Transcription of FLUID FLOW BASICS OF THROTTLING VALVES
1 FLUID FLOWFLUID FLOWBASICSBASICSOFOFTHROTTLING VALVESTHROTTLING hrottlingValveUpstreamDownstreamFLUID FLOW BASICS OF THROTTLING VALVESFLUID PARAMETERS FLUID PARAMETERS The following FLUID parameters are frequently associated with THROTTLING VALVES PSize = P1 - P2 T = T1 - T2 PressuresFlowQuantityThermo-dynamicRelat iveWeight/MassGeometricFluid Resistanceto Flow CONDITION 1 CONDITION 2m1_____Mass Flow Rate _____ m2Q1, 1_____Volumetric Flow Rate _____ Q2, 2P1, PS1, P1(Abs)_____Static Pressure _____ P2, PS2, P2 (Abs)PV1_____Velocity Pressure _____ PV2 PVC Pressure @ Vena Contracta Psize Sizing Pressure DropVapor PressurePVP1, PSAT1 PVP2, PSAT2 Saturation PressureT1, 1(Abs), T1@xx SH_____Temperature _____ T2, 2(Abs)
2 , T 2@xx SHTSAT1_____Saturation Temperature _____ TSAT2H1, h1_____Enthalpy _____ H2, h2 1_____Specific Volume _____ 2 1_____Liquid Density _____ 2 1_____Actual Gas Density_____ 2SG1_____Specific Gravity _____ SG2 1P_____Pipe Diameter _____ 2P 1V_____Valve Body Size _____ 2VZ1_____Elevation Head _____ Z2A1_____Cross-Section Area _____ A2v1V_____Avg. Valve Velocity _____ v2Vv1P_____Avg. Pipe Velocity _____ v2P 1_____Absolute Viscosity _____ 2 1_____Kinematic Viscosity _____ hrottlingValve P = P - P12 UpstreamDownstreamTHE BASICS OF THROTTLING VALVESTHROTTLING VALVESTHROTTLING VALVESV alves that are utilized as FLUID control devices are typically THROTTLING VALVES .
3 Such VALVES experience internal velocity and internal pressure gradients (both positive andnegative) that conclude with a permanent pressure loss ( P) from the inlet pipe-to-outlet pipeconnections. THROTTLING valve trim (plug-seat) experiences relatively high internal velocitiesnearly 100% of operating time. In comparison, ON-OFF automated or manual VALVES experi-ence velocity changes ONLY when being actuated from open-to-closed , or vice versa; afew seconds or s Theorem is the most useful tool inanalyzing what is going on physically withinthe walls of a THROTTLING valve, which includes velocity gradients pressure gradientsThe other important tool is the 1st Law ofThermodynamics which allows analyzing FLUID state thermal effectsBernoulli s principles apply to the following forthrottling VALVES inlet pipe reducer pressure drop to main orifice pressure recovery to outlet outlet pipe reducer-4-When the pressure gradients are graphically shown.
4 One ends up with the rather typical venacontracta curve The velocity gradients form a sort of inverse of the vena contracta curve -5-The depth of the vena contracta dip is primarily a function of a THROTTLING valve s geometry;globe vs. butterfly, etc. The important parameter in determining the PVC is FL LiquidPressure Recovery Factor . As the name implies, the FL factor is a measure of the effective-ness of the reconversion of velocity pressure into static pressure from the main orifice of thethrottling valve (@ vena contracta) to the valve s following graphic attempts to give relative representation of the four major valve stylesused for THROTTLING butterfly and ball VALVES are sub-classified as high recovery VALVES .
5 As a general rule,globe and eccentric plug (rotary globe) styles tend to make better THROTTLING control hrottlingValveUpstreamDownstreamLIQUID(N on-Compressible)GAS VAPOR(Compressible) P = P1 - P2 LIQUIDLIQUIDFLUID STATESFLUID STATESF luid flow is classified into two basic FLUID states at the pressure changes occur within a THROTTLING valve, it is possible to produce 2-phase flow atthe valve s outlet for either a liquid or gas-vapor at the vapor is a gas that is at, or relatively near, its saturation (boiling) conditions of pressureand temperature; saturated vapor or slightly superheated vapor.
