Saudi Aramco DeskTop Standards 30 SIZING CONTROL VALVES FOR FLUIDS WITH DISSOLVED GASSES
Mechanics Of Outgassing And Implications For Valve Sizing Dissolved Gas Defined
Figure 24 shows that gasses can be forced into solution in a liquid. The amount of gas that can be dissolved in a solution is partially dependent upon the fluid pressure and the amount of time that the fluid is under pressure.
Dissolved gasses are typically found in high pressure streams of untreated, multi-component fluids. Crude oil is a common example of a liquid that includes dissolved gasses. An example that is found in daily life is a carbonated drink in a sealed bottle or can.
Mechanics Of Outgassing
Gas molecules may come out of solution (outgas) if the fluid pressure is reduced or if the fluid is agitated. A common example of outgassing occurs when a can or bottle of a carbonated beverage is shaken and then opened. Similarly, pressure letdown and the creation of turbulence are two of the operating mechanisms of a high-pressure separator that is used in oil field operations.
Use Word 6.0c or later to
view Macintosh picture.
Figure 24 Mechanics Of Outgassing
Saudi Aramco DeskTop Standards 31 Outgassing Versus Flashing
Refer to Figure 25 and compare the thermodynamic analysis of a flashing liquid with an outgassing liquid.
With flashing fluids, Pvc must fall below Pv and P2 must remain inside the two-phase dome. With outgassing fluids, P1, Pvc, and P2 may all be on the liquid side of the two-phase dome. A slight reduction in fluid pressure or the occurrence of agitation is all that is required to cause a gas to come out of liquid solution (outgas).
Use Word 6.0c or later to
view Macintosh picture.
Figure 25
Outgassing Versus Flashing
Saudi Aramco DeskTop Standards 32 Outgassing Versus Cavitation
If the local pressure of a liquid falls below the liquid’s Pv and then rises above the liquid’s Pv, the fluid will vaporize and then revert to the liquid state; i.e., the fluid will cavitate. If the local pressure of an outgassing liquid decreases and subsequently increases, the gaseous portion of the fluid may not go back into solution.
Often, additional time is required for the increased pressure to force the gas back into the liquid solution. Refer to Figure 26.
Use Word 6.0c or later to
view Macintosh picture.
Figure 26
Outgassing Versus Cavitation Indicators Of The Presence Of Dissolved Gasses
Stated Pv = P1 - Whenever the stated Pv of a liquid is equal to P1, one may deduce that the liquid includes dissolved gasses that will come out of solution upon any reduction in pressure. The exception is when both the true vapor pressure and the inlet pressure happen to fall on the saturated vapor line of the two-phase dome. In this case, flashing rather than outgassing may be the greatest consideration.
Pv=P1>Pc - Even though it is a physical impossibility for the value of Pv to be larger than the value of Pc, there are two common reasons for the occurrence of such defective data. (1) The value that is given as the fluid’s Pv is actually the fluid’s bubble point. The bubble point is the pressure at which the lightest fluid components will come out of solution, or outgas. (2) The value that is given for the critical pressure is actually the fluid’s pseudocritical pressure. Pseudocritical pressures will be discussed later in this Module.
Saudi Aramco DeskTop Standards 33
Use Word 6.0c or later to
view Macintosh picture.
Figure 27
Fluid Properties As Indicators Of Outgassing
Saudi Aramco DeskTop Standards 34 Implications For Valve Sizing
Valve Capacity As A Function Of Gas Volume Ratio - Pressure reductions and turbulence in the valve can cause varying amounts of dissolved gasses to come out of solution. Depending on the amount of dissolved gas in the liquid and the degree of outgassing that occurs, fluid expansion at the valve vena contracta can have a choking effect on flow capacity. If the effects of outgassing are not considered, the valve may be undersized. The challenges that are encountered during valve sizing are shown in Figure 28 and they are listed below.
• There is no simple method that can be used to accurately determine the amount of gas that is dissolved in the liquid.
• There is no simple method to determine the extent of outgassing that will occur under various service conditions.
• There is no scientific method for precisely calculating the impact of outgassing on valve capacity.
Use Word 6.0c or later to
view Macintosh picture.
Figure 28
Sizing Considerations For Fluids With Dissolved Gasses
Absence Of Sizing Standards For Dissolved Gasses - No standards body (ISA, IEC, etc.) has endorsed a method for compensating for dissolved gasses. Experience and the application of practical techniques are the only guides that are available to the specifier.
