TATA PROJECTS LIMITED

Control Valves and Actuators

A Report

BINIT GOENKA

6/11/2009

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Acknowledgement

An activity can be termed as an accomplishment only when the purpose is

fulfilled. The accomplishment of any activity involves a continuous unflinching

effort, motivation and support from its mentor. I would like to extend my

heartfelt gratitude to my guide and mentor Mr. Neeladri Roy for having

constant faith in me throughout and directing me and supporting me in every

possible way at each step.

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Table Of Contents

Introduction ...................................................................................................................................................... 4

Control Valves ................................................................................................................................................... 5

Classification of Control Valves .............................................................................................................................. 6

Linear Motion Valves ................................................................................................................................................. 7

Double Seated Globe Valve ............................................................................................................................ 8

Single Seated Globe Valve ............................................................................................................................ 10

Three Way Globe Valve ................................................................................................................................. 12

Angle Valve ......................................................................................................................................................... 14

Diaphragm Valve .............................................................................................................................................. 15

Pinch Valve ......................................................................................................................................................... 17

Rotary Motion Valves .............................................................................................................................................. 18

Ball Valves ........................................................................................................................................................... 19

Butterfly Valves ................................................................................................................................................ 21

Eccentric Plug Valves ..................................................................................................................................... 23

Actuators ......................................................................................................................................................... 24

Pneumatic Actuators ................................................................................................................................................. 25

Piston Actuator .................................................................................................................................................. 25

Diaphragm Actuator ........................................................................................................................................ 26

Electric Actuator ......................................................................................................................................................... 27

VMD ........................................................................................................................................................................ 27

Modulating ........................................................................................................................................................... 28

Conclusion ....................................................................................................................................................... 29

Glossary ............................................................................................................................................................ 30

References ....................................................................................................................................................... 34

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Introduction

Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. When we talk about control valves or valves , we are referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which can include positioners , transducers, supply pressure regulators, manual operators, snubbers , or limit switches.

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Control Valve

Control Valves The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator. To do this, the valve must:

Contain the fluid without external leakage.

Have adequate capacity for the intended service.

Be capable of withstanding the erosive, corrosive, and temperature influences of the process.

Incorporate appropriate end connections to mate with adjacent pipelines and actuator attachment means to permit transmission of actuator thrust to the valve plug stem or rotary shaft.

Many styles of control valve bodies have been developed through the years. Some have found wide application, others meet specific service conditions and are used less frequently. The following summary describes some popular control valve body styles. The control valves can be broadly classified into two types

1. Linear Motion Valves 2. Rotary Motion Valves

These are further classified into the different valves which perform the respective type of fluid motion. The classification can be seen in the following table :

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Classification Of Different types of Control Valves

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Linear Motion Valves The linear motion valves are characterised by the following features :

i. TORTUOUS FLOW PATH ii. LOW RECOVERY OF PRESSURE

iii. CAN THROTTLE SMALL FLOW RATES iv. OFFERS VARIETY OF SPECIAL TRIM DESIGNS v. SUITED TO HIGH-PRESSURE APPLICATIONS

vi. USUALLY FLANGED OR THREADED vii. SEPARABLE BONNET

Types Of Closures of Linear Motion Valves are :

i. Globe a. Single Seated b. Double Seated c. Three way

ii. Diaphragm iii. Pinch

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Globe Valves

Double Seated Globe Valve Working : Fig 1. here shows a double-seat globe control valve having two seats 2 and 3 in a body 1 and two disks 5 and 6 on a stem 4. This control valve can regulate a large volume of fluid. Because the force acting on the stem 4 depends on the difference between the projecting areas of the upper and the lower disks on the corresponding seats, the force for driving the stem 4 can be remarkably reduced by substantially equalizing the projected areas of the two disks. Therefore this type of valve is used in large diameter applications. When closed, however, it cannot shut fluid completely off because the disks 5 and 6 have to close the seats 2 and 3 simultaneously. Fluid leakage is much greater than single-seat type. In addition, its intricate interior makes disk motion unstable, which in turn induces erosion and cavitations . Consequently the body and other parts wear out rapidly or break down and noise becomes intolerable.

Fig 1. Fig 2.

