Flammable Gas

Hydrogen and deuterium are colorless, odorless gases and hence not easily detected by human senses. Hydrogen air mixtures are flammable over a large range of relative concentrations from 4 $\%$ to 75 $\%$ H$_2$ by volume. Detonation can occur with very low energy input, less than $\frac{1}{10}$ that required by mixtures of air and gasoline. At temperatures above -250 C hydrogen gas is lighter than (STP) air and hence will rise. At atmospheric pressure, the ignition temperature is approximately 1000 $^\circ$ F but air H$_2$ mixtures at pressures of 0.2 to 0.5 Atm can be ignited at temperatures as low as 650 $^\circ$ F. Hydrogen mixtures burn with a colorless flame [4].

The total volume of liquid hydrogen in the heat exchanger is about 2 $l$. The target cells and their associated plumbing hold an additional 3.4 $l$. Thus the total volume of hydrogen in the target is approximately 5.4 $l$. The volume changes between the liquid state and gas at STP by a factor of about 800. Thus filling the target would require about 4,300 STP $l$ of hydrogen. The hydrogen target is connected to a 1,000 Gallon (about 3,800 $l$) recovery tank. The normal running condition for hydrogen is 26 PSIA. So the total amount needed to fill the target and the tank is about 10,900 STP $l$. For deuterium, the target is about 4,300 STP $l$. The normal running condition for deuterium is 22 PSIA. So the total volume needed to fill the tank is about 5,600 $l$. The total to fill both the target and the tank is about 9,900 STP $l$.

The Hall A inventory of hydrogen and deuterium gas is stored outside the Hall A gas shed, adjacent to the counting house. The current inventory is two A size cylinders of hydrogen ($\approx$ 6,800 STP $l$ each) and four A size cylinders of deuterium ($\approx$ 5,000 STP $l$ each). One bottle of hydrogen and one deuterium bottle will be kept in the Hall in order to fill the targets. These bottles will be placed in a gas rack behind the gas panels.

The basic idea behind safe handling of any flammable or explosive gas is to eliminate oxygen (required for burning) and to prevent exposure to any energy source that could cause ignition. In the Hall A environment, the most likely source of oxygen is of course the atmosphere and the most likely ignition sources are from electrical equipment.