Throttle control valve

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

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. 

In addition to the throttling control valve, other types of process valves can be used to manipulate a process. 

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 only application in which a throttling control valve must shut off tightly is when it is required to do double duty as a remote-controlled block valve. There are a number of instances where this is a legitimate requirement. For example, there are turbine bypass valves that need to conserve energy when closed, but open quickly and throttle when the turbine is tripped off the line. 

The reactor may be adversely affected by events that lead to an abrupt reduction of the coolant flow when there is a risk of a significant mismatch between flow rate and power. These events include MCP seizure, tripping of one or several MCPs, failure of the MCP throttling control valve in the closed position and break-off of the DGH check valve disc. Startup of an idle MCP is a routine action with no hazard to the reactor facility. 

Figure 12-1 shows the basic equipment found in a distillation feed system. Compressed air or nitrogen systems are also used in the feed system. Nitrogen is an inert gas that provides protection from fire or explosion. Compressed air is used to open, close, or throttle control valves. Basic instrument systems used in the feed system include indicators, control loops, recorders, and analyzers. 

A nozzle needle can also influence the motive flow rate of a jet compressor. The installation of such a nozzle needle controlled ejector is shown in Figure 4.11. The needle Is adjusted by a pneumatic or electric actuator, as used for standard control valves. When a nozzle needle control is used, the motive pressure is not influenced. In contrast to the throttle control, described above, the motive mass flow is reduced without reducing the motive pressure that is required for the compression. The efficiency of a nozzle needle controlled ejector for partial load is therefore higher than with a throttle control valve. 

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