Valves

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BP CASPIAN SEA VALVES

VALVES (MD-023)

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REV 1: Valves (MD-023) Page 2 of 42

REVISI0N HISTORY Revision Number Date Comments Rev: 0 29-07-2004

Rev: 1 03-11-2004 Signed off by Douglas Milne 06-11-04 ISSUE 1


TOPICS COVERED INCLUDE

Factors affecting valve selection

Pressure Drop

Flow Regulation

Shut Off Capability

Erosion

Corrosion


CONTENTS

VALVES (MD-023)

1. INTRODUCTION

2. HAZARDS ASSOCIATED WITH VALVES

3. IDENTIFY VALVES & DESCRIBE THEIR FUNCTIONS

4. MAJOR VALVE PARTS

4.1. BODY

4.2. OPENING ELEMENT

4.3. STEM

4.4. HANDWHEEL / HANDLE

4.5. BONNET

4.6. PACKING GLAND

5. TYPES OF VALVES AND THEIR FUNCTIONS

5.1. BUTTERFLY VALVE

5.2. GLOBE VALVE

5.3. BALL VALVE

5.4. GATE VALVE

5.4.1. RISING STEM

5.4.2. NON RISING STEM

5.5. PLUG VALVE - FIGURE 11

6. AUTOMATIC VALVES & THEIR FUNCTION

6.1. CHECK VALVES (NON RETURN VALVES N.R.V )

6.1.1. SWING CHECK VALVES FIGURE 12

6.1.2. BALL CHECK VALVE - FIGURE 13

6.2. FUNCTION OF CHECK VALVES FIGURE 14


6.3. NEEDLE VALVES - FIGURE 15

6.4. SAFETY RELIEF VALVE

7. THE OPERATION VALVES

7.1. MANUAL OPERATION

7.1.1. HANDWHEEL OPERATION

7.1.2. HANDLE OPERATION

7.2. LOCAL OPERATION

7.3. REMOTE OPERATION

7.4. AUTOMATIC OPERATION

7.5. LOCAL AUTOMATIC OPERATION

7.6. REMOTE AUTOMATIC OPERATION

8. VALVE ACTUATORS

8.1. ELECTRIC MOTORS

8.2. PNEUMATIC ACTUATORS

8.3. HYDRALIC ACTUATORS

8.4. SOLENOID / MAGNETIC ACTUATORS

9. GLOSSARY


VALVES (MD-023)

1. INTRODUCTION

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.


2. HAZARDS ASSOCIATED WITH VALVES

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.


3. IDENTIFY VALVES & DESCRIBE THEIR FUNCTIONS

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 Open and Closed Valves

The element changes position to open and close the valve.

Body

Stem

Valve

Seat

Inlet

Handwheel

Closed Open

Opening

Element


Figure 2 Common Valve Parts

Handwheel Nut

Bonnet

Handwheel

Packing

Packing Gland

Body

Valve Seat

Opening Element

Stem


4. MAJOR VALVE PARTS

4.1. BODY

The body is the part of a valve that is attached to pipes. The gases or liquids

flow through the body when the valve is open.

4.2. OPENING ELEMENT

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.

4.3. STEM

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.


4.4. HANDWHEEL / HANDLE

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.

4.5. BONNET

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.

4.6. PACKING GLAND

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.


5. TYPES OF VALVES AND THEIR FUNCTIONS

We will discuss types of valves that are either manually or automatically

operated. They are:

Butterfly Valve

Globe Valve

Ball Valve

Gate Valve

Plug

5.1. BUTTERFLY 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

Open Closed

Disc Disc


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.

Figure 4 Butterfly valve and its Engineering Drawing 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.


5.2. GLOBE 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.


Figure 6 Globe Valve and its Engineering Drawing Symbol

Globe valves are primarily used as control valves to regulate fluid flow.


5.3. BALL VALVE

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.

Open Closed


Figure 8 Ball Valve and its Engineering Drawing

Both ball valves and butterfly valves open and close with a 90 turn.


