By:Chitrakshi Goel

M. Tech. EST

ValveAn integral component in piping systemprimary method of controlling the flow, pressure

and direction of the fluid within a system or process

Valves may be required to operate continuously e.g. control valves, or intermittently e.g. isolation valves, or rarely e.g. safety valves• A valve can be an extremely simple, low cost

item or it may be and extremely complicated, expensive item.

ValveValve controls a system or process fluid flow

and pressure by performing any of the following functions:Stopping and starting fluid flowVarying the amount of fluid flowControlling the direction of fluid flowRegulating downstream system or process

pressureRelieving overpressure in any component

or piping

Parts of Valve1. Body2. Bonnet3. Stem4. Actuator5. Packing6. Seat7. Disk

Valve Bodyis the principal element of a valve assembly

and is the framework that holds everything; all

other parts fit onto the bodyis usually cast or forged into a variety of

shapesinlet and outlet pipes fit onto the valve body

through threaded, bolted or welded jointsresists fluid pressure loads from connecting

piping

Valve BodyFluid passes through the valve body when

the valve is openValve body must be strong enough to take

the maximum pressure of the process fluid and must be made of a material that is not attacked by the fluid

BonnetA bonnet acts as a cover on the valve bodyis the removable cover fitted to the bodySome 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.

During manufacture of the valve, the internal parts are put into the body and then the bonnet is attached to hold everything together inside.

Valve TrimThe internal elements of a valve are

collectively referred to as a TRIM.The trim typically includes:

the opening/closing element (disk): 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

StemThe stem, which connects the actuator and disk,

is responsible for positioning the disk.Stems are typically forged and connected to the

disk by threaded or welded joints.There are two types of valve stems: rising stems

and non-rising stems. These two types of stems are easily distinguished by observation.

For a rising stem valve, the stem will rise above the actuator as the valve is opened. There is no upward stem movement from outside the valve for a non-rising stem design.

ActuatorThe actuator operates the stem and closing

element (disk) assembly. An actuator may be a manually operated

handwheel, manual lever, motor operator etc.In some designs, the actuator is supported by

the bonnet. In other designs, a yoke mounted to the bonnet supports the actuator.

PackingMost valves use some form of packing to

prevent leakage from the space between the stem and the bonnet.

Packing is commonly a fibrous material that forms a seal between the internal parts of a valve and the outside where the stem extends through the body.

Disk and SeatDisk provides the capability for permitting

and prohibiting fluid flow.The seat or seal rings provide the seating

surface for the disk. A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed.

Types of ValvesBlock Valve- stop and start flowFlow Control Valve- control flow rateNon-Return Valve- prevent flow

reversalPressure Control Valve- prevent fluid

pressure exceeding a set maximum

Block Valveeither allow full flow or stop flow completelyshould only be operated in the fully open or fully

closed position If they are only partly opened, they offer a lot of

resistance to flowFluid friction and turbulence cause a loss of

pressure in the fluid and can cause vibration.are not meant to control flow rate

Types of Block ValveGate ValveBall ValvePlug Valve

Gate Valveare used to start or stop a flow completely and should

not be used to control flow rate. Using in a partially open position can damage the

valveFluid flow across the gate causes erosion to the gate

making it impossible to seal well against its seatFluid can flow through most gate valves in either

directionThe closing element in a gate valve is a wedge-shaped

disc or gate attached to the end of the stemThe gate fits into a wedge-shaped seat in the valve

body to stop flow through the valve

Rising Stem Gate Valve

Gate Valveis classed as linear-motion valve as the

closing element moves in a straight line (e.g. down and up) to close and open the valve

can have rising or non-rising stemsIn rising stem gate valve, the stem moves up

and down with the gate. Rising stem is fixed to the gate and can not turn in it.

Non-rising stems are threaded at the bottom and this thread mates with a thread in the gate.

Non-Rising Stem Gate Valve

Ball Valvestarts and stops flow by rotating a ball-shaped

closing elementclassed as rotational-motion valveThe 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.only used in the fully closed or fully open

positionsThe open valve leaves a clear path for flow with

no obstruction.

Plug ValveOperation of a plug valve is similar to the ball

valvealso rotational-motion valvesThe 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.

Plug valves are normally used in non-throttling, on-off operations, particularly where frequent operation of the valve is necessary.

Flow Control ValveControl of flow rate is done by reducing the

area of the flow path through a valve, this also reduces the fluid pressure.

Block valves should not be used to control the flow. The pressure drop across them is too great and the flow becomes turbulent. Turbulent flow can cause many problems in pipelines and equipment. In a valve, it can erode the closing element and valve seat. Turbulent flow also increases the rate of wear.

Flow Control ValveFlow Control Valves are designed to

operate partially opened with little pressure loss and turbulence.

also called regulating valves

Types of Flow Control ValveGlobe ValvesButterfly ValvesDiaphragm ValvesNeedle Valves

Globe Valveis a linear-motion valve and can look very similar

to gate valve from the outsidehas rising stems but, unlike gate valves, the

actuator is fixed to the stem and rises with it. Its design makes it good for flow regulation as

well as starting and stopping flow.Globe valves can have three main types of body

Z-typeAngleY-type

(a) Valve Closed (b) Valve Open

Here, the fluid pressure helps to push the valve open.

used mainly for small-size, low-pressure applications

Z-Type Globe Valve

Angle-Type Globe Valve

Globe ValveIn 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.

