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Comprehensive Guide to Valve Types and Sizing Equations in Control Systems

Learn about different types of valves (linear, rotary) and how to size them using Bernoulli's principle and flow equations. Understand the importance of Kv/Cv values and factors affecting flow rate calculations. Explore valve styles, flow limitations, and factors like temperature, material of construction, and trim considerations.

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Comprehensive Guide to Valve Types and Sizing Equations in Control Systems

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  1. Samson Controls Pvt. Ltd.

  2. Types of Valve Linear Rotary ( On-Off) Globe Gate Diaphragm Ball Butterfly Plug Control & On Off 2 Way/3 way/Angle

  3. Sensor Controller Transmitter i/p convertor Positioner

  4. Valve Bonnet/Yoke Actuator Accessories Valve Components

  5. Valve styles: Linear • Globe valves • Single Seated/Double seated Three-way valves as mixing valve or diverting valve

  6. Valve styles: Rotary Butterfly ValvesRotary Plug valves Ball Valves Segmented Ball Valves

  7. Selection Of Valve • Size • Rating • Temperature • Material Of Construction • Trim • Differential Pressure • Characterstic • Leakage • Rangability • Noise • End Connections…etc

  8. Defination of Sizing Equation Sizing is based on Daniel Bernoulli’s principle As fluid flows through orifice, the square of the fluid velocity is directly proportional to differential pressure across orifice and inversely proportional to specific gravity of fluid . 2 Stagnation Pressure= Static Pressure + ½ ρv Static Pressure = hgρ Where h= diff in height g= Gravity ρ= density 2 2 V2 – V1 = 2 g h

  9. General Flow Equation SymbolsQ = Flow rateAv = CoefficientDp = Differential pressurer = Density

  10. Definition of Kv value • To be able to compare different types of control valves, a uniform procedure for measuring the flow rate (IEC 534, Part 4) has been agreed upon. • The Kv value of a control valve considers the influence factors mentioned above and is determined by: • Water as medium • Onem3 per Hour • Density of 1000 kg/m3Temperature of 5 to 50 °C • Differential pressure of 1 bar • The Cv value : • Water • One US gallon per minute • Differencial pressure of 1psi • Relation 1Kv = 1.17Cv

  11. Flow equation In practice, the measurable flow across a control valve turns out to be slightly smallerthan the theoretical value calculated by means of a previously derived equation. This is caused by the following: Style of the control valve Ratio of opening between the control valve inlet and outlet (with reducers and increasers) Viscosity of the medium(-> Reynolds number) Amount of pressure recovery Expansion of compressible media Flow limitation of liquids

  12. A formula As per IEC 534 for Liquids • with consideration for • Reynold’s Number factor …. Fr • Piping Geometry Factor…… Fp

  13. Pipe reducers and increasers  The factor Fp Same increase in the pipe diameter upstream and downstream of the valve

  14. Types of flow Laminar flowNo exchange between the molecules of a liquid in the flow Turbulent flow Immediate exchange between the molecules of a liquid in the flow laminar turbulent

  15. Reynolds number factor  Can only be determined iteratively due to its complexity Valve style factor Fd: Fd = 1 for standard valves Fd = 0.7 for butterfly valves, double-seated valves, etc.

  16. Determining FR from REV

  17. FlowLimitation Choked Flow means by keeping P1 constant if P2 is reduced flow coefieciant will continue to reduce till it reaches choked flow condition :Critical Flow Choked flow condition occures in liquid due to Cavitation/flashing. In case of choked flow Δp will be replaced by ΔP = Differential pressureFL2 = Valve factor (indicated by the manufacturer)P1 = Upstream pressureFF = Factor to be determined using the diagramPv = Saturated steam pressure

  18. The factor FF

  19. Basic equations for gas As per ISA After replacing value of N6 we get

  20. Assuming ideal gas Since r1 is the density under operating conditions, the "ideal gas equation” must be understood as approach, provided that ideal gases are used: Included in the Kv equation:

  21. Deriving a simplified equation Simplified Kv equation for gases:

  22. Expansion factor Y Standard equation is: By replacing Fg and x, the equation becomes: Where FY = Correction factor & gamma is specific heat ratio

  23. Isentropic factor g (cp/cv)

  24. xT values per IEC 534

  25. Kv equations (2) Compressible medium: Gas Mass flow rate W Unit: kg/h Flow rate Q Unit: m3/h Compressible medium: Steam

  26. Working Equations

  27. Valve Rating In cae of ANSI pressure is tested at temperature of 446 deg C (835 Deg F); Where as in case of DIN it is tested at 120 C; Hence we can say approximataly ANSI 150 Equ PN 16 300 PN 40 600 PN 100 900 PN 160 1500 PN 250 2500 PN 420 .