Saudi Aramco DeskTop Standards 35 Bracketing Approach To Valve Sizing For Dissolved Gas Applications
General Concept
A common approach to valve sizing for outgassing liquids is to perform two or more sizing calculations that are based on different assumptions regarding the state of the fluid. Then, the results of the two calculations are compared and a subjective assessment is made in order to estimate the appropriate valve size. A common technique is illustrated in Figure 29 and it is introduced below.
1. First, in order to determine the smallest possible valve size, the specifier sizes the valve as if it were a pure, non-choked, liquid flow.
2. Next, in order to determine the largest possible valve size, the specifier assumes that the gas that does come out of solution is present at the valve inlet. This is accomplished through the use of the two-phase sizing procedure that was previously discussed.
3. The results of the two sizing calculations are compared and a valve size is selected on the basis of experience and engineering judgment.
Use Word 6.0c or later to
view Macintosh picture.
Figure 29
Basic Concept Of A Bracketing Approach To Valve Sizing
Saudi Aramco DeskTop Standards 36 Calculating The Minimum Valve Size
Assumption: Fluid Remains In Liquid State - In order to determine the smallest possible valve size, the assumption is made that the fluid will remain in a liquid state; i.e., no vaporization will occur. Refer to Figure 30.
Sizing Technique - To size the fluid as a liquid, the value of Pv is set to an arbitrarily low value; e.g., Pv = 0 (or a very low pressure value). After setting Pv to 0, the minimum valve size is calculated with the use of the standard liquid sizing equations. Figure 30 shows that by setting Pv to 0 or a value that is near 0, there is little chance that the pressure at the vena contracta (Pvc) will drop below the fluid vapor pressure (Pv). In other words, the sizing equations will not allow for fluid vaporization (choked flow).
The calculated results will lead to the smallest possible valve size. In fact, if outgassing occurs within the valve, the valve may be undersized.
Use Word 6.0c or later to
view Macintosh picture.
Figure 30
Sizing For The Minimum Valve Size
Saudi Aramco DeskTop Standards 37 Calculating Maximum Valve Size
Assumption: Upstream Gas/Liquid Fraction = Downstream Gas/Liquid Fraction
The maximum valve size is calculated with the use of a two-phase sizing method - most likely the gas/liquid method. In order to calculate the maximum valve size, the gas/liquid fraction or the volume ratio of the gas at downstream conditions must be known. For valve sizing purposes, the gas/liquid volume ratio at the valve inlet is assumed to be equal to the gas/liquid volume ratio at the valve outlet as shown in Figure 31.
Assumption: Pv<P2
The two-phase sizing method will make sufficient allowance for fluid expansion that is caused by outgassing. To ensure that the sizing equations do not make additional allowances for fluid expansion by calculating the choked flow pressure drop, the value of Pv is set to an arbitrary value such as the value of P2.
Use Word 6.0c or later to
view Macintosh picture.
Figure 31
Sizing For The Maximum Valve Size
Saudi Aramco DeskTop Standards 38 Evaluation Of The Minimum And Maximum Cv Calculations
After the minimum and maximum Cv’s have been calculated, they are evaluated according the information that is shown in Figure 32.
Cv min ∼∼ Cv max - If the minimum calculated Cv (Cv min) and the maximum calculated Cv (Cv max) lead one to the selection of the same valve size, that valve size can be selected with reasonable confidence.
Broadly Differing Values Of Cv max and Cv min - If the results of the calculations indicate valve sizes that are significantly different, the specifier should consult with the valve vendor or manufacturer.
Difference In Valve Size That Is Required By Cv min And Cv max
Confidence In Sizing Method
Action
None High Select Indicated Valve Size
1 Valve Size Medium Select The Larger Valve Size
More Than 1 Valve Size Low Contact The Valve Vendor
Figure 32
Interpreting The Results Of The Sizing Technique Valve Style Selection Guidelines
Selection Of Replaceable Trim - Because of uncertainties in the sizing calculations, one should select top-entry valves and cage-style trim in order to allow for field changes of trim size if necessary.
Body And Trim Material Selection - The physical effects of an outgassing fluid are similar to those of a flashing fluid; i.e., when gasses come out of solution, localized areas of high-velocity flow increase the potential for erosion damage. Accordingly, the body and trim materials for an outgassing application should be selected as if the application were flashing; i.e., hardened trim and alloy bodies.
