The most common mechanical device used in process operations is the valve. Valves are used to control the flow of liquids and gases in piping systems.
A wide range of products, pressures, and temperatures are used in processing systems, so valves are made in a wide range of types, shapes and materials.
Valves are used to minimise the hazards associated with the transfer of fluids by providing methods of controlling or isolating the pressure and flow of fluids, It is mainly when valves fail that hazards arise, depending on the chemical and physical nature of the fluid being handled.
Often the failure of a valve will go unnoticed as they are situated within the pipe where their operation cannot be seen. Extra precautions should be taken when performing tasks that rely on the correct operation of a valve.
For example, when pipelines are being dismantled for maintenance, any isolating valves in the system must be closed and if possible, the pipeline drained. Adequate safety equipment should be worn and pipelines opened slowly to prevent large amounts of fluid leaking out in the event of an isolation valve failure.
Any valve that is stuck should not be forced open as this may cause fluid to leak out past the stem and onto operators. Any valves that are known to be leaking, either internally or externally, should be reported and changed as soon as possible.
Any unusual trends in valve operation should also be reported but no adjustments should be made without prior consultation with senior personnel and investigations carried out to discover the reasons for the unusual trend.
Different types of valves perform different functions. Some are used to start or stop flow in pipelines. Other valves are used to regulate flow in pipelines.
Figure 1 Valve open and closed position
The element changes position to open and close the valve.
Figure 2 valve major parts
The body is the part of a valve that is attached to pipesand hold all the parts together. The gases or liquids flow through the body when the valve is open.
This is the part that opens and closes the valve. All valves have an opening element in the valve body. Opening elements are made in various shapes and sizes. They can move up and down on a stem, or rotate around a central pivot point.
The opening element stops or allows flow through a valve. We will discuss types of opening elements further when we look at different types of common valves. When the valve is closed, the opening element fits against the valve seat.
The stem raises or lowers the opening element. In manually operated valves, the steam is usually threaded. In ball valves and butterfly valves, the stem does not go up or down. It turns inside the bonnet. In pneumatic and hydraulic valves, the threaded stem is replaced by a smooth rod that moves up and down in the valve.
On a manual valve, a hand wheel or handle is attached to the stem. The hand wheel or handle allows the operator to turn the stem. The hand wheel is held on the stem by the hand wheel nut.
The bonnet is a separate housing that is bolted tightly to the top of the valve body. The stem passes through the bonnet. The bonnet holds the stem in position.
The packing gland is held in place by bolts, or sometimes it is screwed into place. This gland keeps the packing tight. Packing is a soft material inside the bonnet, placed around the stem to make a tight seal. The packing keeps pressurized gas or liquid in the valve from escaping to the atmosphere.
We will discuss types of valves that are either manually or automatically operated. They are:
- Butterfly Valve
- Globe Valve
- Ball Valve
- Gate Valve
The opening element in a butterfly valve is a flat disc. The disc rotates around a central pivot point and fits against the valve seat when fully closed. The flat disc fills the entire area of the valve body. The butterfly valve offers very little resistance to flow when it is fully open.
Figure 3 butterfly valve
The opening element of a butterfly valve is a flat disc. When the disc is perpendicular to the pipeline, it stops the flow.
Butterfly valves need only one-quarter of a turn to move from a fully closed to a fully open position. A handle is often used instead of a hand wheel to turn the valve stem.
Butterfly valves are used as control valves in low-pressure gas pipelines. They can be set at any position between fully open and fully closed.
Butterfly valves are also sometimes used in low-pressure liquid pipelines.
Butterfly valves are not used in high-pressure pipelines because they do not provide complete shutoff.
A butterfly valve and its engineering symbol are shown in Figure 4 butterfly valve and it’s engineering symbol
Figure 4 butterfly valve and it’s engineering symbol
The vertical lines at either end of the symbol mean that the valve has flanges on the valve body. You may see these lines on any type of valve
The opening element in a globe valve is a plug shaped disc operated by a hand wheel. The disc moves away from the valve seat to open the valve. The disc fits tightly into the seat to close the valve. When closed, the valve completely shuts off the flow of liquid or gas.
