At a coal-fired power project in South East Asia, our team was asked to look at the cooling water pipe during the shutdown of a unit. It had been in service for six months.
The cooling water pipe cools the condensers, so it was important to get the cooling water pipe back up and running as soon as possible.
This pipe was 12 feet (3.7 m) in diameter, with a 1-inch-thick (2.5 cm) wall. The underground pipe was 2 miles (3.2 km) long from the sea water intake to the turbine building. The exterior of the pipe had been protected with a polyethylene coating along with cathodic protection and the interior had been protected with an epoxy lining.
Once the pipe was drained and ventilated, our team walked down the interior of the pipe and the contractor started performing minor repairs. In the sea intake area we noticed nine pitted areas. I tried to take readings with my pit gage and could not because of pack rust. We then did an ultrasonic thickness test on the pitted areas. The results of these tests showed a thickness of remaining steel ranging from 2.5 mm to 3 mm (98.4‒118.1
This unit generates $500,000 dollars a day when in full operation. The client asked us to produce short- and long-term options to repair this pipe; they had an obligation to provide a certain amount of power to the outside grid, so repairs were necessary.
Here were the options in regards to repair of this pipe:
1. Dig out this section and replace. The problem is the fabrication of the pipe, the tie in of the protective polyethylene on the exterior, and the blasting and tie in of the existing interior epoxy lining that would extend the shutdown
2. Puddle weld the pits in the interior of the pipe. This would damage the exterior polyethylene and compromise the cathodic protection
3. Clean the pitted areas using paintable steel putty and plan to replace the section of pipe during a planned shutdown. This seemed to cause the least disruption to the system and the client
The client opted to go with the steel putty. The contractor cleaned all of the pits by means of mechanical cleaning with power tools. They left a minimum of a 1 mil (25.4 microns) profile and filled the pits with steel putty. After the putty cured, they proceeded to install an epoxy lining and then forced-cured the repaired areas.
During our research before getting on site, we noticed that there were no reports of calibration of the holiday detectors. Therefore, while on site during the repairs, we asked for the holiday detector that was used. Not only did the contractor say that they did not have the instrument on site, but they had no calibration certificates on any of the reports.
Our conclusion of the source of pitting problems pertained to the holiday detector and what it wasn’t detecting (aka holidays). Either the contractor had initially used a faulty holiday detector that wasn’t calibrated, they didn’t use the equipment correctly, or they didn’t use one at all.
The pipe had lost 95 percent of its metal within six months of installation. This could be attributed to a breach in the linings due to discontinuities in the coating system and turbulence and high chlorides and sulfurs present in the cooling water. These would have compromised the integrity of the steel pipe.
The importance of following a procedure, verifying that the testing equipment is in good working condition, calibrating equipment (certificates may come from the manufacturer), and reading the manufacturer’s instructions before the use of any testing equipment could have prevented the problems during the installation of the coatings on this pipe. And that could have saved the contractor and client time and money.
About the Author:
Louis Sharp is a senior consultant for Energy Project Resources. He has 23 years of experience in the protective coatings industry. For the past eight years, he has been completing failure analyses on large-scale projects. Sharp has a NACE Level 3 Coating Inspector Certification, belongs to NACE, Society for Protective Coatings (SSPC), and ASTM. For more information, contact: Louis Sharp, firstname.lastname@example.org