Q: Can I limit the fire scenario for Pr. Relief sizing?

[hollerg]

I have read that pool fires consume 3-4 mm of fuel per minute. I am reviewing the relief requirements in a moderately sized batch plant. The area has a drain that is maintained open, but some material will pool until the spilled volume drains away. The liquids in the area are 1B flammables

A) Is it proper to limit the fire scenario to the time to burn off/drain the maximum spill inventory within that containment area, in smaller plants?

Assume that there are remote (hand operated) valves that exist for sources to the area, but that the access to those valves is clear because they are larger but remote, raw material storage tanks.

B) If I compute that the vessel cannot be heated to the disk burst point by that fire scenario, is this no longer a credible case?

C) Is there any design approach for jet fire impingement, or would I need this only when handling Class 1A flammables or pressurized gaseous processes?

D) For a reduced duration fire, does anyone have data on how long a stainless steel over-jacket on 2" of fiberglass insulation can be counted on to last before it has no insulation value?

E) Do any companies say that making allowances for the duration, as described above is considered good engineering practice?

 

[KenA]

Hollerg,
Am sure you'll get plenty of feedback on this subject, but here's by two-penny's worth:
Gut feel is that you're skating on pretty thin ice when you try and convince yourself (and safety authority) that a fire will not overpressure a vessel because the fuel source will burn off/drain before the relieving pressure is reached.
Secondly you don't normally take credit for insulation unless it is fireproof but refer to API-520 Appendix D for a table of environment factors (F) for insulation.
In my (limited) experience you don't normally consider the type or duration of fire for relief on vessels. There's either a fire or no fire. You follow API-520 sizing criteria, which uses vessel surface area to determine heat abdorption and hence relieving rate and required valve area.
Hope this helps - sorry I didn't address your questions one at a time.
k

 

[CHD01]

hollerg: YES! You can consider the maximum duration of a fire! KenA is right that insulation has to be fire proof to take credit for it - but avoid the other comments since they do not cover all situations - and remember that you have to consider the vent rate based on the wetted area; but that's OK he qualified his comments by saying he's had limited experience (I went through the same thing, we all do. The other thing is that we never know it all)

The first issue is certainly whether or not a fire will occur. The next issue is how many vessels/tanks are contained in the fire perimeter. If there are multiple vessels/tanks then the capacity of each and the time required to reach relief conditions is an important factor for each tank/vessel and determines the peak release rate for design of the vent header or flare installed.

Last comment, even for single vessels/tanks; API standards themselves considers this issue of the fire duration for a single vessel/tank. Since if a tank is large enough; an emergency vent for the case of a fire exposure may not even be required for the tank (per API). Further, even if the tank is not large enough, if the tank is greater than 30ft above grade (or 25 ft in some standards) you do not count this against the exposed area to fire either. Thus if you had a vessel on the 5th floor of a plant 100 ft above grade AND a containment area was not beneath the vessel - fire exposure would not be a case; unless you had a flame shooting out of a pipeline or something really wild like that striking the wall of a vessel.

The more you learn, the less you are certain of.

 

[hollerg]

The vessels are 3000 gallon so they are too small to preclude from complete involvement (BTW, I believe the API tank standard precludes a fire case only if a Class IIIB fluid stored at least 30 F below the flash point) and it is completely within 30 foot of grade.

If I assume the spill and fire cover the 1600 sq ft inside the curbed floor, and the fire has 2500 gallons (different vessel) of spill, then for my case the fire would last only 2-8 minutes around any one vessel, which depends on the location and profile of my containment area.

Also, I was only asking about fiberglass because of the 2-8 minute duration of the fire. Although the code does not allow me to claim an environmental factor for the fiberglass insulation, in sizing, I might claim that fire case was not credible if the fire could not result in enough heat gain to burst the disk or pop the valve.

The question dovetails with the question about the length of time the heat is applied. Certainly for a limited time it is preventing heat transfer, even if it is only until the heat of fusion enters and melts the insulation. When all you need is <10 minutes then it might be worth knowing, unless there is a hidden assumption I am missing.

 

[KenA]

Just for my knowledge - perhaps CHD01 could clarify a couple of points.

(1) The original question regarded potential for reducing PSV sizing by taking into account the duration of the pool fire. Whilst I agree that the relief header sizing will of course be affected by the dynamics of the fire (and you may be able to reduce the size by making certain assumptions and calculations for the results of a pool fire) I am still not sure that the individual relief valve sizes will be affected. Once again I say that you're taking a big risk if you rely on calculation to show that the vessel cannot be heated to the disk burst point by that fire scenario, and thus does not require overpressure protection.

(2) Are you refering to API-520 when you talk about API code allowing no relief if a tank is large enough that an emegency vent may not be required for fire exposure.

You're right - The more you learn, the less you are certain of!
k

 

[CHD01]

KenA: I did not mean to offend - apologies offered! I hope I still recognize that I am just as limited in what I know - so this is why I selected my motto on memos; to try to remind me of this.

Yes, I should have stated what API standard I was referencing, it is from API-2000.