Figure 5 globe valve
The opening element is a disc, which moves down to shut off flow. The element can be positioned to allow different rates of flow.
The main function of the globe valve is to regulate the flow of liquids and gases. The valve may be placed in any position between fully open and fully closed.
Globe valves are used mainly as control valves, when the rate, volume, and pressure of flow must be frequently adjusted.
A globe valve and its engineering symbol are shown in Figure 6 globe valve and it’s engineering symbol
Figure 6 globe valve and it’s engineering symbol
Globe valves are primarily used as control valves to regulate fluid flow.
In a ball valve, the opening element is a ball with a hole through the centre. When the valve is fully open, gas or liquid flows through the valve easily. The hole in the ball is the same size as the pipe, so there is no obstruction to flow.
Figure 7 ball valve
The ball element rotates within the valve body to open and close the valve. Large ball valves are operated by hand wheels. Small ball valves have a handle, connected to the stem that turns the ball.
A one-quarter turn of the valve stem will rotate the ball 90°. This is enough to move the valve from a full open to a full closed position.
Ball valves have the advantage that they can be opened and closed more quickly. They are used in high-pressure liquid and gas pipelines as quick shutoff valves.
Ball valves are usually set in a fully open or fully closed position. Because of their construction, ball valves can be opened slowly without damage to the ball. For this reason, they can be used as bypass valves. Bypass valves are opened slowly to allow pressure to build up gradually in a PIpeline. Both ball valves and butterfly valves open and close with a 90° turn.
A ball valve and its engineering symbol are shown in Figure 8 ball valve and it’s engineering symbol below
Figure 8 ball valve and it’s engineering symbol
The opening element is a gate valve is a wedge shaped disc or gate attached to the end of a threaded stem. The gate fits into a wedged shaped seat in the valve body to stop flow through the valve.
Turning the hand wheel raises and lowers the valve gate. When the valve is fully opened, the gate is positioned above the passage in the valve body. This allows full flow in either direction, with little or no restriction.
When the gate valve is fully closed, the gate fills the passage and it stops flow through the valve completely.
Figure 9 rising and non rising stem valves (open and closed)
The gate element is raised and lowered by the hand wheel. When the valve is closed, the gate fits tightly against the valve seat, stopping flow completely.
Large gate valves normally have rising stems. When the hand wheels turned, the stem rises out of the valve bonnet through the hand wheel. The stem shows the position of the disc, and shows if the valve is opened or closed. In small gate valves, the wheel and stem may rise together.
Some gate valves have a nonrising stem. The stem does not come out of the valve body. The gate moves up on the stem. This type of valve is used where there is not enough space for the stem to rise out of the valve body. Most gate valves are manually operated.
Gate valves provide a good seal against flow. For this reason, they are normally used as isolation valves in high-pressure hydrocarbon pipelines. Gate valves are the most commonly used valves in process operations plants.
A gate valve and its engineering symbol are shown in Figure 10 gate valve and it’s engineering symbol
Figure 10 gate valve and it’s engineering symbol
Gate valves can stop flow completely even on high-pressure lines. For this reason, they are the most common valves in process operations.
This multi-turn valve derives its name from the rotating plug, which forms the closing element; the plug may be cylindrical or truncated.
In the open position the fluid flows through the hole in the plug.
Lubricated plug valves rely on a sealing compound injected between the plug and the valve body
Whilst sleeved plug valves are fitted with a soft insert between the plug and the body.
Figure 11 plug valve and it’s engineering symbol
Needle valves are a form of the globe valve and are intended for low flow, fine adjustment conditions. A needle valve and its engineering symbol are shown in Figure 12 needle valve
Figure 12 needle valve
The spindle ends in a tapered needle, which moves into the valve seat to restrict or shut off the flow of fluid through it. A hand wheel operating the spindle controls the ‘in and out’ motion of the needle to and from the valve seat.
