FUNDAMENTALS OF CONTROL VALVES

CONTROL VALVE

A CONTROL VALVE IS A FINAL CONTROL ELEMENT AND A POWER OPERATED DEVICE WHICH MODIFIES THE FLOW RATE IN A PROCESS CONTROL SYSTEM.

ROLE OF A CONTROL VALVE

It is used for The correct distribution and control of Flowing fluid or gases Reduction of Pressure As a variable orifice As a throttling or modulating equipment

CONTROL VALVE PARTS

1. BODY

2. VALVE

3. ACTUATOR

The body is the pressure carrying part and it meets all their applicable pressure, temperature and corrosion requirements.

THE BODY PROVIDES THE PIPING CONNECTING ENDS FLUID FLOW PASSAGE WAY, AND

SUPPORT THE SEATING SURFACE THE VALVE CLOSURE MEMBER

BODY

INLET OUTLET

STEM WITH PLUG

GLAND PACKING

BODY FLANGE

SEAT

BODY- INTERNAL PARTS

TRIM

The trim of the valve consists of all the parts which will be wetted other than the body and the bonnet

Trim is the heart of the valve with a primary function to proportion the valve orifice in such a manner that a prescribed relationship exists between flow capacity and valve plug lift

The secondary function may be shut off tightly.

The trim includes Seat, Plug, Stem, Gland Follower, Gland Nut, Plug, Guide Bushings and Cage

Stuffing box components considered as trim are the packing, follower, Spring, Lantern Ring, and Packing Retaining Ring.

Secondary trim parts are stem to plug attachments , seat retaining ring, seat to body seals and spacers.

.

YOKE

A STRUCTURE BY WHICH THE DIAPHRAGM ASSEMBLY IS SUPPORTED RIGIDLY ON THE BONNET ASSEMBLY

TRAVEL SCALE

SPRING ADJUSTMENT

SPRING BASE NUT

STEM COUPLING POINTER

ACTUATOR BASE

BONNET BASE

YOKE

ACTUATOR

.

ACTUATOR

UTILISES AIR PRESSURE / ELECTRIC POWER HYDRAULIC PRESSURE

TO ACT ON A DIAPHRAGM / PISTON AND DEVELOP A FORCE TO MOVE THE ACTUATOR STEM

• PNUEMATICALLY OPERATED DIAPHARGM

ACTUATOR PNUEMATICALLY OPERATED CYLINDER

ACTUATOR MOTOR OPERATED PISTON ACTUATOR

HYDRAULIC OPERATED PISTON ACTUATOR

ACTUTATOR CLASSIFICATION

DIAPHRAGM

CASINGS

LOCK NUT

STEM CONNECTOR

SPRING ADJUSTER

ACTUATOR STEM

ACTUATOR

SPRING

BONET FLANGE

VENT

SCALE

STUFFING BOX

YOKE

DIAPHRAGM

GLAND STUDS

SPRING SEAT

DIAPHRAGM

PLATE

INPUT SIGNAL

INDICATOR

ACTUATOR PARTS

•GLOBE

•GATE

•DIAPHRAGM

•PINCH OR CLAMP

•GLOBE

•ANGLE

•THREE-WAY

•BALL

•BUTTER FLY

•PLUG

•SEGMENTED BALL

•FULL BALL

•CYLIDRICAL

•TAPERED

•ECCENTRIC SPHERE

•LINEAR

•MOTION

•ROTARY

•MOTION

•VALVE

B a ll B u tte rf ly P lug

R o ta ry M o tion

G lo be A n g le T h re e W ay

G lo be G a te D ia ph ra gm P in ch o r C la m p

L in ea r M o tion

T yp e s o f V a v les

IT IS MOSTLY COMMONLY USED ONE. IT IS A VALVE WITH A LINEAR CLOSURE MEMBER MOST COMMON TYPE CONTROL VALVE CLASSIFIED AS SINGLE SEATED AND DOUBLE SEATED SINGLE SEATED CONFORMS TIGHT SHUT OFF. DOUBLE SEATED VALVE IS HAVING HIGH RESISTANCE TO VIBRATION AND ABRASION.

IT HAVE THE ADVANTAGE OF EASY TRIM REMOVAL .

IT IS TOP ENTRY VALVE WITH UNBALANCED, SINGLE-SEATED TRIM.

THE INNER VALVE PARTS CAN EASILY BE REMOVED AFTER REMOVING THE BONNET , BECAUSE ABSENCE OF INTERNAL THREADS.

•

ANGLE VALVES ARE SUITABLE FOR SLURRY FLUID ,

ADHESIVE FLUID AND HIGH VISCOSITY LIQUIDS.

SUITABLE FOR HIGH PRESSURE APPLICATION.

HAS STRONG RESISTANCE TO VIBRATION AND WEAR

• IT IS SIMPLY DESIGN

EXTENSION OF DOUBLE

SEATED GLOBE VALVE.