Also, my comments where for emergency vents used for exposure to fire; conservation venting for breathing of the tank due to thermal changes and pump-in/pump-out still must be provided for; even if an emergency vent is not required.

But, yes I stand by my comments, and this is supported by recognized good engineering practices.

API2000 standard basically supports the following emergency vent exceptions:

EXCEPTION #1:
Emergency venting for fire exposure is not
required if the tank is larger than 12,000 gallons
US and flash point is not less than 200 Deg F; and
tank is not exposed to other liquids with flash
point <140 Deg F

EXCEPTION #2:
If MAWP of tank is not greater than 1 psig, no
increase in venting is required for wetted areas
greater than 2800 ft2. However, API2000 requires
an increase in venting for Refrigerated Tanks even
if pressure is < 1 psig.

NOTE:
API2000 also does not require additional emergency
venting for tanks with qualifying weak roof-to-shell
attachments.

IN my industry we choose not to use weak roof-to-shell tanks; and rely on API520 and API 650 contruction for tanks with relief devices installed.

In addition to API2000; API520, Part 1, in section 3, 4 and Appendix D is a good source of information on fire sizing;
plus take a look at API521, section 3 (in my older copy its in sections 3.11, 3.12, 3.15, and 3.19). These sections address most of the issues required for fire sizing.

hollerg: Use API520 to assist in determing the amount of heat that the vessel is exposed to, from that determine time required to reach saturated conditions at set point of vent and relief requirements (with and without insulation; you could remove insulation for inspection purposes).

Last, remember that we are responsible for the design, standards that we have discussed are often guidelines (sometimes requirements); when guidelines, we may disagree with them and elect to provide greater protection (or less?). When we make an engineering judgement such as this, we need to document it and defend it if an event occurs. If we are not comfortable doing this then we of course should take the safer route.

The more you learn, the less you are certain of.

 

[CHD01]

One correction, API construction codes are API620 (not
API 520) and API650.

The more you learn, the less you are certain of.

 

[KenA]

CHD01,
No offense taken - just want to learn more.
I see where you're coming from now. API-2000 is for atm storage tanks as opposed to vessels. That's where I was confused. For vessels I don't know of anything which allows you to limit relieving rate due to either vessel volume or fire duration considerations.
I was most concerned with point (B) where it appeared that vessel relief was not required because pool fire was calculated not to exceed set pressure of vessel.
k

 

[CHD01]

API-2000 is for atmospheric vessels (<15 psig); but API520 and API521 is for vessels > 15 psig; so any comments on limiting rates for pressure vessels in API520 and API521 would apply.

To avoid confusion, I prefer to refer to vessels designed per either API620 or API650; and for less than 15 psig as a TANK; and call a vessel a VESSEL when it is designed per the ASME Code Section VIII, for 15 psig or greater. Next, I prefer to limit the term RELIEF VALVE to a device with a set pressure of 15 psig or greater; and use the term VENT for a device with a set pressure of < 15 psig. Very rarely, you will find a RELIEF VALVE set for < 15 psig - but not often. Then for VENTS, it may be either an EMRGENCY VENT, a CONSERVATION VENT, or a PRESSURE or VACCUM VENT.

The more you learn, the less you are certain of.

 

[CHD01]

KenA: I dug up some additional info for you regarding the fire sizing of relief devices for pressure vessels designed for 15 psig or greater that you may find of interest.

Particularly notice how Q varies with the size of the vessel. Again, you can refer to the API standards I mentioned (notice that Figure B-1 in Appendix B of API2000 is used to determine fire exposure for pressure vessels as well). You can also find similar info in many relief vendor catalogs and programs:

The following equations are used:

W = (Q*Kp)/L

w = pph flow, Q = BTUH from fire,
Kp = Protection Factor and
L = latent heat in BTU/lb

SCFHa = ((1100*Q*Kp)/(C*L))*SqRt(z*T/M)

C = 520*{k[(2/(k+1)]^[(k+1)/(k-1)]}
k = sp ht ratio, z = compresibility factor,
T = temperature in Deg R, and M = Mol Wt

Wetted surface area of tank is based on 55% of the total exposed area for a sphere, 75% of the total exposed area
for a horizontal tank (assuming 90% fill), and the first 30 ft (API is 25 ft) above grade of the exposed shell area of a vertical tank.

The following equations are used for fire exposure (again - this is actually in API2000 as well, but recognize there are still differences in fire sizing for tanks vs. vessels):

The following is heat input for pressure vessels:

Q = 21,000*A^0.82, for calculation of heat input for wetted areas > 2800 ft2,

Q = 963400*A^0.338 for 1000-2800 ft2,

Q = 199300*A^0.566 for 200-1000 ft2 and Q = 20000*A for <200 ft2 per API Standards.

Last, note that there are exceptions. One exception is for Ammonia per Ammonia Std. K61.1. Another is for Liquified Petroleum Gases like propylene, propane, etc; per OSHA 1910.110 Standard. It also does not apply to cylinders and portable containers and DOT vessels, nor to vessels relieved at above critical conditions.

The more you learn, the less you are certain of.