Sizing of Pressure Relief Systems

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

Uncertainty is introduced at many stages in the sizing of pressure relief systems for runaway chemical reactions, and the application of a safety factor to offset this uncertainty may be appropriate. This Annex gives some guidance on the selection of an appropriate value for any safety factor. 

In order to predict the end of two-phase relief, the superficial velocity should be calculated based on the rate of depressurisation by the pressure relief system. (The size of the relief system will have to be known or guessed at this stage). 

HA Duxbury, "Comments on Safety Factors to be Applied When Sizing a Simple Relief System", Notes of Short Course on "Pressure Relief The Design of Pressure Relief Systems", Session 9, Loughborough University, 1996 (This was also published with same title as Appendix 13 in F P Lees, "Loss Prevention in the Process Industries", 2nd Edn., Butterworth-Heinemann, 1996, ISBN 0 7506 1547 8. However, note misprinted power in Equation A13.2 the last term should be Zmax1/2.)... 

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. 

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

If a scrubber is installed after PSV-lb and it has a pressure drop of 30 psig, how would this affect the size (qualitatively) of this relief system ... 

Figure 9-2 Viscosity correction factor Kv for conventional reliefs in liquid service. Source API RP 520, Recommended Practice for the Sizing, Selection, and Installation of Pressure-Relieving Systems in Refineries, 6th ed. (1993), p. 35. Used by permission of the American Petroleum Institute, Washington, DC.

In sizing depressuring valves, it should be assumed that heater burners are shut-off, reboilers are shutdown, and normal flow in the vessel has ceased. Vapor depressuring valves should be designed such that the initial, instantaneous depressuring flow rates do not exceed the capacity of the closed pressure relief system and the flare. 

The method employed for pressure relief system sizing will depend on the classification of the reacting system according to ... 

For tempered systems, the pressure relief system will almost always need to be bigger if two-phase flow occurs, and DIERS[1] recommended that two-phase relief should normally be assumed for vent sizing purposes using the type of hand calculation methods given in this Workbook. This is explained in 4.3.2, Subsection... 

To achieve (b), it is necessary to use relief sizing methods that take account of the dynamics of the pressure relief, event. Pressure relief systems for runaway chemical reactions usually discharge a two-phase mixture (see 4.3). If a steady-state calculation were used to size the relief system, then it would be necessary to size it for the volumetric rate of two-phase mixture equal to the volumetric, rate, of gas/ vapour generation at a particular point (e.g. at the relief pressure for vapour systems). This leads to very large calculated relief system sizes. 

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