In this passage, I’m gonna explain to you how the globe, gate, ball, plug, butterfly, diaphragm, check, needle, pinch, and safety valves work. I’m also gonna tell you about the different types of valve classification, how valves get their names, and what valves actually do. So let’s start with the basics.
How do we name a valve?
Almost always, valves get their name from the type of discs used. The disc is the part of the valve that is used to open, close, or regulate the flow through the valve. Regulating the flow is referred to as throttling. Ball valve gets its name because of the ball-like shape of the disc. The butterfly valve is named because of its similar appearance to a butterfly. And the gate valve is named because of its similar appearance to a gate.
There are exceptions to the rule, though. Globe valves are named after the shape of the valve body itself.
What are the valves used for?
Valves are used for stopping and starting the flow, varying the amount of flow, controlling the direction of flow, regulating downstream system or process pressure, and finally, for the relieving system over and under pressure. Valves can be categorized as rotary or linear motion valves. Rotary valves are those which require a 1/4 of a turn in order to change position from fully open to fully closed or vice versa. Rotary valves are fast-acting valves. Examples of rotary valves would include the ball, plug, and butterfly valves. Linear motion valves are slower to operate than 1/4 turn valves. Linear motion valves raise or lower the disc in order to open, regulate, or close the flow through the valve. Examples of linear motion valves include the gate and globe type valves.
Types of actuators
There are four main types of actuators used to operate the valves. These are mechanical, electrical, hydraulic, and pneumatic.
A ball valve is a rotary motion valve that uses a ball-shaped disc to stop or start the flow. When the valve handle is turned to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, the ball is rotated so that the hole is perpendicular to the flow opening of the valve body and the flow is stopped. Ball valves are not suitable for throttling or regulating flow. The pressure drop across ball valves when fully open is very low. Small to medium size ball valves are fast-acting 1/4 turn valves. Larger ball valves employ a planetary gearbox. The planetary gearbox allows the use of a relatively small handwheel and operating force to operate a fairly large valve.
A butterfly valve is a rotary motion valve that is used to stop, regulate, and start the flow. Like all 1/4 turn valves, the butterfly valve is fast-acting. Larger butterfly valves employ planetary-type gearboxes. Butterfly valves possess many advantages over gate, globe, plug, and ball type valves, especially for larger applications. Savings in weight, space, and cost are the most obvious advantages. The pressure drop across the butterfly valve when the valve is fully open is low. Butterfly valves are very well-suited for the handling of large flows of liquids or gases at relatively low pressures.
A diaphragm valve is a linear motion type valve that is used to start, regulate, or stop fluid flow. A flexible diaphragm can be raised or lowered onto the valve seat in order to open or close the valve. A great advantage with the diaphragm type valve is that very few parts are exposed to the flowing medium. The valve is constructed so that only the flexible diaphragm and internal valve flow passages are exposed to the flowing medium. This makes it particularly well-suited for the handling of corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination.
Gate valves are the most common type of valves employed to date. Gate valve is a linear motion type valve used to start or stop the flow. It is not suitable for regulating flow. The name gate is derived from the appearance of the valve disc. The disc of the gate valve is completely removed from the flow stream when the valve is fully open. This allows flow through the valve with virtually no resistance. This gives the valve a very low-pressure drop across the valve. The major advantages with the gate valve are that it is cheap, has a simple design, and there is a very low-pressure drop across the valve when it is fully open. The major disadvantages with gate valves are that they are not suitable for throttling applications. They’re also prone to excessive vibration when only partially open. Compared to a globe valve, they are more susceptible to seat and disc wear and potential leaking.
A globe valve is a linear motion valve used to stop, start, and regulate fluid flow. There are four main globe valve designs: Straight flow, angle flow, cross-flow, and Y-flow. Compared to a gate valve, a globe valve generally yields much less seat leakage. This is because the disc to seat ring contact is more at right angles, which permits the force of closing to tightly seat the disc. Globe valves are almost always installed with the system pressure on the underside of the valve seat. This makes it easier to open the valve, and also removes the pressure on the stem, packing, and bonnet when the valve is closed. The largest disadvantage with a globe valve is that there is a relatively large pressure drop across the valve. In addition, large globe valve sizes require considerable power to operate and are especially noisy in high-pressure applications. Globe valves are also often heavier than other types of valves with the same flow rating.
Pinch valves are inexpensive and are the simplest of any valve design. The pinch control valve consists of a sleeve molded of rubber or other synthetic material and a pinching mechanism. Pinch valves can be used to start, stop, or regulate flow. However, the effective throttling range is usually between 10% and 95% of the rated flow capacity. There is almost no pressure drop across a pinch valve. Pinch valves are ideally suited for the handling of slurries with large amounts of suspended solids. This is because they have a very large seating area. Because the operating mechanisms of the valve are completely isolated from the flowing medium, these valves are very well-suited where corrosion or metal contamination of the flow medium might be a problem.
A plug valve is a rotary motion valve used to start or stop the flow. The name is derived from the shape of the disc, which resembles a plug. The design is very similar to a ball valve, although the shape of the disc is different. In the open position, the passage in the plug lines up with the inlet and outlet ports of the valve body. When the plug is turned 90 degrees from the open position, the solid part of the plug blocks the ports and stops the flow. When the plug valve is fully open, there is a very low-pressure drop across the valve. An important characteristic of the plug valve is that it is easy to adapt for multiport applications. The use of a multiport valve, depending upon the number of ports in the plug valve, eliminates the need of as many as four conventional shutoff valves. This is a considerable cost and space saving. Plug valves are often used in non-throttling, on-off applications, particularly where the valve must be operated frequently.
Check valves are designed to prevent the reversal of flow in a piping system. These valves are activated by the flow of material in the pipeline. The pressure of the fluid passing through the system opens the valve, whilst any reversal of flow will close the valve. The closure is accomplished by the weight of the check mechanism, by back pressure, by a spring, or by any combination of these means. The most common type of check valves is the swing, tilting disc, piston, butterfly, and stop valves. The type of check valve used will depend upon the system pressure, temperature, and flow requirements. For example, swing check valves are very well-suited for medium velocity, high volume flow applications. There is also a relatively low-pressure drop across this type of valve.
A needle valve is used to make relatively fine adjustments in the amount of fluid flow. The most distinguishing characteristic of a needle valve is the long, tapered needle-like point on the end of the valve’s stem. The needle acts as a disc. The longer part of the needle is smaller than the orifice in the valve seat and passes through the orifice before the needle seats. This arrangement permits a very gradual increase or decrease in the size of the opening. Needle valves are frequently used as metering valves. This is because the number of turns of the handwheel can be directly correlated to the amount of flow.
Relief and safety valves prevent equipment damage by relieving accidental system overpressurization. A relief valve gradually opens as the inlet pressure increases above the setpoint. The valve only opens enough to relieve the overpressure condition, whereas the safety valve rapidly pops fully open as soon as the pressure setting is reached, and it will stay fully open until the pressure drops below a reset pressure. Solenoid valves are electromechanically operated valves. They are very well-suited to opening and closing operations. They can be used to start or stop the flow, but are not well-suited for regulating flow. This type of valve is fast-acting. An electromagnet is used to operate the valve when current is supplied to the windings. A spring is used to return the valve to its fail-safe position when the electrical current is no longer present.
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