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.

Heat Treatment Applications

These are the common applications for Heat Treatment

Post Weld Heat Treatment (PWHT)

Post Weld Heat Treatment (PWHT), also referred to as stress relieving, is an application of controlled heat applied after the welding is complete.

PWHT is the relaxation of residual stress caused during the welding or in other processes such as high pressure piping and is also effective for hydrogen removal.

PWHT Time, Temperature and Thermocouple locations are driven by industry codes, Axiom’s knowledge and ability to meet or exceed these requirements are the reason our customers rely on us for a quality driven method.

Preheat

Preheating is a process of heating the metal to a desired temperature and maintaining this before and during the welding process.

Maintaining required preheat and interpass temperatures is critical, both for producing a softer, less brittle microstructure for allowing hydrogen to diffuse out of the weld metal and HAZ during welding and also helps with the cooling effect of the metal after each weld pass.

These temperatures are often driven by code or in certain cases by proven and specified welding procedures, preheat in some cases can be elevated to reduce the need for PWHT in certain metals and processes.

Bake Out (Hydrogen Bake Out) 

Bake Out is a heat treatment process that drives out any hydrogen that may have occurred during welding or has seeped into the parent metal in an in service process pipe, vessel or structure.

Hydrogen occurs in metals on a molecular level when hydrogen gets absorbed into the metal, if left undetected can lead to hydrogen cracking and may lead to mechanical failure of the pipe, vessel or structure.

Post-Heating

Post Heating is the maintenance of preheat after the weld has been completed, which allows hydrogen dissolution from the weld to occur. The post-heat temperature may be the same as, or greater than, the original preheat temperature specified

Often used when a weld has not been complete in heavier wall materials where pre heat does not require to be maintained and/or after welding in these heavier materials. Post Heating is also often used during and after welding in creep strength enhance ferritic steels (CSEF Steel)

Line Thaw

Line Thaw is applied heat to help liquify congealed or solidified product in a process system

Heat is applied externally in conjunction with some means of pressure internally to force the product towards an exit in the system or to free the product enough that the systems can go back to full operations.

Due to some complexities of this type of heating being aware of the product lower and upper critical temperatures to avoid any temperature over runs should be discussed prior to applying heat

Methods

Below are common methods of Heat Treatment

Electrical Resistance

Electrical Resistance heating uses low-voltage ( 80volt) flexible ceramic heating elements placed around the component controlled by multi point heat treatment control consoles.

Power to the consoles can be by temporary generator power or from a main power supply inside the facilities.   

The most utilized method in the industry due to the flexibility and sizes of heating elements (FCP) available and the types and geometry of the applications we work on resistance heating can give a more finite temperature control to the parts being treated.

With code changes in our midst electrical resistance may be the preferred method for many applications.

Induction

Induction heating uses an electromagnetic field to create a disturbance in the molecules of the metal that creates heat, the frequency of the electromagnetic field can be quicker than traditional electric resistance heating, the system is quicker to set up and is a good tool for some heat treatment applications.

Not all metals are equal, some do not take well to the induction frequencies, it is always good to check before proceeding with this method

Often used for bolt heating and stretching, induction rods can help expedite the opening of turbine casings, presses and heat exchanger systems.

These systems often provide quicker heating, faster set up and teardown, and can be a safer option due to cool coils in the vicinity of welders and other personnel.

Due care and attention should be taken with rapid heating in certain applications to prevent material overheating.

Combustion Heat

Is used to heat large structures inside temporary furnaces, inside vessels, lime kilns and other large structures because of its efficiencies. Utilizing gas and air to force the heat into the area requiring the heat treatment creates a uniformity that heats the intended area.

Typical uses are Temporary Furnaces, PWHT of Pressure Vessels and Refractory dry out of lined structures, vessels and lime kilns.

A preferred method for refractory dry out it helps drive heat through the material to remove any chemical water used in the formation of the refractory, also helps with chemical bonding and reduces the chance of spalling increasing the life and efficiency of the material.

Blended Heating and Cooling

Blended Heating & Cooling is a new Heat Treatment method utilizing the use of 1, 2 or all 3 heating methods simultaneously to help maximize temperature dissipation into the heated area and a cooling method once the metal design criteria have been met, reducing costs on the front end and improving your time back to production on the back end.


By utilizing Axiom’s experience and patented technology* we have the means to offer a compilation of heating and cooling methods that help you manage the budget and improve the schedule.
Axiom Heat Treatment is The World Leader in blended methods of heating and cooling

*Pending

Contact us to discuss or to schedule a lunch and learn or demo

Background

Axiom Heat Treatment sought to develop new technology in an industry that has seen very little change over the years. Axiom developed a new and innovative technology to reduce power consumption and treating time cycles for field heat treating services.

Axiom began a process of developing and testing a new methodology which included design, engineering and fabricating of temporary specialty convection sections that would be formed and installed over the total required heated area. These convection sections could utilize existing conductive heat from applied electrical resistance heating elements, capturing thermal energy inside the sections with the intent to dissipate the applied heat throughout these convection sections while still achieving the required temperature profiles required to meet the industry standards and specifications.

In theory, the thinking was that by taking this action it could capture the thermal through wall conductive heat and dissipate it throughout the formed sections and would enable convective heat to be captured and applied within the sealed formed sections. This would dissipate the heat into the metal thus reducing the total amount of heat required in the thermal gradient areas.

It was also thought that the convection sections, following heat cycles, could also be utilized to facilitate a cryogenic solution that could help expedite the cooling process through engineered manifolds that could be placed inside the convection area with an applied cooling solution used to force cool the metal after design temperatures have been met and which usually is in an uncontrolled state, usually around 800 – 600 degrees F.

Through some detailed research, engineering, and discussion with vendors and manufacturers, a path forward was formulated that would enable the research and development to begin to prove the heating and cooling concept was viable. The convection section panel manufacturing began, specialty coating suppliers as well as other support services and consulting continued as the path to proof of concept

Project Objective

The objective of the R&D project was to prove the safety and efficacy of the pending new patent equipment and process to reduce power consumption and decrease cooling time of a heat treatment cycle

Key Metrics:

  1. Prove that the soak and thermal gradient temperatures could be achieved using reduced KW for required time for Post Weld Heat Treatment and Pre Heat and Bake Out.
  2. Prove results with internal panels and external heat set up.
  3. Prove 800 degrees F to 120 degrees F in 4 hours or less using a select cryogenic solution.
  4. Prove similar results with external panels and internal heat set up.
  5. Prove installation fixing methods for brackets and panels

Three trials were performed to prove the concepts: 

Trial 1: External Heat – Internal Mimicked Convection Box

Trial 2: External Heat – Internal Convection Box

Trial 3: Internal Heat – External Convection Box

Results & Analysis

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