The valves used on a plant can be grouped into two main types.
Shut Off Valves: Shut off valves can be operated by hand or operated automatically using an actuator. Their purpose is either to allow full flow through the valve or shut off the flow completely. They must not be used to control the amount of flow.
Control Valves: Control valves can also be operated by hand. However, they are normally operated by an actuator automatically. Their purpose is to control the amount of fluid passing through the valve and to act as the correcting element for a control loop.
There are many different valve designs. They can be very simple (e.g. a water tap). They can be very complicated (e.g. low noise valves for the control of high pressure gas). The following notes are given as an introduction to the valve types available.
The Gate Valve
The gate valve is the common shut-off valve for a pipeline. It is designed to be either fully open or fully closed. Any position between the two can cause a lot of damage to the valve.
Rising Stem: This is a simple device. As the handle is turned the screw thread on the stem pulls up or pushes down the disc gate. The valve is designed for two positions only, fully open or fully closed.
Non Rising Stem:The handle turns the stem. The stem fits into the sleeve which has an inside thread. As the stem turns, the inside thread causes the sleeve and gate to move up or down. This type of gate valve is used for higher pressures. The gate is split so that both sides are forced tight against the two seats. This gives a tight shut off.
Note: The rising stem valve is normally turned back a 1/4 of a turn after setting fully open or closed. This stops it from sticking if it is left for long periods in one position. This must never be done with a non rising stem valve.
THE GLOBE VALVE
This is the most common type of valve used to control the flow of fluid. The simplest type is the water tap. The flow can be controlled by hand. A simple diagram of how it works is in Figure.
The tap body is ball-shaped. The circular plug controls the flow of water by adjusting the gap between the plug and the seat ring. This action is called “throttling”; squeezing fluid through a smaller and smaller gap to reduce the flow.
The Industrial Globe Valve.
Industrial globe valves come in all shapes and sizes. However, they can be split into two main groups; the “shaped plug” and the “plug and cage”.
The single plug valve is used for flow control at low pressures. High pressure control is difficult as the line pressure pushes against the plug. Therefore, extra force must be applied to the stem to hold the plug in position. The double plug system (see Figure 4-3b) overcomes the problem of line pressure by providing two controlled flow streams. The pressure on the top plug forcing the stem up is balanced by the pressure on the bottom plug forcing the stem down. So, less force is needed to move the stem and you have good control at high pressures. The diagram also shows the two types of valve body. Figure (a), is direct. As the stem rises the flow increases. Figure (b) is reverse. As the stem rises the flow decreases.
The flow through the valve is via holes cut in the cage. The amount of open hole, and thus the flow, depends on the position of the plug. The plug is held in position by a force on the stem. Most of the globe valves used in the oil/gas industry are of the plug and cage type. They are cheaper to manufacture and service, and provide a balanced action with a simple hole through the plug. The pressure at the top of the plug balances the pressure at the bottom of the plug. The diagram above shows a direct acting body. A reverse acting body is also available but it is very unusual.
The Butterfly Valve
The butterfly valve is a thin disc which rotates across a pipe flow stream. A typical butterfly valve is shown in Figure 4-5
The valve is rotated by an actuator. This rotates the disc a 1/4 turn (900) from full closed to full open as shown in the simple diagram below.
The discs in butterfly valves can have different shapes. Different shape discs can improve throttling characteristics and provide a tight shut-off. A popular model is the Fisher Fishtail. Figure 4-6 shows the shape of a Fishtail butterfly disc. The sectional drawing shows how a tight shut-off can be obtained.
BALL AND PLUG VALVES
Ball and plug valves are 1/4 turn valves which operate in the same way as a butterfly valve. The only difference between the two is the shape of the part being rotated in or out of the flow steam. Figure below shows a typical ball and plug valve.
The ball valve is a sphere with a hole drilled in it. The plug valve is a tapered cylinder with a hole drilled in it. All these valves are designed so that the actuator (handle) puts the hole in line for full flow as shown in the diagram below.
These valves are popular as shut-off valves, particularly in air lines and process lines to instruments. They are also used as control valves. There are many different designs. Two of the common ones used in the oil/gas industry are the Fisher “V” ball and the Masoneilan eccentric plug (eccentric means off-centre). The Masoneilan eccentric plug valve is shown in Figure below .
The actuator turns the plug a 1/4 turn, from fully closed to fully open. The plug is shaped to provide flow control similar to a globe valve.
Ball/plug valves are often used to provide three/four way connections. This is done by drilling the hole in the ball in different ways. Some examples are given in Figure below .
THE PINCH VALVE
The pinch valve is used to control the flow of very corrosive liquids, eg. acids. The flow passes through a flexible pipe or diaphragm. The pipe must be made of material which does not corrode easily. The pipe is squeezed or pinched to throttle the flow. There are two basic types which work in much the same way.
The SAUNDERS VALVE is a patented device. From the simplified diagram (see Figure 4-10) it can be seen that the actuator throttles the flow by pressing the diaphragm closer to the weir. The body itself is insulated from the corrosive liquid by a glass or plastic coating. The stem is connected to an actuator (either hand or automatic control).
The PINCH VALVE shown in Figure above uses a mechanical linkage to squeeze top and bottom together. The flexible sleeve is made of a synthetic rubber. When you turn the hand wheel it throttles the flow to the rate you want. An automatic actuator can be fitted to this device if required.
THE NEEDLE VALVE
The needle valve is used to control very low flows (e.g. chemical dosing of pipelines to stop corrosion). A typical example of a needle valve is shown in Figure below .
The flow is controlled by a needle plug which fits into a small hole in the seat. These valves can have manual or automatic actuators. When it’s made from a solid block of stainless steel it can give very low flow control at very high pressures (e.g. 100,000 psi).
The Check Valve
The check valve is a non-return valve. It allows fluid to flow in one direction only in the same way as a diode allows an electrical current to flow in one direction only. There are two types of check valve; swing check and lift check.
This is the method used for large flow rates through big pipe lines. Figure 4-13 shows a typical swing check valve.
The swing check valve consists of a disc assembly which is free to rotate on the pivot pin. In the open position the fluid pressure on the disc swings it into the position shown. This allows full flow through the valve. In the reverse direction the pressure on the disc forces it hard against the valve seat and no flow is possible.
This is the method use for smaller flow rates and pipelines. Figure below shows a typical lift check valve.
PRESSURE RELIEF (SAFETY) VALVE
Pressure Relief (Safety) Valves are mechanically set valves which open to relieve pressure when automatic control is lost. There are many different designs for this type of valve, divided into two basic groups.
A) Liquid pressure relief, normally called “Pressure Relief Valves ” (PRV).
B) Gas pressure relief, normally called “Pressure Safety Valves” (PSV).
The basic difference is in the speed of operation. PSV’s must relieve the pressure much faster than PRV’s.
- Under normal conditions the valve is closed because of the force applied by the spring.
- The inlet pressure is applied to the under side of the disc.
- If the inlet pressure reaches the relief pressure the disc lifts against the spring. The valve opens and the excess pressure is released; in this example, to the flare.
- The device is calibrated using a dead weight tester. The relief pressure is set by using the adjusting bolt to control the force applied by the spring.
Note: The relief valve shown is only an example. Only certified technicians can calibrate safety relief valves. Each manufacturer holds special certification courses for its safety valves. You will do these special courses during training on the job.