Hydrostatic Test Window on a 12-Inch Section
A 6.2-mile, 12.75-in. OD by 0.375-in. wall X52 transmission section is ready for hydrostatic test. MAOP is 2,200 psi. The profile rises 210 ft from the low point to the high point. Company procedure limits test hoop stress to 95% SMYS at every point and requires the high point to see at least 1.25 times MAOP. The test crew fills with water and pressurizes from the high point. After 8 hours the high-point gauge reads 2,772 psi, down from 2,790 psi; the low-point gauge reads 2,881 psi. Water temperature at the high point dropped 7°F over the same period. The gauge at the low point was calibrated 30 days ago; the high-point gauge was calibrated 14 months ago. The crew asks whether the test can be accepted or whether the section must be depressurized and investigated.
1. Calculate the allowable high-point test-pressure window and state whether the observed readings are acceptable from a pressure-limit standpoint.
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The elevation difference creates a static head of 210 ft × 0.433 psi/ft = 90.9 psi, so the low point runs about 91 psi above the high point. The maximum allowable low-point pressure at 95% SMYS is P = 2 × (0.95 × 52,000) × 0.375 / 12.75 = 2,906 psi. Therefore the maximum high-point pressure is 2,906 − 91 = 2,815 psi. The minimum high-point pressure is 1.25 × 2,200 = 2,750 psi. The window is 2,750 to 2,815 psi at the high point. The observed high-point reading of 2,790 psi is inside the window, and the low-point reading of 2,881 psi is below the 2,906-psi limit. The low-point hoop stress is 2,881 × 12.75 / (2 × 0.375) = 48,977 psi, or 94.2% SMYS, which is under the 95% limit. From a pressure-limit standpoint the readings pass, but the high-point gauge calibration issue still has to be resolved.
2. The pressure dropped 18 psi while the high-point water temperature fell 7°F. How should the crew evaluate whether this is a leak or thermal contraction?
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A closed, restrained test section loses pressure as the water cools. For this pipe size a practical planning value is about 2 to 3 psi per °F, so a 7°F drop can explain roughly 14 to 21 psi. The 18-psi drop is therefore within the thermal range and is not by itself evidence of a leak. The crew should not accept it on a rule of thumb alone. Plot pressure and temperature together, correct the pressure to a reference temperature, or hold until the temperature stabilizes. At the same time, check exposed valves, flanges, and the low-point drain for weeping, and confirm that no air was trapped during filling. If pressure continues to fall after the temperature is stable, or if the corrected high-point pressure falls below 2,750 psi, the section should be investigated. Also confirm the low point never exceeded 2,906 psi during the hold.
3. The high-point gauge was last calibrated 14 months ago. What is the disposition of the test?
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The high-point gauge is the controlling gauge for the minimum test pressure, so an out-of-calibration gauge cannot be used for acceptance. A 14-month interval likely exceeds the company or procedure interval, which is commonly six months or one year. Do not apply an informal field correction without calibration data. Replace or recalibrate the gauge, then either repeat the test or continue the hold using calibrated gauges while documenting that the section did not fall below the minimum test pressure. The low-point gauge being in calibration helps verify pressure, but it does not replace a calibrated high-point reading. If the test is accepted with the low-point gauge only, the record should show why that gauge controls and how the elevation head was accounted for.