- Very precise flow control at low rates
- Needle valves impose a high restriction to the fluid flow, so there is a very high pressure-loss,
- The small flow passage between the needle and seat restricts the use of this valve to low flow rate
- The seat and needle are prone to damage if the fluid being handled contains suspended
Automatic valves, once installed, operate by themselves. They are not opened and closed by an operator or actuator.
Check valves are used to permit flow in only one direction in a pipeline. There are two types of check valves:
- Swing Check Valves
- Ball Check Valves
In a swing check valve, the opening element is a disc attached to a hinged arm. Figure 13 swing check valve
Figure 13 swing check valve
Upstream pressure causes the hinged disc to rise, opening the valve. Higher downstream pressure will cause the valve to close.
The pressure of liquid or gas flowing through the valve holds the disc open, allowing full flow. If flow stops, or the pressure downstream of the valve becomes higher than the pressure upstream, the hinged arm will swing down. The disc will cover the passage and seal the valve. The higher downstream pressure will press against the disc to keep it tightly closed. None of the liquid or gas that has already passed through the valve will be allowed to flow back through the valve. Swing check valves can be installed vertically (the disc must point up in the flow direction) or horizontally. Gravity will close the valve if flow stops.
The opening element in a ball check valve is a ball.
The ball check valve works on the same principle as the swing check valve. The pressure of liquid or gas in the pipeline opens the valve by raising the ball from the valve seat. If flow stops or reverses direction, the ball drops back into the seat. The higher pressure of the liquid or gas downstream holds the valve closed.
The ball check valve shown in Figure 14 ball check valve open and closed, must be installed in a horizontal position. Some ball check valves are made so they can be installed vertically. That is, flow from below raises the ball to allow liquid or gas to pass through the valve.
There will usually be an arrow on the outside of the check valve body to indicate the direction of flow.
Figure 14 ball check valve open and closed
Higher downstream pressure will cause the element to seat, closing the valve.
Check valves are used to control the direction of flow in a pipeline. Swing and ball check valves can be used in both liquid and gas pipelines. They are often installed downstream of reciprocating pumps and compressors.
A check valve and its engineering symbol are shown in Figure 15 check valve and its engineering symbol
Figure 15 check valve and its engineering symbol
The circle on the symbol indicates the upstream side of the check valve. The arrow on the valve casing indicates the direction of flow for installation.
Safety relief valves prevent excess pressure, which may cause serious accidents.
The opening element in the safety relief valve shown in Figure 16 safety valve is a disc held closed by a spring. The amount of tension on the spring is adjustable. Maintenance personnel set the tension on the spring when the valve is installed.
The safety relief valve pressure setting is always higher than the normal operating pressure of the system it is protecting. It is always lower than the pressure, which could damage the system.
Figure 16 safety valve
Safety relief valves prevent damage to equipment from overpressure. When fluid pressure at the inlet exceeds the spring tension, the valve disc will rise, opening the valve.
For example, a safety relief valve is often used to protect a positive displacement pump or compressor. If the normal discharge pressure is 150psi, the safety relief valve might be set at 180psi. If the pressure in the discharge pipeline goes up to 180psi, the pressure will overcome the tension of the valve spring. The valve disc will lift up off its seat. The liquid or gas causing the high pressure will then escape through the valve as shown in Figure 17 safety relief valve operation
Figure 17 safety relief valve operation
The valve opens when the fluid pressure at the inlet is higher than the setting on the relief valve.
Once the pressure in the pipeline falls below 180psi, the spring will force the valve disc back down in its seat.
Safety relief valves are used in both liquid and gas pipelines. These valves are also installed on process vessels and tanks to protect them from excess pressure build-up.
Some safety relief valves will allow the liquid or gas to escape to the atmosphere. Hydrocarbon liquids and gasses that escape through safety relief valves are piped to a burn pit or flare.
Flow through the relief valve will only occur under high-pressure conditions.
