Hydrostatic testing is a crucial step in ensuring the integrity and safety of pressure vessels before they are delivered to clients. Here are the guidelines for conducting a hydrostatic test 

1. Bill of Material Check

    • Verify that all components listed in the fabrication drawing are physically present on the vessel.
    • Ensure that loose parts or parts meant to be welded later (e.g., on a pad plate) are listed.
    • Check all drawings related to the vessel for completeness.

2. Welding Completion Check

    • Visually inspect all welds on the vessel.
    • Butt welds should be checked for profile uniformity, smooth merging with parent metal, reinforcement, undercuts, and even ripples.
    • Fillet, socket, and corner welds should also be inspected for size, contour, and evenness.
    • Any welding defects beyond acceptable limits should be addressed through rework, including welding and dressing/grinding.

3. Non-destructive Testing (NDT) Completion Check

    • Ensure that all required NDT has been successfully conducted.
    • Verify compliance with control charts provided in the relevant NDT procedures.
    • Indicate satisfactory NDT completion in the pre-hydro inspection report.

4. Nonconformity Clearance Check

    • Clear any non-conformance reports (NCRs) generated during fabrication.
    • Ensure that NCR resolution adheres to the applicable code.
    • Third-party inspection agencies should review and approve NCR resolutions.

5. Surface Examination

    • Conduct visual inspections of the entire inside and outside surface of the vessel, including nozzles and flanges.
    • Address any issues such as arc strikes or tack welds.
    • Perform Magnetic Particle Testing (MPT) or Liquid Penetrant Testing (LPT) if necessary, and maintain records of these inspections.

6. Pad Air Test

    • Test all reinforcement pads of nozzles using air pressure (around 22 psi) with soap solution applied around the weld, both inside and outside.
    • This test must be conducted before post-weld heat treatment (PWHT), as no welding repairs are permitted after PWHT.
    • Especially important for thin-walled vessels.

7. Gauges

    • Install an indicating-type pressure gauge directly on the vessel.
    • Consider using two gauges to account for potential gauge errors.
    • For large vessels, use a pressure recorder.
    • Pressure gauge range should be double the test pressure but not less than one and a half times or more than four times the test pressure.
    • Calibrate gauges and recorders against a deadweight tester or calibrated master gauge, ensuring valid calibration on the test date.

8. Hydrostatic Test Procedure

    • The test pressure should be 1.3 times the design pressure, with temperature correction if applicable.
    • When testing in a vertical position, ensure that the pressure at the highest point meets the required value.
    • Gradually build up pressure to half the test pressure and allow it to stabilize for about 5 minutes.
    • Increase pressure in steps of 1/10 of the test pressure, with each step stabilized for 2 to 3 minutes.
    • Maintain the vessel at the test pressure until all weld joint examinations are completed.
    • It is recommended to hold the pressure for at least half an hour, sufficient for inspection.

9. Post Hydrostatic Test

    • The vessel must be thoroughly dried and cleaned to a satisfactory level.
    • Any internals that were removed to facilitate the hydro test must be reinstalled or sent as loose parts with proper documentation.
    • Inspect the inside of the vessel for debris and sediments that may have accumulated during testing.
    • If any coating is specified, it should be applied after thorough cleaning.
    • Coating application may not fall under the scope of fabrication and may require a separate process.
    • Once cleaning and, if necessary, coating are completed, reinstall any fittings that were removed during testing.
    • Ensure all internals are correctly positioned if applicable.
    • If everything is in place, the vessel can be prepared for packaging.
    • Leave open nozzles where piping is expected, but seal them properly to prevent rainwater ingress.
    • Use temporary blind flanges made of at least 5-mm-thick plates.
    • Secure the blind flanges with at least four bolts and include a rubber gasket between them.
    • To prevent humidity from causing corrosion inside the vessel during storage, hang a few bags of silica gel inside the vessel.
    • Silica gel helps absorb moisture and maintain a dry environment.

Following these guidelines ensures that the pressure vessel undergoes a thorough hydrostatic test, providing confidence in its integrity and safety for operation under design conditions.

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