Tanks and Burst Disks

There is a myth that leaving a SCUBA tank in the trunk of a car places the tank in serious danger of blowing a burst disk. This is not likely, especially with new equipment.  This would only happen if the disk is fatigued, and well out-of-spec on the low end of its rating. The purpose of the burst disk is not to protect the tank from solar heat in a trunk of a car. Its purpose is to protect the tank if left unattended while filling it from a running compressor, and possibly protecting the compressor, though most have their own pressure relief valve. They are designed to burst at well above the tank rating - often around 40% above nominal. For an aluminum 80, that would be:

3000 PSI x 1.4 = 4200 PSI.

Charles' Law states that: (Pressure x Volume)/Temperature is constant, meaning that pressure rises with temperature to keep their ratio a constant if the volume remains the same.  This can be expressed as:

P1/T1 = P2/T2

Many people believe that a 140 degree F trunk vs. a 100 degree F fill temperature would yield a 40 percent increase in tank pressure:

3000/100 = P2/140, or  P2 = (3000/100) x 140 = 4200 PSI

BUT!!!..... It doesn't work that way because the pressures are relative to "absolute zero" temperature (-460 degrees F).  Degrees Rankine are Fahrenheit degrees in size, but measured relative to absolute zero. Since the equation for Charles' Law is based on degrees Rankine, the example above is correctly calculated as follows:

3000/(460+100) = P2/(460+140)

3000/560 =P2/600

P2= (3000/560) x 600 = 3215 PSI,

which is well below burst disk concerns. The burst pressure is rated at nearly 1000 PSI above the expected 215 PSI rise in a 140 degree Texas trunk in the 100 degree summer day (assumed temperature for rated pressure in the summer).

If you fill the tank in an 80 degree room/water tank, the rise would be greater relative to the filling pressure, but still well under 4200 in the example.

3000/(460+80) = P2/(460+140)

3000/540 =P2/600

P2= (3000/540) x 600 = 3333 PSI,

which is still only about 11% over rating, and within tank safety margins.

A tank tip - Store your tank with very low pressure (100 PSI) to keep water out and material stress low. If that isn't practical, store it full. In a house fire, the burst disk may blow from heat combined with pressure if the storage pressure is high. If the aluminum tank is half full, the tank will likely give way before the burst disk because aluminum is severely weakened by extreme heat, and the pressure rise might never reach the the rated burst temperature before catastrophic failure of the tank.

Press here for the menu These are my opinions and are not those of any particular agency.

Copyright © 1998 Randall C. Allen
All rights reserved
Comments to: rallen@searover.com
visitor # 14484 since 6/22/98