6 A gas is a FLUID that doesnot liquify at reduced temperatures, or is a highly superheated VALVES operate as steady state, steady flow devices. The entering and exiting massflow rates are the same; flow is continuous , and the Continuity Equation is applicable It is a thermodynamic principle that whenever there is a phase change between a throttlingvalve s entering and exiting FLUID state, there is also a temperature change ( decrease orcooling) in all such applications T1 > T2 LIQUIDS. For simple liquid-in and liquid-out flow there is no density change of the liquid 1 = 2 This constant density results in other parameters being typically affected.
7 M1 = m2(EQ. #1)1A1v1 = 2A2v2(no phase change)1A1v1 = 2VA2Vv2V + 2LA2Lv2L(2-phase outlet)VaporLiquidm1 = m2A1 = A2T1 = T2 no phase changev1 = v2 1 = 21 = hrottlingValveUpstreamDownstreamGAS-VAPO RS. For simple gas-vapor-in and gas-vapor-out flow, there is a density change( decrease) of the gas-vapor as the FLUID decompresses ( expands) 1 > 2 This changing density results in other parameters being typically affected GAS-VAPORGAS-VAPOR h = h1 - h2 h = 0 THERMODYNAMIC PRINCIPLESTHERMODYNAMIC PRINCIPLESTHROTTLING PROCESS. In looking into the thermodynamic principles of a THROTTLING pro-cess , we know hrottlingValve P = P - UnitsMetric Unitsh1= Valve Inlet EnthalpyBtu/#kJ/kgh2= Valve Outlet EnthalpyBtu/#kJ/kgm1= Inlet Mass Flow#/Hrkg/Hrm2= Outlet Mass Flow#/HrKg/HrTHE CHANGE IN ENTHALPY ACROSS A RESTRICTION IN A PIPE ORIFICE, REGULA-TOR, CONTROL VALVE IS ZERO FOR A THROTTLING the continuity equation m1 = m2m1h1 m2h2 = 0 OR(EQ.)
8 #2)m(h1 h2) = = m2A1 < A2T1 T2 GAS or HIGHLYSUPERHEATED VAPORv1 > v2 1 < 2T1 > T2 VAPOR1 > 2-8-It is the use of FLUID thermodynamic data and the thermodynamic principles of the constantenthalpy THROTTLING process that THROTTLING VALVES experience which allows an accurate determi-nation of a FLUID s state while internal to the valve as well as at the valve s outlet. In particular,we want to know what the FLUID is physically doing at the THROTTLING valve s main orifice (plug-seat); what is occurring at the vena contracta and elsewhere within the valve?
9 SATURATION STATE. A FLUID is said to be saturated when Liquid - when at the boiling temperature Tsat for a given pressure PsatExamples: water @ Psat = psia Tsat = 212 FPsat = BarA Tsat = 100 CWater @Psat = 145 psig Tsat = FPsat = 10 BarA Tsat = C Vapor - when at the boiling temperature Tsat for a given pressure PsatExamples: Steam @ Psat = psia Tsat = 212 FPsat = BarA Tsat = 100 CSteam @ Psat = 145 psig Tsat = FPsat = 10 BarA Tsat = CRestating the above examples, we have both saturated liquid water (condensate) and satu-rated steam at the same Psat and Tsat.
10 Further, for any given FLUID in its saturation state, whenwe have its pressure (Psat), we KNOW its temperature (Tsat). To say a FLUID is saturated is togive a property of the FLUID . Only two extensive properties of a FLUID will locate the FLUID in thephysical universe. We know exactly where a FLUID is when we say the FLUID is saturated water at Psat = 29 psia = BarA, we know that Tsat = F = C. saturated steam at Tsat = 212 F = 100 C, we know that Psat = psig = VAPOR. A FLUID is a superheated vapor when its temperature is greater thanTsat corresponding to the flowing : Steam @ P1 = 145 psia & T1 = 425 F SH(Tsat = F)P1 = 10 BarA & T1 = 219 C SH(Tsat = C)-9-To say a vapor is superheated does NOT give an extensive property of the FLUID ; so, a secondproperty must also be known to physically locate a superheated vapor in the LIQUID.