Impacts of Cycling - Turbine Water Induction Events
Protect your assets by preventing water induction events
29 September 2015
Water induction events at both nuclear and fossil power plants are a continuing problem for the power industry, with an estimated two dozen events every year in the United States alone. This is an extremely damaging situation and continues to be poorly understood by many system owners.
These events have increased as units are required to operate flexibly - the vast majority of induction events occur during startup, shutdown, unit trips, and load swings. As the risk of these events increases, operator training and a defense-in-depth approach becomes more important.
The damage that can occur due to water in the turbine at operating speed includes thrust bearing failure, damage to rotating and stationary blades, and thermal cracking of components. The stress on components can also lead to secondary effects as tie wires and shrouds break off. The additional thrust combined with quenching of the rotor and warping of the shell can lead to rub damage on diaphragms and packing as well as permanent distortion of the casing if the elastic limit of the metal is exceeded – meaning a long-term operational problem for the unit.
Several factors have been identified that indicate a unit is at higher risk for a water induction event. Leaking condensate or feedwater heater tubes, inoperative header drains, damaged non-return valves, and high drum levels or a history of water hammers should raise a red flag that an issue exists and should be addressed quickly. Plant operators should be trained to identify the signs of a water induction event and to act accordingly because immediate action is required to prevent damage. Be aware that step changes in vibration, thrust bearing alarms, rotor short conditions and asymetrical casing expansion combined with hammering in steam lines and sharp drops in metal temperatures indicate water damage is imminent. After an event, consequences include increased vibration, decreased MW output and stage efficiency, and an unusually rapid coastdown due to rubbing.
If a water induction event is suspected during startup, operators should shut down and identify the source. When water induction occurs at rated load, do not trip the unit unless vibration and differential expansion are outside limits. Instead, the source of water must be identified and isolated. Once a unit trips, and the stop valve closes, casing pressure will drop, and there is nothing to resist large amounts of additional water entering through the extraction lines and causing more severe damage.
While identification and training is important, preventing water induction is the preferred strategy. A proactive defense scheme involves a means of detecting water before it enters the turbine, procedures to isolate and remove any accumulated water, and most importantly, the systems that prevent water induction should be designed so no single-point vulnerability exists. Section 2 of ASME TDP-1 outlines methods for detection, isolation, and removal of water through manual and automatic means.
Water induction events are highly damaging, yet preventable. Addressing this issue requires a proactive approach, understanding of the systems that contribute to induction events, and support from system owners and management, but these incidents can be reduced or eliminated entirely. Water induction is a tremendously damaging event, so attention on the front end is necessary to protect your asset and ensure high availability long into the future.
Grant Lanthorn P.E. is a Project Engineer with Intertek’s Asset Integrity Management Group. He has seven years of experience in the power industry and provides consulting on topics including plant operations and maintenance, proactive lubrication practices, and fleet cycling readiness. Before joining Intertek, Grant worked as a turbine engineer for an R&D firm and focused on developing innovative solutions for nuclear and fossil utilities worldwide.