Precision weld

Published : Aug 14, 2009 00:00 IST

A.K. Pal, Chief Construction Engineer, KKNPP: "Welding this type of steel has not been done so far in the country."-A. SHAIKHMOHIDEEN

A.K. Pal, Chief Construction Engineer, KKNPP: "Welding this type of steel has not been done so far in the country."-A. SHAIKHMOHIDEEN

THE reactor building of Unit-1 of the KKNPP is a maze of pipelines and other massive equipment. Among them, one pipeline, the main coolant pipeline, stood out as much for its size as for the engineering feat that made it possible.

This pipeline, which weighs 272 tonnes, circulates light water from the RPV to the steam generator and then to the reactor coolant pumps and the pressurisers before returning to the RPV. The light water, a coolant, is used for the lifetime of the reactor after periodic purification.

A.K. Pal, Chief Construction Engineer of the KKNPP, who took me into the reactor building, explained that the pipeline, which runs to a length of 140 metres in a closed loop, was in fact made up of 32 pieces welded together. Not a simple task, considering that some of the pieces weigh around 12 tonnes. Each piece of pipe is 8 cm thick and has a diameter of 85 cm. These massive pipes had to be welded at their locations inside the building, at elevations ranging from 8m to 21m.

The pipes are padded with stainless steel inside to prevent corrosion and each joint has about 90 kg of electrode deposition, that is, fused metal. The pipes are made of special alloy steel (P-21 modified steel) that have unique properties. This meant that the welding process had to be done at a specific temperature to prevent cracking.

We did a pioneering job in welding together the 32 pieces of the main coolant pipeline. It was a highly risky job. Welding this type of steel has not been done so far in the country. We could not lower the temperature of the weld material [pipes] below 150 Celsius because of specific restrictions, Pal explained.

Several conditions had to be taken into account before the welding could begin. This included the movement of steam generators and dilation of pumps.

The sequence had to be decided, that is, which piece should be welded first and which one last, to avoid stress during welding. The shrinkage of the pipes had also to be taken into account.

We studied all this and arrived at specific stress-relieving curves We did mock-ups. We even had to get the welders to qualify for doing this job, Pal said. The alignment of the pipes was difficult. The entire job took nine months for the first unit of KKNPP and eight and a half months for the second unit. It was an enormous job that had to be done with precision, Pal said.

T.S. Subramanian
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