Orifice Area Calculations

AP = Pj — P2 = upstream pressure, psig (set + overpressure) — total backpressure, psig 

ASME Code valves Board Certified for liquids only. 

U = viscosity flowing temperature, Saybolt Universal Seconds (See Appendix A-12 and A-13) 

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 Kv and Kw. With variable backpressure, use maximum value for P2 [33a, 68]. 

To apply the viscosity correction K, a preliminary or trial calculation should be made for the areas required using the equation of paragraph (e) above or the modified equation (still ASME conformance [33] but not 

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Kc equals 1.0, if there is no rupture disk at PRV inlet. Kc equals 0.9, if there is a rupture disk at PRV inlet. (0.9 is uncertified Kc certified Kc is available from the National Board of Boiler and Pressure Vessel Inspectors). Kv is viscosity correction factor. At initial orifice area calculation (Eq. (9a)), let Kv equal to 1.0. Based on calculated orifice area, select a PRV orifice. Based on this orifice area, recalculate Re, kv, and A using Eq. (9c), Eq. (9b), and Eq. (9a). Repeat these calculations until selected PRV orifice doesn t change anymore. 

Calculate individually the orifice area required to pass the flashed vapor component, using Equation (5a), (3b), (4), (5), or (6), as appropriate, according to service, type of valve and whether the back pressure is greater or less than the critical flow pressure. 

Calculate individually the orifice area required to pass the unflashed hquid component, using Equation (8). The pressure drop term Pj should be made equal to the set pressure minus the total back pressure developed by the vapor portion at critical flow pressure, except when the critical flow pressure is less than the calculated total back pressure (superimposed plus built-up), considering the combined liquid and vapor flow. In the latter case, P should be made equal to set pressure minus the calculated total back pressure. 

Add the areas calculated for the vapor and liquid components to obtain the total orifice area required. This may be somewhat conservative, since flashing does not take place instantaneously across the PR valve orifice. 

Sizing for Mixed-Phase Vapor and Liquid Service - PR valve sizing for a mixture of hquid and vapor at inlet conditions may be calculated using the sum of the orifice areas required for each phase individually, in the same way as described above for flashing liquid service. 

Number of Valves Required - Normally a manufacturer s standard PR valve with Drifdce area equal to or larger than the calculated requirement is specified. In some cases, e.g., large reheving rates or to prevent chattering, two or more valves are necessary. Likewise, if there is an appreciable difference between the calculated orifice size and the available size, multiple PR valves are desirable to more nearly match the available area to the required orifice area. The column for "spares" indicates the requirement, if any, for spare PR valves installed on the equipment. Normally, this applies only in the case of refinery preference or local regulations, but is required in many European countries. 

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-... 

There are no precise formulas for calculating orifice area for two-phase flow. The common convention is to calculate the area required for the gas flow as if there were no liquid present and the area required for the liquid flow as if there were no gas present. The two areas are then added to approximate the area required for two-phase flow. 

For final calculation use from Figures 7-23 or 7-24 and substitute in the above equation. Determine the needed Reynolds number, Re, using the next size larger orifice. Area is determined from that made in the first trial calculation [33]. 

Step 1 — Calculate IMsI Orifice Calculate the trial required orifice area from the liquid equation on page 40 (Alternate used in this example). 

Select Ihe next larger orifice size, or an M" orifice with 360 sq in. orifice area. (This should be about 20% greater than the calculated area to allow lor reduction of capacity due to Ihe viscosity correction factor "Ku")... 

After the value of R is determined, the factor Kvt is obtained from the graph. Factor Kv is applied to correct the "preliminary required discharge area. If the corrected area exceeds the chosen standard orifice area", the above calculations should be repeated using the next larger standard orifice size. 

Due to gas expansion from external fire, the API code [10] provides for calculation of the pressure relief valve orifice area for a gas containing vessel exposed to external fire on die unwetted surface ... 

The following multi-step method may be used for calculating the required orifice area for SRVs on vessels containing liquids that are potentially exposed to fire. (Reference API Recommended Practice 521, Fourth Edition.)... 

A = Calculated safety relief valve (SRV) orifice area (in2)... 

After determining the required orifice area necessary to flow the required capacity, the appropriate valve size and style may be selected. It should have a nominal effective area equal to or greater than the calculated required effective area. API effective areas and coefficient of discharge for different manufacturers can be found in their respective catalogues or in their comprehensive sizing software. ... 

In this section, we will describe the techniques that may be used for calculating the required effective orifice area for an SRV application on two-phase flow based on the work done by the Design Institute for Emergency Relief... 

Relieving Temperature Units 49. Calculated Orifice Area (in square in.) ... 

Both reactions were studied by Brittain, Lau, Knittel, and Hildenbrand102 using a torsion-effusion method in the temperature range 800 to 900 K. They found the S03 pressure over both the sulfate and oxysulfate derived from extrapolation of steady-state pressures to the zero orifice area to be a factor of five lower than those calculated from established thermodynamic data. This indicates that the products are formed in a finely divided metastable state. 

Determination of the required capacity of the pressure safety valves (PSVs) involves a calculation that considers the approximated amount of heat that a fire would send into the storage tank and the properties of the tank contents as they are being vaporized by that heat and generate the vapors to be relieved through the valve. The details of this calculation are beyond the scope of this discussion, and the reader is advised to review Chapter 7.15 in the fourth edition of Volume 1 of the Instrument Engineers Handbook if that information is needed. Once the required PSV capacity is determined, the orifice area required to handle that capacity can be obtained graphically. 

For the maximum flow of 4 1/s, G = 4 kg/s. The largest practicable height of a water manometer will be taken as 1 m and equation 6.21 is then used to calculate the orifice area A0. If the coefficient of discharge CD is taken as 0.6, then ... 

GAS CONSTANT DETERMINED BY RATIO OF Cp/Cv CALCULATED VALVE ORIFICE AREA, in 2 ... 

From a compressibility factor chart, Z = 0.86. The rate of heat input due to fire and flow rate of the vapor release can be determined by Equations 5-53 and 5-45. Table 5-12 gives the input data and results of vinyl chloride monomer. The results show that the calculated relief valve orifice area is 2.172 in. The nearest standard orifice size is L, with an orifice area of 2.853 in. ... 

CALCULATE THE MAXIMUM FLOWRATE FROM THE MANUFACTURER S ORIFICE AREA... 

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