Self-operating Reducing Valve
Principle of a self-actuated pressure control valve
When the pressure downstream reaches the control valve and increases beyond the setpoint, process fluid exerts increased pressure on the diaphragm thus closing the valve. Closing of the valve stops process flow thus reducing the valve downstream pressure.
This type of valve is installed with the actuator below the pipe when used with steam and has a water seal pot to stop high steam temperatures from reaching and damaging the actuator’s flexible diaphragm, which is commonly made out of neoprene.
Oversizing should be avoided with all types of control valve and this is equally true of reducing valves. A valve plug working close to its seat when passing wet steam can suffer wiredrawing and premature erosion. In addition, any small movement of the oversized valve plug will produce a relatively large change in the flow through the valve, making it more difficult for the valve to control accurately.
Many reducing valve problems are caused by the presence of moisture or dirt. A steam separator and strainer with fine mesh screen, if fitted before the valve, will help to prevent such problems. The strainer is fitted on its side to prevent the body filling with water and to ensure that the full area of the screen is effective. Large isolation valves will also benefit from being installed on their side for the same reason.
All upstream and downstream pipework and fittings must be adequately sized to ensure that the only appreciable pressure drop occurs across the reducing valve itself. If the isolating valves are the same size as the reducing valve connections, they will incur a larger pressure drop than if they are sized to match the correctly sized, larger diameters of the upstream and downstream pipework.
If the downstream pipework or any connected plant is incapable of withstanding the maximum possible upstream pressure, then a safety valve or relief valve must be fitted on the downstream side. This valve should be set at, or below, the maximum allowable working pressure of the equipment, but with a sufficient margin above its normal operating pressure. It must be capable of handling the full volume of steam that could pass through the fully open reducing valve, at the maximum possible upstream pressure.
Select the installation location making sure that the valve is installed at a distance of at least six times the nominal size (DN) away from pipe fittings or instruments that cause flow turbulence (e.g. pipe bends, manifolds, pressure measuring points or other valves). They can change the flow conditions which may lead to an instable control process especially in applications with gases, air or steam.
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