DIVIDES FLOW IN TWO

DIRECTIONS

CONVERGES TWO

STREAMS OF FLOW TO

ONE

APPLICABLE WHERE STRAIGHT THROUGH FLOW PATH , TIGHT SHUT OFF AND EASE OF OPERATION ARE REQUIRED.

MOST SUITABLE FOR HANDLING SLURRIES AS IT MINIMISES THE TENDENCY OF THE FLUIDS TO EITHER SETTLE OR STAIN .

SUITABLE FOR HIGH FLOW CAPACITY..

.

IT OFFERS THE ADVANTAGES OF SIMPLICITY, LOW COST, LIGHT WEIGHT,AND SPACE SAVING.

VALVE IS SUITABLE FOR LOW PRESSURE DROP APPLICATIONS

ACTUATOR TORQUE REQUIRED IS LOWEST AS COMPARED TO OTHER VALVES .

IT HAS MAXIMUM FLOW CAPACITY .

INSTALLATION AND HANDLING IS EASY .

•The most common type of rotary valve used for control is the BUTTERFLY valve.

•The typical application range is in sizes from 2” through 36” or larger, for low or moderate pressures,or on unusual applications involving large flows at high static pressures ,but with limited pressure drop.

IT IS THE RELATION BETWEEN THE FLOW RATE THROUGH THE CONTROL VALVE AND PLUG TRAVEL , WHEN A CONSTANT PRESSURE DIFFERENTIAL IS MAINTAINED ACROSS THE VALVE

THIS IS ACHIEVED BY VARYING THE FLUID FLOW AREA WITH RESPECT TO THE SHAPE OF THE PLUG.

THE DESIGN OF A CONTROL VALVE TRIM IS BASED ON THE FLOW CHARACTERISTICS .

DIFFERENT CHARACTERISTICS WHICH ARE COMMONLY SELECTED TWO TYPES

1. TWO POSITION (ON-OFF)CONTROL

2. THROTTLING CONTROL

FLOW CHARACTERISTICS DECIDED BY SHAPE OF PLUG OR CAGE SLOTS.

•THE PLUG IS THE MOVING COMPONENT OF THE VALVE WHICH

THROTTLES FLOW BY POSITIONING ITSELF WITHIN THE SEAT

ORFICE AND SHUTS OFF FLOW BY CONTACTING THE SEAT.

• THE PLUG IS MOVED AGAINST DYNAMIC FLUID FLOW

FORCES BY STEM FORCE TRANSMITTED FROM THE ACTUATOR.

• THE PLUGS (OR CAGE SLOTS) ARE SHAPED TO THROTTLE

FLOW WITH GIVEN CHARACTERSTICS SUCH AS

•QUICK OPENING,

• LINEAR,

•PARABOLIC OR EQUAL PERCENTAGE .

THREE TYPES OF VALVE CHARACTERESTICS ARE

•QUICK OPENING :

This type of characteristics provides maximum change in flow rate at lower valve travels, as the plug bottom is flat with a fairly linear relationship , opening with a small signal will give the maximum flow rate

Additional increase in valve travel gives sharply reduced changes in flow rate .

•The common application of the valve is ON -OFF

•LINEAR CHARACTERSTICS:

This type of valve provides change in flow rate which is linear with the valve lift.

That is this proportional relationship produces a characteristics with constant slope so that with constant pressure drop , the valve gain is the same at all flow rates.

Q = KX

•EQUAL PERCENTAGE :( =% )

Equal increments of valve travel produce flow changes which are equal percentage of existing flow .

The change in flow rate is always proportional to the flow rate that exits just before the change in valve position is made .

Q = Q0 emx

Q0 = minimum controllable flow rate .

m = ln R/T = constant for a particular valve

R= rangeability , T= max. valve lift

Equal

Percent = (

Present value

- Previous

value) /

Previous

value.

Q = Flow

Rate

QO =

Min

Flow

QM =

Max

Flow

X = Valve

Travel

m = ln R/T m X R/T R=QM /

QO

T = 1

2.96 2 100 0.1 3.91 0.39 50 50 1

0.48 4.37 2 100 0.2 3.91 0.78 50 50 1

0.48 6.47 2 100 0.3 3.91 1.17 50 50 1

0.48 9.56 2 100 0.4 3.91 1.56 50 50 1

0.48 14.14 2 100 0.5 3.91 1.96 50 50 1

0.48 20.91 2 100 0.6 3.91 2.35 50 50 1

0.48 30.92 2 100 0.7 3.91 2.74 50 50 1

0.48 45.73 2 100 0.8 3.91 3.13 50 50 1

0.48 67.62 2 100 0.9 3.91 3.52 50 50 1

0.48 100.00 2 100 1.0 3.91 3.91 50 50 1

LIFT(%) LINEAR EQUAL % Q.O

10 20 30 40 50 60 70 80 90 100

10 20 30 40 50 60 70 80 90 100

2.96 4.37 6.47 9.56 14.14 20.91 30.92 45.73 67.62 100

07 24 45 68 83 92 95 98 100 100

•Linear

•Equal %

FLOW

•% of valve opening

•10

•20

•30

•40

•50

•60

•70

•80

•90

•100

•10 •20 •70•60•50•40•30 •90•80 •100•0

•It is the number of US gallons per minute of water at 60

degree F that will pass through flow restriction at

maximum opening maintaining a pressure drop of 1 psi.