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The following figure depicts a reverse-acting double seat globe style

valve :

Applications :

1. Used for high flow and high pressure services.

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Single Seat Globe Valve

Working :

Fig 3. Shows a single-seat globe control valve having a seat 8 in a body 7 and a

disk 10 on a body 9. Contrary to the above described double-seat globe control

valve, this type has a good shut-off and dynamic characteristics. However, the

force acting on the 9 depends on the product of the projected area of disk 10

on the seat 8 and the difference between the two pressures at the inlet and

the outlet of the valve. Accordingly it requires more force than the double –

seat type. When the output of the valve actuating system or a operating

machine is limited , the allowable differential pressure of the fluid becomes

small. Consequently, this type of valve is available only for small diameter

applications.

Fig

Fig 3. Fig 4.

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The following figure depicts a single-seat globe control valve :

Application :

To control large range of process parameters, specially for oil and gas

production, power generation, chemical, petrochemical, fertilizer,

pharmaceutical, processed food and other process industries.

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Three Way Globe Valve

Working:

Three-way globe valves can be used for either mixing or diverting service

depending upon the plug and seat arrangement inside the valve. Here, the

actuator pushes a disc or pair of valve plugs between two seats (Fig 5.),

increasing or decreasing the flow through ports A and B in a corresponding

manner.

Fig 5.

The three way globe control valves have a high flow capacity and a high

rangeability. The leakage capability of the three way globe control valves are

consistent with the normal single seat globe control valve types.

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The following figure depicts a three-way globe control type valve :

Applications :

Used for high temperature fluid and particularly for the textile induatries, to

mix or divert fluids.

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Angle Valves

Angle valves are nearly always single ported. They are commonly used in boiler

feed water and heater drain service and in piping schemes where space is at a

premium and the valve can also serve as an elbow. The angle valve gives a

normal flow, have a high flow capacity and a tight shut off.

Applications :

Used for very high pressure drop and also used where the fluids contain solid

particles.

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Diaphragm Valves

Diaphragm valves constitute the third major type of linear motion valves. The stem of the valve is used to push down a flexible diaphragm, which in turn blocks the path of the fluid. There are two different classifications of diaphragm valve based on the geometry of the valve body:

Weir type - A weir is cast into the body, and when closed, the diaphragm rests on the weir, restricting the flow.

Straight-through type - The bore runs laterally through the body and a wedge shaped diaphragm is used to make the closure.

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Figure below depicts a Diaphragm Valve :

Applications :

The main advantage of a diaphragm valve is the fact that the diaphragm

isolates the moving parts of the valve from the process fluid. They are

therefore suitable for handling aggressive fluids and for those containing

suspended solids. In addition, as the bonnet assembly is not exposed to the

fluid, it can be made from inexpensive materials such as cast iron, thereby

reducing the overall cost. The development of new diaphragm materials

enables diaphragms to be used on most fluids. Their application is however

limited by the temperature that the diaphragm can withstand - typically less

than 175°C. Diaphragm valves are generally used on process fluid applications.

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Pinch Valve

Pinch valves used for fluids usually employ a device that directly contacts

process tubing. Forcing the tubing together will create a seal that is equivalent

to the tubing's permeability. Major components of a pinch valve consists of

body and a sleeve. The sleeve will contain the flow media and isolate it from

the environment hence reducing contamination to the environment.

Applications :

Generally used for slurries or processes with entrained solids, because the

flexible sleeve allows the valve to close droptight around solids—solids that

would typically be trapped by the seat or stuck in crevices in globe, diaphragm,

butterfly, gate, or ball valves. The sleeve material can be selected upon the

corrosiveness and abrasiveness of the flow media, a suitable synthetic polymer

can be chosen. A pinch valve may be the best type of valve for flow control

application if the operation temperature is within the limit of the polymer.

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Rotary Motion Valves

The linear motion valves are characterised by the following features :

i. STREAMLINED FLOW PATH ii. HIGH RECOVERY

iii. MORE CAPACITY iv. LESS PACKING WEAR v. CAN HANDLE SLURRY AND ABRASIVES

vi. FLANGELESS vii. INTEGRAL BONNET viii. HIGH RANGEABILITY

Types Of Closures of Linear Motion Valves are :

i. Ball

ii. Butterfly

iii. Plug

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Ball Valves

Working:

Fig 6. shows a ball valve consisting of a spherical ball located between two

sealing rings in a simple body form. The ball has a hole allowing fluid to pass

through. When aligned with the pipe ends, this gives either full bore or nearly

full bore flow with very little pressure drop. Rotating the ball through 90°

opens and closes the flow passage. Ball valves designed specifically for control

purposes will have characterized balls or seats, to give a predictable flow

pattern.