A safety relief valve and its engineering symbol are shown Figure 18 safety valve and its engineering symbol
Figure 18 safety valve and its engineering symbol
In this part of the module, you will learn how valves are operated.
- Valves can be operated manually or automatically.
- Valves can be operated locally or by remote cont
Manual operation occurs when an operator opens, closes, or adjusts a valve. Figure 19 manual operation shows one method of manual operation. The operator turns a hand wheel or moves a handle that changes the position of the valve disc or ball.
Figure 19 manual operation
The operator turns the hand wheel clockwise to close the valve. He opens the valve by turning the hand wheel counter clockwise. (Usually can be opposite)
In process operations, all valves with hand wheels operate the same way; you turn the hand wheel in a counter clockwise direction to open the valve and in a clockwise direction to close the valve. Generally the hand wheels have arrows marked on the surface to show the direction for opening and closing the valve.
There are three important points to remember for hand wheel operation of gate valves:
- Grease the stem after operati This helps to lubricate the stem, prevent corrosion, and keep sand out of the threads. In some plants you will put a plastic hose over the threads after greasing them.
- Always turn the hand wheel ¼ turn closed after fully opening the va This prevents the valve from seizing or sticking in the open position.
- Always turn the hand wheel ¼ turn open after fully closing the valv This prevents the valve from seizing or sticking in the closed position.
In general, to open a handle-operated valve you put the handle in line with the pipe work. To close the valve you turn the handle 90 to the pipe work. The movement from open to close is 90.
Figure 20 shows local operation of a motor-operated valve (MOV).
Local operation occurs at the valve. If the valve is operated by a motor, the controls for the motor are located near the valve.
Figure 20 local operation of an MOV
The local controls are located close to the valve, in the field. The operator in Figure 20 local operation of an MOV is pressing a button that operates an electric motor. The electric motor does the work of opening or closing the valve. This is local operation of the valve.
Remote means at a distance, or away from. Remote valve operation is usually performed by control room personnel. A remotely operated valve is equipped with a valve actuator. This is a mechanical device that provides force to open, close, or adjust a valve.
Valve actuators may be operated electrically, pneumatically, or hydraulically. In figure 20, the operator is using an electrically operated actuator to operate a valve.
InFigure 21 remote operation, electric wires connect the valve actuator mechanism to a pushbutton switch in the control room. A valve position indicator will be located near the pushbutton switch that operates the valve. The indicator shows whether the valve is open or closed. A control room operator can operate the valve by pressing the pushbutton switch while he watches the position indicator.
Figure 21 remote operation
Remote valve operation is normally performed in the plant control room.
Automatic operation of a valve is when an instrument control system is used to open, close, or adjust a valve Figure 22 controlling water level compares automatic control with manual control.
Figure 22 controlling water level
Figure 22 controlling water level shows an example of automatic control. The valve position changes automatically to adjust the water level in the tank.
The objective in each system is to control the water level. The system below is a manual system. An operator opens and closes the valve in the inlet line as necessary to keep the water level at the set point. In this case, the set point is 50%. The operator observes the level indicator and adds water when the level drops below 50%.
The system in figure 22 is an automatic system. A sensing instrument continuously monitors the water level in the tank. Another instrument, called a transmitter, sends an electronic or pneumatic signal corresponding to the water level from the sensor to a controller. A controller is an instrument that takes the place of the operator in adjusting the valve.
The controller is preset to a specific set point. The controller compares its set point with the information it receives from the transmitter about the actual water level. If the water level is not exactly at the set point, the controller automatically adjusts the control value to bring the level back to the set point.
Local automatic operation means that a process instrument control system is located near the process control valve. A plant operator sets the instrument controller to a required set point. The automatic controller operates the valve as necessary to control the process.
In remote automatic operation, the controller that operates the valve is located in a control room. The control room may be quite far from the valve. A control room operator sets a required set point on the controller. The controller opens and closes the valve to maintain the process at set point.