ASME B31.3 (Process Piping) treats double-walled piping—commonly referred to as jacketed piping—as two (2) distinct pressure systems. Because the failure of the inner pipe (core) or outer pipe (jacket) carries different risks, the code requires specific sequences and methods to ensure the integrity of both.
The requirements generally fall under ASME B31.3, Chapter VI, specifically sections addressing jacketed piping (e.g., Paragraph 345). The internal pipe MUST be tested before the jacket is closed or before the jacket welds obscure the core’s joints.
- Sequence: The core pipe is typically hydrostatically or pneumatically tested prior to the completion of the jacket. If you wait until the jacket is sealed, you cannot visually inspect the core welds for leaks.
- Pressure: Standard hydrostatic test pressure is 1.5× the design pressure, or pneumatic at 1.1×.
- Visual Inspection: All joints (welds or flanges) must be exposed during this test. If a leak occurs, it must be visible to the inspector.
Once the core pipe has passed its test and the jacket is fully welded and sealed, the jacket itself must be tested.
- Standard Test: The jacket is subjected to a hydrostatic or pneumatic test based on its own design pressure (Pj).
- Differential Pressure Warning: You must ensure that the test pressure in the jacket does not exceed the external collapse pressure of the core pipe. In many designs, the core pipe is kept under internal pressure (equalizing pressure) during the jacket test to prevent the core from collapsing inward.
Depending on the service (e.g., lethal substances vs. steam), the code allows for different methods:
| Method | Application | Code Reference |
|---|---|---|
| Hydrostatic | Standard method using water or other non-flammable liquid. | Para. 345.4 |
| Pneumatic | Used when water contamination is a risk or systems are dry. | Para. 345.5 |
| Sensitive Leak Test | Often used for the jacket space; utilizes Gas Mass Spectrometer (Helium) or “bubble” test at low pressure (15 psi or 25% of design pressure). | Para. 345.8 |
| Alternative Leak Test | Used if Hydro/Pneumatic is impractical; requires 100% Radiography (RT) or Ultrasonic (UT) of all welds. | Para. 345.9 |
- Weld Examination: ASME B31.3 requires that any weld not “exposed” during the leak test must be examined by alternative means (usually 100% RT or UT). Since double-walled pipe often hides welds, the timing of your NDE (Nondestructive Examination) is critical.
- Closure Welds: The final “closure” weld that connects the jacket to the core pipe is often a “final closure” weld. If this weld cannot be pressure tested traditionally, it must undergo 100% surface (PT/MT) and volumetric (RT/UT) examination.
- Holding Time: The test pressure must be maintained for at least 10 minutes before the examination begins, and long enough for the inspector to view every joint.