Cavitation Considerations
In a majority of applications where the gas/volume ratio is relatively high, the presence of dissolved gasses can have a cushioning effect on any cavitation that may be occurring within the valve and piping. Many specifiers assume that the dissolved gas will absorb most of the energy that is released during cavitation and the issue of cavitation is ignored. This is the case with many unstabilized crude oils. However, if the gas/liquid volume ratio is very low (VR < 0.2) and cavitation is predicted, cavitation resistant trim should be considered.
Saudi Aramco DeskTop Standards 39 SIZING CONTROL VALVES FOR HYDROCARBON MIXTURES
Introduction
Many hydrocarbon fluids are mixtures. For example, crude oil may contain several different liquid components and several different gaseous components. Most natural gasses are actually mixtures of several different components (methane, ethane, propane, etc.).
Unique Sizing Problems With Liquid Mixtures Multiple Pressure-Enthalpy Diagrams
Each component of a liquid mixture has a unique saturated liquid line, a unique critical point, and a unique saturated vapor line. The challenge to the valve specifier is that the valve sizing equations require a single value for the mixture vapor pressure and a single value for the mixture critical pressure.
Use Word 6.0c or later to
view Macintosh picture.
Figure 33
Multiple Pressure-Enthalpy Diagrams For A Liquid Mixture
Saudi Aramco DeskTop Standards 40 Defining Fluid Properties
In order to establish single values for the fluid properties of mixtures, several different averaging techniques may be applied.
Bubble Point Versus Vapor Pressure - For a single-species fluid at a specific temperature, the fluid vapor pressure defines a relatively precise pressure at which the fluid will begin to vaporize. With mixtures, however, each mixture component may begin to vaporize at a different pressure. The pressure at which the lightest component of a mixture begins to vaporize is often referred to as the bubble point. If a vapor pressure for a liquid mixture is given as a single value, that value often is the bubble point. The bubble point may be determined by testing, with the use of computer simulation programs, or by calculation. When evaluating liquid mixtures, the saturated liquid line on a pressure-enthalpy diagram may be replaced with a bubble line as shown in Figure 34.
Use Word 6.0c or later to
view Macintosh picture.
Figure 34
Pressure-Enthalpy Diagram For A Liquid Mixture
Saudi Aramco DeskTop Standards 41 Pseudocritical Pressure and Pseudocritical Temperature - Each component in a liquid mixture has a unique critical pressure and a unique critical temperature. For a mixture, the critical point is defined by the intersection of the “pseudocritical” pressure and the “pseudocritical” temperature. Refer to Figure 34. One method of determining the pseudocritical pressure and the pseudocritical temperature is through the use of a molar averaging technique. The pseudocritical properties are determined by:
1. Multiplying the mole fraction of each component times the values of Pc and Tc of each component. The result is the pseudocritical property of each component.
2. Summing the values of the pseudocritical properties of each component to obtain the pseudocritical properties of the mixture.
Pseudocritical pressures and temperatures may also be determined by test, by calculation, or with the use of process simulation software. Although the pseudocritical point may not fall exactly on the phase line, it does provide information that is reasonably effective in the valve sizing calculations.
Dew Line Vs. Saturated Vapor Line - For mixtures, there is not a single saturated vapor line on the pressure-enthalpy diagram. Instead, the “dew line” describes the conditions under which the mixture begins to change from the gaseous to the two-phase state and vice versa. Refer to Figure 34.
Saudi Aramco DeskTop Standards 42 Common Anomalies In The Values Of Fluid Properties
P1=Pv - When one is performing valve sizing calculations for mixtures, it is common to receive data that indicates that P1=Pv. If P1=Pv, then one of two conditions may be present:
1. The fluid includes dissolved gasses which will come out of solution upon any reduction in
pressure. In this situation, the stated vapor pressure is actually the bubble point. Refer to Figure 35.
2. Pressure P1 is at or very near the vapor pressure of the lightest component in the mixture. In this instance, fluid vaporization rather than outgassing is the result. Depending on the conditions, fluid vaporization may be accompanied by flashing, by cavitation, and/or by choked flow. Refer to Figure 35.
3.
Unless a complete thermodynamic analysis is performed, it is difficult to distinguish between the two conditions that are described above. However, a vapor pressure value that is equal to the valve inlet pressure is the “classic” indication of the presence of an outgassing fluid.
