Valve flow area

This equation defines the flow coefficient, Cv. Here, SG is the fluid specific gravity (relative to water), pw is the density of water, and hv is the head loss across the valve. The last form of Eq. (10-29) applies only for units of Q in gpm and hv in ft. Although Eq. (10-29) is similar to the flow equation for flow meters, the flow coefficient Cv is not dimensionless, as are the flow meter discharge coefficient and the loss coefficient (Af), but has dimensions of [L3][L/M]1/2. The value of Cv is thus different for each valve and also varies with the valve opening (or stem travel) for a given valve. Values for the valve Cv are determined by the manufacturer from measurements on each valve type. Because they are not dimensionless, the values will depend upon the specific units used for the quantities in Eq. (10-29). More specifically, the normal engineering (inconsistent) units of Cv are gpm/ (psi)1/2. [If the fluid density were included in Eq. (10-29) instead of SG, the dimensions of Cv would be L2, which follows from the inclusion of the effective valve flow area in the definition of Cv]. The reference fluid for the density is water for liquids and air for gases. 

To eliminate the need for new capacity tables, revised catalogues and so on, the ASME/NB allowed manufacturers to use the KD figures as K values on the condition that the relief valve flow areas would be increased by at least 10%. The manufacturer can show any K and any A (orifice area) as long as their advertised KA is equal or smaller to the certified ones. 

Amplitude of controlled variable Output amplitude limits Cross sectional area of valve Cross sectional area of tank Controller output bias Bottoms flow rate Limit on control Controlled variable Concentration of A Discharge coefficient Inlet concentration Limit on control move Specific heat of liquid Integration constant Heat capacity of reactants Valve flow coefficient Distillate flow rate Limit on output Decoupler transfer function Error... 

In valve trays, perforations are covered by liftable caps. Vapor flows lifts the caps, thus self creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays. 

Critical and Subcritical Flow - The maximum vapor flow through a restriction, such as the nozzle or orifice of a pressure relief valve, will occur when conditions are such that the velocity through the smallest cross-sectional flow area equals the speed of sound in that vapor. This condition is referred to as "critical flow" or "choked flow . 

In addition to valve velocity, the manufacturer can furnish the effective flow area of the valve. This area is determined by measuring the pressure drop across the valve with a known flow rate and then calculating an equivalent orifice area that provides the same pressure drop. Valves with larger effective flow areas have less pressure drop and better efficiencies. The effects of the seat area, the lift area, and the flow paths are automati-... 

Calculations of Orifice Flow Area for Conventional Pressure Relieving Valves, and Flow is Critical (sonic) Through Part of Relieving System, i.e., backpressure is less than 55% of the absolute relieving pressure (including set pressure plus accumulation). See Figure 7-7A, use... 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

The coefficient of discharge, Ko, is the actual flow divided by the theoretical flow and must be determined by tests for each type or style and size of rupture disk as well as pressure-relieving valve. For rupture disks, the minimum net flow area is the calculated net area after a complete burst of the disk, making allowance for any structural members that could reduce the net flow area of the disk. For sizing, the net flow area must not exceed the nominal pipe size area of the rupture disk assembly [1]. 

Hypercoagulable states, in turn, have been traditionally associated with venous thrombosis. Consequently, attention has been paid to alterations of the hemostatic balance. Although this is a systemic variable, focality is favored due to the contribution of decreased blood flow, as confirmed by the preferential development of venous thrombi at the level of valves, an area of stasis where low-velocity flow is moderately turbulent. 

Chamcteristics. By changing the shape of the plug and the seat in the valve, different relationships between stem position and flow area can be attained. The common flow characteristics used are linear trim valves and equal-percentage trim valves as shown in Fig. 7.7. The term equal percentage comes from the slope of the /Ji, curve being a constant fraction off. 

