Back pressure sizing

Figure 12. Variable or constant back pressure sizing factor Kb for balanced bellows safety relief valves (vapors and gases).

Figure 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

VARIABLE OR CONSTANT BACK PRESSURE SIZING FACTOR... 

Figure 5-5. Constant back pressure sizing factor, for conventional safety relief valves in vapor or gas service. (API Recommended Practice 520, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1, 5th ed., 1990. Reprinted courtesy of the American Petroleum Institute.)...

The dehvery port can be sized sufficiently large that the back pressure is negligibly small. Equation 4 then reduces to equation 5, where k represents the osmotic water permeabihty of the membrane and S represents the solubihty of the dmg at saturated concentration. 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

Case A. The calculated back pressure at the lowest set relief valve on a header is much smaller than its MABP. Reduce header size. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Pilot-operated valves may be satisfactorily used in vapor or liquid services up to a maximum back pressure (superimposed plus built-up) of 50% of set pressure, provided that the back pressure is incorporated into the sizing calculation. At higher back pressures, capacity becomes increasingly sensitive to small changes in back pressure. As an exception, back pressure up to 7% of set pressure may be used, provided that this disadvantage is recognized. 

Back pressure is included as a factor in PR valve selection and sizing accordingly ... 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

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... 

It is important to note that back pressure affects balanced PR valve capacities in the same way as for conventional valves, and appropriate factors are included in the sizing procedures. They are subject to the same recommended limits of maximum total back pressure (superimposed plus built-up) as conventional valves. In the case of balanced bellows valves, mechanical considerations must also be evaluated, since they may limit the maximum permissible back pressure. 

We shall first consider the design of elevated flare systems. Sizing of flare systems is a function of maximum allowable back pressure on safety valves and other sources of release into the emergency systems. 

A trial and error estimate is made for determining the diameter of the flare header based upon the maximum relieving flare load and considering the back pressure limitation of 10 percent for conventional valves and 40 percent for balanced type valves. Note, however, a single main header in most cases turns out to be too large to be economically feasible. Line sizing procedures are discussed in detail in the next subsection. 

When the maximum vapor-relieving requirement of the flare system has been established and the maximum allowable back pressure (as just described) has been defined, line sizing reduces to standard flow calculations. 

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. 

In summary, the back-pressure for relief valves should be limited to the following values unless the valve is compensated. We do not recommend using a relief valve with higher back-pressure than shown below without consulting a person knowledgeable in relief valve sizing and relief system design. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

The advantages of monosized chromatographic supports are as follows a uniform column packing, uniform flow velocity profile, low back pressure, high resolution, and high-speed separation compared with the materials of broad size distribution. Optical micrographs of 20-p,m monosized macroporous particles and a commercial chromatography resin of size 12-28 p,m are shown in Fig. 1.4. There is a clear difference in the size distribution between the monodispersed particles and the traditional column material (87). 

A trend in chromatography has been to use monosized particles as supports for ion-exchange and size-exclusion chromatography and to minimize the column size, such as using a 15 X 4.6-mm column packed with 3-/rm polymer particles for size exclusion chromatography. The more efficient and lower back pressure of monosized particles is applied in the separation. 

---