Leaking Tube-to-Tubesheet Joints on a New Exchanger
A shell-and-tube exchanger, TEMA type AES, is being built for a refinery. The shell is SA-516 Gr. 70, 0.625 in., designed for 250 psi; the tubes are SA-249 TP316L, 3/4 in. 16 BWG, 121 tubes; the tubesheet is SA-516 Gr. 70 with seal-welded tube-to-tubesheet joints. After the shell-side hydrotest at 1.3 times the shell design pressure, weeping appears at three tube-to-tubesheet seal welds (tubes 22, 23, and 57). The shop proposes to plug those three tubes plus three more as a precaution, seal weld the plugs, re-test, and ship on schedule. The client's mechanical specification references TEMA and states that tubes may not be plugged without owner approval. No tubes have been plugged yet and the exchanger has not been shipped.
1. Is plugging the correct disposition for a new exchanger? What alternatives and information do you need?
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Plugging is a capacity-reducing change, not a normal fabrication fix, so it should not be the first response. For a new exchanger, a leaking seal weld signals a process problem: tube hole clearance, surface finish, expansion practice, weld parameters, cleanliness, or thermal damage during welding. Three leaks out of 121 joints suggests the population is at risk, and a re-test after plugging only three may reveal more.
Alternatives, in order of preference:
- Excavate and repair the three joints - remove the seal weld, re-expand, re-weld per a qualified procedure, and re-test. This restores design surface area.
- If repair fails, re-tube the affected tubes.
- Plugging only if the owner accepts the thermal and hydraulic penalty.
Information needed before deciding: whether the leak path is through the weld, the expanded joint, or a tube defect; the joint detail and expansion procedure used; whether the other 118 joints were tested with equal sensitivity; and confirmation of the exchanger's required duty margin. Without that, plugging is a guess.
2. If the owner were to approve plugging, what conditions, testing, and documentation would you require?
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Approvals: written owner approval, since the mechanical specification prohibits unapproved plugging, plus process engineering confirmation that reduced surface area (6 of 121 tubes is about 5 percent) still meets duty.
Technical conditions:
- A qualified plugging procedure and qualified welder, matching the tubesheet material and tube material.
- Properly designed tapered plugs set to the correct depth, with a seal weld that does not damage adjacent tube holes or the tubesheet ligament.
- Penetrant examination of each plug seal weld.
- Re-test both shell side and tube side at the specified test pressures, and a sensitive leak test between sides (helium or the specified method) to prove there is no cross-contamination path.
- Re-check the remaining tube-to-tubesheet joints, since the failures indicate a systemic issue.
Documentation: plug locations recorded on a tube map, repair procedure and welder identification, examination and test records, and a note in the manufacturer's data report or its remarks section. Plugging tubes does not normally change the maximum allowable working pressure, so the nameplate stays as designed, but the record must show the exchanger as delivered does not have all tubes in service.