Tube-side feed water heater (FWH) – Heat Exchanger Inspection

Background

A feed water heater (FWH) is a specialized heat exchanger used primarily at nuclear and fossil fuel electric power plants. The FWH improves the plant’s “heat rate” or thermal efficiency by using the steam turbine’s exhaust steam energy (heat) to preheat the boiler feed water coming from the condenser and before it gets returned to the boiler.

Large (>300MW) plants can have up to 14 FWHs normally configured in two parallel strings ranging from high-pressure to low-pressure. Each FWH in a string is responsible for incrementally lowering the exhaust steam temperature while incrementally raising the boiler feed water temperature.

Control room operators can tell that something is wrong with a FWH when temperature, pressure, flow or heat rate changes. FWH’s can be isolated during normal plant operation (for inspection and maintenance), but at the expense of heat rate.

Inspection

 A customer identifi ed a FWH tube leak in a newly installed unit. After isolating and opening the FWH channel head, direct visual observation showed leaks in 4 – 6 clustered tubes. RVI was used to determine the exact axial location of the leak and the root cause of the tube failure.

The RVI data convinced plant management that there were manufacturing defects in the new FWH. The root cause of the failure was over-rolling of the tube into the tube sheet which caused both localized thinning and a stress riser at the tube OD/tube sheet interface.

The customer used the RVI data to receive warranty claims from the FWH manufacturer.

Top of tube sheet in lower left corner. Failed tube on right side. Undamaged tube in upper left.	A steel-stay-road inserted into the failed tube to prevent vibration and subsequent damage to the adjacent tubes.