Pv>Pc - Specifiers may receive data that indicates the fluid Pv is greater than the fluid Pc; however, it is a physical impossibility for Pv to be greater than Pc. When these conditions are given, some of the mixture components are above their critical temperatures (are not liquid) and they are dissolved in heavier liquid components. The value that is given for Pc is probably the pseudocritical pressure. The true critical pressure of some of the components will be higher. In addition, the value of the Pv that is given may be the bubble point; i.e., the pressure at which the lightest components begin to vaporize.
Use Word 6.0c or later to
view Macintosh picture.
Figure 35
Common Anomalies In Fluid Properties
Saudi Aramco DeskTop Standards 43 Sensitivity Of Sizing Calculations To Accurate Fluid Properties
Sensitivity Of Valve Sizing Calculations To Pc - The valve sizing procedure for liquid flows requires a value for the critical pressure (Pc) in order to permit calculation of the choked flow pressure drop (∆Pallow or ∆Pchoked).
∆Pallow or ∆Pchoked = FL2 (P1-rcPv) where:
rc = 0.96 - 0.28 (pv/Pc).
Recall that rc (the critical pressure ratio) is a measure of pressure reduction below the vapor pressure at the vena contracta that provides the energy that is required to vaporize an amount of liquid. Fluid vaporization can have a significant impact on the valve size that is required. In addition, choked flow may be accompanied by flashing or cavitation. Given the significance of proper valve sizing and of flashing and cavitation, the determination of useful values of Pc and Pv for the purpose of valve sizing is essential.
Impact of Pc on ∆∆Pchoked (∆∆Pallow) - If all other parameters are fixed and the value of Pc is increased, the choked flow equation will predict that choked flow will occur at smaller and smaller pressure drops. As a result, the Cv that is calculated will increase. Similarly, if the value of Pc decreases while other parameters remain constant, larger and larger pressure drops may occur before the flow becomes choked and a smaller value of Cv will be calculated.
Sensitivity Of Valve Sizing Calculations To Pv - If all other parameters are held constant and the value of Pv is increased, the equation will predict that choked flow will occur at smaller pressure drops.
If a Pv is given which actually describes the mixture bubble point, the actual liquid Pv may be much lower. The result is that the choked flow equation will predict total vaporization and choked flow when, in fact, only a portion of the fluid stream will be vaporizing or outgassing. The result is that the sizing equation will calculate a conservative Cv; i.e., the valve will not be undersized.
Saudi Aramco DeskTop Standards 44 Liquid Mixture Sizing Techniques
Many different techniques have been developed in order to address the challenges of sizing control vales for liquid mixtures. Only a few will be presented in this Module. The techniques that are presented below are designed to given an estimate of the valve size that will be required. For mixtures and other difficult sizing problems, specifiers should always seek assistance from valve manufacturers and others who can apply advanced sizing tools and techniques.
When Pv<Pc<P1
The relative values of Pv, Pc, and P1 that are shown in Figure 36 are what one would expect to find in a normal liquid sizing situation. Because Pv does not equal P1, dissolved gas is not present. And, Pv is below the value of Pc, which is to be expected. Because the fluid is known to be a mixture, the assumption is that the Pv is actually the bubble point and the Pc is actually the pseudocritical pressure. This mixture would be sized with the use of the standard liquid sizing equations. If choked flow or flashing are not indicated, the fluid remains in the liquid phase and the Cv that is calculated is accurate. If choked flow or flashing are indicated, the Cv that is calculated will be conservative because only a portion of the mixture will actually vaporize. The major problem is that it is difficult to precisely determine the extent of fluid vaporization.
Use Word 6.0c or later to
view Macintosh picture.
Figure 36 When Pv<Pc<P1
Saudi Aramco DeskTop Standards 45 When P1=Pv<Pc
In this example, because P1=Pv, we know that either (1) dissolved gasses are present, or (2) P1 is at the vapor pressure of the mixture component with the highest Pv. If dissolved gasses are present, some outgassing will occur upon any reduction of pressure at the vena contracta. If a fluid component’s vapor pressure is equal to P1, then some flashing or cavitation could occur. However, the intensity of the flashing or cavitation may not be significant if only a small fraction of the fluid is vaporizing.
The recommended sizing technique is the bracketing method that was previously discussed, i.e.:
1. Size the mixture as a non-choked liquid by setting the value of Pv to 0 or 1.
1. Size the mixture as a non-choked liquid by setting the value of Pv to 0 or 1.