CONTROL VALVE

INTRODUCTION

The Control Valve plays a very important role in the automatic control of modern plants. Controls like exchange of energy, reduction of pressure or simply to fill a tank depends on some form of Final Control Element to do the job. Final control element may be considered as the muscle of automatic control. The furnish the necessary power amplification between the low energy levels in the controllers and the higher energy levels needed to perform their function in controlling flowing fluids.

The control valve is most widely used type of final control element. The control valve functions as a variable resistance in the pipeline.It provides a pressure drop by changing the turbulence in the process fluid.

DEFINITION OF CONTROL VALVE

• Control valve is a device which controls fluid passing through a restriction by regulating the travel of stem or plug.

• Regulation of the plug travel can be done by various methods :- Manually, Pneumatically, Hydraulically and Electrically operated actuator or any combination of these operating devices.

POSITION OF CONTROL VALVE IN A CONTROL LOOP

REC

CONTROLLER

TRANSMITTER

ORIFICE

CONTROLVALVE

I/ P CONVERTER

Primary element

(Final control Element)

PARTS OF CONTROL VALVE1. Actuator2. Diaphragm3. Stem Guide4. Spiral Guide Plate5. Ball Bearing6. Multiple Springs7. Spring Adjustment Screw8. Yoke9. Travel Indicator10. Glands Packing11,12 Plug13. Seat rings14. BonnetTRIM: Includes all the parts that are in flowing contact with the process fluid except the body, BONNET, and body flanges and gaskets. The plug, seats,

stem, guides, bushings, and cage are some parts of the trim. 

CONTROL VALVE FAIL SAFE POSITION

CONTROL VALVE FLOW CHARACTERSTIC

Trim design will affect how the valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. THE RELATIONSHIP BETWEEN VALVE CAPACITY AND VALVE TRAVEL IS KNOWN AS THE FLOW CHARACTERISTIC OF THE VALVE. Valve trims are specially designed, or characterized, in order to meet the large variety of control application needs. These curves are based on CONSTANT PRESSURE DROP across the valve and are called INHERENT FLOW CHARACTERISTICS..

When valves are installed with a pump, pipes, fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel. When the actual flow in a system is plotted against valve opening, the curve is called the INSTALLED FLOW CHARACTERISTIC.

The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.• Linear - Flow capacity increases linearly

with valve travel. They are used :# For slow processes

# When more than 40% of the system pressure drop occurs across the valve

• Equal Percentage -Flow capacity increases exponentially with valve trim travel, equal increments of valve travel produce equal percentage changes in the existing Cv. The are used :

# For fast processes # When high rangeability is required

• Quick Opening – # For On / Off control # When max. valve capacity must be

obtained quickly.• Modified Parabolic Characteristic is approximately midway between linear and

equal-percentage characteristics. It provides fine throttling at low flow capacity and

approximately linear characteristics at higher flow capacity.

TYPES OF CONTROL VALVE

There are two basic types of control valves: rotary and linear.

Linear-motion control valves commonly have globe, gate, diaphragm, or pinch - type closures.

Rotary-motion valves have ball, butterfly, or plug closures. Each type of valve has its special generic features, which may, in a given application, be either an advantage or a disadvantage.

Linear Valve Features

• TORTUOUS FLOW PATH

• LOW RECOVERY • CAN THROTTLE

SMALL FLOW RATES • OFFERS VARIETY OF SPECIAL TRIM

DESIGNS • SUITED TO HIGH-

PRESSURE APPLICATIONS • USUALLY FLANGED OR THREADED • SEPARABLE BONNET

                                    

Rotary Valve Features

• STREAMLINED FLOW PATH

• HIGH RECOVERY • MORE CAPACITY • LESS PACKING

WEAR • CAN HANDLE

SLURRY AND ABRASIVES

• FLANGELESS • INTEGRAL BONNET • HIGH RANGEABILITY

                              

Control Valve ClassificationIn addition to linear and rotary, control valves are also classified according to their guiding systems and the types of services they are used in.