Both floating and trunnion-mounted designs are available with other design variations that include metal seied valves, soft seated valves, top-entry valves, end-entry valves, and split body valves. Valves in all these design variations are available as either full port or reduced port. Port refers to the round-bore fluid flow area through the ball. Full port valves have a bore that is approximately equal to the inside diameter of the mating pipe. Reduced port valves have a bore that is approximately equal to the inside diameter of pipe one size smaller than full bore. 

Valves with pocket areas or closing devices (e.g., ball, plug, gate, and globe) that move into and out of flow area should be avoided. 

In order to calculate a relief flow area the two-phase mass flow rate per unit area, G, needs to be calculated. Since relief is to be via a safety valve, friction can be neglected and.the equilibrium rate model (ERM) can be used to calculate G (see 9.4.2) . ... 

Note that it is also worthwhile to find the actual nozzle flow area for a particular valve, since there is some variation in this between different manufacturers, even for standard nozzle sizes. The standard nozzle sizes (rather than the actual sizes) for safety valves are defined in references 19 and 22. 

The flow rate used for this calculation should be the best estimate flow rate for the safety valve. BS 6759 19] requires that any safety factors used in determining G for relief system sizing, including the 10% de-rating of the safety valye discharge coefficient, should be removed. The actual flow area through the valve should be used., ... 

Sizing formulae based on equations (A6.4) to (A6.6) are given in many safety valve manufacturers catalogues. Where possible, the method given by the manufacturer of the particular safety valve should be used to find its capacity because this will contain the correct values of flow area and discharge coefficient for the valve. (This is not usually true in the USA where National Board certification information should be used.) The relief lines upstream and downstream of the safety valve also need sizing. Further information is given in 9.7 and references 2 and 4. 

The required flow area of the safety valve can now be found using equation (A6.1) ... 

The flow capacity for checking the upstream pressure drop and back pressure for the valve is obtained by using the actual flow area and multiplying by 1.1 to remove the 0.9 de-rating factor in the discharge coefficient ... 

For relief-protection dump valves, relief valves, and finally rupture discs are used. The sizing of the relief devices is based on the rate of pressure-rise. The flow area should be large enough to stop the pressure rise as soon as the set pressure is reached. 

Simulation of Mouth Conditions for Flavor Analysis The RAS is not intended to simulate the size or structure of the mouth. The conditions in the mouth expected to affect volatility—i.e., temperature, breath flow, mastication, and salivation—are simulated. Temperature iscontrolled with a waterjacket (37°C). Gas (N2 or purified air) flow is controlled with a variable-area needle-valve flow meter (20 ml/sec). The shearing resulting from mastication is implemented with blender blades and a high-torque variable-... 

The primary components of the RAS are a 1-liter stainless-steel blender container, a stainless-steel jacket that water flows through, a variable-speed motor with controller, modified lid with inlet and outlet for gas flow, and a variable-area needle-valve flow meter. The large volume allows for the collection of sufficient volatiles to concentrate trace components for GC/MS analysis. Figure Gl.7.2 shows a diagram of the RAS. 

F igure 14-36 illustrates the pressure drop of a typical moving valve tray as a function of gas velocity. At low velocities, all valves are closed. Gas rises through the crevices between the valves and the tray deck, with increasing pressure drop as the gas velocity rises. Once point A, the closed balance point (CBP), is reached, some valves begin to open. Upon further increase in gas velocity, more valves open until point B, the open balance point (OBP), is reached. Between points A and B, gas flow area increases with gas velocity, keeping pressure drop constant. Further increases in gas v ocity increase pressure drop similar to that in a sieve tray. 

Reduced bore PRV A PRV in which the flow path area bdow the seat is smaller than the flow area at the inlet to the valve, creating a venturi effect. 

Actual discharge area The lesser of the curtain and effective discharge or flow areas. The measured minimum net discharge area determines the flow through a valve (Figure 3.4). 

To size the SRV, calculate the minimum area necessary in order to flow the required flow. When selecting the SRV, choose the next API orifice letter up in practice, this is always a safety factor with normal API valves. Nevertheless, some manufacturers can customize valves to the exact required flow area (A). Oversizing valves is not good practice. Undersizing valves is simply dangerous. 

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