Unit 3 Valves

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UNITS IN THIS COURSE

UNIT 1 PIPING SYSTEMS

UNIT 2 PIPE FITTINGS

UNIT 3 VALVES

TABLE OF CONTENTS

Para Page

3.0 COURSE OBJECTIVE 3

3.1 INTRODUCTION TO VALVES 4

3.2 VALVE IDENTIFICATION AND FUNCTION 5

3.3 VALVE PARTS 6

3.4 BLOCK VALVES 7

3.5 PLUG VALVES 11

3.6 BALL VALVES 12

3.7 THROTTLING OR REGULATING VALVES 13

3.8 CHECK VALVES

18

3.9 SPECIAL VALVES 22

3.10 STUFFING BOX AND VALVE PACKING 26

3.11 CONTROL VALVES 30

3.12 AUTOMATIC CONTROL VALVES 30

3.13 VALVE ACTUATORS 31

3.14 PNEUMATIC ACTUATOR OPERATION 32

3.15 ELECTRIC MOTOR ACTUATOR 35

3.16 HYDRAULIC VALVE ACTUATORS 36

3.17 SOLENOID / MAGNETIC ACTUATORS 37

3.18 VALVE INSPECTION 38

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3.0 COURSE OBJECTIVE

This course introduces the students to all major items ofstatic equipment relating toa plant site. Upon completion of the course the trainees will have a general

understanding ofthe following.

Equipment Terminology.

Theory of operation.

Equipment construction.

Hands on operation.

& Safety features.

Equipment interaction with the overall process.

3.1 INTRODUCTION TO VALVES

A valve is a device installed in a piping system which controls the flow of a gas orliquid. Valves have an opening through which gas or fluid can flow. They can closeoff (isolate) the piping system to stop the flow and they can control the amount offlow.

Processing systems handle different products at different pressures andtemperatures. So, there are many different types of valves A valves shape andmaterial depends upon the product and its operating pressure and temperature.

Valves carry the same rating system for pressure as flanges; i.e. 150-300-600 etc.

In a gas plant or refinery, valves will be made from the following materials:

Stainless steel. Carbon steel. These areused for process fluids Cast iron.

Brass. These are used for utilities / fire water Bronze.

Plastic These are used for air chemical lines.

3.2 VALVE IDENTIFICATION AND FUNCTION

Table 1 shows the four main types of valves and their function.

VALVE TYPE EXAMPLES FUNCTION

Block Valve Gate Valve To stop or start the flow of aBall Valve fluid. Used in the fully open or

fully closed position.

Throttling / Regulating Globe Valve To control how much fluidModule

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Valve Butterfly Valve flows through a valve. Used inNeedle Valve any position from fully open to

fully closed.

Check / Non Return Ball Check Valve Allows fluid to flow in oneValve Piston Check Valve direction only. Fluid flowing in

Swing Check Valve the other direction will close

the valve.

Safety / Relief Valve Balanced bellows To release excess pressure.Pressure Safety To protect system from tooValve (PSV) much pressure.Pressure Relief Valve(PRV)

3.3 VALVE PARTS

Figure 3-1 Major Valve Parts

The major parts common to most valves are as follows: See Figure 3-1

Body Valve Disc (Opening Element). Stem. Handwheel / Handle. Bonnet.Packing Gland

BODY

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The body is the part of a valve that is attached to the pipework flange. The gases orliquids flow through the body when the valve is open.

VALVE DISC (OPENING ELEMENT)

This is the part that opens and closes the valve. All valves have a valve disc in thevalve body. Valve discs are made in various shapes and sizes. They can move upand down with the stem, or rotate around a central pivot point.

The valve disc stops or allows flow through a valve. We will discuss types of valvedisc further when we look at different types of common valves. When the valve isclosed the valve disc fits against the valve seat.

STEM

The stem raises or lowers the valve disc. In manually operated valves the stem isusually threaded. In ball valves and butterfly valves the stem does not move up ordown. It turns inside the bonnet. In pneumatic and hydraulic valves the stem is asmooth rod that moves up and down in the valve body.

