Relief sizing

The relief sizing calculation procedure involves, first, using an appropriate source model to determine the rate of material release through the relief device (see chapter 4) and, second, using an appropriate equation based on fundamental hydrodynamic principles to determine the relief device vent area. 

The relief vent area calculation depends on the type of flow (liquid, vapor, or two-phase) and the type of relief device (spring or rupture disc). 

Experimental data at the actual relief conditions are recommended for sizing relief vents for runaway reaction scenarios. As always, manufacturers technical specifications are used for selection, purchase, and installation. 

In this chapter we present methods for calculating the relief device vent areas for the following configurations  

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TWO-PHASE FLOW RELIEF SIZING FOR RUNAWAY REACTION ... 

Pressure relief must be adequate to prevent internal pressures from rising over 10% above the maximum allowable working pressure, except when the excess pressure is developed by external fire or other unforseen heat source. See design details in later paragraph. Papa [85] proposes an improved technique for relief sizing. Also see Figures 7-7A and 7-7B. 

The relief size should be as large as possible, ideally occupying the whole of one side, and minimum vent area = 60 x area of side/EniaxCm ) (where P ax is the pressure that the plant can withstand without damage - if unknown, assume 140mbar). 

Explosion calculations, 499-504 Estimating destruction, 501 Overpressure, 502 Pressure piling, 501, 504 Relief sizing, 505 Scaled distance, 502, 503 Schock from velocity, 503 TNT equivalent, 499-504 Explosion characteristics of dusts, 515 Explosion suppression, 518 Explosion venting, gases/vapors, 504 Bleves, 504 Explosions, 482 Blast pressure. 496 Combustion, 482 Confined, 482 Damage, 498-501 Deflagration, 482 Detonation, 483... 

In this chapter we introduce relief fundamentals and the steps in the relief design procedure. Relief sizing methods are covered in chapter 9. 

For each specific relief all possible scenarios are identified and cataloged. This step of the relief method is extremely important The identification of the actual worst-case scenario frequently has a more significant effect on the relief size than the accuracy of relief sizing calculations. 

Physical property data and sometimes reaction rate characteristics are required for making relief sizing calculations. Data estimated using engineering assumptions are almost always acceptable when designing unit operations because the only result is poorer yields or poorer quality. In the relief design, however, these types of assumptions are not acceptable because an error may result in catastrophic and hazardous failures. 

The data acquired with this calorimeter are shown in Figures 8-9 and 8-10. Results of particular importance for relief sizing calculations include the temperature rate (dT/dt)s at the set... 

After the relief type has been chosen and the relief size computed, the engineer takes the responsibility for completing the design of the relief system, including deciding how to install the relief in the system and how to dispose of the exiting liquids and vapors. 

Referring to Problem 8-7, qualitatively describe the algorithm you would use to compute the relief size for this system. 

The first step in the relief sizing calculation for two-phase vents is to determine the mass flux through the relief. This is computed using Equation 4-104, representing choked two-phase flow through a hole ... 

Determine the relief size for spray dryers operating under the following conditions ... 

Consider Problem 9-9, part a. This time use alcohol as a liquid medium with a thermal expansion coefficient of 1.12 X 10 3/°C. The heat capacity of the alcohol is 0.58 kcal/kg °C, and its density is 791 kg/m3. Determine the relief size required. 

A horizontal vessel, 10 ft long and 3 ft in diameter, contains water. What relief size is required to protect the vessel from fire exposure Assume the following vapor relief only, M AWP of 200 psig, conventional spring-operated relief. 

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