5.4. GATE VALVE

The opening element is a gate valve is a wedge shaped disc or grate

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 (a) Rising Stem Gate Valves

Closed Open


Figure 9 (b) Non-Rising Stem Gate Valves

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.

5.4.1. RISING STEM

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.

5.4.2. NON RISING STEM

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.

Stem

Gate

Inside

Thread

Closed Open


Gate valves are the most commonly used valves in process operations

plants.

Figure 10 Gate Valve and its 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.


5.5. PLUG VALVE - FIGURE 11

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

Wrench

Sealant Groove

Body

Sealant Fitting

Stem

Gland Flange

Plug


6. AUTOMATIC VALVES & THEIR FUNCTION

Automatic valves, once installed, operate by themselves. They are not

opened and closed by an operator.

6.1. CHECK VALVES (NON RETURN VALVES N.R.V )

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

6.1.1. SWING CHECK VALVES FIGURE 12

In a swing check valve, the opening element is a disc attached to a hinged

arm.

Figure 12 Swing Check Valve

Closed Open

Hinged Disc


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 or horizontally. Gravity will

close the valve if flow stops.

6.1.2. BALL CHECK VALVE - FIGURE 13

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 12 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 13 Ball Check Valve (Top is open, bottom is closed)

Higher downstream pressure will cause the element to seat, closing the valve.


6.2. FUNCTION OF CHECK VALVES FIGURE 14

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 14.

Figure 14 Check Valve and its Engineering Drawing 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.


6.3. NEEDLE VALVES - FIGURE 15

Needle valves are a form of the globe valve and are intended for low flow, fine

adjustment conditions (see Figure 15).

Figure 15 Needle Valve and its Engineering Symbol

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.

Advantages

Very precise flow control at low rates.


Disadvantages

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 rates.

The seat and needle are prone to damage if the fluid being

handled contains suspended solids.

6.4. SAFETY RELIEF VALVE

Safety relief valves prevent excess pressure, which may cause serious

accidents.

The opening element in the safety relief valve shown in figure 16 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 Relief 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.

Spring

Base

Inlet

Disc

Outlet

Adjusting

Screw


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 16.

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.

Inlet

Valve Fully Open

Inlet

Valve Partially Open

Inlet

Valve Closed


Figure 18 Relief Valve and its Engineering Drawing Symbol

Flow through the relief valve will only occur under high-pressure conditions.


7. THE OPERATION VALVES

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 control.

7.1. MANUAL OPERATION

Manual operation occurs when an operator opens, closes, or adjusts a valve.

Figure 19 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.


7.1.1. HANDWHEEL OPERATION

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 operation.

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 turn closed after fully opening

the valve.

This prevents the valve from seizing or sticking in the open

position.

Always turn the hand wheel turn open after fully closing the

valve.

This prevents the valve from seizing or sticking in the closed

position.

7.1.2. HANDLE OPERATION

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.


7.2. LOCAL OPERATION

Figure 20 shows local operation of a motor-operated valve (MOV).

Local operation occurs close to 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 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.


7.3. REMOTE OPERATION

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.

In figure 21, 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.


7.4. AUTOMATIC OPERATION

Automatic operation of a valve is when an instrument control system is used

to open, close, or adjust a valve. Figure 22 compares automatic control with

manual control.

Figure 22 Controlling Water Level

Set Point Pointer

Level Indicator

Float

Automatic

Control Valve

Sensor

Water Level

Outlet Line

Manually

Operated Valve

Inlet Line

100%

Transmitter

50%

Controller

0%

Desired Water

Level

Inlet Actual Water

Level


Figure 22 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 on the

right 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.

7.5. LOCAL AUTOMATIC OPERATION

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.

7.6. REMOTE AUTOMATIC OPERATION

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.


8. VALVE ACTUATORS

A valve actuator is a device that provides the force to open, close, or adjust a

valve.

8.1. ELECTRIC MOTORS

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

motor-operated valves, or MOVs.