  28. Pressure Temperature Rating as per ANSI

  29. Temperature To decide Rating Special Services like Cryogenics High Temperature Ext Bonnet etc

  30. Material Of Construction In Casting In Forging Bar stock To decide Suitability with rating (Pressure & Temperature) Suitability with Medium

  31. Trim • Plug, Seat, Stem, Guide, Cage etc • FTO/FTC, Lift, MOC, Shape, Leakage, Characterstic, Port(Full/Reduced) Microflow, Balanced plug • Temperature, Pressure, Flashing, cavitation, Viscosity, Solids

  32. Trim Flow To Open / Flow To Close Flow under the plug / Flow above the plug Change in FL value In case FTO FL = 0.95 FTC FL=0.75 Low FL / XT will bring early critical flow condition Cavitation Index KC = FL heavier cavitation in case of FTC Advantage: Smaller actuator in case of stem extend position 3

  33. Trim Plug Shapes

  34. Plug Lift

  35. Rangeability = Kvs/Kvr From 30:1 to 50:1 with Kvs 100%: Rangeability Kvr-value (%) 50/1 => 2 30/1 => 3,3 Rangeability

  36. Pressure measurement When measuring the upstream and downstream pressure for use in the sizing equations, the following applies:

  37. Pressure development in a pipeline at 3 different valve positions Three different valve positions Pipeline

  38. Pressure ratios within the system Valve An easy example The heating system in which the pressure prevails includes: Heat generator Pump Valve Heat consumer Consumer Pump Heat generator

  39. Valve Characterstic Gain = Change in output/Change in input Valve Gain = Change in flow/change in travel Which is also known as Characterstic

  40. Characterstic Inherent Installed Inherent characterstic is characterstic Installed characterstic is characterstic of only valve as it is manufactured of entire system, including valve + piping or or Characterstic at constant pressure drop Characterstic at variable pressure drop 1) Quick Opening 2) Linear 3) Equal Percent

  41. Quick Opening: Provides maximum change in flow rate at initial lift Linear : Flow rate is directly proportional to the valve travel Equal Percent : Equal increments of valve travel produce change in flow which is equal percent of existing flow Q = Q0 e m = logn Qo = Flow at 0% lift i. e. 2% m x valve travel in fraction Rangability/ Stroke

  42. Linear & Equal Percent Characterstic Difference Linear Q = K X Eq Percent Q=Qo e mx m = Lognat R/T

  43. At 20% lift Q = Qo x e Q = 2 x e Q = 2 x e Q = 2 x e Q = 2 x 2.1814 Q = 4.36 At 30% lift = 6.47 At 50% lift = 14.14 At 60% lift = 20.91 At 80% lift = 45.72 At 90% lift = 67.62 Rangability/Stroke Logn x fractional valve travel 50/1 Ln x fractional valve travel 3.9120 x 0.2 0.78

  44. 6.47- 4.37 = 2.1 & 4.37/2.1 = 2.08 20.91-14.14 =6.77 & 14.14/6.77 = 2.08 67.62-45.72 = 21.9 & 45.72/21.9 = 2.08 67.62 45.72 20.91 14.14 6.47 4.37 10 20 30 40 50 60 70 80 90 100 Lift or travel

  45. To calculate percentage opening of control valve at various flow rates H = Logn (Cvc/Cvs x 50) ÷0.03192 for 50:1 rangability H = Logn (Cvc/Cvs x 30) ÷ 0.034012 for 30:1 rangability

  46. To understand Installed characterstic, we must understand pressure profile in entire system In pumped system, Pump head Pressure will start decreasing with increase in flow Which means dynamic losses will increase, Incase of increase in flow capacity system will become unstable,unless extra dp is designated To the valve, (As shown in picture B)

  47. 1) Linear Q = K X 2) = % Q = Qo . e 3) Mod Parabolic Q = K X Where Q = Flow Rate Qo= Minimum Controllable Flow X = Valve Position m = Log nat R/T R = rangability T = Maximum valve travel 0….1 mx 2 2

  48. Pump Head = 170 psi Maximum Flow = 200 gallons/min Required Pressure = 80 psi 80 psig PUMP At maximum flow of 200 gallons pump head is 100 psi, Since required pressure at system is 80 psi, We have only Differtial pressure of 20 psi available at valve, Now Calculated Cv = Q √ Density/dp, Assume this is water Cv = 200 √ 1/20 Cv = 45

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