Y-type globe valve have the seat at about 45° to the flow direction, hence straightens the flow path and reduces the pressure drop. It can be used for high-pressure applications.

Y-Type Globe Valve

Butterfly Valveis rotational-motion valveLike ball and plug valves, it needs only a

quarter turn (90°) to fully open or close itIt can start, stop and regulate flow, although

it is not very good at completely stopping the flow.

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.

(a) Valve Closed (b) Valve Open

Butterfly ValveButterfly 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 one has to push it against the fluid pressure.

Larger valves usually have geared actuators to make operation easier.

Diaphragm ValveThe closing element of a diaphragm valve is not

a solid disc. Instead, it has a sheet of flexible material called a diaphragm.

This diaphragm completely separates the valve trim from the fluid flowing through the valve. This means that the fluid does not contact the trim and the stem does not need any gland packing.

Diaphragm valves are rising-stem, linear-motion valves. As the actuator turns, the stem screws into or out of the sleeve attached to the actuator.

Diaphragm Valve Positions

Needle ValveNeedle valves are linear-motion valves.can make very small adjustments to flow rate.Its name comes from the long, tapered shape

of the bottom of the spindle that forms the closing element.

Non-return (Check) ValveNon-return valves, also called check valves, stop

flow reversal in a pipe.only allow fluid to flow in one direction.The pressure of the fluid passing through the valve

in the correct direction opens it automatically.If the flow tries to reverse, the valve closes

automatically.There are a number of designs of non-return valve.

Some rely on the weight of the closing element and fluid flow only to close them. Others have a spring to help close them.

Types of NRVSwing Check ValvesLift Check ValvesPiston Check ValvesBall Check ValvesStop Check Valves

Swing Check ValveIn this type, the valve disc is hinged at the

top. When there is no flow, the weight of the disc closes the valve. This valve must be mounted in a horizontal pipeline, with the disc hinge at the top to allow gravity to close it.

Lift Check ValveThese valves have a similar valve body and

seating arrangement to globe valves.Flow must enter from under the seat to lift

the closing element.Flow in the reverse direction pushes the

closing element against its seat.The closing element may be free to fall under

its own weight, or it may be helped by a spring

Piston Check ValvePiston check

valves are similar to lift check valves.

Instead of a valve disc there is a piston that slides in a cylinder.

This gives a smoother motion during operation.

Ball Check ValveThis type of

valve has a spherical (ball-shaped) closing element.

Like the other check valves, the closing element may operate by gravity or the flow pressure or it may be spring-loaded.

Stop Check ValveA stop check is a non-return globe valve.It is similar to a globe valve but the valve disk is

free to slide on the stem.With the valve stem raised, it acts as a lift check

valve allowing flow only from below the disc.If there is no flow, or if flow reverses, the disc

drops into the seat.When the stem is lowered to the closed position,

the disc can not lift and flow is stopped in both directions.

Pressure Control ValvePressure control valves can be divided into three

main types:pressure reducingpressure reliefpressure safety

Pressure reducing valves operate where a pressure drop is needed between two parts of a process.

Pressure relief valves maintain fluid pressure below a maximum allowable value for a process.

Pressure safety valves protect the plant from damage caused by overpressure.

Pressure Reducing ValveReducing valves automatically reduce liquid or gas

pressure to a pre-set value.One common use is to control the pressure of gas

leaving gas bottles and vessels, for example pressure reducing valves on gas welding equipment.

During operation, the valve continuously opens and closes to maintain a flow of fluid at the reduced pressure.

The operation of this valve depends on the balance between the fluid pressures acting above and below a piston, and a spring force.

Pressure Reducing ValveWhen the force of the low pressure fluid plus

the spring force pushing down on the piston is more than the force of the high pressure supply fluid pushing up, the piston closes the valve.

When the force of the low pressure fluid drops, the new lower pressure plus the spring force pushing down on the piston becomes less than the force of the high pressure fluid pushing up and the piston opens the valve.

Pressure Relief Valveis used mainly to relieve overpressure of liquids.This happens when a liquid in a closed container

or pipeline expands as its temperature increases.Under normal operating conditions, a spring

holds the PRV closed. Fluid pressure pushes against the spring to open the valve.

The fluid pressure needed to push the valve open is called the set-point pressure. The setpoint pressure is usually the maximum normal operating pressure of the liquid.

• When the liquid pressure exceeds the set-point pressure, the valve opens slowly. It releases just enough liquid to bring the pressure down to the normal operating pressure.

• The spring then closes the valve slowly so that normal operations can continue. The outlet from the valve is connected back into the inlet of the equipment so that no liquid is lost.

Pressure Safety Valveis used mainly to relieve overpressure of gases and

vapours (e.g. steam).The set-point pressure is greater than the

maximum normal operating pressure of the process fluid but less than the maximum safe working pressure of the equipment.

When the fluid pressure exceeds the set-point pressure, the valve pops fully open. This happens very quickly to release overpressure as quickly as possible.

The pressure at which the valve closes again is lower than the opening set-point pressure.

The valve outlet diameter is greater than the inlet. This allows fluid to escape quickly to bring pressure down to normal (both in PRV and PSV).

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