Written by Gerry McWeeney on December 27, 2021

Vessel Considerations During Post Weld Heat Treatment

Proper Planning

When planning PWHT of any pressure vessel the unequivocal number one concern should be vessel integrity during the heat cycle as this is where any damage is going to happen. The cost of having a bad PWHT and causing damage to the material, internals, externals, or the vessel integrity itself could have irreversible damage. It is therefore prudent to run engineering calcs, finite element analysis or wind and weight load forecasts prior to the post weld heat treatment.

There are a few different ways to carry out PWHT on pressure vessels. The most common is by electrical resistance method, where heating pads are applied direct to vessel wall internal or external to cover the required soak band, (SB) heat affected zone (HAZ) and thermal gradient areas then encapsulated both inside and out with insulation. The other less common but more cost effective is the fuel fired method, where the vessel is encompassed with insulation, Hi Velocity burners are placed though firing ports or manways along with distribution tubes to utilize the vessel as its own furnace.

The PWHT governing codes and recommended practices that drive the PWHT conditions can mostly be found in API, ASME Section VIII and NBIC. Recently some clients are insisting on using WRC452 recommendations for PWHT which is referenced in ASME 2016 Edition.

Where a crane is required, someone will have worked engineering calcs, for this exercise calcs are assumed.

Vertical Vessel Considerations:

  1. Get crane company involved and start line of communication between HT Contractor for logistics and scheduling, weight, crane span, overheat restrictions, load weight, growth, etc.
  2. Vessel grounding is a must do if attaching a crane to support especially during electrical resistance PWHT, the vessel is normally grounded, is a must do double check.
  3. If the skirt is attached to the foundation with bolts these require loosening to allow skirt to grow with vessel.
  4. If the area being heated is in close proximity to the skirt weld there needs to be consideration of heat being applied on the skirt to allow skirt and shell to grow together and prevent any tearing of the skirt to shell weld.
  5. Maintaining a steady load and allowing for growth should be crane companies key concern and responsibility. Crane co will need vessel weight and load specs. It is advisable to have a digital load calculator between crane jib and vessel that can be monitored from the crane cab for monitoring and adjustment when needed.
  6. Vessel will continue to grow after soak period, consideration of this needs to be discussed as part of the crane support plan.
  7. Vessel Internals required to be removed from within the heated area plus 6’ above and below, preference is to have no internals at all.
  8. Ladders should be unbolted from the vessel to allow growth. Platforms should be removed entirely from heated area, platforms 3’ above heated area should be loosened.
  9. Platforms above heated area should have no load bearing during the heat treatment. (Often Overlooked)
  10. Attached piping should be unbolted from nozzles.
  11. Nozzles over 2” require additional heat applied to meet code requirements.
  12. Piping that restricts growth and are welded direct to vessel need to be cut and rewelded afterwards.
  13. Growth is not one dimensional, each type of material has different growth patterns when heat is applied.
  14. The growth will be radial as well as longitudinal and needs to be factored when assessing appendices to remove prior to PWHT.

Horizontal Vessel Considerations:

All of the above can be applied to the PWHT of a vessel in horizonal position. For vessels with a span of more than 15’ between saddles it is advisable to add additional support.

Where saddles are bolted to the foundation these bolts need to be loosened. (Similar to skirt scenario above)

You may have to cut slits in the saddle base and have plates inserted between foundation and base of saddle to allow vessel to slide / grow during the heat cycle.

Heat treatment of vessels are in general the most difficult of heat treatments and most often fall within critical path, pressure intensifies. There is no room for error. It takes a team effort to ensure everything is lined out and executed flawlessly. Having SME’s along with a contractor that has experience as well as technical and practical nous is advised.

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