Closed valve control valves

In some plants, the operators have free reins to govern the flow in the pipes by opening and closing flow control valves. Strangling a valve reduces the flow and inerea.scs the resistance and pressure in the system... 

A process waste heat boiler was damaged when the control operator inadvertently closed the cooling water valve when he intended to open it. The manual control knob turned in the opposite direction from most of the other valves in the plant—the L, cooling water valve was air to close—most control valves were air... 

Snap-in tabs, above each end of the output meter, indicate the direction the final control element moves for a change in the output signal. Tabs normally read "open-close" for control valves and "slow-fast" for variable-speed motors, or other appropriate designations. 

TTie pH signal is fed via a pH meter to a potentiometric recorder and to a neutrahzation controller, which compares the electrode e.m.f. with pre-selected values and opens or closes gas-control valves accordingly. Depending on the pH, it is possible to obtain a flow of air, ammonia or a mixture of both gases. By careful adjustment of the potentiometers and the flow-rates of ammonia and air, it is possible to control the final pH and also to keep the digest within a fairly closely defined pH range during the neutrahzation process. 

Bowl does not close although control valve is turned to position 3 Check 1. That tank contains water 2. That cock in conduit from tank is open 3. Control valve 310-1 4. Paring disc device 310-2 5. Control means of bowl Nozzle C15 in bowl body. Plugs C21 in operating slide 300-4... 

The effect of this on the reactor operation would have to be considered. This would be brought out in the operability study on the reactor it would be a possible cause of no chlorine flow. Since the flow controller does not know that steam flow has been lost, chlorine will continue to be pumped into the vessel until the high-level alarm sounds and the high-level shutdown closes the control valve. A secondary consequence is that the vessel is now filled with liquid chlorine that must be drained to a safe level before operation can be resumed. The operating procedures must include instructions on how to deal with this scenario. 

Bypass - An alternative path. In a heating duct or pipe, an alternative path for the flow of the heat transfer fluid from one point to another, as determined by the opening or closing of control valves both in the primary line and the bypass line. 

Closing the control valve resulted in compression of the bubbly mixture column. Pressure sensors PI and P2 registered the local pressure in time. 

A control loop TIC-1 using a temperature transmitter TT-1 that closes the control valve CV-1 using its positioner in response to increased temperature. 

Controllers can be operated in manual, automatic, or cascade control. During plant start-up, the controller is typically placed in the manual position and left there until the process has lined out. This process initiates a setpoint on the final control element however, it does not utilize a controlling function. It only opens the valve 50% or 25% and keeps it there until the technician changes the mode. This keeps the process from swinging up and down during start-up. After the process is stable, the operator places the controller on automatic and allows it to supervise the control loop function. At this point, the controller will open and close the control valve to maintain the setpoint. Cascade control describes how one control loop controls or overrides the instructions of another control loop in order to achieve a desired setpoint. In this case, a control loop s controller may use all five elements of another control loop as its final control element. 

As an example, with a sudden rise in the vessel pressure, the cascade flow controller would immediately close the control valve to keep the bottom flow constant. With normal level control the flow would increase, and the control valve would not begin closing until the level controller sensed a drop in the bottom level. 

An operator may decide to close a control valve partially with the hand jack. He sets the hand jack in the one-half closed position and then... 

We can see that, if the molecular weight increases, the PVof the flow controller will now increase. The controller will respond by now closing the control valve. 

Controller fails and closes valve 4. Same 4. Place controller on critical DAC 1/93... 

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. 

Fig. 9. Control valve and actuator (a) nomenclature (b) fail-open (F/O) or air-to-close (A/C) action and (c) fail-closed (F/C) or air-to-open (A/O)...

Fig. 10. Use of solenoid valve for shutdown. Upon signal to shutdown, the solenoid closes a controller air signal port and opens a vent port.

Pneumatic Controllers The pneumatic controller is an automatic controller that uses pneumatic pressure as a power source and generates a single pneumatic output pressure. The pneumatic controller is used in single-loop control applications and is often installed on the control valve or on an adjacent pipestand or wall in close proximity to the control valve and/or measurement transmitter. Pneumatic controllers are used in areas where it would be hazardous to use electronic equipment, in locations without power, in situations where maintenance personnel are more familiar with pneumatic controllers, or in applications where replacement with modern electronic controls has not been justified. 

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

Trip Valves The trip valve is part of a system that is used where a specific valve action (i.e., fail up, Fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls befow a preset level. Trip systems are used primarily on springless piston ac tuators requiring fail-open or fail-closed acrion. An air storage or Volume tank and a check v ve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instabihty when the trip pressure and the reset pressure settings are too close to the same value. 

Close minimum-flow loop or control valve byj)ass valve. 

Although such control devices are usually automatically operated, manual operation is satisfactoiy for some services. When manual operation is provided, it often consists of a valve or valves to open and close clearance pockets. In some cases, a movable cyhnder head is provided for variable clearance in the cylinder (see Fig. 10-92). 

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