Emergency pressure relief

An essential point, when considering emergency rehef, is that as the relief system suddenly opens, reaction mass may be entrained by the gas or vapor, leading to two-phase flow that decreases the relief capacity of the system. Thus the 

The design of emergency pressure relief systems usually proceeds in three steps, described in the next subsections. 

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Designers of emergency pressure relief systems should be famihar with the fohowing hst of regulations, codes of practice, and industiy standards and guidehnes. 

Figure 7-39A. Emergency pressure relief valve vent using guided weight. By permission, The Protectoseal Co.

First K. E. and J. E. Huff, Design Chaits for Two-Phase Flashing Flow in Emergency Pressure Relief Systems, Plant Operations Progress, Vol. 8, No. 1, 1989, p. 40. 

Shaw, D. A., S,VFIRE Program for Design of Emergency Pressure Relief Systems, CEP, 14—17, July 1990. 

Emergency Pressure Relief Fires and Explosions Rupture Disks... 

In addition to the adiabatic dewar, several adiabatic calorimeters are commercially available that allow emergency pressure-relief system sizing. These include ... 

It will then be necessary to check whether or not the credible maloperations can lead to exothermic runaway. Information on this is given in reference 1. Where it is shown that runaway can occur, and it is decided that emergency pressure relief may be used as part of the basis of safety, then it will be necessary to carry out further work to identify the worst case for relief system sizing. ... 

It may be possible to minimise the time spent on relief system assessment and design if the basis of safety for a multi-purpose reactor can be changed to prevention rather than emergency pressure relief. For example, if it can be arranged for all the reactions to operate in semi-batch mode with no significant reactant accumulation, then the use of a trip system of sufficient integrity may provide a suitable basis of safety. (This may not always be possible.)... 

A G Keiter, "Emergency Pressure Relief Discharge Control by Passive Quenching", International Symposium on Runaway Reactions, 425-450, AlChE, 1989, ISBN 0-8169-0460-X... 

Emergency Pressure Relief Systems", Plant/Operations Prog, 8 (1), 40-54, 1989... 

A pressure relief valve is attached to the roof. This valve is opened automatically when pumping product to, or withdrawing product from, the tank. The valve shuts when pumping stops so that vapour losses from the tank are contained. A bursting disc on the roof also provides emergency pressure relief for sudden pressure rises. The final tank specification is summarized in Table 12.1. 

Therefore, no special measure is required for this class of scenario, but the reaction mass should not be held for a longer time under heat accumulation conditions. The evaporative cooling or the emergency pressure relief could serve as a safety barrier as far as their design is appropriate. 

After loss of control of the synthesis reaction, the technical limit will be reached (MTSR > MTT) and the decomposition reaction could theoretically be triggered (MTSR > TD24). In this situation, the safety of the process depends on the heat release rate of both the synthesis reaction and decomposition reaction at the MTT. The evaporative cooling or the emergency pressure relief may serve as a safety barrier. This scenario is similar to class 3, with one important difference if the technical measures fail, the secondary reaction will be triggered. 

This measure is different from emergency pressure relief. A controlled depressurization is activated in the early stages of the runaway, while the temperature increase rate and the heat release rate are slow. 

The key factor of success in the design of emergency pressure relief systems lies in a good understanding of the behavior of the reaction under relief conditions. The first point in this context is the cause of pressure increase. This may be the vapor pressure of the reaction mass, the so-called tempered system. Pressure increase may also be due to gas release by a reaction, the so-called gassy system. There are also cases where the pressure stems from both vapor pressure and gas release, the so-called hybrid system, which may or may not be tempered. 

Computer programs are available for the dynamic simulation of venting [22, 23]. Leung [31,32] developped simplified methods for hand calculation for this purpose. The design method for safety valves is detailed in the work of Schmidt and West-phal [33, 34]. The design of emergency pressure relief systems is a complex matter and will not be treated in more detail in this book. 

For an open system, the vapor or gas velocities serve as assessment criteria. These velocities may be adapted and so are different of those encountered in emergency pressure relief, since here the aim is to control a runaway before it becomes critical. Criteria are provided for the velocities in a piping system and for the velocity across the liquid surface in a vessel, by assessing the swelling effect... 

Stoessel, F. (2006) Novel approach to emergency pressure relief design using calorimetric methods, in STK-Meeting (Swiss Society of Thermal Analysis and Calorimetry), STK meeting, Basle. 

Huff, J. E, A General Approach to the Sizing of Emergency Pressure Relief Systems, Reprints of Int. Symp. on Loss Prev. and Safety Promotion in the Process Ind., Heidelberg, Germany, 1977, p. IV 223, DECHEMA, Frankfurt (1977). 

An accelerating rate calorimeter (ARC) can be used to provide design values for emergency pressure-relief flow requirements of runaway systems. The ARC is a device used to obtain runaway history of chemical reactions in a closed system (DeHaven, 1983 Huff, 1982,1984a Townsend and Tou, 1980). The experimental technique is fairly straightforward, but considerable engineering expertise is required to do the calculations needed to design a relief system from the ARC data. 

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