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steam quantity from a tank

steam quantity from a tank

(OP)
Hi,

If I have a simple vessel with water in it, say 1000mm dia by 2000mm o/heads.
Under any and all operating or fault conditions that we can identify the PRV, if set at the MAWP pressure for the tank, would never operate.
However, if there is a fire the water will get heated and could turn to steam and so pressurise the tank, is there a standard way to calculate what steam would get produced so that I can estimate the correct PRV size ?

Thanks in advance for any guidance.

RE: steam quantity from a tank

What is the fire loading for the room? If you know that, you can probably figure out heat in to the boiler with basic heat transfer equation. Once you have heat in to the boiler, you can calculate steam production rate.

RE: steam quantity from a tank

(OP)
MFJewell - Unfortunately I don't know the rating of the room. The customer has brought this up after the event as we supplied a vessel with their specified size of PRV on the vessel, now they are asking us to help them justify what they asked us to do ! The "room" is a pump house of steel frame construction/roof with electrically driven pumps in, I don't know how to rate this is, there some spec./ code I can look ? I have RP520 and RP521 which provide guidance, but these specifications don't talk about room rating, but come at it from a different angle.

RE: steam quantity from a tank

I've never had to perform fire load calculations as that was always done by a fire protection specialist (I was a fire service engineer at a nuke plant though). If I remember right, NFPA has some guidance to figure fire load. You will probably have to make some assumptions since amounts of combustible materials and their individual fire ratings will probably be needed.

RE: steam quantity from a tank

You're right that ASME doesn't have a detailed method for sizing the relief valve for fire conditions. I'd recommend that you take another look at API 521. Heat absorption rates are provided in charts based on the wetted area of the vessel. From this you should be able to figure out the steam rate, and size the relief valve appropriately.

If this doesn't work, I think you may be looking at a more detailed analysis based on the room fire load to develop a reasonable heat generation rate.

I hope this helps!

Jim Breunig P.E.
XCEED Engineering
Mechanical Engineering
FEA Consultants

RE: steam quantity from a tank

API-2000 includes assumptions about fire exposure and heat input from fires around large outdoor tanks, unfortunately, I don't know of any way to confirm if that is adequate for an inside application.

RE: steam quantity from a tank

Why not just use a bursting disc? So long as you vent it to somewhere safe it's only steam not a flammable vapour cloud.

If it's that rare why bother trying to size this?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: steam quantity from a tank

Dumb question maybe...if a bursting disc is used to discharge the produced steam into the protected space could it not serve as smothering steam to extinguish the fire that produced it in the first place? Obviously for safety reasons prior evacuation of the protected space by all personnel would be a necessity...perhaps a tank pressure alarm switch could activate a warning horn to advise the internal pressure of the tank is approaching the release point...dumb question perhaps because this isn't at all my area of expertise; just spit-balling.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: steam quantity from a tank

According to the size of the tank and water contain, the most heat absorption and amount of steam is in the approx. 200000Btu.hour and 200 lbs.steam per hour. So a STD 3/4 in pressure relief valve will do. You choose the pressure depending g of the system pressure plus 10% . I would use a 400,000 BTU STD pressure and temperature 125 psi valve yr water heater prv.

General Blr. CA,USA

RE: steam quantity from a tank

The OP should give us more information about the location of the tank in relation to the potential fire risk, otherwise, to answer the question is futile. Sure there are guidelines about sizing relief valves due to external fire exposures but these guidelines are specific to actual physical conditions and tank content.

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