Process control valves, solenoid actuated

As with process-control valves, actuators used to control fluid-power valves have a fundamental influence on performance. The actuators must provide positive, real-time response to control inputs. The primary types of actuators used to control fluid-power valves are mechanical, pilot, and solenoid. 

By themselves, valves cannot control a process. Manual valves require an operator to position them to control a process variable. Valves that must be operated remotely and automatically require special devices to move them. These devices are called actuators. Actuators may be pneumatic, hydraulic, or electric solenoids or motors. 

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. 

In the process industries the most common final element is a remote actuated valve. This assembly usually consists of a pneumatic/hydraulic control assembly, an actuator and a valve (Figure 11-1). The control assembly may be as simple as a three-way solenoid, a smart partial valve stroke box or a complex electro-pneumatic assembly with solenoids, test switches, pneumatic booster relays and quick exhaust valves. 

Reactor A process hazards analysis identifies a hazard associated with a reactor dump valve. If the dump valve opens when the reactor is not depressurized, downstream equipment is overpressured. An SIF is implemented to prevent the reactor dump valve from opening when the pressure is high, using a solenoid-operated valve that controls the instrument air supply to the dump valve actuator. To support batch operation, the SIF energizes the solenoid-operated valve for each batch when the reactor pressure is low. However, from a safety perspective, the SIF prevents a hazardous event only when the control system fails and tries to open the control valve when the reactor is under pressure. This failure is estimated to be approximately 1 in 10 years. The SIF is operating in demand mode with regard to the hazard. 

Another common interlock configuration is to locate a solenoid switch between a controller and a control valve. When an alarm is actuated, the solenoid trips and causes the air pressure in the pneumatic control valve to be vented consequently, the control valve reverts to either its fail-open or fail-close position. Interlocks have traditionally been implemented as hard-wired systems that are independent of the control hardware. But, for most applications, software implementation of the interlock logic via a digital computer or a programmable logic controller is a viable alternative. Programmable logic controllers (PLCs) used for batch processes are considered in Chapter 22 and Appendix A. 

The trip valve is provided as a second line of defense in case of overspeed The trip valve is frequently equipped with a trip-actuating solenoid which can be operated by push button, by low od pressure, or by some other process upset. When the speed control functions as described above, the trip will not be actuated by load dump. 

Son very small linear stem-motion valves. A solenoid is usually gned as a two-position device, so this valve control is on/off. Special solenoids with position feedback can provide proportional action for modulating control. Force requirements of medium-sized valves can be met with piloted plug designs, which use process pressure to assist the solenoid force. Piloted plugs are also used to minimize the size of common pneumatic actuators, especially when there is need for high seating load. 

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