Machinery's Handbook, 31st Edition
2788 VALVE TYPES mechanism. Automatic valves can be equipped with manual overrides which permit the operator to actuate the valve manually. Valve Types.— There are many valves types commercially available. The most common types are check valves, foot valves, ball valves, gate valves, globe valves, butterfly valves, relief valves, and directional control valves. Check Valves and Foot Valves: Check valves are constructed to allow flow in one direc tion and stop flow in the other direction. This type of valve is essential in pipe and tube systems where backflow could cause damage or flooding. The two most common types of check valves are ball type and swing type. Ball check valves create a higher energy loss due to restriction around the ball. Check valves have a “minimum cracking pressure” that is the pressure required to open the valve and allow flow. This may be expressed as a mini - mum pressure or a minimum flow velocity, depending on the application. Foot valves are commonly used on the intake lines of pumps that draw fluid from an open reservoir. They are comprised of a check valve outfitted with a low-loss strainer to keep out debris. The strainer should be cleaned regularly to prevent suction line restriction. Restriction Valves: There are several types of valves that are used to reduce flow or to stop flow entirely. Ball valves contain a sphere with a hole through it inside the valve body. When the sphere is turned, the through-hole can be aligned to allow full flow or to obstruct flow partially or completely. These valves are fully actuated with one quarter turn, and are typically used for on/off control rather than throttling. Gate valves have a blade or gate that is cranked into or out of the flow path to allow full flow, partial flow, or no flow. Globe valves are true throttling valves, and can be used even in high flow applications. Due to their design, they cause a significantly higher pressure drop than other types of valves, even when fully open. Butterfly valves consist of a disk that can be turned to fully or par tially block flow. When fully open, the disk is still in the flow path, but is turned to present its edge to the flow. Relief Valves: Pressure relief valves are used to protect the fluid circuit and its compo nents from dangerously high pressures. A pressure relief valve opens to relieve pressure at a predetermined set point. These valves can be selected such that they close again when the system pressure has returned to a safe level. Since pressure relief valves are safety de- vices, their design and use is covered by various codes. The information and calculations required to properly size pressure relief valves is quite extensive and beyond the scope of this text. Most major pressure relief valve manufacturers also offer sizing software. In liquid pumping systems, the relief valve should discharge to the suction reservoir in a way that prevents gases from entering the suction line to the pump. Directional Control: Directional control valves are used to change the direction of flow. They can also be used to stop and start flow. Directional control valves are functionally described in terms of the number of ports, the number of positions (usually 2 or 3), the number of paths (passing and non-passing) the fluid can take through the valve, and the valve’s de-energized state. The number of paths the fluid can take through the valve is said to be the number of “ways.” Three way valves have one output and four way valves have two outputs. Generally, three way valves operate single acting linear actuators and four way valves operate double acting linear actuators. Some of the most commonly used directional control valves are: • Two port, two position (2/2) two way valves have one inlet and one outlet port. They perform a basic on/off function. • Three port, two position (3/2) two way valves have one inlet, one outlet, and one exhaust port. These valves switch flow between inlet and exhaust, and are commonly used to pressurize and exhaust a single actuator supply line. • Five port, two position (5/2) four way valves are typically used to control pressure and exhaust to two output lines. This is ideal for controlling double acting cylinders. 5/2 valves are also available with four ports. Five ported valves have separate exhaust
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online