Valves for on/off applications

Solenoid Valves The electric solenoid valve has tw o output states. Wlien sufficient electric current is supplied to the coil, an internal armature moves against a spring to an extreme position. This motion causes an attached pneumatic or hvdraiilic valve to operate. Wlien current is removed, the spring returns the armature and the attached solenoid valve to the deenergized position. An intermediate pilot stage is sometimes used when additional force is required to operate the main solenoid valve. Generallv, solenoid valves are used to pressurize or vent the actuator casing for on/off control-valve application and safetv shutdown applications. 

A globe valve uses a linear stem to lift a plug assembly on and off the seat to permit or stop flow. While many globe valves are fitted with sophisticated trim designs used for various control capabilities, in safety instrumented function applications a simple trim is used for on-off functionality. Various t5 pes of seat materials are available with Class IV leakage standard. Tight shut-off to Class VI is available in some manufacturers primarily with soft seat material. Globe valves have operational preferences to flow and pressure direction. 

As a general rule, system with large sy tetn pressure drop should use equal pooentage contro] valve, and system with little system pressure drop should use linear control valve. (If not sure, common practice is to use equal percemage control valve, since it is used in most applications.) For on-off service, quick open control valve is usually used. 

Four different types of tasks are performed by automation. Two involve the sequencing of valves and pumps Involved 1n the setup and completion of the designed experiment through the operation of the test and hydraulic fluid systems. The other tasks involve the control of the temperature bath and data collection. To perform these tasks, a1r-actuated solenoids and optically coupled sol Id-state relays are used. These devices are controlled by an electrical circuit consisting of the device connected 1n series with a power supply and a channel on the actuator card In the HP 3497. The power supply 1s either 24 VDC for use with the solenoids or 5 VDC for the solid-state relays. The actuator output channel acts as a simple on/off switch which allows power to be supplied to the solenoid or relay when closed. The logic of the circuit 1s controlled by application programs running on the local HP 1000. 

Isolation A means for pressure-isolating control valves, pumps, and other piping hardware for installation and maintenance is another common application for an on/off valve. In this application, the valve is required to have tight shutoff so that leakage is stopped when the piping system is under repair. As the need to cycle the valve in this application is far less than that of a throttling control valve, the wear characteristics of the valve are less important. Also, because many are required in a plant, the isolation valve needs to be reliable, simple in design, and simple in operation. 

The simplest valve function is on-off service. Examples of this valve type are gate and ball valves, shown in Figure 8.2. A ball valve is used for tight shutoff. One application is a drain valve on a tank, where it is required to have the... 

Control valves form an important equipment item in process plants, and therefore, a lot of money can be saved by ensuring that the right valves are installed. Selecting the right control valve for any application starts with defining the valve s functions. For example, if an on-off interlock valve is to be used, then the primary criteria for selection are reliability and simplicity. If very fast and precise modulation is required, such as for a compressor antisurge vent, a high quality valve is chosen. 

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