HANDWHEEL / HANDLE

On a manual valve a handwheel or handle is attached to the stem. The handwheelor handle allows the operator to turn the stem. The handwheel is held on the stemby a handwheel nut.

BONNET

he 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.

PACKING GLAND

he packing gland is held in place by bolts or sometimes it is screwed into place. Thisgland keeps the packing tight. Packing is a soft material inside the bonnet placedaround the stem to make a tight seal. The packing stops pressurised gas or liquidin the valve from escaping to the atmosphere.

3.4 LOCK VALVES

The three most commonly used block valves in process systems are:

Gate Valve.

Plug Valve.

Ball Valve.

GATE VALVES

There are two main types of gate valves used in the process system:

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Rising stem gate valve.

Non-rising stem gate valve.

The valve disc in a gate valve is a wedge shaped disc or gate. It is attached to theend of a threaded stem. The gate fits into a wedged shape seat in the valve body

to stop flow through the valve.

Turning the handwheel raises and lowers the valve disc. When the valve is fullyopen the gate is positioned above the passage in the' valve body. This allows fullflow in both directions through the ports. There is little or no restriction and nopressure drop across the valve.

When the gate valve is fully closed the gate fills the passage and it stops the flowthrough the valve completely. Gate valves are only used in the fully open or fullyclosed position to start and stop flow. You must turn the handwheel to many timesto move the valve to the fully open or fully closed position.

RISING STEM GATE VALVE

Figure 3-2 Rising Stem Gate Valve

Large gate valves normally have rising stems. When the handwheel is turned thestem rises out of the valve bonnet through the handwheel. The stem shows theposition of the valve disc and shows if the valve is fully open or fully closed. Insmall gate valves the wheel and stem may rise together.

The threaded part of the stem is above the valve body. It is not in contact with thefluid flowing through the valve. "this type of valve has less corrosion problems. The

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rising stem gate valve requires more space and more head room.

NON RISING STEM GATE VALVE

Figure 3-3 Non Rising Stem Gate Valve

Some gate valves have a non rising stem. The stem does not come out of the valvebody. The valve disc moves up and down the stem. With this type of gate valvethere is more chance of corrosion as the valve stem is in contact with the liquidflowing through the valve. This type of valve is used where there is not enoughspace above the valve for the stem to rise out of the body.

Gate valves provide a good seal against flow. For this reason they are normallyused as isolation valves in high pressure hydrocarbon pipelines. The gate valve isthe most commonly used valves in the petroleum industry.

If the gate valve is partially closed there may be vibration. This can cause noise andpossible damage to the valve disc and the seating surfaces. For this reason gatevalves should only be used for throttling (restricting flow) in an emergency.

There are three main types of gate designs for gate valves: (See Figure 3-4)

Solid Wedge Gate.

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Split Gate (parallel disc).

Flexible Gate.

Figure 3-4 Gates for Gate Valves

SOLID WEDGE GATE

The solid wedge valve disc was the first type that was used in gate valves. It doesnot allow for expansion or contraction of the valve body and stem material withlarge temperature changes. This type of wedge is not used in high temperatureprocesses where there is thermal expansion and contraction of the valve body.(See Figure 3-4)

SPLIT GATE

The split gate valve disc is split up the centre. This allows the element to expand orcontract. Therefore, the disc has a better fit in the valve seat over a wide range ofpressures and temperatures. The wedge can adapt itself to small amounts ofdistortion caused by seat wear or piping strain. (See Figure 3-4)

FLEXIBLE GATE

Like the split gate valve disc the flexible gate valve disc is split up the centre. Thisallows the element to expand and contract and to give a very tight seal in the valveseat over a wide range of pressures and temperatures. This type of wedge is usedin high temperature processes. (See Figure 3-4)

3.5 PLUG VALVES

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Figure 3-5 Plug Valve

The plug valve is one of the simplest types of valve. It is also known as a petcock ora plug cock. Like the gate valve it is only used in the fully open or fully closedposition. The plug is a tapered round plug. It has a rectangular hole in the centrethat lines up with the ports in the valve body.

