Valves

Abbreviations and Terminology:
FC: Fail closed
FL : Fail locked
FO : Fail open
MOV: Motor Operated Valve
PRV : Pressure Relief Valve
PSV : Pressure Safety Valve
SOV : Solenoid Operated Valve


Armature: Part of an electric machine that strengthens magnetic fields.
Bonnet: Removable cover giving access to valve trim.
Cast: Method of producing shaped components by pouring molten material into a mould.
De-energised: Having no electric current passing through it.
Dynamic seal: A seal between parts that move relative to each other.
Energised: Having electric current passing through it.
Erosion: A slow wearing away, usually by a fluid flowing over a surface.
Fail closed: Describing a valve that automatically closes when the system fails.
Fail open: Describing a valve that automatically opens when the system fails.
Fail safe: Describing something that goes to a safe condition when the system fails.
Flow rate: Volume of fluid flowing past a point in unit time; SI units litres/sec.
Forged: Method of producing components by hammering hot metal into shape, usually between shaped dies.
Gear Ratio: The ratio between diameters or numbers of teeth on meshing gears.
Overpressure: Pressure that is higher than the specified value.
Perimeter: The distance all around the outside of an area.
Pig (pigging): A plug that is pushed through a pipeline by a gas or liquid to perform various functions such as flushing, inspection and cleaning.
Port: An entry or exit through which fluids pass.
Regulating valve: Another name for a throttling or flow-control valve.
Setpoint: pressure A predetermined pressure value at which a component is set to operate.
Turbulence: Flow that is not smooth or regular.
Yoke: External part of a valve that supports stem bushing.


Valves control the flow of fluids through pipes by:

Starting and stopping flow—to control process or isolate part of a pipeline
Changing the flow rate—allowing more or less fluid to flow
Re-directing flow from one line to another at a pipeline
junction
Allowing flow in one direction only
Reducing fluid pressure
Keeping the pressure in a container or pipeline below a fixed maximum
Preventing accidents by relieving overpressure in a container or pipeline
.


It is very important to use the correct:
 
• Valve type—to suit the task it performs, as described above
Size—to suit the pipe size and the flow rate required
Material—to suit the fluid passing through it and to avoid corrosion
.

Basic Valve Components:

The design of different types of valves and some of the components used may vary. There are, however, some basic parts that are common to most valves. These parts are shown in Figure.


The valve body is the main part of the valve. All other parts fit onto the body. It is usually cast or forged and the shape varies with the type of valve. Inlet and outlet pipes fit onto the valve body through threaded, bolted (flanged) or welded joints. The fluid passes through the valve body when the valve is open. The valve body must be strong enough to take the maximum pressure of the process fluid. It must also be made of a material that is not attacked by the fluid.

The valve bonnet is a removable cover fitted to the body. Some bonnets support the moving parts of the valve. Others just close the hole in the body through which the moving parts pass for assembly and dismantling.

The valve trim is the name give to the parts inside a valve. This normally includes:
The opening/closing element—closes the fluid path through the valve body
The valve stem—connects the actuator to the closing element
The valve seat—makes a seal with the closing element when the valve is closed
Sleeves—sometimes used to guide the stem


Valve packing was described in detail in the earlier module in this course: Gland Packing. It allows the valve stem to pass into the valve body without loss of fluid or fluid pressure from the valve. It forms a dynamic seal between the valve stem and the bonnet.


The actuator operates the stem and closing element assembly. The simplest actuator is the manually operated handwheel shown in the Figure . Other actuators may be operated by:


Electric motor—motor operated valve (MOV)
Electric solenoid—solenoid operated valve (SOV)
Air—pneumatically operated valve
Oil—hydraulically operated valve


Valves can be divided into four classes:
 
Block valves—stop and start flow
Throttle valves—control flow rate
Non-return (or check) valves—prevent flow reversal
Pressure control valves—prevent fluid pressure exceeding a set maximum
.


Block Valves:

Block valves either allow full flow or stop flow completely. They should only be operated in the fully open or fully closed position. If they are only partly opened, they offer a lot of resistance to flow. Fluid friction and turbulence cause a loss of pressure in the fluid and can cause vibration.
Block valves are not meant to control flow rate.


