes through an I/P transducer and is converted to a stream of pressurized air. This air pushes against one side of a diaphragm and works against a spring to move the valve stem to the desired position.

If the air signal is lost, the spring drives the valve to its failure position. Depending on whether the spring is above or below the diagphragm valves can be Fail Closed (a.k.a. Air-to-Open or AO) or Fail Open (Air-to-Close or AC). The failure position of a valve is a significant safety consideration and is determined early in the control system design.

The valve, actuator, and I/P transducer all influence the system behavior (Riggs lumps them together as the "actuator system").

Other types of valves (notably rotary or butterfly valves) are used, but the globe type control valve described is most common. Other final control elements include furnace dampers, variable speed drives, etc.

A valve positioner is a pneumatic device which precisely positions a valve. It is essentially a small controller which senses the stem position, compares it to the controller output, and adjusts the pressure to the actuator. Positioners can be used to overcome stem friction, high fluid pressure, viscous or dirty fluids, or to improve response when system dynamics are slow (cf. valve position controllers).

Sizing Valves

Undersized control valves cannot pass the required flow. Oversized valves cost more than is necessary and may not make a large enough impact on the system. Neither case permits precise regulation of the process, hence control valve sizing is an important engineering task.

The valve sizing equation should look familiar -- after all, a valve is just a flow restriction.

The size coefficient has two parts -- a constant value that relates to the maximum flow the valve can pass and a characteristic function that describes how the valve open area varies with stem travel. This function goes to 1.0 when the valve is wide open and to 0.0 if the valve is completely closed.

Source: facstaff.cbu.edu

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That makes sense.

2013-04-27 08:33:43 by eeieeyeoh

However the manual recalibration does not. If all the actuators are working and the battery ground cable is disconnected for 10 minutes, upon reconnecting the battery ground cable will automatically calibrate the auto climate control microcontroller for designed use. The only things that needs to be reprogrammed then are the radio station presets and clock. Perhaps the manual calibration procedure will allow one to override the auto cal procedure to get defrost function working though if that actuator works. Especially important if windshield is fogging up often which shouldn't happen much w/auto cli cont properly working keeping vehicle interior normally dry

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