The valve has a handle for manual operation. It is opened or closed by a quarterturn of the valve handle. When the valve handle is pointing in the direction of thepipe the valve is in the fully open position. When the valve handle is pointingacross the pipe the valve is in the fully closed position. .

A special grease is forced into and around the plug to act as a seal and a lubricant.These valves are a simple design. They do not take up much space and do nothave high yokes, bonnets or valve stems. Normally these valves are manuallyoperated but they can be operated automatically if required.

3.6 BALL VALVES

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Figure 3-6 Ball Valve

This type of valve is opened or closed by a ball. Like the plug and gate valves, theball valve is used only in the fully open or fully closed position. The ball has a holethrough the centre which is the same size as the valve ports in the valve body. Thevalve moves from the fully open position to the closed position by moving the valvehandle a quarter of a turn.

The position of the valve handle indicates whether the valve is fully open or fullyclosed. When the valve handle is pointing in the direction of the pipe the valve is inthe fully open position. When the valve handle is pointing across the pipe the valveis in the fully closed position.

The ball is carefully machined so that it fits exactly into the valve seats. It gives avery tight shut off with metal to metal seal. Ball valves are very good when they areused in gas processes at high and low pressures and temperatures.

3.7 THROTTLING OR REGULATING VALVES

Throttling or regulating valves are used to control the amount of flow They can onlyreceive flow from one side. (Except the butterfly valves)

There are three main types of throttling valves that are used in the petroleumindustry.

Globe Valves.

Needle Valves.

Butterfly Valves.

GLOBE VALVES

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Figure 3-7 Globe Valve

A globe valve is used to control the amount of flow. The valve can be operated inany position from the fully closed position to the fully open position. The valve disc

is globe shaped (circular). The valve operates by raising and lowering the valvedisc into a circular seat. There is no contact between the seat and disc ends whenthe flow through the valve starts. Only a few turns of the handwheel are needed tofully open or close the valve. (The gate valve needs many turns to open or close it.)

There are partitions inside a globe valve. The fluid flowing through the valve mustmake two 90 degree turns. The flow is "under and over" to prevent wear to theseat. Therefore, there is a high pressure drop in the valve. It is easier to control thiskind of flow.

Globe valves are usually smaller if they are hand controlled and need to be openedand closed often. The direction of the fluid flow through the valve keeps the seat

and disc clean.

NEEDLE VALVES

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Figure 3-8 Needle Valve

The needle valve is similar to the globe valve. It has a needle shaped plug and seatinstead of globe shaped ones.

When the valve is fully open the needle valve does not allow a full flow. This isbecause the hole in the seat is much smaller than the pipe flow area.. The threadson the stem of a needle valve are very close together. This allows very exactcontrol of the flow because the needle can be adjusted very precisely. Needlevalves are used in small sizes, only up to 1 ". They are mainly used oninstrumentation systems.

BUTTERFLY VALVES

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Figure 3-9 Flanged Butterfly Valve

Figure 3-10 Lug Wafer Type Butterfly Valve

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Figure 3-11 Wafer Type Butterfly Valve

Modern butterfly valves usually have rubber or elastomeric seats. These provide a

tight shut off like any other valve.

This valve design is very good for installations where space considerations areimportant. This type of valve is good for large piping systems since they can bemade at almost any size. Basically, the valve consists of the valve body, shaft andbutterfly disc, sealing gland and valve actuator

Because the design is simple, this valve works equally well in three different valvebodies; flanged butterfly valve, the lug wafer butterfly valve and the wafer typebutterfly valve., (See Figures 3-9, 3-10, 3-11).

The butterfly valve moves from fully open to fully closed. in a quarter turn of the

valve handle. These valves are used to control the amount of flow. The sealingmechanism is a circular disc made of rubber or metal. It is difficult to shut off theflow if the sealing mechanism is made of metal; rubber gives a better seal. Butterflyvalves usually have some kind of automatic actuator because they are fairly hardto close manually. Butterfly valves can be used for fluid flow in either direction.

