Scuba Diving club, Southern
California
Sea Sabres
Tanks and Burst Disks
By Randall C.
Allen
www.seaover.com
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.
These are the authors opinions and are not those of any particular agency
William
John MacQuorn Rankine the person
Randy’s Scuba
Page (the Author)
Posted April 11, 2003
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