Relief System Design

[sgoel]

Hi

What is the basis for sizing of a relief header and flare for a petrochemical unit. Specifically speaking:

1. Should the design load be equal to summation of all relief duties attached to header or should it be based on maximum release connected?
2. What should be the criteria for selection of relief load, should it be based on fire case only. If it is to be sized for utility failures, than should two utility failures at a time be considered?
3. Relief duty in case of utility failure may be much lower than fire case, [This is because of self cooling with in the towers etc]. What should be design criteria in such cases?

Thanking you in anticipation.

regards

 

[Dijla1]

Get yourself a copy of API-RP-521, it gives all the information you need about sizing a relief header and flare.

 

[MikeClay]

1. The relief rate should be based on the maximum release from any credible scenario. Whether it be fire, utility failure, blowdown...

2. If sized for fire, assuming the system is not blownd down, then the system should be sized for the relieving of all vessels in the section. The API code allows for a reduction of flow from other involved areas due to possible timing considerations, but this should be looked at closely.

3. Only common mode utility failures need to be looked at. For example, if an electrical failure takes down your cooling water circulation pumps and air compressors, then you would have to include all three.

 

[ihg]

And if you want to get scared, try reading the recent UK Institute of Petroleum (now Energy Institute)guidelines at

http://www.portlandpress.com/pcs/books/prod_det.cfm?product=0852932790

which discuss jetfire, etc.

"Mitigation of the impact of severe fires on hydrocarbon, petrochemical and chemical processing plants is critical to minimise the risk to personnel, to reduce damage and to limit capital loss.

These new guidelines are intended for design and process engineers concerned with large, essentially fully enveloping pool fires and jet fire impingement on pressure vessels, their associated pipework, valves, flanges and other equipment, referred to collectively as pressure systems.

The scope of the guidelines covers a wide range of steel pressure vessels used both onshore and offshore. These include process vessels, fixed storage vessels and transportable vessels such as road and rail tankers whilst at the loading/unloading facility. (These guidelines do not apply to small portable pressure vessels such as gas cylinders as these already have specific requirements related to their portability).

These guidelines are intended for use primarily for designing new facilities and specifically deal with fires that are more severe than the open pool fires currently covered in other documents. These guidelines are therefore intended to be used in conjunction with the existing codes and recommended practices such as API RP 520 and 521, which cover the design and fire protection required for all other fire scenarios."
API 520/521 are NOT necessarily conservative documents.