Fig 6.

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Ball valves are an economic means of providing control with tight shut-off for

many fluids including steam at temperatures up to 250°C (38 bar g, saturated

steam). Above this temperature, special seat materials or metal-to-metal

seatings are necessary, which can be expensive. Ball valves are easily actuated

and often used for remote isolation and control. For critical control

applications, segmented balls and balls with specially shaped holes are

available to provide different flow characteristics.

Applications :

Used in paper, pulp and textile industries.

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Butterfly Valves

Working :

Fig 7. is a simple schematic diagram of a butterfly valve, which consists of a disc

rotating in trunnion bearings. In the open position the disc is parallel to the pipe wall,

allowing full flow through the valve. In the closed position it is rotated against a seat,

and perpendicular to the pipe wall. Traditionally, butterfly valves were limited to low

pressures and temperatures, due to the inherent limitations of the soft seats used.

Currently, valves with higher temperature seats or high quality and specially

machined metal-to-metal seats are available to overcome these drawbacks. Standard

butterfly valves are now used in simple control applications, particularly in larger

sizes and where limited turndown is required.

A fluid flowing through a butterfly valve creates a low pressure drop, in that the valve

presents little resistance to flow when open. In general however, their differential

pressure limits are lower than those for globe valves. Ball valves are similar except

that, due to their different sealing arrangements, they can operate against higher

differential pressures than equivalent butterfly valves.

Fig 7.

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The figure bellow depicts a butterfly valve :

Applications :

Steel, Sugar and textile industries and specially for fluid containing slurry

particles. Used for on-off as well as control applications.

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Eccentric Plug Valve

Working:

Fig 8. shows a typical eccentric plug valve. These valves are normally installed

with the plug spindle horizontal as shown, and the attached actuator situated

alongside the valve.

Plug valves may include linkages between the plug and actuator to improve the

leverage and closing force, and special positioners that modify the inherent

valve characteristic to a more useful equal percentage characteristics.

Fig 8.

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Actuators

The operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve. The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal. The actuator accepts a signal from the control system and, in response, moves the valve to a fully-open or fully-closed position, or a more open or a more closed position (depending on whether 'on / off' or 'continuous' control action is used). There are several ways of providing this actuation. This Tutorial will concentrate on the two major ones:

1. Pneumatic.

a) Piston actuators b) Diaphragm actuators

2. Electric.

a) VMD ( Valve Motor Drive ) b) Modulating

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Pneumatic Actuators

Piston Actuators:

Piston actuators are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small. The compressed air is applied to a solid piston contained within a solid cylinder. Piston actuators can be single acting or double acting, can withstand higher input pressures and can offer smaller cylinder volumes, which can act at high speed.

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Diaphragm Actuators

Diaphragm actuators have compressed air applied to a flexible membrane

called the diaphragm. Figure 6.6.2 shows a rolling diaphragm where the

effective diaphragm area is virtually constant throughout the actuator stroke.

These types of actuators are single acting, in that air is only supplied to one

side of the diaphragm, and they can be either direct acting (spring-to-retract)

or reverse acting (spring-to-extend).

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Electric Actuators

VMD ( Valve Motor Drive )

This basic version of the electric actuator has three states:

1. Driving the valve open. 2. Driving the valve closed. 3. No movement.

Figure below shows the VMD system where the forward and reverse travel of the actuator is controlled directly from any external 3-position or two 2-position switch units. The switches are rated at the actuator voltage and may be replaced by suitable relays. Limiting devices are fitted within the VMD actuators to protect the motors from over-travel damage. These devices are based on either the maximum motor torque or physical position limit switches. Both devices stop the motor driving by interrupting the motor power supply.

Position limit switches have the advantage that they can be adjusted to limit valve strokes in oversized valves.

Torque switches have the advantage of giving a defined closing force on the valve seat, protecting the actuator in the case of valve stem seizure.

If only position limit switches are used, they may be combined with a spring-loaded coupling to ensure tight valve shut-off.

A VMD actuator may be used for on/off actuation or for modulating control. The controller positions the valve by driving the valve open or closed for a certain time, to ensure that it reaches the desired position. Valve position feedback may be used with some controllers.