The valve disc in a butterfly valve is a flat disc. The disc rotates around a centralpivot. When fully closed the flat disc fills the entire area of the valve body. Thebutterfly valve offers very little resistance to flow when it is fully open. Butterflyvalves are used as low pressure control valves in gas and liquid pipelines.

GAUGE COCK VALVE

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Figure 3-12 Gauge Cock Valve

A gauge cock valve is a special type of valve. It is used between a vessel and avessel gauge glass. Its function is to isolate the gauge glass automatically if thegauge glass begins to leak or break.

The gauge cock valve has a small metallic ball inside. If the gauge glass breaks, thefluid inside the vessel will flow out through the valve and out of the gauge glass.The fluid movement pushes the metal ball into the outlet hole of the valve. Thiscloses the valve and so stops the leak.

A gauge cock valve must always be in the fully open position when it is in service or

it cannot protect the system. If it is only part open the valve stem will stop the ballfrom sealing off the outlet hole.

If the gauge glass must be isolated for cleaning etc. the gauge valve can bemanually operated.

3.8 CHECK VALVES

Check valves are also known as non return valves. This type of valve allows thefluid to flow in one direction only.

All check valves operate in the same manner. The pressure of the fluid flowing inone direction holds the valve open. Fluid pressure from the opposite directioncloses the valve against a seal. Most check valves are marked with an arrow toshow the direction of flow through the valve. There are four main types of checkvalve:

Swing Check Valve.

Horizontal Lift Check Valve.

Vertical Lift Check Valve.

Ball Check Valve.

Check valves are installed on the discharge lines of pumps and compressors.

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SWING CHECK VALVE

Figure 3-13 Swing Check Valve

Swing check valves are so named because of the way they work. They swingopen to allow flow in one direction. When they close they stop the flow from theopposite direction. They are widely used in general industry as they offer littleflow resistance and they don't go wrong.

In a swing check valve the valve disc is attached to a hinged arm. The pressureof gas or liquid flowing through the valve holds the disc open, allowing full flow. If

the flow stops or the pressure downstream of the valve becomes higher than thepressure upstream of the valve, the hinged arm will swing down. The disc willcover the passage and close the valve. The back pressure will press against thedisc to keep it tightly closed. None of the liquid or gas that has already passedthrough the valve can flow back through the valve.

Swing check valves can be installed horizontally or vertically. Gravity will closethe valve if the flow stops. They are usually installed on the discharge lines ofpumps and compressors.

BALL CHECK VALVE

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Figure 3-14 Ball Check Valve

The valve disc in a ball check valve is a ball. This type of check valve is used -tocontrol the flow direction of heavy fluids. They are available in horizontal, verticaland angle design. Ball check valves also work well where particles are present in a

fluid.

The ball in this type of check valve rotates. This ensures that the ball wears evenlyand makes a good seal on the valve seat. A certain pressure is required to openthe ball valve. When the pressure falls, the ball valve will close by gravity.

LIFT CHECK VALVES

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Figure 3-15 Lift Check Valve

Lift check valves are sometimes built in globe valve bodies. They are also calledpiston check valves. (See Figure 3-15) They have a guided disc shaped plug whichis lifted by the pressure of the fluid. This type of check valve does not shutsuddenly as the swing check valve does. It is better for piping systems where theflow is irregular or often changes direction. The disc is seated horizontally so thefluid must change direction twice as it passes through the valve.

3.9 SPECIAL VALVES

SAFETY AND RELIEF VALVES

Figure 3-16 Pressure Relief Valve

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Figure 3-17 Safety Relief Valve

A safety or relief. valve is a very important protective device. It is designed toavoid accidents by relieving pressure when something goes wrong in the

system.

Safety valves and relief valves respond to high pressure in a pipe system. Thesevalves relieve the pressure before it can cause an accident or damage.

Safety valves are used for protection on steam and gas production facilities.Relief valves are generally used on liquid production systems.

