CONTROL VALVE PRESSURE DROP AND SIZING

Transcription

1 CONTENT Chapter Description Page I Purpose of Control Valve II Type and Main Components of Control Valve 3 III Power 5 IV. Pressure Drop Across Control Valve 7 V. Symbols and Units 10 VI. Unit Conversion 11 Table 1. Typical Flow Factors for single ported C.V. 1 Table. Typical flow factor for rotary type C.V. 13 Table 3. Properties of Fluids 14 1

2 I. PURPOSE OF CONTROL VALVE Control valve is required to control capacity or pressure of fluid where is flowing in piping system. Pressure control is included liquid level such as in level control. Control valve is also used as an instrument to control temperature where capacity of mixing fluid in an equipment or piping is handled by control valve. Figure 1 shows typical application of control valve. Temperature control Level control Flow controller Figure 1. Typical application of control valve

3 Typical pressure controller Figure 1. (continue) Typical application of control valve II. TYPE AND MAIN COMPONENTS OF CONTROL VALVE Figure shows main components of control valve and figure 3 shows several type of control valves. Figure. Main component of control valve 3

4 Figure 3. Types of control valve when divided into number of port, plug & seat type, number of connection and valve types 4

5 Figure 3. (continue) Types of control valve III. POWER Control valves are powered by manual, pneumatic system or electric motor. Instrument air is commonly used for pneumatic power. Figure 4 shows typical of control valve power system. Figure 4. Types of actuator and power system for control valve 5

6 Figure 4. (continue) Types of actuator and power system for control valve 6

7 IV. PRESSURE DROP ACROSS CONTROL VALVE The following figure is schematic illustration to show fluid flow at around port and plug of control valve. Figure 5. Illustration of fluid flow inside the control valve Pressure drop of fluid where flows across control valve can be determined as the following equations. For liquid, SG. Q Cv PLOSS = kg/cm (1) For gas.3136 MW. To. Z. Qn Y. Cv. Po.10 7 P/Po = () Or by other method, P/Po = C arc sin Cg. Po..483 Qn 333 SG. To 0.5 (A) Where Cv is flow coefficient of valve or valve sizing coefficient given by manufacturers. Q is volume flow in m3/hr, Qn is volume flow at normal condition in Nm3/hr, Po and To is fluid pressure and temperature in kg/cma and K, MW is fluid and SG is specific gravity. Y is expansion coefficient, C1 universal flow coefficient, Cg is gas flow coefficient also given by manufacturers. Typical Y is, 7

8 1.4( P / Po) Y = 1 (3) 3. k. X T k is gas adiabatic exponent, XT is pressure ratio factor when valve is installed without fitting (elbows or reducers very close to valve). XT is given by manufacturers. When control valve is installed with fittings, replace XT by XTP, XTP = X F T P X T 1 d V Do 1 d 4 V 4 Do Cv d V 1 (4) And equation (3) become, 1.4( P / Po) Y = 1 (5) 3. k. X TP FP is piping construction factor and dv is nominal valve size in mm (it is not port diameter, d but casing connection size) are given by manufacturers. Do is pipe diameter upstream fitting in mm. If control valve constructed between identical reducers, FP = d V Do Cv d V 0.5 (6) If all fittings pressure drop installed in the piping system has been calculated such as in article Fluid flow in pipe in this blog, it will more simple to use equation (3) without XTP and FP factor. For liquid-gas mixture SG. Q Y. Cvm PLOSS = kg/cm (7) Cvm = (Cv+Cg)(1+Fm) (8) Fm = RQ for RQ < 0.6 and Fm=1.334 RQ 0.01 for 0.6< RQ <0.9 (9) RQ = Qg / (QLIQ + Qg) Qg is gas volume flow, QLIQ is liquid volume flow and RQ is gas-liquid volume ratio. Limitation and correction Viscosity correction For viscous liquid such as oil, Cv in equation (1) shall be corrected become, 8