SINGLE SEATED GLOBE VALVE

ADVANTAGES :

• High Rangeability

• Tight Shutoff

• Frequently used in sizes<2”

DISADVANTAGES :

• Unbalanced design usually requires large actuator

• Low pressure recovery characteristics

DOUBLE SEATED GLOBE VALVE

ADVANTAGES :• High flow capacity compared to single

seated globe valve of same size

• High Rangeability

• Balanced Design requires smaller actuator

• Frequently used in sizes > 2”

DISADVANTAGES :• Relatively high leakage rate.

• Low pressure recovery characteristics.

• Erosion may occur due to leakage characteristics

THREE WAY GLOBE VALVE

ADVANTAGES :• Good for blending & diverting

applications.

• Can replace 2 two way valve in certain application

• Frequently used for temperature control systems at heat exchangers.

DISADVANTAGES :• Can not control total flow

• May requires different size ports

ANGLE VALVE BODY

ADVANTAGES :• Good control characteristics

• High Rangeability

• Tight shutoff

• Minimizes erosion problems

• Good for flashing services

DISADVANTAGES :• Normally made in sizes 2” &

above.

SAUNDERS (DIAPHRAGM) VALVE

ADVANTAGES :• High capacity

• Low cost

• Good for slurries

• Provides tight shut off if pressure is low

DISADVANTAGES :• Poor control characteristics

• Low rangeability

• Short diaphragm life

• Slow response speed

BUTTERFLY VLAVE

ADVANTAGES :• High Capacity• Economical specially in larger size• Low pressure drop.• Does not permit sediment buildup-

Good for slurries services

DISADVANTAGES :• Operating torque is high- Requires

large actuator• Tight shutoff depends on use of

resilient seat which is temperature dependant.

BALL VALVE

ADVANTAGES :• High Capacity

• Good control characteristics

• Good Rangeability

• Good for Slurry services

DISADVANTAGES :

• Limited operating pressure

• Not good for high pressure drop

PINCH VALVE

ADVANTAGES :• High capacity

• Economical

• Good for slurries

• Flowing medium does not contact working parts

DISADVANTAGES :• Poor control characteristics

• Not good for high pressure drop application

MATERIAL OF CONSTRUCTION OF CONTROL VALVE

The most common materials for valve body are iron, bronze, carbon steel and alloy steels.

Stainless Steel 316 is most commonly used material for valve plugs, seats & stems.

Monel, Hastelloy , Aluminum and plastics are used when corrosion is a problem.

Stellite No. 6, 440 C Stainless steel , Colmony No.6 and carbides are used in erosive services requiring the use of Hardened Trim.

COMMONLY USED TERMS RELATED TO CONTROL VALVE

RANGEABILITY :Rangeability of the Control Valve can be defined as the Ratio of maximum controllable flow to the minimum controllable flow .

FLOW COEFFICIENT (Cv):Flow Coefficient can be defined as Gallons per minute of water flow at room temp. which will pass through the flow restriction with pressure drop of one psi and temperature 60 deg F .

Where q = Vol. Flow (Gallons / minute)

Cv = q G G = Specific Gravity

P P = Diff. Pressure across the valve

EROSION :

A wearing action on valve trim and body resulting from high velocity of fluids and fluids containingsolid particles. Erosion occurred in steam service, in flashing service, and where high pressure drops occur across valve body .

CORROSION :

It is chemical reaction between valve body material and fluids carrying in it and causes deterioration of valve material.

If the speed over the valve is high enough, the pressure in the liquid drop to a level where the fluid may start bubble or flash. The pressure recovers sufficiently and the bubbles collapse upon themselves.

Cavitation may be noisy but

is usually of low intensity and low frequency. 

This situation is extremely destructive and may wear out the trim and body parts of the valve in short time. 

                                                                                     

                                                 

CAVITATION & FLASHING :

HOW TO AVOID CAVITATION :

Cavitation can be avoided by using more than one control valve or more convenient a multistage control valve.

