Saudi Aramco DeskTop Standards 24 Determining Flow Rates At The Valve Inlet
Establishing Service Conditions At The Valve Inlet - It should be clear from the previous discussion that in order to apply the two-phase sizing equation, the flow rates of the liquid and the gas
components at the valve inlet must be known. For most applications, the respective flow rates of the liquid and the gas at the valve inlet are assumed to be the same as the flows at the downstream conditions as shown in Figure 19. Although flow rate information is often difficult to obtain, it should be available from process engineers and experienced operations personnel. In many instances, the flow rates can be determined by studying the downstream process.
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Figure 19 Inlet Flow Assumptions
Fm Values - The values of Fm range from 0 to 1.0. When the value of Vr is near 0, the fluid is mostly liquid and the correction factor Fm is small. As the volume ratio increases (the flowstream consists of increasing amounts of gas or vapor), the correction factor Fm also increases. At the maximum value of Fm, the Cvr is double that of the uncompensated value of Cv. Notice that when the volume ratio is greater than 0.9, the correction curve displays a steep downward slope. The steep slope is partially related to the phenomenon of slip; i.e., the much higher velocity of a gas or vapor as compared to the velocity of a liquid under the same conditions.
Saudi Aramco DeskTop Standards 25 Unique Problems For Vapor/Liquid Flows
Transfer Of Mass And Energy Between Phases - Refer to Figure 20 and note that the horizontal traverse of the constant temperature line across the two-phase dome can be viewed as an indicator of quality, x. When the quality (x) of a fluid is 0, the fluid is entirely in the liquid state and when the quality is 1.0 the fluid is entirely in the vapor state. If only the pressure and temperature of a vapor are known, the fluid state could be defined by any point on the horizontal temperature line that passes across the two-phase dome. For this reason, either the value of x or the enthalpy must be known in order to determine the density of a vapor. Because the enthalpy value is often difficult to obtain, the value of x is commonly used for valve sizing purposes.
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Figure 20
Enthalpy And x Versus Pressure And Temperature
Available Data Vs. Actual Conditions At The Valve Inlet - The service conditions that are provided to the valve specifier - including the value of x - are often the conditions that were determined for the fluid at some upstream location. Because of pressure losses, heat losses, and changes in operating parameters, the pressure conditions, temperature conditions, and the value of x at the valve inlet may be substantially different than the given data. Any such discrepancy can result in erroneous
calculations for Vr and Cvr. When sizing valves for vapor/liquid flows, specifiers should make an additional effort to ensure that accurate data is available.
Saudi Aramco DeskTop Standards 26 Computer Sizing Two Phase Flows
Method Selection Criteria
Refer to Figure 21 and note that there are two two-phase sizing methods within the Fisher Sizing Program: a vapor/liquid method and a gas/liquid method. The criteria for selection of a particular method is discussed below.
Units For Fluid Density - If the fluid density is given in units of mass (M, lb/ft3, kg/m3, etc.), the vapor/liquid option should be selected.
Gas And Liquid Chemical Structure (Single Species Vs Binary Fluid) - As a generalization, the vapor/liquid method is selected for single species fluids (water/steam) while the gas/liquid method is commonly selected for binary fluids.
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view Macintosh picture.
Figure 21
Fisher Sizing Program Menu Screen
Saudi Aramco DeskTop Standards 27 Inputs To The Vapor/Liquid Two-Phase Sizing Method
The sizing screen for the vapor/liquid method is shown in Figure 22 . The entry fields and the calculated results are discussed below.
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Figure 22
Fisher Sizing Program Screen For The Vapor/Liquid Sizing Method
Vapor Phase Information - The fluid data that is required to size the vapor phase of the flow is shown in Figure 22. The required inputs are:
• The vapor name.
• The density of the vapor, lb/ft
3.
• The flow rate, lb/hr.
Liquid Phase Information - The fluid data that is required to size the liquid phase of the flow is shown in Figure 22. The required inputs are:
• The liquid name.
• The liquid critical pressure, P
cin psia.
• The liquid vapor pressure, P
vin psia.
• The liquid specific gravity, SG.
• The liquid flow rate, Q lb/hr.
Saudi Aramco DeskTop Standards 28 Mixture Service Conditions
The mixture service conditions that must be entered are:
• The inlet pressure, P
1, in psia.
• The pressure drop, dP, in psid.
• The temperature of the mixture at the valve inlet, T, in degrees F.
Note: The measurement units for many of the entry fields can be changed by pressing the F8 key, and, then, selecting the desired units from the list that is shown.
Valve Specifications - The valve specifications that must be input are:
• The value of K
m(recall that F
L= K
m)
•
The value of C
1(recall that C
1= C
g/C
vNote: The program can be used to size non-Fisher control valves by converting the pressure recovery coefficient FL to Km with the use
of FL2 = Km, and by calculating the value of C1 with the use of C1=Cg/Cv.
Calculated Results With The Vapor/Liquid Two-Phase Sizing Method
The software automatically calculates the valve size and displays the results of the various calculations.
Calculated Parameters - The results that are calculated and displayed include the following:
• dP Critical - The flow-limiting pressure drop for the gas phase.
• dP allowable - The flow limiting pressure drop for the liquid phase.
• r
c- The critical pressure ratio that defines the vena contract pressure.
• C
g- The flow coefficient for the gas or vapor phase.
• C
v- The flow coefficient for the liquid phase.
• Quality - The value of x (x = the weight fraction of vapor in a vapor-liquid mixture).
• V
r- The volume ratio (the percent by volume of gas to liquid).
• F
m- The correction factor that is included in the two-phase sizing equation.
• C
vr- The required control valve C
v, corrected for two-phase flow.
Saudi Aramco DeskTop Standards 29 Notes On Other Sizing Parameters - Many of the sizing options that are normally available by pressing the F3 key are not active in the two-phase sizing methods. For example: