Last updated: 2021-11-23 | 134 จำนวนผู้เข้าชม |
A wide variety of valve types and control operation exist. However, there are two main forms of action, the sliding stem and the rotary.
The most common and versatile types of control valves are sliding-stem globe, V-notch ball, butterfly and angle types. Their popularity derives from rugged construction and the many options available that make them suitable for a variety of process applications. Control valve bodies may be categorized as below:
Gate valves are general service valves primarily used for on/off, non-throttling service. Specifically, gate valves are used in applications requiring a straight-line flow of fluid with minimum restriction is desired. Gate valves operate when the user rotates the stem in a clockwise to close (CTC) motion or a clockwise to open (CTO) motion. The gate moves up or down on the threaded step when an operator moves the stem, which is why it is a multi-turn valve; the valve must turn several times for it to go from open to closed, and it is the slow operation that prevents water hammer effects. Engineers also utilize gate valves when minimum pressure loss and a free bore are required. Typical gate valves have no obstruction in the flow path, which results in a minimal loss of pressure.
A linear motion valve, globe valves stop, start, and regulate flow. Globe valves initiate closure via a plug featuring a flat or convex bottom that is lowered onto a horizontal seat situated in the center of the valve. When a user opens the valve, the plug raises to allow fluid to flow. Globe valves are used for on/off and throttling applications because the disk of the valve can be removed from the flow path completely or it can completely close the flow path. While this type of flow control valve does produce slightly higher pressure drops than straight-through valves like gate, plug, and ball valves, they are applicable in situations where the pressure drop through the valve is not a controlling factor.
A butterfly valve is operated by rotating a disk within the flow area and, due to this design, it does not have linear flow characteristics. This makes these valves less precise than the more common flow control valve types above. For this reason, it can often be dismissed as a flow control valve choice even though it is useful in some applications that do not require a very high degree of accuracy. They are also a very affordable valve option, which makes it worthwhile to consider them in the right applications.
Ball valves are commonly used in flow systems across numerous industries due to their low cost, durability, and excellent shutoff capability. Similar to butterfly valves, they are not as effective for flow control applications that require a high degree of accuracy and control. One of the reasons for this is that a ball valve requires a high degree of torque to open and close that prevents an operator from making fine adjustments. There is also a certain amount of “play” between the stem and the ball which can make finding specific flow rates difficult. For flow control applications where a ball valve is possible, such as filling a tank to a reasonable degree of accuracy, a trunnion or v-port ball valve design is usually the best choice.
Plug valves come in a variety of configurations and are operated by rotating a cylindrical or cone-shaped plug within the valve body to regulate the flow through a hollow area of the plug. For flow control applications the most common design is an eccentric plug valve, which uses a half plug to create a higher seating force with minimal friction as it is opened and closed. This has the advantage of greater shut off capability which is ideal for flow control situations.
A cost-effective flow control valve, pinch valves are ideal for applications of slurries or liquids containing significant amounts of suspended solids. Pinch valves seal using one or more flexible elements like rubber tubes that become pinched to turn off the flow. These rubber sleeves are the valve’s only wetted part, and their flexibility allows pinch valves to close tightly around entrapped solids. Air or hydraulic pressure is placed directly on the elastomer sleeve to actuate pinch valves. A pinch valve’s body acts as a built-in actuator, which eliminates expensive hydraulic, pneumatic, or electric operators and results in the cost-effectiveness of this type of flow control valve.
Check Valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, and inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.
An important concept in check valves is the cracking pressure which is the minimum differential upstream pressure between inlet and outlet at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.