It is great when I can convince an organization to pressure test and leak test its process piping. However, if this task is not done according to the book, we may have created a serious hazard. Two (2) safety-critical devices are often overlooked during pressure and leak testing, and missing either one or both can lead to disastrous consequences.
- Pressure Relief Device
- Calibrated Pressure Gage
Let’s examine each of these devices and understand how and why they are “safety-critical” during these tests.
345.5.2 Pressure Relief Device. A pressure relief device shall be provided, having a set pressure not higher than 110% of the test pressure, except that for test pressures lower than 700 kPa (100 psi), the set pressure may be as high as 70 kPa (10 psi) above the test pressure, but not higher than 1.5 times the rating of the weakest component in the system.
As with all PRDs, they are our last line of defense. We can overpressure the piping in pressure testing piping; hence, the code ASME B31.3 requires a PRD with a set point of not higher than 110% of the test pressure. Early in my career, we over-pressured piping simply by having a 5,000 psi N2 cylinder as the source of our test gas, and the pipe run was miscalculated. We put all 5,000 psi into a run of pipe that should have seen only 440 psig. Luckily, the pipe run was not in a unit, and everyone was clear when it failed. Had the PRD been in place, we would have lifted the PRD rather than blowing up the pipe (which was a costly error!)
(d) Test Gages
(1) One or more pressure-indicating gages shall be connected to the piping in a manner in which they will receive full test pressure.
(2) Dial indicating pressure gages used in leak testing should be graduated over a range of about double the intended test pressure, but in no case shall the range be less than 1 ½ nor more than 4 times the test pressure.
(3) Digital reading pressure gages having a wider range of pressure may be used, provided the readings give the same or greater degree of accuracy as obtained with dial pressure gages.
(4) The level of accuracy for pressure test gages should be specified in the engineering design.
(5) Each gage shall be calibrated against a standard dead weight tester or a calibrated master gage. The gage shall have been calibrated within 12 months prior to each test, or any time there is reason to believe the gage is in error. Calibration may exceed 12 months with owner's approval.
The other safety-critical device(s) is a calibrated pressure gage between the pressure test gas and the piping to be tested and a calibrated gage on the piping being tested. Both gauges must be located so that the person controlling the test gas/pressure can read them. BOTH gages need to have been professionally calibrated within the past 12 months PRIOR to the pressure test. BOTH gauges need to have a range of about DOUBLE the intended test pressure, but in no case shall the range be less than 1 ½ nor more than 4 times the test pressure.
This information MUST be included in our written pressure and leak testing procedure. Although these two items may seem minor to some, they both play a critical role in safely executing our pressure testing.