Machinery's Handbook, 31st Edition
VALVE TYPES 2789 ports for each supply port. Flow controls can be installed on each exhaust port and thus the speed of an actuator can be controlled in each direction. • Five port, three position (5/3) four way valves also can control pressure and exhaust to two output lines. These valves often have the center position normally closed when the valve is de-energized. This valve is also excellent for control of double acting cylin ders. The closed position makes it possible to ‘jog’ actuators. Directional control valves can be either manually controlled or automatically controlled. Manual control valves are outfitted with buttons, levers, or knobs. Automatic control valves are electrically operated solenoids. Solenoid valves are most commonly actuated using spools or poppets. These valves can be single or double acting. Single acting valves normally have spring returns. Double acting valves use fluid pressure on either side of the valve to actuate back and forth. Automatic control valves can be either direct acting or pilot operated. Direct acting sole noid valves open and close the valve. They are used in systems with low flow capacities or low pressure differentials across the valve. The sealing surface that opens and closes the main valve orifice is connected to the solenoid plunger. Direct acting valves can oper - ate with a zero pressure differential, or up to the maximum rated pressure differential (MOPD) regardless of fluid pressure. Pressure drop across the valve is not required to hold the valve open. Pilot operated valves use system fluid pressure to open and close the valve. In a normally closed valve, the sealing surface is opened when the pilot is energized. When a normally open valve is energized, fluid pressure builds up behind the sealing surface, forcing it to close the valve. Pneumatic solenoid valves are generally pilot operated. When vacuum or air with insufficient pressure is the medium, separate pilot lines must be provided with sufficient positive pressure to actuate the valve. Energy Loss in Valves.— Energy is lost in all valves due to restriction, friction, and changes in direction of flow. The type of valve and its specific geometry will determine the amount of energy lost. Manufacturers will generally provide equivalent length of straight pipe values or resistance coefficients for their valves. Equivalent length is added to the pipe circuit when calculating pressure drop through the system. Resistance coefficient K is used in previous equations for friction loss. Some values for equivalent lengths are given in Table 26 and Table 27. Valve Selection and Sizing.— Valves should be selected by first considering their function and selecting a type. Compatibility with the fluid must be carefully evaluated, since most valves include seals and mechanical components of different materials. The temperature and pressure range of the system should be taken into account. The cleanliness of the fluid matters when selecting valves with small or sensitive internal components. A fluid filter (with its own pressure drop) may be required. Pressure drop across the valve should be examined at the operating flow rate to determine if it meets the valve’s minimum pressure differential requirement in the case of pilot operated valves. Valves are usually sized based on Coefficient of Velocity ( C v ). These values are nor mally listed in manufacturer’s catalogs. C v and its calculation is covered in Coefficient of Velocity, Cv on page 2760. Using C v to size valves is industry standard practice, but be aware that it is only an approximation. Energy loss in the circuit due to pipe and tubing runs, fittings, and other components will cause deviation of system performance form ideal. Valve switching response time can also be a factor to consider when selecting a valve. To select the proper size valve for an actuator to perform a specific task, first calcu - late the C v of the actuator, and then select a valve that has a corresponding or greater C v .
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