Safety valves are commonly called PSV's (Pressure Safety Valves). Relief valvesare commonly called PRV's (Pressure Relief Valves).

A safety valve on a gas processing system is usually connected to a flare vent

system. The gas escapes to a flare where it is burnt. A safety valve on a steamsystem will vent to the atmosphere.

Safety valves are set to open (pop) completely at a pre-set pressure. This pre-setpressure can be adjusted. The safety valve will stay open until the pressuredrops back to the pre-set lower pressure. The valve should then shutimmediately.

The outlet ports on safety valves and relief valves are always larger than the inletports. This allows the excess pressure to flow through the valve faster and bringthe pressure down to normal quickly.

The safety valve is spring loaded. The spring is adjusted before it is installed. Atthe pre-set pressure the spring loaded disc is forced open. It is designed so thatthe flow pushes the disc fully open. It stays in the fully open position until the

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pressure falls to below the pre-set pressure. The spring then snaps the disc tothe closed position where it is firmly seated.

The relief valve is normally spring loaded. When the pressure goes above thepre-set pressure the disc does not go fully open immediately. The amount itopens depends on the amount of excess pressure. The more excess pressurethere is, the more the valve opens.

When the pressure falls to the pre-set pressure, the valve will close gently. Thereis no loss of liquid because the liquid that escapes is recycled to the suction ofthe pump or inlet of the vessel it came from, These valves are generally used asrelief valves for the thermal expansion of liquids in pipelines.

Safety valves and relief valves are very important for the safe operation of a pipingsystem. They are normally inspected and tested once a year or during plant turnarounds.

RUPTURE DISCS

Figure 3-18 Rupture Disc

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Figure 3-19 Ruptured Rupture Disc

Figure 3-20 Rupture Disc Installation

Rupture discs (bursting discs) are another pressure relief device. A rupture disc isdesigned to break if the pressure is too high. Different discs are designed fordifferent pressures. After they break they must be replaced.

DIAPHRAGM VALVE

Figure 3-21 Diaphragm valve

A diaphragm valve is not like the other valves. It does not have a valve disc. Thediaphragm valve uses a flexible diaphragm as its opening / closing element. When

the diaphragm is forced downward it completely closes the valve. When it iscompletely open the diaphragm is out of the line of flow so it does not restrict it.There is no valve stem in the flow path. No stem packing is required. These aretypes of valve require less maintenance than the conventional valves.

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The diaphragm is usually made of a rubberised compound so these valves are oftenused where highly corrosive materials are present. Figure 6-63 shows the threeposition of a diaphragm valve.

Figure 3-22 Positions of a Diaphragm Valve

3.10 STUFFING BOX AND VALVE PACKING

The valve packing inside the stuffing box has two main functions. It keeps theprocess fluid inside and the atmospheric atmosphere outside the pipe. The packingalso makes it easier to open or close a valve because it lubricates the valve stem.

The valve packing must be able to withstand the following:

High temperature.

Low temperature.

High pressure.

Vacuum.

Many different types of fluids.

Abrasive materials.

Compression forces.

Good packing must be:Module

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Good lubricant.Compressible.Long Lasting.Tough.Flexible.

One type of packing cannot handle all the processes or be suitable for all thedifferent types of valves. Because of the wide variety of valves and process fluids,a wide variety of stuffing boxes and packings are used. Stuffing boxes aredesigned to hold a number of different types and sizes of packing. Four of the mostcommon types are shown below:

PACKING NUT WITH GLAND

Figure 3-23 Packing Nut With Gland

This type uses ring type packing. The packing is secured and compressed by anormal single packing nut with a free gland.

BOLTED GLAND

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Figure 3-24 Bolted Gland

This type compresses the packing into the stuffing box with a gland follower (Seefigure 3-24). The follower has two eye bolts through These are tightened down bythe nuts to compress the packing.

LANTERN TYPE

Figure 3-25 Lantern Type Packing.

This type has two sets of packing in the gland separated by a lantern ring. Thelantern ring is connected to the outside of the valve by two holes with a threadedinlet and outlet. This will allow the following:

A cooling fluid can be circulated in and out of the gland so as to keep it cool.

