Hydrogen Explosion cont'd

DOE-HDBK-3010-94

7.0 Application Examples; Reduction Line Example

initial baseline for evaluating the process-specific context of the explosive hazard.

Seriously contemplating a detonation involving all 37 g of hydrogen would be

inappropriate, as discussed below.

At an ambient temperature of 27 oC, one mole of hydrogen gas ideally occupies a

volume of 24.5 l, and 18.5 moles would occupy ~ 450 l. Allowing for 25%

equipment occupancy, the gas volume of the reduction glovebox is 30,600 l. The

dispersed hydrogen concentration is therefore 1.5% by volume. The lower

flammability limit of hydrogen is 4.0%. Accordingly, even if the reduction glovebox

had an air atmosphere, the possibility of a detonation in the glovebox volume is

essentially nonexistent. Even attaining a localized concentration to support a small

deflagration in the glovebox volume is dubious.

The same basic examination can be applied to the maintenance room for this

operation, where calcium metal is stored. A maximum of 9 kg (226 moles) of

calcium, the amount needed for processing the output of hydrogen fluoride from one

entire production cycle, is allowed in the storage locker. To completely react all of

this calcium would require 552 moles of water (8 l) and would generate 226 moles of

hydrogen (5500 l at 27 oC). Discounting space occupied by equipment, the overall

maintenance room volume is at least 170,000 l. The dispersed hydrogen

concentration is therefore 3.2%. Exceeding 4% would not have been of major

significance, but the inability to do so under such an extreme set of assumptions again

indicates that a large hydrogen deflagration in the room is very unlikely.

For the sake of example, the reduction glovebox is assumed to have an air

atmosphere, as opposed to a nitrogen atmosphere, and a localized hydrogen

accumulation above the lower flammable limit is assumed. The demarcation point for

ARF and RF values for deflagrations occupying less than 25% of the available free

gas volume is 0.17 MPag (25 psig). Glovebox confinement would easily fail well

below this value, but it is also not even physically attainable from a grossly

conservative calculation.

The combustion of 18.5 moles of hydrogen (68,317 cal/mole) yields 1.3E+6 calories.

At 27 oC, the remaining 30,100 liters of gas space holds ~ 1230 moles of air, or

35,600 g. The constant volume heat capacity of air is ~ 0.17 cal/g oC. The heat

liberated by the combustion of all the hydrogen that can be generated from complete

reaction of a calcium charge therefore theoretically raises gas temperature by 215 oC.

Using the ideal gas law, this temperature increase would only raise the glovebox

pressure to 0.07 MPag (10.5 psig).

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