The "vena contracta" is much lower for a single stage valve than a multi stage valve. Depending on the pressure drop and the temperature of the fluid its possible to avoid cavitation conditions using more than one stage.

                                           

                                                                                       

SEAT LEAKAGE CLASSIFICATION

There is no such thing as "Bubble Tight."   Control valves are designed to throttle. However, this is not a perfect world, and control valves are also usually expected to provide some type of shut-off capability. A control valve's ability to shut off has to do with many factors. The type of valves for instance. A double-seated control valve will usually have very poor shut-off capability. The guiding, seat material, actuator thrust, pressure drop, and the type of fluid can all play a part in how well a particular control valve shuts off.

There are actually six different seat leakage classifications as defined by ANSI/FCI 70-2-1976.

But most common are: CLASS IV and CLASS VI.

CLASS IV is also known as METAL TO METAL.

CLASS VI is known as a SOFT SEAT classification. SOFT SEAT VALVES are those where either the plug or seat or both are made from some kind of composition material such as Teflon.

Class I. Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.

Class II. Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.

Class III. Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.

Class IV. Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to

0.01% of rated valve capacity.

Class V. Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X

10 ml per minute per inch of orifice diameter per psi differential.

Class VI. Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The

leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.

CONTROL VALVE ACCESSORIES

• Air Pressure Regulator• Valve Positioner• I/P Converter• Booster Relay• Ratio Relay• Reversing Relay• Lock up relay• Hand Wheels• Limit Switch• Solenoid Valve

Air Pressure Regulator

  The most common accessory for a control valve is the air set or air filter cum pressure regulator. It is designed to furnish reduced air supply pressure to the valve positioner ranging from 1.4 kg/cm² or more. The filter portion in the regulator removes the particulate matter large enough to clog instrument nozzle and orifices.

VALVE POSITIONER

     A valve positioner is basically a relay that senses both an instrument signal and a valve stem position. Its primary function is to ensure that the valve plug position is always directly proportional to its controller output signal. For example if the positioner receives a 35% signal, it will supply sufficient pressure to the actuator to cause it to stroke 35% of its travel. It is used to split range valves, to reverse the signal to a valve, to overcome friction forces within a valve on high-pressure drop application and on application requiring fast, accurate control. It is normally mounted on the control valve.

I / P CONVERTOR

     The electro-pneumatic (I/P) signal converter is used as a linking component between electric or electronic and pneumatic systems. It converts standard electric signal 4-20mA into the standard pneumatic signal of 0.2 kg/cm² to 1.0 kg/cm² or 3-15 psi.

BOOSTER RELAY

   The “Booster relay” is used to increase the volume of the signal to overcome the time lay due to large volume of the actuator of the control valve or due to large distance between the transmitter and receiver instrument. With this the speed of transmission is improved. The amplifying pilot provides a gain of one so that out put pressure is the same as the input pressure. The booster relay is simply inserted in the output line. Its advantage derives from the large part of the pilot valve so that the large flow of air is allowed when change of pressure takes place.

REVERSING RELAY

Reversing relays are used when the action from controller need to be reversed. For example, split range valve may be operated from a controller where one valve is air to open while the other is air to close. A reversing relay can be used on one of the valves to achieve the desired action.

LOCKUP RELAY    Certain application may required a diaphragm control valve to remain in its last controlled position in the event of air supply pressure failure. This purpose is achieved by

incorporating lock valve in the air output line from valve positioner to actuator of that valve.

HANDWHEELHand wheel may be supplied for manual operation of control valves for emergency use, during start up or in the event of the air failure. They are used infrequently & primarily in critical services or when block & bypass valves are not provided.

LIMIT SWITCH

A switch that activates when the position of a device crosses a given physical limit.

SOLENOID VALVE

A Solenoid valve is an electromagnetic device, which moves a plunger up or down when a current is passed through a coil that surrounds the plunger.

2/2 way normally closed solenoid valve 3/2 way normally closed solenoid valve