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
CONTROL VALVE PARTS
1. BODY
2. VALVE
■
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
INLET OUTLET STEM WITH PLUG
GLAND
PACKING BODY FLANGE
SEAT
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 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
.
ACTUATOR
■ UTILISES AIR PRESSURE / ELECTRIC POWER HYDRAULIC PRESSURE
■ TO ACT ON A DIAPHRAGM / PISTON AND DEVELOP A FORCE TO MOVE THE
• ■ PNUEMATICALLY OPERATED DIAPHARGM ACTUATOR
■ PNUEMATICALLY OPERATED CYLINDER ACTUATOR
■ MOTOR OPERATED PISTON ACTUATOR HYDRAULIC OPERATED PISTON
ACTUATOR
ACTUTATOR
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 t t e r f ly P lu g R o t a r y M o t io n G lo b e A n g le T h r e e W a y G lo b e G a te D ia p h r a g m P in c h o r C la m p L in e a r M o tio n T y p e s o f V a v le s
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
•
■ ANGLE VALVES ARE SUITABLE FOR SLURRY FLUID ,
ADHESIVE FLUID AND HIGH VISCOSITY LIQUIDS.
■ SUITABLE FOR HIGH PRESSURE APPLICATION.
•
■ 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 .
•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
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
•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,
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 .
•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 .
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 10 20 30 40 50 60 70 80 2.96 4.37 6.47 9.56 14.14 20.91 30.92 45.73 07 24 45 68 83 92 95 98 •Linear •Equal %
F L O W •10 •20 •30 •40 •50 •60 •70 •80 •90 •100 •10 •20 •30 •40 •50 •60 •70 •80 •90 •100 •0
•It is the number of US gallons per minute of water at 60 degree F that will pass through flow restriction at
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
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
•ORIFICEPLATE •-- -- ---- -- ---- -- ---- -- ---- -- ---- -- ---- -- ---- -- ---- ---•PRESSURE •VELOCITY •VENA CONTRACTA •P1 •PV •P2
•Control valves give problems mainly in three areas •1. Cavitation with associate noise and vibration •2. Flashing
•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
•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
•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
•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.
USAGE OF VALVE POSITIONER:
• SMOOTH CONTROL
• CHANGING OF OPERATION MODE • ACHIEVING SPLIT RANGE
• REDUCING TIME LAG
• CHANGING VALVE CHARACTERSTICS
BELLO WS SPRING RESTRICTION I/P SIGNAL NOZZLE EXHAUST VALVE STROKE C.V.STEM BELLOWS DEFLECTION RELAY VALVE
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
