In previous articles I have discussed the ins and outs of NFPA 58, that connecting a hot head torch to a bulk tank is against code and therefore illegal etc ad nauseum…but I have not presented any “proof” of exactly WHY it is hazardous to do so.
There is a mathematical formula which converts flow of a particular gas in a line to a volume. Here is the formula:
V = 58 * SQRT((p * (d^5))/(W*L))
V equals volume flow in CFM
p equals difference in pressure between two ends of the hose in PSI
d equals ID of the hose in inches
W equals weight in pounds of one cubic foot of the gas
L equals the length of the hose in feet.
For the purposes of an example, let’s assume a 1/4″ hose, 20 feet long, running propane (0.1162 pounds per cubic foot in gasous form). Let’s also assume a leak of 1 PSI – this is a typical small leak usually caused by a loose fitting.
Some other numbers to keep in mind:
A 20# propane tank contains about 4.7 gallons of liquid propane when full. Propane converts to vapor at a 1 to 36 ratio, so that 4.7 gallons of liquid will convert to 169.56 cubic feet of propane vapor (gas).
The explosive level of propane is 2.1% propane in normal room air.
Let’s also assume an 8 x 8 room with a 8 foot ceiling, resulting in a space with 512 cubic feet. The explosive level in this room is 10.75 cubic feet (512 * 0.021).
Plugging the numbers into the formula, (assuming the 1 PSI leak) we get a volume flow of 1.19 cubic feet of propane gas per minute. In 9 minutes, the room will reach explosive limits (10.75/1.19). The tank will be empty in about 2 and a half hours.
Next, let’s assume a broken supply line on a regulated torch, such as a Nortel Minor or GTT Bobcat, with a regulated flow of 5 PSI. This scenario will produce a flow of 2.66 cubic feet per minute. The room will reach explosive limits in 4 minutes and the tank will empty in just over an hour.
Another scenario would be a broken supply line on a regulated large torch, with a flow of 15 PSI. This will produce a flow of 4.60 cubic feet per minute. The room will reach explosive limits in about two and a half minutes and the tank will empty out in just under 40 minutes.
Lastly, let’s assume a broken supply line on a hot head torch, free flowing at 120 PSI. This provides a flow of propane of 13.2 cubic feet per minute. The room will reach explosive limit in less than a minute, actually about 50 seconds (10.71/13.02)*60. Additionally, the tank will be empty in approximately 13 minutes.
The average response time for a fire department across the country ranges from 5 minutes to over 15 minutes, depending on how far you live from the fire hall, whether the fire department is full time or volunteer.
When you are using a bulk tank with a hot head torch, you are relying on the hose and all connections to be safe and secure. You are relying on the hose manufacturer to have properly manufactured and tested the hose assembly, but remember that all manufacturing processes are prone to error. Are you willing to have your life depend on someone elses quality control standards?