Any excess fluids passing the lower packing can be piped away without anyleaking or loss.

A flushing fluid can be pumped in so as to flush out any corrosive or abrasive

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materials before they reach the top packing. This ensures a good seal.

INJECTION TYPE GLAND

Figure 3-26 Injection Type Gland.

The injection type gland uses a sealing compound instead of packing material. Thecompound, e.g., special grease is injected into the stuffing box. This grease actsas a packing to seal the process fluid and to help the valve turn easily.

The operator must check the following on these types of valves:

Check the valve for leaking fluids.Check the gland for tightness.Check how much thread is showing through the nuts. This will tell you how

much life is left in the packing materialCheck for the smooth operation of the valveCheck that the valve is lubricated.

3.11CONTROL VALVES

The function of a control valve is to control the amount of fluid which flows throughit. By controlling the flow of fluids we can also control other process variables, i.e.pressure, temperature and liquid levels. The types of control valves are globevalves and butterfly valves. They have a valve actuator attached.

Control valves are important in any piping system where you want automatic control.Modern plants use a lot of automatic control from a centralised control room; so dopipeline and tank farm operations.

Manually operated valves can control fluid flow in any piping system. Control valvesare specially designed to provide a variable resistance in a piping system. This iscalled throttling.

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Valves can be operated Manually or Automatically, Valves can be operated Locallyor by Remote Control

3.12 AUTOMATIC CONTROL VALVES

Automatic valves are made in the same wide range of valve body styles, materials,

pressure ratings and end connections as manual valves. However, there are somedifferences and certain types of valve bodies are better than others. The mostcommon automatic control valve is the globe type, unbalanced contoured plugvalve. This is very good for throttling actions. A similar plug type globe valve is thebalanced tight shut off plug. This is also available with a balanced trim. (seeFigures 3-27 and 3-28 ).

Figure 3-27 Unbalanced Contour Valve

Figure 3-28 Balanced Tight Shut Off Plug

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3.13 VALVE ACTUATORS

Valve actuators are devices that provide the force to move a valve. They open,close, adjust or maintain a set valve position by pulling, pushing or rotating the valvestem. There are a number of different types of valve actuators that can be used:

Manual Valve Actuators:

Handwheel: Gear operated.Handwheel: l Chain operated.

Automatic Valve Actuators:

Pneumatic: Air driven.Electric : Motor driven.Hydraulic: Liquid driven.

Valve actuators are used on valves for the following reasons:

Remote Operation:

Valve actuators are used on valves so that the operator can open, close or controlthe flow from the control room. An operator can control the valve from a safelocation, e.g., at ground level.

Valve Size:

Large valves can be too large to be opened / closed by hand. A valve actuator isquicker and easier. A valve actuator is more accurate for setting a desired flowrate.

Safety

A valve actuator gives a remote operation for valves that are in hazardous areas.

3.14 PNEUMATIC ACTUATOR OPERATION

Most control valves use a pneumatic valve actuator. Air pressure from the actuatorpositions the valve stem. A pneumatic valve actuator is attached to a valve. Thevalve stem connects the valve disc to a diaphragm inside the valve actuator.

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Figure 3-29 Major Components of a Typical Control Valve

The actuator has a rod which connects the actuator diaphragm to the valve stem ofa globe or butterfly valve. The diaphragm moves by increasing or decreasing theair pressure pushing on the diaphragm. The air pressure is increased or decreasedby the operator in the control room when he moves the valve controller.

Figure 3-30 Typical Reverse Acting Diaphragm Actuator

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In Figure 3-30 the air pressure from below the diaphragm is increased and it pushesthe diaphragm up to open the valve. To close the valve the air pressure against thediaphragm is reduced and the diaphragm moves down. It is the spring that holds thediaphragm down.

