Orifice areas, relief valves

NPSH (available from system, A), 160 NPSH (required by pump, R), 160 Operating pressure, 408 Operational check-list, safety relief, 428 Orifice areas, relief valves, 437 Sharp edge, 440 Orifice, flow, 82, 83, 119 Air, table, 107 Overpressure, 403 Causes, 427... 

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 . 

The chart given in Figure 16 can be used in the following manner in order to size a relief valve for liquid service. First, determine the area required, A, without any viscosity correction (i.e., for K = 1). Then select the next larger standard orifice size from manufacturer s literature. Determine the Reynolds number, based on the following definition ... 

Sizing methods for pilot-operated pressure relief valves are in accordance with the accepted formulas described above, utilizing the appropriate discharge coefficients and effective orifice areas as recommended by the valve manufacturer. The following points should be noted ... 

Relief valves are most often sold using the standard orifice sizes shown in Table 13-3. Once a required area is calculated from the applicable formulas, the next larger standard orifice size is specified. 

When a relief valve is sized for viscous liquid service, it is suggested that it be sized first as for nonviscous type application in order to obtain a preliminary required discharge area, A. From manufacturer s standard orifice sizes, the next larger orifice size should be used in determining the Reynold number R, from either of the following relationships ... 

Pressure Relief Valve Orifice Areas on Vessels Containing Only Gas, Unwetted Surface... 

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 ... 

Pipe Orifice Area of a Single Safety Relief in Vapor Service/ Should Not Exceed 2X of the Cross Sectional Area of the Protected Line. 0 Multiple Valves with Staggered Settings May be Required. 

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

The actual KD, K and A (orifice area) values are published for all code stamped relief valves in the NB-18 (red book). 

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. 

API Standard 526, Flanged Steel Pressure Relief Valves This is a purchase specification for flanged steel PRVs. Basic requirements, such as orifice designations and area, materials, pressure-temperature limits and centre-to-face dimensions, inlet and outlet, are given for both spring-operated and pilot-operated PRVs (Figure 4.13). 

From API Std. 526 we would select an H orifice relief valve with effective orifice area 0.785 in. A size 2H3 carbon steel relief valve will allow a set pressure up to 740 psig in the expected range of operation temperature. In practice, however, we would have to consider two-phase flow due to entrainment of boiling liquid with the vapor and might select a larger orifice size after more detailed design. 

The requisite orifice area for a safety relief valve used in liquid service (nonviscous) requiring liquid capacity certification is ... 

The required orifice area for a relief valve sizing for air at constant back pressure is ... 

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. ... 

DETERMINE THE RELIEF VALVE ORIFICE AREA FOR VAPOR CONDITION CALL VAPOR (WG, SPl, OPl, T1, Zl, MWl, HI)... 

DETERMINE THE RELIEF VALVE ORIFICE AREA FOR STEAM CONDITION CALL STEAM (WS, SP2, OP2, KSH, T2, KV72, N2)... 

DETERMINE THE RELIEF VALVE ORIFICE AREA FOR LIQUID CONDITION CALL LIQUID (GPM, SP3, PB, SPL, VISL, KV, KW, KD)... 

DETERMINE THE RELIEF VALVE ORIFICE AREA FOR FIRE CONDITION. 

DETERMINE THE RELIEF VALVE ORIFICE AREA FOR FIRE CONDITION CALL FIRE (F, A, LAT, SP5, OPS, T, Z, MW5, NS)... 

The pressure drop allowed through the inlet and discharge lines is unlimited as long as the capacity of the line is adequate for the relief requirements. That is, at the required flow rate the vessel pressure must not exceed the maximum allowable accumulated pressure. In sizing a safety disc, it is usually assumed that the entrance loss at the nozzle is the governing restriction insofar as capacity is concerned. Thus the effective orifice area is considerably larger than the effective orifice area of a safety valve of the same pipe size. Consequently,... 

When prelimitwy PRV sizes are calculated for various relief cases, select the largest one as the required PRV orifice area. From Manufacturer s catalog or API 26, we will be able to select which relief valve to use, and what arc its inlet and outlet nozzle sizes. In general, PRV is designated with a letter 1o indicate its size. Commonly used PRV and its effective orifice area arc as follows ... 

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