BASIC FLOW RATE FORMULA FOR LIQUIDS IS:

Q = K* A* SQRT {(P1-P2)/G}

Q=Flow rate in gpm or Kg/hr

p1=Upstream pressure in psig or Kg/cm2

p2=Downstream pressure in psig or Kg/cm2

G=Specific gravity at referred temperature

K= DISCHARGE COEFFICIENT, A = AREA

IF G =1 AND P1-P2 = 1, THEN Q = K*A

AS PER THE ABOVE DEFINITION,

Cv = K*A

NOW Q = Cv * SQRT {(P1-P2)/G}

WHICH IS THE RELAVENT FORMULA OF FLOW

COEFFICIENT OF CONTROL VALVE IN CASE OF

LIQUIDS .

THE SAME PROCEDURE WILL BE ADOPTED IN CASE

OF GAS FLOW , STEAM FLOW ETC. WITH SUITABLE

MODIFICATION OF WORKING FORMULA.

•It is effectively an index of pressure recovery in a control valve.

•i.e. High Cf values indicate low pressure recovery where a low Cf value indication of high pressure recovery .

•when fluid flows through the valve orifice , there is a marked increase in velocity . This velocity increase is accompanied by proportional decrease in pressure . Velocity reaches maximum and pressure a minimum at the smallest cross sectional flow area downstream of the orifice and that particular point is called vena contracta.

•ORIFICEPLATE

•--------------------------------------------------------------------------------------------------

•PRESSURE

•VELOCITY

•VENA CONTRACTA

•P1

•PV•P2

•VENACONTRACTA

•Control valves give problems mainly in three areas

•1. Cavitation with associate noise and vibration

•2. Flashing

•3. Leakage of fluid around valve stem

•Control valve selection is in step wise

•step 1 : calculate CV

•1.select valve type

•step 2 : select valve size , trim size and characteristics.

•Step 3 : select end connections

•1. End connection type

•2. End connection rating

•Step 4 : select seat leakage

•that is class1/2/3/4/5.

•Step5: select actuator type and size

•step 6: select valve materials

•1. Valve body material

•2.trim material

•3 seat construction as dictated by seat leakage

•4. Guided bushing

•5.soft seat materials for class vi leakage

•Step 7 : select packing material and bonnet type

•1.packing material a)standard Teflon asbestos

•b)Teflon v- ring c)Grafoil

•2. bonnet type

•standard / extended / bellows seal extension

•step8: select valve accessories

•1.positioner

•2.sov / volume booster / air set / lock up valve / volume tank / limit switch / travel stop ………..etc

•IN GENERAL, ALL MANUFACTURERS HAVE THEIR

OWN DESIGN DATA SHEETS, CONSIDERING THE

CRITICAL FLOW , CAVITATION , FLASHING AND

VALVE NOISE OF THE GIVEN PROCESS CONDITION

•WHEN THE CALCULATED Cv VALUE IS KNOWN,

SELECTION AND SIZING OF CONTROL VALVE IS

EASY.

•AIR FILTER REGULATOR

•VALVE POSITIONER

•VOLUME BOOSTER

•QUICK EXHAUST

•AIR LOCK RELAY

•LIMIT SWITCHES

•SOLENOID VALVE

•POSITION TRANSMITTERS

•The main function of positioner is to ensure that the control

•valve position is always proportional to the value of controller

•output signal regardless of packing box friction,actuator hysteresis

•the unbalanced forces of the plug.

•POSITIONER :

USAGE OF VALVE POSITIONER:

• SMOOTH CONTROL

• CHANGING OF OPERATION MODE

• ACHIEVING SPLIT RANGE

• REDUCING TIME LAG

• CHANGING VALVE CHARACTERSTICS

•ACTION CAN BE REVERSED BY CHANGING THE PILOT.

BELLOWS

SPRING

RESTRICTIONI/P SIGNAL

NOZZLE

EXHAUST

VALVE

STROKE

C.V.STEM

BELLOWS DEFLECTION

RELAY VALVE

FROM NOZZLETO BELLOWS

CLASS I

CLASS II

CLASS III

CLASS IV

CLASS V

CLASS VI

NOT SPECIFIED

0.5% RATED VALVE CAPACITY

0.1 % RATED VALVE CAPACITY

0.01% RATED VALVE CAPACITY

5*10-4ml per min. OF WATER PER INCH OF ORIFICE DIAMETER PER PSI DIFFERENTIAL

LEAKAGE EXCPRESSED IN ML PER MINUTE VERSUS PORT DIAMETER

4

2

5

4

5

2

1

3

AVOID AVOID