Truth is in the Details ™

The Case of the Hasty Hydro Repair

Sometimes, haste indeed makes waste. The financial pressure to repair major revenue producing machinery can sometimes substantially increase the revenue lost. Contractors who succumb to the owner’s pressure often bear the bitter fruit of their haste.

A control malfunction in a hydroelectric generating facility resulted in rapid opening and closing of the valve that admits water to the turbine, causing a turbine shaft seal to leak. Repairs were completed at a cost of about $1/4 million. On completion of repair, the generator was restarted and leaked again, resulting in another $1/4 million repair.

TI was requested to separate the cost of the original repair from that of the second, and to determine the cause of the second incident of shaft seal leakage. The involved shaft seal was a face seal that consisted of a phenolic stationary member and a ceramic member that rotated with the shaft. Because of the large diameter of the shaft, the seal was split so that it could be removed for repair. Design of the turbine drive required a limited amount of axial movement for the turbine shaft and in order to accommodate this movement the seal components were spring loaded which allowed the shaft to slightly slide back and forth through the seal. In order to prevent leakage, a large O-ring slid within an assembly consisting of a fixed, inside diameter on which the O-ring slid and a movable outside diameter, clamped to the turbine shaft, on which the O-ring was mounted in a groove. Repair work was overseen and it was observed that when the turbine shaft seal was removed and disassembled there was severe damage to the large O-ring seal.

The O-ring was replaced in the initial repair. Inspection of both the outer sliding surface and inner O-ring groove found that both were devoid of grease, which is extremely important not only for operation but for damage-free assembly. In its haste to repair the turbine the contractor had failed to grease the surfaces for the O-ring seal. Costs of the first and second repair were separated and the owner paid only for the first repair.

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