Hydrostatic Test Pressure Drop and High-Point Pressure
An 8-inch, 3.2-mile new gas gathering line has MAOP 1,480 psig. The hydrostatic test is held at 1,850 psig using water from a nearby pond. The line was filled with 1,100 bbl and pigged to remove air. After a 2-hour stabilization, pressure is 1,850 psig at 06:00. At 08:00 it is 1,828 psig; the water temperature fell from 62°F to 58°F, and the steel temperature fell similarly. The test head is at the low end; elevation difference across the section is 140 feet. No visible leaks. The contractor wants to sign off because the drop is 'only 1.2%.' Company procedure requires an 8-hour hold with no more than 5 psi drop after temperature correction, and requires the test to be witnessed. The gauge is a 0-3,000 psi digital unit calibrated six months ago; its certificate is on file.
1. Is the 22 psi drop acceptable? Show the reasoning.
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Not automatically. The drop is 1.2% of 1,850 psi, which exceeds the procedure's 5 psi limit after temperature correction, but the raw drop is not the right measure. A 4°F water temperature fall can cause a significant pressure decrease even with no leak, so temperature correction is required. However, the elevation difference creates another problem: 140 ft of water is about 60 psi, so if the gauge at the low end reads 1,850 psi, the high point is only about 1,789 psi. That is below the required test pressure at the high point if 1,850 psi is the minimum test pressure. The test is therefore not acceptable as run, regardless of the drop. Re-check gauge location, high-point pressure, temperature stabilization, and air content before signing off.
2. What steps should be taken before the test is accepted or repeated?
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Verify gauge calibration and location, install or confirm a high-point pressure gauge, and calculate the elevation correction. Confirm the water and steel temperatures have stabilized; a falling temperature trend means the pressure will continue to drop. Check for air entrainment and verify the line was properly pigged and vented. Inspect the entire test section for leaks, including temporary connections, flanges, and vents. If the high-point pressure is inadequate, re-pressurize from the low end while respecting the high-point limit, or relocate the test head. Hold for the full 8 hours after stabilization and apply temperature correction. A witness must be present. Document all readings, corrections, and any repairs. Do not accept a test that fails either the pressure-hold or the high-point pressure requirement.
3. If a leak is suspected, what is the safe sequence?
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Stop pressurizing, isolate the test section, and reduce pressure in a controlled manner. Do not walk or drive over the test section looking for leaks while it is at pressure. Depressurize, drain as needed, and repair the leak. Then re-fill, pig or vent to remove air, stabilize, and repeat the full test with witnesses. Investigate whether the leak indicates a defective weld, fitting, or pipe, and repair per procedure. Record the failure, location, cause, and repair. Re-test at the specified pressure and hold time. If the pressure drop was thermal or due to air, correct the test setup rather than treating it as a leak. The key is that acceptance requires both pressure retention and adequate pressure at the high point.