Mannings Thermal & Environmental Engineers of Southport have successfully completed the heat treatment of two giant steel pressure vessels, which are probably the largest ever, built in the Middle East.
The vessels are identical 19 metre diameter spheres, which will be used for storing liquefied propane at the Abu Dhabi National Oil Companies LPG Blending Facilities at Ruwais. Each sphere weighs approximately 500 tonnes and has a shell thickness of over 50mm (2"). Post weld heat treatment reduces internal stresses in the metal and is a requirement of the design code. This process had to be carried out immediately after the fabrication work at site was complete.
To achieve the desired results on each vessel the temperature of the whole sphere had to be raised gradually to 620 degrees centigrade and held at this temperature for a set period of time before being cooled at a specified rate. During this heat treatment cycle it was essential that temperature uniformity was maintained over all areas of the entire construction within set limits. Since it is obviously not possible to use a conventional furnace to carry out the heat treatment on large site built vessels such as these, the answer was to turn the vessel itself into a huge furnace by thermally insulating the outside and to use the internal fired technique.
Mannings were awarded the contract by Al Khaja Pimex who are a specialist heat treatment company based in Dubai and who were responsible for the entire job of Preheating and Post Weld Heat Treatment. Al Khaja Pimex connected over 60 thermocouples at intervals around the sphere surface so the temperatures could be carefully monitored and recorded. Mineral wool insulation was then applied to the outside to surround the shell and this was firmly secured in place. The heat sources used to raise the internal temperature were Mannings Gas Fired Portable High Velocity Combustion units, which are specially designed for post, weld heat treatment and refractory curing applications.
Each combustion unit is a self-contained system and incorporates the gas burner, flame failure protection, combustion air fan, gas flow regulation and control system. Remarkably, only two burners were required for the entire process. One of the burners was arranged to fire into an access way at the top of the sphere and the other fired into an access at the bottom. Additional openings were used as chimneys to exhaust the flue gases.
The secret of achieving the controlled, even temperature distribution and avoiding localised hot spots lies in the design of the burner. This produces a stream of heated air and combustion gases at high velocity to promote turbulence and give good convective heat transfer. Because of the extremely high turndown capability of the burner, it is possible to keep the airflow constant and regulate the heat output by simply adjusting consumption of the fuel gas.
All of the temporary services were put in place such as cranage, scaffolding, lighting and power and two of Mannings specialist combustion engineers then carried out the final preparations before commencing the heat treatment process, which took three days to complete on each sphere. The gas, used to fire the burners, had to be delivered to the site by tankers and approximately 28 tonnes of fuel gas was required on the project.
Ian Adamson who is International Sales Manager for Mannings Thermal & Environmental Engineers said "to be able to raise over 500 tonnes of steel to a temperature of 620 degrees centigrade and keep the uniformity to within 15 degrees is an excellent achievement and the successful completion of this contract is a credit to all concerned".
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