Figure 3-31 Typical Direct Acting Diaphragm Actuator

In Figure 3-31 the air pressure from above the diaphragm is increased and it pushesthe diaphragm down to open the valve. To close the valve the air pressure againstthe diaphragm is reduced and the diaphragm moves up. It is the spring that holds

the diaphragm up.

The position of the valve depends on the air pressure on the diaphragm. The valvewill move from the fully open position to the fully closed position with an airpressure change of from 3 psig to 15 psig.

In most process systems there are fail safe control valves. These protect parts ofthe system from too much pressure if something goes wrong. A large spring isattached to the actuator rod and diaphragm. Its function is to fully open or fullyclose the valve if the instrument air supply to the valve actuator cuts off.

In Figure 3-30 if the air supply fails the valve will close. In Figure 3-31 if the air

supply fails the valve will close, these are called "Air Fail Closed" control valves.

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Figure 3-32 Butterfly Valve with Actuator

3.15 ELECTRIC MOTOR ACTUATOR

Some valves use an electric motor as an actuator. These valves are called motor

operated valves or MOV's.

Figure 3-33 Motor Operated Valve

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screw stem. The motor uses two bevelled gears to move the valve stem up anddown. The drive gear is attached to the shaft of the motor through speed reducinggears. The stem gear is threaded in the centre. The valve stem is screwed throughthe gear.

As the motor turns the stem gear, the valve stem threads itself inside the stemgear. This threading action moves the stem up or down depending on the direction

of motor rotation.

A motor operated valve is also equipped with a handwheel. The handwheel allowsthe valve to be manually operated if the motor should fail.

3.16 HYDRAULIC VALVE ACTUATORS

A hydraulic actuator uses the force of a pressurised liquid, (usually oil) to positionthe valve stem.

In a hydraulic valve actuator a cylinder assembly is attached to a valve. A pistoninside the cylinder moves up and down. The piston is attached to the valve stem.

The pressure of hydraulic fluid in the cylinder causes the valve stem to rise or fall.

When the hydraulic oil is pumped to the top side of the piston the valve will close.When hydraulic oil is pumped to the bottom side of the piston the valve will open.

Figure 3-34 Hydraulically Actuated Control Valve

3.17 SOLENOID / MAGNETIC ACTUATORS

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Figure 3-35 Solenoid Valve Operation

Solenoid operated valves are not very large. They are commonly used to shut offinstrument air supply to control valves. They are often used in emergencyshutdown systems (ESD's).

Solenoid valves use the principle of electromagnetism to operate. As long as the

coil is energised, electromagnetism holds the core up. The valve remains open andinstrument air reaches the control valve. In an emergency the coil is deenergised.Then the coil moves down and shuts off the instrument air to the control valve. Thecontrol valve can be set up to open, close or remain in its position.

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Figure 3-36 Magnetic Valve Operation

Electromagnetic force from the coil holds the solenoid core down, allowinginstrument air to the , valve actuator. When power to the coil is shut off the solenoidcore moves up shutting off the instrument air supply to the valve actuator. At thesame time the system air is vented from the actuator (See Figure 3-36).

Control valves act in one of two ways:

Air pressure to close, "Direct Acting". An increase in air pressure closes the valve.

Air pressure to open, "Reverse Acting". An increase in air pressure opens the valve.

3.18 VALVE INSPECTION

Valves should always be checked regularly by the operators for flange stem orbonnet leaks. A stem leak may indicate the valve needs new packing or the

packing nut needs tightening. Bonnet leaks may be caused by loose bolts due tostrains in the line over a long period of time. A flange leak may also be due toloose bolts.

No small leak should be overlooked or ignored as a small leak may develop into alarge leak.

You should lubricate valves regularly. Regular lubrication will prevent frictional wear(galling). Lubrication of the valves will also prevent the valves seizing.

Seizing occurs when metal to metal surfaces stick to each other because of heator pressure.

If any type of valve is left in one position for a long period of time, corrosion andforeign particles may accumulate. This is especially true on stem threads.Periodic checking and lubrication should be done regularly. Maintenance shouldbe told of leaking valves by the operations staff. Operations staff are the eyes ofthe maintenance staff.

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