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Modulating

In order to position the control valve in response to the system requirements a

modulating actuator can be used. These units may have higher rated motors

(typically 1 200 starts/hour) and may have built-in electronics.

A positioning circuit may be included in the modulating actuator, which

accepts an analogue control signal (typically 0-10 V or 4-20 mA). The actuator

then interprets this control signal, as the valve position between the limit

switches.

To achieve this, the actuator has a position sensor (usually a potentiometer),

which feeds the actual valve position back to the positioning circuit. In this way

the actuator can be positioned along its stroke in proportion to the control

signal. A schematic of the modulating actuator is shown in Figure below.

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Conclusion

The control valve is used in plant processes to regulate the rate of fluid flow as the position of the valve plug or disk is changed by force from different types of actuators available.

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Glossary Actuator*: A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or close a valve. Actuator Stem: The part that connects the actuator to the valve stem and transmits motion (force) from the actuator to the valve. Actuator Stem Force: The net force from an actuator that is available for actual positioning of the valve plug. Angle Valve: A valve design in which one port is co-linear with the valve stem or actuator, and the other port is at a right angle to the valve stem. Bonnet: The portion of the valve that contains the packing box and stem seal and can guide the stem. It provides the principal opening to the body cavity for assembly of internal parts or it can be an integral part of the valve body. It can also provide for the attachment of the actuator to the valve body. Typical bonnets are bolted, threaded, welded, pressure-seals, or integral with the body. Closure Member: The movable part of the valve that is positioned in the flow path to modify the rate of flow through the valve. Capacity* (Valve): The rate of flow through a valve under stated conditions. Controller: A device that operates automatically by use of some established algorithm to regulate a controlled variable. The controller input receives information about the status of the process variable and then provides an appropriate output signal to the final control element.

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Control Valve Assembly: Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc. Diaphragm: A flexible, pressure responsive element that transmits force to the diaphragm plate and actuator stem. Diaphragm Actuator: A fluid powered device in which the fluid acts upon a flexible component, the diaphragm. Disk: A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or closure member. Fail-Safe: A characteristic of a valve and its actuator, which upon loss of actuating energy supply, will cause a valve closure member to be fully closed, fully open, or remain in the last position, whichever position is defined as necessary to protect the process. Fail-safe action can involve the use of auxiliary controls connected to the actuator. Flangeless Valve: Valve style common to rotary-shaft control valves. Flangeless valves are held between ANSI-class flanges by long through-bolts (sometimes also called wafer-style valve bodies). Globe Valve: A valve with a linear motion closure member, one or more ports, and a body distinguished by a globular shaped cavity around the port region. Globe valves can be further classified as: two-way single-ported two-way double-ported Positioner*: A position controller (servomechanism) that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal.

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Rangeability: The ratio of the largest flow coefficient (Cv) to the smallest flow coefficient (Cv) within which the deviation from the specified flow characteristic does not exceed the stated limits. A control valve that still does a good job of controlling when flow increases to 100 times the minimum controllable flow has a rangeability of 100 to 1. Rangeability can also be expressed as the ratio of the maximum to minimum controllable flow rates. Reverse Flow: Flow from the shaft side over the back of the disk, ball, or plug. Some rotary-shaft control valves are capable of handling flow equally well in either direction. Other rotary designs might require modification of actuator linkage to handle reverse flow. Relay: A device that acts as a power amplifier. It takes an electrical, pneumatic, or mechanical input signal and produces an output of a large volume flow of air or hydraulic fluid to the actuator. The relay can be an internal component of the positioner or a separate valve accessory. Seat: The area of contact between the closure member and its mating surface that establishes valve shut-off. Sensor: A device that senses the value of the process variable and provides a corresponding output signal to a transmitter. The sensor can be an integral part of the transmitter, or it may be a separate component. Transmitter: A device that senses the value of the process variable and transmits a corresponding output signal to the controller for comparison with the set point. Travel*: The movement of the closure member from the closed position to an intermediate or rated full open position.

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Trim*: The internal components of a valve that modulate the flow of the controlled fluid. Yoke: The structure that rigidly connects the actuator power unit to the valve.

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References

1. Control Valve Handbook by Fisher. 2. www.Wikipedia.com 3. www.Google.com