A volume booster is a mechanical relay that amplifies the flow of air.
Valve positioners, controllers and transmitters have limited flow capacity. To ensure accuracy, the internal nozzles, relays and other mechanical devices must be kept small to maximize responsiveness. Without external amplification, the output capacity of these instruments can be quite low.
Valve actuators, on the other hand, often become quite large as capacities and/or pressures increase. These large actuators contain much greater volumes of air that must be loaded or exhausted as the valve moves through its travel. To handle the higher capacities, valve positioners often include an output relay or high-capacity spool valve to amplify flow capacity. Even with this built-in amplification, however, the response of large valves can be slower than required.
The solution is an external volume booster. It provides a 1:1 amplification of flow while keeping the output pressure as close as possible to signal pressure. Since the booster has a separate supply connection, it is not limited by the capacity of the devices ahead of it in the loop.
Volume boosters used on control valves are normally applied with a bypass or gain adjustment to provide stability. The volume booster normally has a built-in deadband where a certain amount of signal change is necessary to activate the volume booster. The bypass or adjustment works by mixing the positioner output with the pressure in the actuator, allowing small signal changes to flow through the bypass without opening the main plug within the booster. Controlling the amount of bleed adjusts the sensitivity of the booster, and prevents overshoot that can be caused by excess capacity, ensuring smooth, controlled responses to small changes.
MANUAL OVERRIDE (HANDWHEEL/HANDJACK)
A manual override is an emergency backup device that allows manual operation of a control valve in the event power and/or air supply is lost. This should not be confused with a manual operator. Unlike a manual operator, an override device is not intended for continual use. Instead, the override is a limited-use device, often designed to counter the built-in spring return within the actuator to push the valve to its fully open or closed position, depending on fail action.
Several designs are available to accommodate various valve and actuator types and force requirements. They range from a simple wheel and drive screw attached to the top of a spring diaphragm actuator to a mechanical wheel and lever system connected to the valve stem or hydraulic handjacks (pumps) that attach to or are integral within a piston operator.
Control valves are indeed essential to the operation of any process facility. Equally important, however, are the accessories that allow a control valve to perform optimally. This article has only scratched the surface, touching on the most commonly used accessories. Numerous other options are available, giving control system designers and plant operators a virtually bottomless toolbox to draw from in meeting the needs of any application.
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