There are four main types of block valve used on the plant:
• Gate valves
• Slide valves
• Ball valves
• Plug valves


Gate Valves:

They are used to start or stop a flow completely. They should not be used to control flow rate. Using a gate valve in a partially open position can damage the valve. Fluid flow across the gate causes erosion to the gate making it impossible to seal well against its seat.
Fluid can flow through most gate valves in either direction.
The closing element in a gate valve is a wedge-shaped disc or gate attached to the end of the stem, as shown in Figure. The gate fits into a wedge-shaped seat in the valve body to stop flow through the valve, as shown in Figure.



Turning the handwheel raises and lowers the gate. When the gate valve is fully closed, the gate fills the passage and stops the flow through the valve completely.
When the valve is fully opened, the gate is positioned above the passage in the valve body. This allows full flow through the valve, with little or no obstruction. There is very little pressure drop across the valve.
Gate valves are classed as linear-motion valves as the closing element moves in a straight line (e.g. down and up) to close and open the valve.
Gate valves can have rising or non-rising stems. The valve shown in Figure  has a rising stem. The stem moves up and down with the gate. A rising stem is fixed to the gate and can not turn in it. The upper part of the stem is threaded and screws into a mating thread in a bushing. The bushing is held in a yoke located at the top of the
bonnet as shown in Figure . The actuator turns the bushing in the yoke, screwing the stem into or out of the valve body.


Non-rising stems are threaded at the bottom. This thread mates with a thread in the gate as shown in Figure . Left-hand threads allow clockwise rotation of the handwheel to lower the gate and close the valve.

The stem is fixed to the actuator and turns with it, as shown in Figure . The stem can rotate in its housing but does not move axially.

An open gate valve allows anything that can pass through the pipeline to pass through the valve. Sometimes it is necessary to send solid objects along a pipeline. The object sent is called a pig and the process is called pigging. A pipeline is pigged to flush pipes, clear blockages or for inspection purposes. Gate valves allow these operations.

The gate, or disc design may be:

Solid wedge
Flexible wedge
Split wedge
Parallel disc


Most gate valves have solid-wedge discs.

This is the simplest and strongest type of disc.

The flexible-wedge is also made in one piece. It has a groove cut around its perimeter that allows it to bend a little to fit the shape of the seat more easily. These discs may also have recesses cast into them to increase flexibility.


Flexible-wedge discs are used for valves in steam lines. When the temperature of a closed valve rises, solid-wedge discs can expand and stick in their seats. Flexible wedge discs can compress mere easily and are less likely to distort.
Figure 7  shows a rising-stem, flexible-wedge gate valve.

Figure 7
Split-wedge discs are made in two separate halves. This allows the wedge angle between their outer faces to adjust to fit the seat. This is especially useful if a solid particle is stuck between the disc and its seat. Split-wedge discs are used for gases,especially corrosive gases.
Parallel slide valves also have split discs. Their faces are parallel, not wedge shaped,as shown in Figure 8. A spring between the disc halves pushes them against theirseats. When the valve is closed, the disc on the outlet side is also pushed against its seat by the fluid pressure on the inlet side.



Figure 8
As the valve opens and closes, the sliding action keeps the disc faces clean but causes wear to discs and seats. When fully open, the discs are completely clear of the bore giving no obstruction to flow through the valve. 

Gate valve seats may be integral with the valve body or separate seat rings. Integral seats are cut into the valve body and are part of the body. These seats can not be replaced. They can be repaired by lapping with grinding paste. Seat rings may be pressed or screwed into the body. These can be of a different material and can be replaced when worn or damaged.

Knife gate valves, have a simple, one-piece closing element. It is a parallel-sided plate that may move clear of the flow path to open or may have a hole that moves into the flow path. 

These two types are shown in Figure 9(a) and (b). 





Ball Valves
Ball valves start and stop flow by rotating a ball-shaped closing element. They are classed as rotational-motion valves. The ball has a hole through it of the same diameter as the pipeline. The valve is open when the hole lines up with the inlet and outlet of the valve body. Figure 10 shows a ball valve with part of the body cut away to show the closing element. 

Cutaway Ball Valve
                                               Figure 10
The valve above is shown partially open to show the hole in the ball. This is not the normal valve position; a ball valve is normally only used in the fully closed or fully open positions.

Figure 11 shows the same ball valve looking through the valve inlet. In Figure11(a) the valve is in the closed position. In Figure 11(b) the valve is in the open position.
Figure 11

The open valve leaves a clear path for flow with no obstruction. These valves can be pigged.