The motor-operated valve has two main parts: the valve itself and a motor

driven screw stem. The motor uses two bevelled gears to move the stem up

and down.

Figure 23 Motor Operated Valve

A motor drives a bevelled gear, which in turns a gear on the stem. The

rotation of the gears lowers or raises the stem.

The drive is attached to the shaft of the motor through speed reducing gears.

The stem gear is threaded in the centre. The valve stem screwed through

the gear.

Closed

Speed

Reducing Gears

Bevelled Drive

Gear

Valve

Threaded

Stem

Actuator

Bevelled

Stem Gear

Motor

Valve Gate

Motor


As the motor turns the stem gear, the valve stem threads itself inside the

stem gear. This threading action moves the stem up or down, depending on

the direction of the motor rotation.

A motor operated valve is also equipped with a hand wheel. The hand wheel

allows the valve to be manually operated if the actuator fails.

8.2. PNEUMATIC ACTUATORS

A pneumatic actuator operates by air pressure. A pneumatic valve actuator is

attached to the valve. The valve stem connects the valve disc to a

diaphragm inside the actuator.

Figure 24 Pneumatically Actuated Control Valve

Most controlled valves use pneumatic actuators. Air pressure from the

actuator positions the valve stem.

In figure 25 (a), air pressure from below the diaphragm pushes the diaphragm

pushes the diaphragm up to open a valve.

In figure 25 (b), air pressure from above the diaphragm pushes the diaphragm

down to open a valve.

Stem moves up

Stem moves down

A B

Air Supply

Air pushes down on Diaphragm

Air Supply

Diaphragm

Air pushes up on Diaphragm


A strong spring inside each valve will fully open or fully close the valve if the

air supply fails.

8.3. HYDRAULIC ACTUATORS

A hydraulic actuator uses the force of a pressurized liquid; usually oil, to

position the valve stem.

In a hydraulic valve actuator a cylinder assembly is attached to a valve. A

piston inside the cylinder moves up and down. The piston is attached to the

valve stem.

When hydraulic oil is pumped to the topside of the piston the valve will close.

When hydraulic oil is pumped to the bottom side of the piston, the valve will

open.

Figure 25 Hydraulically Actuated Control Valve

The pressure of hydraulic fluid in the actuator causes the valve stem to rise or

fall.

Outlet Port

Seat

Inlet Port

Body

Disc Bonnet

Valve Stem

Actuator Stem Actuator

Piston

Cylinder

Valve Disc

Down

Up


8.4. SOLENOID / MAGNETIC ACTUATORS

Solenoid operated valves are not very large. They are commonly used to

shut of the instrument air supply to control valves. They are used most often

for emergency shutdown systems (ESDs). Figure 26 shows a photograph of

a solenoid valve.

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

the coil is energized, electromagnetism holds the core down. The valve

remains open, and instrument air reaches the control valve. When the coil is

deenergized, as the result of an emergency, for example, the solenoid shuts

off the instrument air to the control valve. Depending on how it is set up, the

control valve will open, close, or remain in its position.

Figure 26 Solenoid Valve


Figure 27 Solenoid Valve Operation

Electromagnetic force from the coil holds the solenoid core down, allowing

flow of instrument air to the coil is cut off, the solenoid core moves up, cutting

off the instrument air supply to the actuator.

Spring

Expands

Instrument

air

Spring

Compressed

To exhaust

Coil denergized

valve moves

up to close

Instrument air to

actuator

Coil energized valve moves down to open


9. GLOSSARY

Check: To stop; to hold back

Pneumatic: Moved or worked by air pressure

Regulate: To fix or adjust the time, amount, degree, or rate

Remote Control: Controlling indirectly or from a distance

Vent: An opening for the escape of a liquid or gas for the relief of pressure

Gland: The movable part used in a stuffing box to compress the packing

Hydraulic: Operated or moved by means of a liquid

Pivot: A shaft or pin on which something turns

Set Point: A setting on a controller that you want a process variable to be

Valve Actuator: A mechanical device that provides the force open, close or adjust a valve

Valves

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