Publication Details

Technical Paper TP16


Why API Recommended Practice May Lead to Unsafe Design of Pressurised Systems

Authors: S Medonos, Petrellus Ltd, Dr G Berge, Petrell as. Proceedings of the Major Hazard Offshore Conference, 2001, London, United Kingdom.

Abstract

Pressure vessels that are exposed to fire have a substantial impact on safety in all process industries. In the oil and gas industry offshore and onshore API 521 and API 520 Recommended Codes of Practice have been the basis for design. Protection systems that have been applied are emergency shutdown systems (ESD) combined with blowdown (BD), bursting discs and pressure relief valves with set relief pressure, and passive fire protection coatings. The code of practice being used is not well documented and decisions whether or not pressure vessels should be fire-protected have been made on the basis of insufficient criteria.

The main problem with the applied recommended codes of practice is that they use far too low fire loads. The codes were originally developed for refineries and they are based on pool fires with no regard to stress distributions in the vessels. In reality, due to a higher heat load transferred into the vessel, the strength of vessel steel is reduced faster than it is applied in the codes. The operator has to rely on that the ESD/BD system is functional and activated to protect the vessel by depressurisation, but the ESD/BD system may not be sufficiently robust to depressurise quickly enough. The pressure relief valve (PSV) is not effective and the pressure vessel bursts before the valve opens, due to the weakening of the material in the vessel shell as temperature increases. In order to maintain the vessel integrity the vessel has to be fire protected, but whether or not the fire protection is sufficient to prevent the boiling of the vessel inventory is not known. A burst of pressure vessel may lead to escalation and a cataclysmic “domino effect” fire throughout the plant.

The problem is further augmented by the fact that in the current design practice plant safety systems are designed in isolation from the actual behaviour of the plant when attacked by a fire.

There are more than 30 variables that need to be taken into consideration when designing a pressurised system with a blowdown. The inter-dependency of these variables is complex, which makes the use of graphs or nomograms practically impossible. A sufficiently robust design can only be achieved by a time history simulation of coupled processes of heat transfer, thermodynamics of vessel inventory and stress.

The API outlines the sources of overpressure and combined effects of pressure, temperature and inventory composition. The Recommended Practice also contains a caveat that in some fire attack circumstances a more rigorous (detailed) approach is recommended. This seems, however, to be ignored by the engineering community. Whenever appropriate, the engineering community should use rigorous methods and be familiar with the methods before the methods are applied.

This Paper gives up-to-date information on these subjects together with recent research initiative and results from response simulations of pressure vessels attacked by a fire.


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