9 CVR = Cv.Fv (10) Fv is viscosity correction for Cv. For pressure drop calculation, Fv is given as the following equations. If Re < 35, Fv = 18.1 Re If 35<Re< 3000, Fv = Re Re.10-5 If 3000<Re<60,000, Fv = Re If Re > 60,000, Fv = 1 (11) Choked Flow and Cavitation High velocity of liquid at vena contracta will reduce static pressure. If static pressure at the vena contracta is lower than vapor pressure of liquid, vaporation will accour (flashing) and than cavitation will also accour at downstream of vena contracta or surrounding valve plug and other parts nearby where vapor bubbles are become liquid again when static pressure is back to higher than vapor pressure. If bubbles due to flashing occurrence are so much, these bubbles will crowd space at downstream of valve port and limit liquid to flow. This occurrence is called choked flow. To prevent from above condition, design of pressure drop of liquid across control valve shall be limited at the following equations. For globe type valve, PLOSS-MAX-GLOBE =FL (Po rc.pv ) kg/cm (1) Km (Po rc.pv) FL is valve recovery coefficient given by manufacturers. Po is upstream pressure and Pv vapor pressure of liquid in kg/cma. rc is critical pressure ratio rc = (Pv/Pc) 0.5 (13) where Pc is thermodynamic critical pressure of liquid. If valve installed with fittings (reducers or elbows), equation (1) become, PLOSS-MAX-GLOBE = FL (Po rc.pv )/Fp kg/cm (14) For ball and butterfly (ROTARY) type valve, PLOSS-MAX-ROTARY = FL (Po rc.pv ) or = FL (Po rc.pv )/Fp kg/cm (15) FL can also be replaced by Km. Ball and butterfly valve is more tend to cavitation. Use following equation to prevent cavitation. PLOSS-MAX-ROTARY = Kc(Po Pv ) (16) Fc is approximately = 0.67 FL or see table. 9

10 Choked condition will also accour for gas, steam and vapor if velocity at vena contracta reach the sound velocity. Port pressure drop of gas or steam at 0.8 x sound velocity is, P sonic = 0.4 Po.k kg/cm (17) Cg MIN = Qn.(SG To) 0.5 /Po (18) V. SYMBOLS AND UNITS Unless otherwise noted, the following symbols and units are used in this manual. Symbol Description Unit Cv Cvm Cg Cs Flow coefficient Flow coefficient for liquid-gas mixture Gas sizing coefficient Steam sizing coefficient D or Do Inside pipe diameter mm dv, d Nominal control valve size, port dia. mm Fm Fp Fv Liquid-gas mixture factor Installation factor Viscosity correction factor g Gravity 9.81 m/s FL Valve recovery coefficient L Pipe length m Viscosity cp (centipoise) Po Upstream pipe pressure kg/cm A Pv Vapor pressure kg/cm A Pc Thermodynamic critical pressure kg/cm A P or PLOSS Differential pressure/pressure drop kg/cm Q Volume flow (see note) m3/hr Qg & QLIQ Gas and liquid volume flow m3/hr 10

11 RQ Gas-liquid volume ratio Fluid density kg/m 3 Re rc SG Reynold Number Critical pressure ratio Specific gravity T Absolute temperature 0 K V Fluid velocity m/s Y Gas net expansion factor VI. UNIT CONVERSION Designation Unit to be converted Factor Unit to be used Length ft mm inch 5.4 mm Pressure psi kg/cm bar kg/cm atm kg/cm Pa (Pascal) x 10-5 kg/cm Temperature F (Fahrenheit) (tf -3) x (5/9) C R (Rankin) (5/9) K C (Celcius) tc + 73 K Velocity ft/s m/s ft/min (fpm) m/s Volume flow GPM (US) 0.7 m 3 /hr CFM m 3 /hr Mass lb kg Power HP kw 11

12 Head ft m Enthalpy kcal/kg kj/kg BTU/lb.36 kj/kg Gas constant kcal/kg.k kj/kg.k Specific heat BTU/lb.R kj/kg.k Density lb/ft kg/m 3 Specific volume ft 3 /lb m 3 /kg Viscosity N.s/m 1000 cp lbf.s/ft cp Kinematic to absolute viscosity, = SG. in cst (centistokes), in cp Note : American Standard State condition is condition where pressure at bar A and temperature at 15.5 C. In volume, is common written as SCF. Normal condition is at bar A and 0 C. In volume, is common written as Nm 3 (Symbol for flow in this article is Qn) Table 1. Typical Cv, FL and XT for Single Ported Globe Style Valve bodies 1

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14 Table. Typical Cv, FL and XT for rotary shaft valve 14

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17 Table 3. Properties of some fluids CONTROL VALVE 17

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