Effect of solids on relief system flow capacity

3 Effect of solids on relief system flow capacity 

Two possible effects of solids on the relief system capacity are  

If solids are allowed to accumulate on the underside of a bursting disc or safety valve, then it is likely that the relief device will not operate when required, at least not at. the required set pressure. Safety valves may be more vulnerable in this respect than bursting discs, and it is common practice to fit a bursting disc upstream of the safety valve to protect it. Further information is given in 10.2.1 and reference 1. 

Provision should also be made to minimise solids build-up under a bursting disc. If the solid may be present due to freezing, then heat tracing may be a solution. In other cases, it may be beneficial to take the relief system off a liquid inlet nozzle (of suitable size) so that the nozzle is washed clean of solids by the liquid. 

If it is not possible to be confident that the relief system will operate without blockage, then, consideration should be given to installing further measures to prevent runaway occurring and/ or the use of alternative measures to mitigate its effects (see Annex 1). Unfortunately, the same mechanisms by which a relief system may be expected to block may also cause blockage of pressure or temperature measurement points within an instrumented protective system, so care should be taken, in such cases. 

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