7 replies to this topic

[manizheh]

Hi Dear Friends,

In case of fire in a unit related PSVs of equipment located in fire zone will be discharged.What are the most credible fire senarios which may happen and related radia that will be impacted.I mean How I divide a unit to fire zones?

Thanks for your advise
Manizheh

[gvdlans]

I cannot give a general answer to such a general question. You have to look at your specific situation, use your engineering judgement and then come up with credible fire scenarios and related relief cases for that specific situation. period.

My recommendation is to get a copy of an article in Chemical Engineering by Wing Y. Wong, "Fires, Vessels and the Pressure Relief Valve", May 2000, pages 84-92.

[pleckner]

I have to second gvdlands for part of his response. If you have the fire triangle, then you have a fire scenario and only you can determine this.

I can tell you that you only need to consider a given elevation above a sustainable fire. By this I mean the fuel must be able to pool within a certain distance below or around the vessel. If the vessel is on a grating, then any spills will fall through the grating to the next solid floor below. That is where the elevation is determined from.

What this elevation is depends on a number of sources. However API RP521 recommends 25 feet. In other words, you consider the first 25 feet above the sustainable fire to determine the exposed surface of the vessel engulfed in the fire.

As far as plot area goes, API RP521 suggests (but is not limited to) a plot area of 2,500 to 5,000 square feet.

[latexman]

A tank of flammable or combustible material can be overfilled, and the pool formed in the dike can ignite.

A pump seal fails, leaks, and the pool formed can ignite.

A gasket in a line can fail, leaks, and the pool formed can ignite.

While doing a crane lift over equipment or piping that contains flammable or combustible material, the equipment or piping is accidently hit. It leaks and the pool formed can ignite.

Backflow from the process can overfill a tank of flammable or combustible material. The pool formed in the dike can ignite.

Following maintenance, a valve is inadvertantly let open. A pool forms and ignites.

The above are just some examples. There are many, many others.

In all these cases, dikes and/or curbs and drains (or the lack of these things) will define the dimensions of the pool fire. These dimensions coupled with an acceptable separation from the fire (see pleckner's reply) will give you the answers you seek.

[manizheh]

I cannot give a general answer to such a general question. You have to look at your specific situation, use your engineering judgement and then come up with credible fire scenarios and related relief cases for that specific situation. period.

My recommendation is to get a copy of an article in Chemical Engineering by Wing Y. Wong, "Fires, Vessels and the Pressure Relief Valve", May 2000, pages 84-92.

Dear gvdlans,

Could you please provide me with your recommended article?

Thanks for your help

[gvdlans]

I am not a library but can recommend you: http://www.lindahall.org/

[manizheh]

I have to second gvdlands for part of his response. If you have the fire triangle, then you have a fire scenario and only you can determine this.

I can tell you that you only need to consider a given elevation above a sustainable fire. By this I mean the fuel must be able to pool within a certain distance below or around the vessel. If the vessel is on a grating, then any spills will fall through the grating to the next solid floor below. That is where the elevation is determined from.

What this elevation is depends on a number of sources. However API RP521 recommends 25 feet. In other words, you consider the first 25 feet above the sustainable fire to determine the exposed surface of the vessel engulfed in the fire.

As far as plot area goes, API RP521 suggests (but is not limited to) a plot area of 2,500 to 5,000 square feet.

Dear pleckenr,


Thanks for your very useful reply.What I underestand is,I shall consider the souce of fires for example vessels including flammable liquids for the case of leak and forming pool fire .I shall cosider 25ft above the level of sustaining pool fire,and upto 5000 ft2 of surface around the fire source.If any equipment which has PSV and located in this height(25 ft) and surface(5000 ft2) of fire source shall be considered in case of external fire relief scenario.What about the other fire scenarios,I mean just pool fire shall be considered?or other fire scenarios shall be considered too.For example jet fire,explosion and so on?And what about the overlaps of the fire zones?If one equipment located in two fire zones and zones have overlap with each other the PSV load shall be considered in both? and for the headres sizing the load which is the most among all fire zones is consedered for sizing?

Thanks for yoy advice
Manizheh

[pleckner]

Let's not get too carried away with these fire cases. Should I design for an airplane crashing into my facility? If there is an explosion in the area, my equipment is destroyed, fire relief is the least of my concerns.

The fire case is meant to be a sustainable fire, not a momentary fire-ball as such. If I can't have a pool fire or be engulfed in flames from a near-by fire of combustible material, then I do not have a credible fire scenario.

Multiple and overlapping fire zones is a definite possibilty and very common. The fire calculation is the same for a given vessel but the way it affects the headers (if the PSVs from multiple equipment are indeed connected) will be different for each zone and each zone must be handled independently. Assuming overlapping fire zones but a single header, then the header size is determined from the zone producing the greatest flow.