Valve control devices

There are three basic requirements that Hquid level control devices are designed to satisfy alarm functions, pump/valve control, and transmitted output signal to track level continuously. Alarm devices provide warning or shutdown functions when process levels pass a predeterrnined point in the vessel pump /valve control devices turn on/off pumps or open/close valves at predeterrnined levels in the vessel and transmitters provide a proportional output signal over a predetermined span to send to a local meter or signal back to a control room. 

The resulting motion of the beam is detected by the pneumatic nozzle amphfier, which, by proper sizing of the nozzle and fixed orifice diameters, causes the pressure internal to the nozzle to rise and fall with vertical beam motion. The internal nozzle pressure is routed to the pneumatic relay. The relay, which is constructed like the booster relay described in the Valve Control Devices subsection, has a direct hnear input-to-output pressure characteristic. The output of the relay is the controller s output and is piped away to the final control element. 

A control valve consists ot a valve, an actuator, and possiblv one or more valve-control devices. The valves discussed in this section are applicable to throttling control (i.e, vviiere tlovv through the valve is regulated to anv desired amount betvv een maximum and minimum limits). Other valves such as check, isolation, and relict valves are addressed in the next subsection. As defined, control valves are automatic control devices that modify the tliiid tlovv rate as specified bv the controller. 

Valve-Control Deviee.s Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve-control devices. In some applications, several auxiliary devices are used together on the... 

As most throttling control valves are still operated bv pneumatic actuators, the control-valve device descriptions that follow relate primarily to devices that are used with pneumatic actuators. The function of hydraulic and electrical coimteiparts are very similar. Specific details on a particular valve-control device are available from the vendor of the device. 

Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve control devices. In some applications, several auxiliary devices are used together on the same control valve. For example, mounted on the control valve, one may find a current-to-pressure transducer, a valve positioner, a volume booster relay, a solenoid valve, a trip valve, a limit switch, a process controller, and/or a stem position transmitter. Figure 8-80 shows a valve positioner mounted on tne yoke leg of a spring and diaphragm actuator. 

Eressure difference—see Fig. 8-71). Dynamic stiffness is usually igher with piston designs than with spring and diaphragm actuators see Positioner/Actuator Stiffness. Failsafe action, if necessary, is achieved without a spring through the use of additional solenoid valves, trip valves, or relays. See Valve Control Devices. ... 

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