huangyhg

methane (blast) containment pit
need to design a concrete cover for a concrete methane spill containment pit (5'x25'x4'deep). it's sized for 3' of spilled methane with 1' extra to the bottom of any vents.
is there an industry generated code for the possible explosion pressure? what about depth of concrete cover for fire protection of reinforcing steel? we would like to place vents along two sides to direct flames away from the storage tank and pumps. how big should the vents be?
you probably need to involve a engineer more familiar with thermodynamics to determine the amount of "explosive" force that might be associated with the expanding volume of gas. i seems to me that heat would not be the significant factor in the initial "explosion". in other words, one load case would involve self wt of the cover and the pressure of the expanding gas, and another load case would involve self wt of the cover and exposure to high temperatures.
with regard to fire resistant structural design, you'll have to do some research (there's plenty on the web). you'll have to consider:
what is the temperature of the fire (depends on the type of fuel - methane)?;
how long will the cover be exposed to the heat after the initial reaction (explosion)?;
do you expect the cover to remain serviceable after exposure to a "containment event" (chances of the event occurring vs. cost of the cover)
if the cover design indicates that the self wt of the cover is not sufficient to counteract the pressure of the explosion, and if it's not economically feasible to design the cover to that effect, then vents would probably be designed considering fluid dynamics with regard to flow rate, pressure, vent opening, serviceability, etc.
where safety against personal injury is a concern, you should increase any safety factors accordingly.
it's an interesting question, and i wish i could have been of more (or some) help.
good luck.
i haven't performed that kind of design, but have seen the effects of that kind of event. rowefff"> is correct - the blast (pressure) is the biggest problem. the event is generally over damn quick, so the heat has very little time to act.
at least that's what i have seen in the aftermath of pipeline explosions, anyway.
they've abandoned the lid portion of the design, but we now need to build a cast-in-place concrete wall adjacent to the pit to shield other equipment from possible explosions.
the stored material is methanol (not methane, my mistake) and i have found references to methods for quantifying explosion pressures in nfpa 30, 68 and 69. they mention determining an "energy-equivalent mass of tnt, but i haven't found a reference yet that relates methanol to tnt. and, of course it makes a difference how big the vapor cloud is and how close you are to it.
focht3 is right, it won't burn for long, and nfpa gives clear cover recommendations for concrete exposed to heat (fire), so i feel certain that i can protect the reinforcing. still, i need to come up with a reasonable and rational design pressure.

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