Pressure, absolute relief

A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia ( ) of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum pressure of 2.6 bara = 1.6 barg. 

It is recommended that the chosen sizing method be evaluated for a range of overpressures. This will give an indication of the likely consequence of undersizing, in terms of how far the pressure may be expected to rise above the maximum permitted for the vessel. For moderate overpressures (typically 10-30% of the absolute relief pressure), the consequence of undersizing may sometimes be small. 

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. 

A 3,500-gal reaetor with styrene monomer undergoes adiabatie polymerization after being heated inadvertently to 70°C. The maximum allowable working pressure (MAWP) of the reaetor is 5 bar absolute. Determine the relief vent diameter required. Assume a set pressure of 4.5 bara and a maximum pressure of 5.4 bara. Other data and physieal properties are given as follows [12] ... 

The percent absolute back-pressure for conventional and pilot-operated safety relief valves is ... 

Always install a relief valve that is capable of bypassing the full-load capacity of the compressor between its discharge port and the first isolation valve. Since helical-lobe compressors are less tolerant to over-pressure operation, safety valves are usually set within 10 per cent of absolute discharge pressure, or 5 psi, whichever is lower. 

Facility is provided to bring the equipment up to the desired absolute pressure without subjecting the test unit to excessive pressure difference between its interior and exterior. This is accomplished by first filling the test cell with water by means of a hand operated hydraulic pump(17 in Figure 1) to a suitable value as indicated on the Bourdon dial gauges(P in Figure 1). The pressure thus developed is used also to control the appropriate back-pressure relief valve... 

Leung25 reported on the computation of the required relief area for a spherical propane vessel exposed to fire. The vessel has a volume of 100 m3 and contains 50,700 kg of propane. A set pressure of 4.5 bars absolute is required. This corresponds to a set temperature, based on the saturation pressure, of 271.5 K. At these conditions the following physical property data are reported ... 

Pressures are converted to bara in this example because absolute pressure is required for relief sizing calculations.)... 

Leung s method (as given in equation (6.5) below) is applicable if all the above assumptions are true. Assumption (d) above, regarding the use of an average rate of heat release, tends to be the most limiting in terms of the maximum difference that can be allowed between the relief pressure and the maximum pressure. The absolute overpressure (often referred to simply as the "overpressure") has been sometimes used to characterise this. This is given by ... 

The evaluation of the two-phase mass relief capacity per unit area, G, is discussed in Chapter 9. The additional parameters which are required for tempered hybrid systems are PJP, the ratio of the vapour pressure to the absolute pressure, and , the closed vessel temperature rise as the pressure rises from the relief pressure to the maximum pressure. 

This is normally the gauge pressure increase above the set pressure of a safety valve (see 5.2). The absolute overpressure is the pressure increase above the relief pressure of any relief device (see equation (6.1))... 

Accordingly, the evening humidity is only about 20% that of the hot daytime humidity, a great relief (Note that relative humidity expresses vapor pressure Pu2 o as a percentage of the saturation vapor pressure or absolute humidity for given T.)... 

Backpressure has no effect on the set pressure or flow capacity of pilot-operated pressure relief valves except when the flow is subcritical (ratio of absolute backpressure to absolute relieving pressure exceeds 55%). In this case, the flow correction factor Kb (see Appendix B) must be applied. If the ratio of absolute backpressure to absolute relieving pressure is less than 55%, no correction factor is required, Kb = 1. 

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. 

In the second, test pressure measurements were also obtained from the system expansion tank. This pressure rose smoothly to a level corresponding to the pressure relief setting. Again the pressure at the sodium inlet and outlet behaved as though they were independent. The pressure at the outlet was moderate and is not shown in Figure 2. The negative pressure recorded at the inlet is an absolute... 

The major areas of investigation have been successfully concluded, and a laboratory model of a cryogenic-solid cooler has been fabricated and tested. An absolute-pressure-relief valve has been developed which will maintain constant pressure over a coolant through a pressure range of 20 to 6 torr. 

When liquid nitrogen is allowed to stabilize at a pressure of approximately two atmospheres, as used for this system, it will solidify when exposed to low ambient pressure, This solidification will occur at an altitude of 53,000 feet or 1.46 psia. Once the liquid has changed to a solid, the temperature is difficult, if not impossible, to control. To overcome this problem, the outlet for the direct liquid transfer system is provided with an absolute pressure relief valve. This maintains a constant pressure on the liquid in the tubing and the detector, preventing the liquid from changing to the solid form. This relief valve will be adjusted to approximately one atmosphere of pressure,... 

One potential problem with the use of rupture disks below relief valves concerns the possibility of pinhole leaks in the rupture disk. If such a leak occurs, then the pressure on the downstream side of the disk will be roughly equal to the process pressure. Therefore, the absolute process pressme required to rupture the disk may be almost twice as high as expected. Moreover, the existence of the pinhole leak may lead to the development of the problem that it was intended to avert, e.g., the relief valve could be plugged with polymer without anyone being aware of the situation. Because of the dangers associated with pinhole leaks, it is good practice to install a pressure gage in the spool piece between the rupture disk and the relief valve. 

To get around this difficulty, some relief valves are designed to open at a specified absolute pressure. The PHA team should check which approach is being used in the facility that is being analyzed and should consider the implications of multiple relief valves opening simultaneously (such as would happen if there were a large fire on the unit). 

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