12 replies to this topic

[Nasiruddin]

Dear Friends,

I am working on Fire Relief PSV sizing for finger type slug catcher (plot area approx 30m x 60 m = 1800m2), to calculate wetted area for Fire relief PSV sizing, anyone please will share what pool fire zone area should be considered for this purpose.

Regards,
Nasiruddin

[fallah]

Nasiruddin,

Your query isn't so clear. The whole fingers of the slug catcher is normally considered in a single fire zone. The wetted wall is normally calculated based on full condensate level within 8 m height. Hope this can help you out.

Fallah

[Nasiruddin]

Dear Fallah,

Thank you for your response. As per API 521 Section 7.1 for fire relief PSV load calculations, fire zone area considered is 230m2 to 460m2. In my case the plot area is 60*30=1800 m2, which is almost 4 times more than the API specified maximum area (460m2). Will you please provide any reference to consider whole finger length.

Slug catcher I am considering having 12 fingers (diameter of each finger is 48") and length of each finger (separation+storage section) is approx 57 m.

Regards,
Nasiruddin

[fallah]

Dear Nasiruddin,

The extent of a fire zone is normally a function of the design and installation lay out such that to be able to confine all possible fires within the given zone which in your case for mentioned slug catcher would be around 1800 m².

IMO, extent of a pool fire itself is another story and as you mentioned in API 521 Section 7.1 can be confined to a ground area of 230-460 m². Then whole slug catcher are to be considered in a single fire zone but in order to calculate wetted wall area in fire relief load determination you should consider mentioned limited ground area (normally a cylinderical volume of 20 m diameter and 8 m height).

Fallah

Edited by fallah, 01 October 2012 - 04:20 AM.

[paulhorth]

Nasiruddin,
I agree with Fallah's first post. A slugcatcher is a very large single item, one which always represents a major fire risk. You cannot limit a fire zone to include only a quarter of a single item of equipment!
You should not allow the wording of an industry code to override your duty to think about the hazards in your system. What API RP521 actually says is:

To define the combined relieving loads under fire exposure, the probable maximum extent of a fire should be
estimated. As a conservative approach, in the absence of any other governing factors, consideration of a fire impact
area is frequently limited to a ground area of 230 m2 to 460 m2 (2 500 ft2 to 5 000 ft2).

This does NOT say that the maximum extent of any fire is 460 m2. It does say that it is your responsibility to make the estimate. In the case of a typical slug catcher, such as yours, the probable maximum extent of the fire, in my view, has to be the entire layout area of the equipment item. What could limit a pool fire to a smaller area? There is certainly enough liquid in the slugcatcher to cover the entire 1800 m3 to a depth of about 0.4m, making a fine pool fire. If you want to use a smaller extent, you would have to demonstrate that the ground under the equipment is partitioned by bunding and that each bund is sufficiently well drained, that, in an emergency with no power for the drain pumps, it is not possible for a liquid pool to fill more than one of these sections.
I have worked on a number of projects with slug catchers and have never seen such a practice adopted.
Conclusion: your fire relief load has to be based on the wetted area of the entire unit. Don't forget also that there will be a fire case blowdown to consider also, based on this same heat input. The blowdown load from a slugcatcher is usually the largest blowdown load on the plant and often sets the required capacity of the flare system.

Paul

[fallah]

Paul,

My comments on your post are in blue as follows:

What API RP521 actually says is:

To define the combined relieving loads under fire exposure, the probable maximum extent of a fire should be
estimated. As a conservative approach, in the absence of any other governing factors, consideration of a fire impact
area is frequently limited to a ground area of 230 m2 to 460 m2 (2 500 ft2 to 5 000 ft2).

This does NOT say that the maximum extent of any fire is 460 m2. It does say that it is your responsibility to make the estimate.
Please note that the above statement of API 521 will be continued as: "...A more detailed analysis can show a smaller fire impact-area..."
In the case of a typical slug catcher, such as yours, the probable maximum extent of the fire, in my view, has to be the entire layout area of the equipment item. What could limit a pool fire to a smaller area?
Please note that the 230-460 m² is estimated based on a conservative approach...
There is certainly enough liquid in the slugcatcher to cover the entire 1800 m3 to a depth of about 0.4m, making a fine pool fire. If you want to use a smaller extent, you would have to demonstrate that the ground under the equipment is partitioned by bunding and that each bund is sufficiently well drained, that, in an emergency with no power for the drain pumps, it is not possible for a liquid pool to fill more than one of these sections.
I have worked on a number of projects with slug catchers and have never seen such a practice adopted.
Conclusion: your fire relief load has to be based on the wetted area of the entire unit.
IMO, no need to do so and based on above it is adequate to consider the mentioned limited area as per API 521.
Don't forget also that there will be a fire case blowdown to consider also, based on this same heat input. The blowdown load from a slugcatcher is usually the largest blowdown load on the plant and often sets the required capacity of the flare system.
Finger type slug catcher is considered as a pipeline (governing code B31.8) rather than an equipment; hence it shall not be equipped with an EDP system but instead a remote opening valve with flow orifice would be provided to vent the slug catcher. Of course it shall be sized such that not to exceed the maximum load of HP flare network means it might not be a base for required capacity of the flare system.

Fallah

Edited by fallah, 01 October 2012 - 06:52 AM.

