Earlier this month, steel fabricator and welding specialist Hydra-Arc finished the post weld heat treatment (PWHT) and pressure test of the third of four drums on an order for a client in the sugar industry.

The order – procured in August last year, which will be supplied and commissioned by August this year – entailed the replacement of two steam drums and two mud drums to be used in new steam generation boilers.

PWHT is a controlled process in which a material that has been welded is reheated to a temperature below its lower critical transformation temperature.

The material is then held at that temperature for a specified amount of time which is done to alleviate residual stresses, increase the strength, increase or decrease the hardness, and reduce the risk of cracking.

“Notably, the PWHT and pressure testing of the two mud drums was finalised in April and the first steam drum was successfully tested last month,” says Hydra-Arc divisional engineering manager Matthew Alfonso.

He explains that the manufacturing of the two mud drums and two steam drums, included the rolling of 50 mm and 80 mm plates into 1 m and 2 m drums respectively with a total length of over 8 m a drum.

After welding, the drums had to undergo PWHT, pressure testing and then the company will undertake the final drilling, boring and grooving of over 1 600 holes for each drum.

Hydra-Arc is fortunate enough to own one of the longest fully computer numerical controlled boring mills in the southern hemisphere, which has accelerated the drilling of over 1 600 holes a drum.

The large boring mill allows the whole drum to be machined on a single setup using state-of-the-art machine tooling.

In the past the machining of such a long drum would usually be done with on-site machine tools with an average time of 15 minutes a hole.

Having the advanced hole making mill has meant that Hydra-Arc is able to complete a hole in under five minutes, this is a reduction in schedule of over 65% on the machining portion of the work.

“The biggest challenge has been encountered on the machining of the drums, as once PWHT has been done on the metal it cannot be repaired by welding, therefore accuracy is key,” says Alfonso.

Alfonso explains that, the fact that the drums had already been treated removed any margin for error for machining, as after heat treatment no repairs are allowed to be made on the drums.

Consequently, Hydra-Arc designed a special set-up and testing jig and ran numerous test runs on mock drums until the process was perfect, and machining could begin on the drums.

“Using the learnings from the test runs and with strict quality assurance procedures, Hydra-Arc is now able to execute the machining work with 100% confidence,” he concludes.