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Hydrotest procedures for pipeline?

asked Dec 18, 2015 in Engineering by Rejean Zhang (515 points)
Hello,

Please recommend a Hydrotest procedures for pipeline.

Thanks much in advance.

2 Answers

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answered Dec 19, 2015 by NewtonTishle (505 points)

Hydrostatic testing of the pipeline including riser should follow the below procedures:

  • The pressure should be raised to the test pressure and allowed to stabilize then held for 15 minutes.  The rate of pressure increase should not exceed 100 kPa (15 psi) per minute during all operations.
  • The pressure should be lowered to approximately 25 percent of test pressure, allowed to stabilize then held for 15 minutes.  Rate of pressure decrease should not exceed 200 kPa (30 psi) per minute during all operations.
  • The pressure should be raised to an amount approaching the test pressure and again allowed to stabilize before it is finally raised to the test pressure.  The pressure should not decrease more than 0.2 percent after compensating for temperature.  None of this or the foregoing should be included in the test duration. The allowable, unaccounted pressure loss of 0.2 percent is based on a maximum allowable air content of 0.2 percent.  Air content can be evaluated by precisely measuring the volume of water (Vi) injected between the measured pressures P1 and P2 and comparing Vi with the theoretical volume that would cause a pressure change from P1 to P2.  The difference between the measured and calculated volumes can be attributed to air content.  The same evaluation can also be made by withdrawing water.
  • Water should be injected or withdrawn from the test section to maintain pressure within the specified limits.  The volume of these injections and/or withdrawals should be measured and recorded.
  • Pressure-volume-temperature (PVT) calculations should be performed to show any observed pressure loss due to a temperature change or a leak.
  • A pressure recorder and temperature recorder should be used to continuously record the test pressure, fluid temperature and ambient temperature.  In addition, pressures should be read and recorded to the nearest 7 kPa (1 psi), using a deadweight tester furnished by the Contractor.  The pressure should be recorded every 15 minutes until it has stabilized; then it can be recorded every 30 minutes.

Note:  Hydrostatic tests for flexible pipelines should require longer pressurizing times and longer test periods.  Flexible pipe will expand more and trap air pockets, making it difficult to reach a stabilized test pressure.  Also, the pipe is subject to creep that will result in a slow reduction of test pressure over time.  The test should begin at a pressure high enough to ensure that the pressure at the end of the test will still be above the minimum test pressure. Flexible pipe materials are subject to cold extrusion.  There are documented cases of field failures after successful shop hydrostatic tests.  Therefore, the test pressure should be maintained for 12 to 24 hours to ensure that the pipe will not fail from cold extrusion in the field.

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answered Apr 27, 2016 by Tadd Pham (5,620 points)

If you involve a contractor to provide the hydrotest, they should be involved rather early in the project stage to ensure the following steps (not in particular order) are met:

  • Upon the completion of successful stabilization period, the pressurization pump should be shut down and disconnected from the pipeline system.
  • Time, test water temperature and deadweight pressure gauge readings made during the hydrotest period should be recorded in tabular form.  Any change in pressure should be fully documented.
  • Should the pressure rise to the maximum permissible test pressure due to temperature increase or other reasons, it should be reduced by bleeding off the fluid slowly into a calibrated container.  Accurate recordings of the volumes removed to prevent pipeline over pressurization should be recorded.  Data should include time of day, pressure before and after bleed-off, temperature range, and liquid volume removed.
  • Should the pressure drop below the minimum test pressure at any time during the test period, it should be repressurized and the 4-hour stabilization period should be completed before the test hold period begins again.  The final 2 hours of the hold period should be without pressure drop other than that caused by temperature change.
  • For any pressure loss during the 8-hour test period, the Contractor should evaluate how much pressure loss can be attributed to temperature change and to free air dissolving in the water.  The test should only be accepted if such pressure drop is supported by calculations and can be explained.  If the Contractor’s explanation for pressure drop is not accepted, Contractor should extend the hydrotest period beyond 8 hours, until a completely satisfactory hydrotest is obtained.
  • Pump should be checked following the completion of pumping operation.  The pump counter should be reset to zero and a calibrated vessel should be filled to recheck the counter for pump volume output at atmospheric pressure.  The final counter figure and volume output should be recorded.
  • After the successful hydrotest is accepted, Contractor should relieve the pressure on the completed test section at a maximum rate of 29 psi (2 bars) per minute.
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