Critical backing pressure

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... 

Vapor vacuum pumps. Part 2 Measurement of critical backing pressure 12/89... 

At the high pressure end of the S vs pMet curve, (here labelled, overload region also known as the forepressure break-down region), S declines from iSmax until a critical pressure is reached in the backing line (critical backing pressure or forevacuum tolerance) which, if exceeded, causes the pumping action of the diffusion pump to cease. 

After closing valve 4, valve 5 is opened and the system is roughed out. (During this operation, it is necessary to monitor the backing line pressure (gauge 3) to ensure that it remains below the critical backing pressure.)... 

A diffusion pump has a pumping speed of 1000Ls-1 at pm < 10 3mbar, falling to 400 L s 1 at 10"2 mbar. The critical backing pressure for the pump is 4 x 10 1 mbar. 

The maximum permissible backing pressure (the critical backing pressure, pcrit) for a diffusion pump (pumping speed = Seff Dp) is 2 x 10 l mbar. If the pump is used only at inlet pressures (/ in) of 10 3mbar or below, calculate the speed of the backing pump (Sback) in terms of Seff DP. 

In Example 3.12, the diffusion pump is to be operated for 1 h with an inlet pressure of 10 5mbar with the backing pump switched off (and the backing valve closed) to eliminate the effect of vibration from the backing pump on the system. Calculate the volume of the backing line to achieve this if the critical backing pressure of the DP is 0.6 mbar. 

Critical backing pressure (vacuum technology) The foreline pressure above which a high... 

Back pressure reduces the pressure drop across the orifice of any type of PR valve. This results in reduced discharge rates in the case of vapors, if the back pressure exceeds the critical flow pressure. For liquids, any back pressure reduces the pressure drop and results in a lower discharge rate. 

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... 

In PR valve design, it is desirable to select a PR valve discharge location at a low enough pressure to permit designing for critical flow conditions, so that the relieving rate will be independent of minor back pressure fluctuations. 

For the exceptional cases of subcritical flow (e.g., where a PR valve is designed for a low set pressure and the total superimposed plus built-up back pressure exceeds the critical flow pressure), the following equation may be used applied ... 

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. 

For gases with specific heat ratios of approximately 1.4, the critical pressure ratio is approximately 0.5. For hydrocarbon service, this means that if the back-pressure on the relief valve is greater than 50% of the set pressure, then the capacity of the valve will be reduced. In other words, if the pressure in the relief piping at the valve outlet is greater than half (he set pressure, then a larger relief valve will be required to handle the same amount of fluid. 

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. 

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. 

For conventional valves, pressure drop or variations in back pressure should not exceed 10% of set pressure. Because most process safety valves are sized for critical pressure conditions, the piping must accommodate the capacity required for valve relief and not have the pressure at the end of vent or manifold exceed the critical pressure. Designing for pressure 30% to 40% of critical w ith balanced valves, yields smaller pipes yet allows proper functioning of the valve. The discharge line size must not be smaller than the valve discharge. Check the manufacturer for valve performance under particular conditions, especially with balanced valves w hich can handle up to 70% to 80% of set pressure as back pressure. 

For non-critical flow the maximum back pressure must be set and pressure drop calculated by the usual friction equations. 

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]. 

Kt, = vapor or gas flow correction factor for constant back pressures above critical pressure (see Figure 7-26). 

Verify critical pressure from Equation 7-7 and establish actual back pressure for relieving device. 

R = temperature, absolute, degrees Rankin r = rc = ratio of back pressure to upstream pressure, P2/Pi, or critical pressure ratio, Pc/Pi rj = relative humidity, percent S = maximum allowable stress in vessel wall, from ASME Code, psi., UCS-23.1-23.5 UHA-23, UHT-23... 

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Case II. Back-pressure reduced (curves 11). The pressure falls to the critical value at the throat where the velocity is sonic. The pressure then rises to Pei — Pb at the exit. The velocity rises to the sonic value at the throat and then falls to 2 at the outlet. 

This is illustrated by conditions (a) to (c). In each case the pressure PE at the exit plane is equal to the back pressure PB. Flow is subsonic throughout the nozzle. This type of behaviour in which the flow rate increases as the back pressure is reduced (P0 held constant) continues until a critical value of the pressure ratio P/P0 is reached at the throat of the nozzle, condition (d). At the critical pressure ratio PJPo, the gas reaches the speed of sound at the throat. It will be shown that PJPo is a function of y only. 

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