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It is imperative that process piping engineer should be familiar with all types of valves and the advent of any new designs. Thorough knowledge of the operation, maintenance, and adjustment of valves is equally important since the success of the process plant and its total activity will depend on it. In this article we are going to study the overview of few most common valves used for isolation and regulation purpose.
These include the gate valve, globe valve, needle valve, butterfly valve, ball valve, plug valve, and check valve. The gate valve, shown in Fig. In the closed position, the disc seats against two faces to shut off the flow. Gate valves are not suitable for throttling service because excessive wear, due to wire drawing erosion , occurs on the gate and gate seats. They are suitable as stop or isolating valves, where conditions require either full flow or no flow.
They have the advantage that, when fully opened, the resistance to flow is low with a minimum of pressure drop, as the fluid flow moves in a straight line. Figure 1 Gate Valve. The globe valve, shown in Fig. The disc and the seat are parallel to the main flow path, and the disc is moved toward, or away from, the seat by means of a threaded stem. Due to its construction, the globe valve is ideal for throttling or regulating flow with a minimum of wire drawing and seat erosion.
Another advantage of the globe valve compared to the gate valve is that it is cheaper to manufacture. On the other hand, the globe valve offers much more resistance to flow than does the gate valve.
The unbalanced single disc type of globe valve shown in Fig. Needle valves, shown in Fig. Its name is derived from the sharp pointed disc and matching seat.
They are extensively used for continuous blowoff or chemical feed control services. The stem threads are finer than usual so that considerable movement of the hand wheel is required to increase or decrease the opening through the seat.
Usually, these valves have a reduced seat diameter in relation to the pipe size. A butterfly valve, shown in Fig. The body is frequently a solid ring type, which is mounted between pipe flanges. The disc is generally cast in one piece. Correct alignment of this valve is required to prevent binding of the swing-through disc. The thickness of the disc is determined by the pressure drop across the valve throttling or closed position.
The valve is fitted with a lever for manual operation. A power actuator is required to position the disc for bigger sizes because large pressure differentials can exist across the disc. The valve shown in Fig. The butterfly valve is commonly used in thermal and hydroelectric power stations, oil and gas processing industries, oil and gas transmission, and in water and sewage plants.
They have the following advantages: relatively light weight, ease of operation, self cleaning, and negligible pressure drop across the valve when it is fully open. Figure 5 Power Operated Butterfly Valve. The ball valve, shown in Fig. The basic type of ball valve requires a quarter turn from the fully open to the fully closed position. The valve can be operated by means of a lever, which also serves as an open or shut indicator, or by the use of an automatic actuator.
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