There may come a time that we need to pressure test one of our pressure vessels after a repair or alteration and our FIRST CHOICE of methods to use should be Hydrostatic as it is MUCH SAFER than pneumatic. However, in some cases, we may NOT be able to perform a hydrostatic test for a couple of common reasons:
- the vessel is not capable of holding the weight of our hydrostatic test media, typically water, and/or
- the process is highly reactive to any trace of water
But when we have to use the pneumatic method, our risks increase substantially as filling a vessel up with gas above its maximum allowable working pressure (MAWP) can lead to catastrophic results. SAFTENG members can see these results in the member’s area.
So when we need to perform a pneumatic test, the code has seen fit to require some other matters to be dealt with BEFORE the pressure test…
The vessel MUST first be examined according to the requirements of UW-50.
UW-50 NONDESTRUCTIVE EXAMINATION OF WELDS ON PNEUMATICALLY TESTED VESSELS
On welded pressure vessels to be pneumatically tested in accordance with UG-100, the full length of the following welds shall be examined before the pneumatic test is performed, for the purpose of detecting cracks:
(a) all welds around openings;
(b) all attachment welds, including welds attaching non-pressure parts to pressure parts, having a throat thickness greater than 1/4 in. (6 mm).
This paragraph REQUIRES that welds around openings and attachments be examined by
- magnetic particle testing (MT) or
- liquid penetrant testing (PT),
BEFORE pressuring the vessel.
Except for certain vessels, the pneumatic test pressure at every point in the vessel shall be 1.1 TIMES THE MAWP; multiplied by the ratio of the allowable tensile stress value at test temperature; divided by the allowable tensile stress value at design temperature.
The metal temperature of the vessel or component shall be AT LEAST 30°F above the MDMT of the vessel.
The test pressure shall be gradually increased to NO MORE THAN HALF OF THE FULL TEST PRESSURE and then increased in steps of 1/10 of the test pressure until the full test pressure is reached.
The pressure shall be reduced to the test pressure divided by 1.1 AND ALL areas are to be examined.
All other requirements for hydrostatic testing shall be observed, including the waiving of the visual examination, provided the same limits are met.