Relief Design Scenarios

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

For steady-state design scenarios, the required vent rate, once determined, provides the capacity information needed to properly size the relief device and associated piping. For situations that are transient (e.g., two-phase venting of a runaway reactor), the required vent rate would require the simultaneous solution of the applicable material and energy balances on the equipment together with the in-vessel hydrodynamic model. Special cases yielding simplified solutions are given below. For clarity, nonreactive systems and reactive systems are presented separately. 

Gustin, J. L., Choice of Runaway Reaction Scenarios for Vent Sizing Based on Pseudo-adiabatic Calorimetric Techniques, Int. Symp. on Runaway Reaction, Pressure Relief Design, and Effluent Handling, AIChE, pp. 11-13, 1998. 

Unwanted reaction Clean and inspect equipment after each use Design with compatible materials contaminants. Maintain integrity of the system Design emergency relief system (ERS) for runaway scenario CCPS G-13 CCPS G-22 CCPS G-23 CCPS G-29... 

When designing reliefs for gas or dust explosions, special deflagration data for the scenario conditions are required. These data are acquired with the apparatus already described in section 6-13. 

Containment within the vessel for the credible worst-case scenario reducing the design requirements for the emergency relief system this step is frequently too expensive and impractical in a multipurpose facility. 

The design of a relief system often involves iteration and recycle. The flow chart in Figure 2.2 shows that possible recycle in the design process may involve changing the assumptions about the worst case relief scenario or changing the sizing method used. 

The procedure to determine both the basis of safety for the reactor (see Annex 1) and the worst case scenario for that basis of safety is iterative. The same screening tests which help determine the worst case for pressure relief sizing may lead to the conclusion that pressure relief is not the best basis of safety. The results of screening tests may also indicate that it is worthwhile to seek a more inherently safe solution by designing out the possibility of certain maloperations or system failures (for example, if the screening indicates that a very large relief system would be required). 

Is relief system design for the worst case scenario acceptable in terms of confidence in the design, cost and reliability ... 

The RSST calorimeter (see Annex 2) is a pseudo-adiabatic, low thermal inertia calorimeter, intended for screening purposes. It can identify the system type and measure adiabatic rate of temperature-rise and rate of gas generation by the reacting mixture. It is therefore well-suited to the task of selecting the overall worst case scenario from a small number of candidates. Alternatively, a calorimeter designed to obtain relief system sizing data may be used for this purpose (see Annex 2). 

The relief size obtained can be reduced by optimising the design parameters, such as relief pressure (see 6.2 below), vessel design pressure, or even changing the worst case relief scenario by designing out certain possibilities (see Chapter 3 and Annex 1). 

A reactor of volume 3.5 m3 has a design pressure of 14 barg. A worst case relief scenario has been identified in which a gassy decomposition reaction occurs. The mass of reactants in the reactor would be 2500 kg. An open cel test has been performed in a DIERS bench-scale apparatus, in which the volume of the gas space in the apparatus was 3,800 ml, and the mass of the sample was 44.8 g. The peak rate of pressure rise was 2,263 N/m2s at. a temperature of 246°C, and the corresponding rate of temperature rise was 144°C/minute. (These values include corrections for thermal inertia.) The pressure in the containment vessel corresponding to the peak rate was 20.2 bara. 

Note e.g. increase design pressure, take measures to change relief scenario. Check that design changes do not alter whether or not the hybrid is tempered, e.g. increasing the maximum accumulated pressure could cause a reacting mixture to no longer temper. 

For the purposes of the Workbook, the worst case scenario is the credible combination of equipment failures and maloperations that gives rise to the largest calculated relief size compared with other credible scenarios. See Chapter 3. The worst case scenario is the basis for the relief system design. 

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