The valve shown has a lever actuator that turns through 90o between the fully closed and fully open positions. The lever is in line with the pipeline when the valve is open.


To Be Continued.....

Plug Valves

Operation of a plug valve is similar to the ball valve; they are also rotational-motion valves. The main difference is the shape of the closing element, which is a tapered plug of circular section. The plug has a hole called a port. Figure 12 shows a plug valve that is lined with PTFE to protect it from corrosion and allow lubricant-free operation. 



Figure 12 
Single-port plug valves are used to start and stop flow. Multi-port plug valves redirect flow from one pipeline to another. Figure 13 shows an example of a multiport plug valve.

Figure 13  

 Flow Control (Throttle) Valves:
 
The control of flow rate by reducing the area of the flow path through a valve is called throttling. Throttling a fluid also reduces its pressure.


Block valves should not be used to throttle flow. The pressure drop across them is too great and the flow becomes turbulent. Fluid flow can be either smooth (laminar), or not smooth (turbulent) as shown in Figure


Fluid Flow Patterns

Turbulent flow can cause many problems in pipelines and equipment. In a valve, it can erode the closing element and valve seat. Erosion was described in the earlier module in this course: Bearings. It is the slow wearing away of a solid material by a fluid passing over it. Turbulent flow increases the rate of wear. Figure  shows smooth and turbulent flow in rivers. 

Water Flow in Rivers
Throttle valves are designed to operate partially opened with little pressure loss and
turbulence. Throttle valves are also called regulating valves.
There are four main types:
Globe valves
Butterfly valves
Diaphragm valves
Needle valves
 

Globe Valves:
 
Globe valves are linear-motion valves and can look very similar to gate valves from the outside. Globe valves have rising stems but, unlike gate valves, the actuator is fixed to the stem and rises with it. Figure shows a globe valve in the fully closed
and open positions.
 

Figure :Globe Valve

Globe valve design makes them good for flow regulation as well as starting and stopping flow. In most designs, the flow direction is as shown in Figure. Here, the fluid pressure helps to push the valve open. The packing is not under pressure
when the valve is closed and this helps it to last longer.


 Figure : Flow through Z-type Globe Valve

The flow direction is often marked on the valve body. Make sure that you fit the valve the correct way around.
Globe valves can have three main types of body.

• Z-type
• angle
• Y-type
 
The valves shown in Figures above have Z-type bodies. The name is given because of the path the fluid has to take as it passes through the valve. It changes direction twice, like the letter Z.
 
Z-type globe valves are used mainly for small-size, low-pressure applications. In large, high-pressure lines, the changes of flow direction cause a large pressure drop and turbulence that can damage the trim.

Figure below shows an angle-type globe valve. The flow changes direction only once and the pressure drop is less than for the Z-type. It can be used for medium-pressure applications. 

Figure: Angle-type Globe Valve 

Figure below shows a Y-type globe valve. Having the seat at about 45o to the flow direction straightens the flow path and reduces the pressure drop. This type of valve can be used for high-pressure applications.

Figure:Y-type Globe Valve 
 
 Most globe valves use one of three types of disc:

Ball
Plug
Composition


Ball discs have a curved lower surface. They seal on a tapered seat that has a flat
surface, as shown in Figure below. They are used mainly for low-pressure and low temperature applications.


Globe Valve Discs 

Plug discs come in different shapes but are all tapered. The seat has a matching taper as shown in Figure (b).

Composition discs have a hard backing piece with a soft face as shown in Figure(c). Hard particles trapped between the disc and the seat push into the soft face, maintaining a good seal. Composition discs are replaceable. 

Butterfly Valves:

Butterfly valves are rotational-motion valves. Like ball and plug valves, they need only a quarter turn (90o) to fully open or close them. They can start, stop and regulate flow, although they are not very good at completely stopping flow. Figure below shows a typical butterfly valve. The lever is in line with the pipeline when the valve is open. 

Lever-operated Butterfly Valve 
The closing element is a circular disc of a similar diameter to the ID of the pipe. The disc turns to open and close the valve. The disc or seat may be made of a polymer (plastic) to give a better seal.

Butterfly valves are simple and take up little space. This makes them especially good for use in large pipelines or where there is not much space. Operating a butterfly valve can take a lot of force as you have to push it against the fluid pressure. Larger valves usually have geared actuators to make operation easier, as shown in Figure below. 

Geared Butterfly Valves 
 
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