Prakyash,
Both tests are carried out on completion of an installation in order to:
1) Prove that the installation has the strength to withstand the operating
conditions plus a code determined margin. In other words to prove that it
will not fail in service.
2) Prove that the installation does not leak.
The test can be carried out by pressurising the system with water or air.
Using water it is called a hydrostatic test. Using Air it is called a
pneumatic test.
The hydrostatic test is inherently safer because if the system fails the
release of water will depressurise the system very quickly and generally
very little damage is done, not that being close to a rupture is not
dangerous. If a system is pressurised with air to the same pressure and
it fails, the consequences are much greater. Air is compressible so there
is very much more air in the system than water. In the event of a rupture
the release is violent and goes on for a long time. It also sets up a
pressure wave that can do a lot of damage some distance from the failure.
Bombs work in the same way. A lot more damage is caused by the pressure
wave from a bomb than the shrapnel it throws around.
So in general you would avoid using a pneumatic test. The codes in
general also encourage the use of hydrostatic tests and caution against
the use of pneumatic tests.
Pneumatic tests have one big advantage however, and that is that air is much thinner than water so small leaks are much more readily detected using them. Using hydrostatic tests one normally looks for leaks visually by inspecting for wet marks or drips, or by monitoring the pressure in the system. Using pneumatic tests one searches for leaks by spraying a foam solution on all joints, and where there is a leak, even a very small one, the foam solution breaks out into bubbles. You always know when you have found a leak on a pneumatic system.
I suspect in your case the specification calls for two tests, first the hydrostatic test at high pressure to test the strength of the system. It then most probably calls for the system to be drained and then pressurises with air to say 10% of the system pressure and checked for leaks using foam solution. That arrangement gives you the best of both tests, the safety of the hydrostatic test, and when the strength of the system is proved, the leak sensitivity of the pneumatic test.
Gordon Reddek
Gordon Reddek
Specialist Mechanical Engineer
Alcan Engineering, Level 3, 443 Queen St, Brisbane, Qld 4001, Australia.
Tel: +61 7 3328 6424
Fax: +61 7 3328 6990
Email: gordon.reddek@alcan.com
"prakyash" <prakyash@yahoo.com>
01/06/2004 03:32 PM
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[PipingDesign] difference between pneumatic test & hydrostatic test
Dear All,
What is the difference between pneumatic test & hydrostatic test? For some of the components/parts why both the tests are carried out?
Please guide me.
regards,
prakash
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Received on Tue Jun 01 02:27:00 2004
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