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.
- 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.