[Nasiruddin]

Dear Fallah and Paul,

Thanks a lot for your input it really helpful for me.

Regards,
Nasiruddin

[paulhorth]

Fallah,
With respect I must disagree with you on two points.
Fire case relief
I repeat that unless there are physical means to limit the extent of a pool fire under the slug catcher, the fire exposure clearly extends to the whole unit, regardless of the numbers quoted in API RP 521. What is to stop it? The fire will spread wherever it can with the available supply of fuel.
Blowdown of a slug catcher
I have worked on at least 5 gas terminal projects which have included slug catchers, which have progressed through to construction and operation, and also on several more through the FEED stage. In every case, the slug catcher was fitted with automatic depressuring (blowdown) valves, actuated from the ESD system, even though the mechanical design was to the pipeline code B31.8. This was the clear requirement of the client operating companies. The time criterion for the blowdown was typically extended for longer than the customary 15 minutes for process vessels,, to 30 minutes or maybe 1 hour, to limit the peak flowrate, but in every case this time extension had to be justified by a study to show that the walls would not reach rupture stress during this longer exposure time. In one case, there were no fewer than eight blowdown valves which opened in sequence controlled by a timer. The control logic for this sequence had to meet stringent reliability criteria and be separate from other control systems in the plant.
The slug catcher is a major hazard on any gas terminal. Although it may be designed to a pipeline mechanical code,It is different from a pipeline in at least three respects:

(1) It is abve ground and thus exposed to fire, which a pipeline usually is not

(2) It is designed to be full of liquid, which a gas pipeline is not

(3) It can be blocked in, while a pipeline is typically open at one or both ends and has a much greater volume.

To Nasiruddin I would say that it is the responsibility of the process engineers to demonstrate that their design meets the risk criteria of their clients and good industry practice, and the profession demands more of us than just applying some generic estimated numbers quoted in a code .

Paul

[RoyenG]

Dear Nasirudin,

there is a project we have done before which is installing finger type slug catcher due to the high capacity of the fluid.

fire case is applicable, but blocked discharge might be the governing case. the wetted area for fire case can be considered from the collecting section at the end of the pipe.

for blowdown, staged blowdown with 3 blowdown valves installed to handle the slugcatcher including piping, and 20 minutes for each blowdown (depend on the pipe class). Blowdown orifice are adjusted so the total flowrate while three blowdown valve opened does not more than maximum flare capacity.

Regards,
Royen

[fallah]

Paul,

I respect to your belief regarding the maximum extent of a fire and blowdown system in slug catcher area; but i have to submit my own opinion in this regard as follows:

Fire case relief

In most projects the clients normally specify the fire exposure area as a design guide for uniformity. If there would be no such guideline, for fire relief load calculation as per API 521 it can be assumed around 230-460 m² (a cylindrical volume with diameter of around 20 m). Of course there may be some exceptions: e.g. if flammable liquid can be able to be contained within an enclosed diked area, let say for a tankage system, whole mentioned diked area regardless of to be in above range or not is to be considered as fire exposure area.
It should be noted that naturally the most credible scenario for slug catcher is jet fire and a pool fire would be occured just in its spillage pit if there was such facility there.

Blowdown of a slug catcher

What i mentioned in my previous post would do the same job as you stated for slug catcher depressurizing.

Fallah

Edited by fallah, 02 October 2012 - 05:11 AM.

[kkala]

Following this interesting topic, found extract from a local refinery practice says in brief that:
Single fire risk areas in a plant, after consideration of layout and drainage capabilities, should be selected in such a way as to limit the extent of each area to 5000 ft2 or less. However, the extent of the fire risk area must be based on the actual drainage and spacing that is available and may result in areas greater than 5000 ft2.
This seems to be an interpretation of API RP521 extract* (with additions in wording), agreeing with paulhorth on the pool fire area to be considered.

* note: probably of older version, 2500 ft2 min area is not reported.

Edited by kkala, 06 October 2012 - 09:01 AM.

[fallah]

Kkala,

The below API 521 statement is so clear such that no need to any interpretation:

"To define the combined relieving loads under fire exposure, the probable maximum extent of a fire should be
estimated. As a conservative approach, in the absence of any other governing factors, consideration of a fire impact
area is frequently limited to a ground area of 230 m2 to 460 m2 (2 500 ft2 to 5 000 ft2). A more detailed analysis can show a smaller fire impact-area."

Indeed, API mentioned "frequently" means: it is obvious that if there would be no drainage facility (nowadays cannot be found) the pool fire can be extended to the area confined by the relieving fluid.

Fallah

[kkala]

Recommended practices, Fallah, often leave a window open to alternatives. I think this is presently expressed mainly through the condition "in the absence of any other governing factors", the latter being max area covered by leaking liquid plus affectable area around it; and secondarily through "frequently limited" (smaller areas usually result, but bigger ones are not excluded).
If this slug catcher was a new installation, one could place the equipment in four dinstict fire zones (each containing 3 figures); this would affect layout and drainage systems, but would seriously reduce fire water demand.
Note: http://www.cheresour...-slug-catchers/ '> http://www.cheresour...-slug-catchers/ advises on slug catcher containment.

Edited by kkala, 11 October 2012 - 04:20 PM.