Relief critical flow

When the pressure relief device is set to open at greater than 15 psig (critical flow will result), it is normally not uecessaiy to be concerned about the pressure drop in the separator. If the hquid is to be drained from the separator during the emergency blowdown, a vortex breaker and false bottom should be used (Fig. 26-18, view BB). 

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

If the superimposed back pressure is less than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is unaffected and back pressure is not a factor. However, builtup back pressure on a conventional pressure relief valve will affect its flow capacity and operating characteristics, and should not exceed 100% of its set pressure. If total back pressure (superimposed plus built-up) is greater than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is affected, and total back pressure is incorporated into the sizing procedure. Any back pressure reduces the capacity of a conventional PR valve in liquid service, and... 

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 flow of a compressible fluid through an orifice is limited by critical flow. Critical flow is also referred to as choked flow, sonic flow, or Mach 1. It can occur at a restriction in a line such as a relief valve orifice or a choke, where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at... 

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

Note The curves above represent a compromise of the values recommended by a number of relief valve manufacturers and may be used when the make of the valve or the actual critical f ow pressure point for the vapor or gas is unknown. When the make is known, the manufacturer should be consulted tor the correction factor. These curves are for set pressures of 50 pounds per square inch gauge and above. They are limited to back-pressure below critical flow pressure for a given set pressure. For subcntical flow back-pressures below 50 pounds per square inch gauge, the rnanufacturer must be consulted tor values of Kk. 

Six-tenths factor, 47 Yearly cost indices, 47 Critical flow, safety-relief, 438 Back pressure, 440 Sonic flow, 438 Critical flow, see Sonic Cyclone separators, 259-269 Design, 260-265 Efficiency chart, 263 Hydroclones, 265-267 Pressure drop, 263, 264 Scrubber, 269 Webre design, 265 Deflagration venting nomographs,... 

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

Sonic flow, safety relief, 438 Rupture disk, 460, 461 Sub-sonic flow, 461 Sonic or critical flow, 115, 125 Calculations, 125 Velocity, 126 Specific speed, 194-197 Impeller designs, 194 Upper limits, chart, 195-197 Specifications,... 

Often, if both high and low pressure relief valves need to relieve simultaneously, parallel high and low pressure headers terminating at the flare knockout drum are the economical choice. Be sure to check for critical flow at key points in the high pressure header. 

For most vapor discharges through spring reliefs the flow is critical. However, the downstream pressure must be checked to ensure that it is less than the choked pressure computed using Equation 4-49. Thus for an ideal gas Equation 4-50 is valid ... 

Any consistent set of units may be used for pressure as long as the absolute pressure is used, not the gauge pressure. The ratio PcfIPi is called the critical pressure ratio. Typical values of this ratio are given in Table 13.6. If the downstream pressure is less than the critical flow pressure, then critical flow will occur in the nozzle. It can be seen from the table that this will be the case whenever the upstream pressure is more than two times the downstream pressure. Since most relief systems are operated close to atmospheric pressure, critical flow is the usual case. 

For preliminary estimates, the coefficient Kj can be taken as 0.975 for a relief valve and 0.62 for a bursting disk. The back-pressure correction factor, Ky, can initially be assumed to be 1.0 for critical flow. The combination correction factor, K, is used when a rupture disk is used upstream of the relief valve (see next section), in which case it is 0.9. If no rupture disk is used, then is 1.0. For vessels designed in accordance with AS ME BPV Code Sec. VIII, Pj = 1.1 times the maximum allowable working pressure. 

The expression used to determine the relief area for vapor discharge when the back pressure is less than the critical flow pressure is ... 

The discharge reactive force is based on the assumption that critical flow of the gas or vapor is obtained at the outlet of the relief device and the discharge is horizontal to the atmosphere. For any gas or vapor, the reactive force can be expressed as ... 

A common prerequisite to the application of all these equations is the evaluation whether the ratio of set pressure of the relief device to the counterpressure is high enough to ensure critical flow conditions in the vent area. 

The procedure of vapor relief valve sizing depends on the type of vapor flow. Sizing for critical flow is different from fliat for subcritical flow. It is therefore required to establish the flow behavior first. 

As long as the pressure ratio exceeds the critical-pressure ratio, the throughput will vary with the inlet pressure and be independent of outlet pressure. For example, a relief valve set at 100 psi will have the same gas flow through it as long as the back-pressure is less than approximately 50 psi. 

Thus, if the dowmstream or backpressure on tlie valve is less than 53%-60% (should be calculated) of the values of P, note above, critical (sonic) flow wll usually exist. If the downstream pressure is over approximately 50% of the relief pressure, P, the actual critical pressure should be calculated to determine the proper condition. Calculation of critical pressure [29] ... 

Figure A8.7 CRITICAL PRESSURE RATIO FOR FLOW THROUGH RELIEF ...

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