With the advancements in combined cycle, steam generation and co-generation technology systems, ASME has published an updated version of its "Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation - Fossil Fueled Plants," ASME TDP-1 2006. TDP-1 was originally published in 1972 and subsequently revised in 1979, 1985 and 1998. The new standard was rewritten to include combined-cycle configurations, multiple steam generator configurations and cogeneration technologies. The standard also was revised to address plant cycling and modern plant instrument and control systems. This article will summarize the major changes included in the updated standard.
TDP-1 was initially developed in response to a rash of water induction incidents in the 1960s as power plants scaled up above 150 MW. TDP-1 now includes conventional steam (Rankine) cycle and Combined-Cycle power plants. Nuclear power plants are covered under TDP-2.
TDP-1 is a Recommended Practice and not a mandatory code - if you want the features put forth in TDP-1
Water induction damage
Water induction can damage steam turbines in several ways. The damage can be caused by the impact of large slugs of water or by the quenching effect of cold water on hot metal. The severity of water damage can vary from minor seal rubs all the way to catastrophic damage to the turbine. Generally, water damage falls into the following categories:
- Thrust bearing failure
- Damaged blades
- Thermal cracking
- Rub damage
- Permanent warping distortion
- Secondary effects
Secondary effects include items such as seal packing ring damage, pipe hangar and support damage, damage to instrumentation and controls, etc.
Sources of water induction
Water can be inducted into a steam turbine from several sources. The following are some of the most common sources of water:
- Motive steam systems
- Steam attemperation systems
- Turbine extraction/admission systems
- Feedwater heaters
- Turbine drain system
- Turbine steam seal system
- Start-up systems
- Condenser steam and water dumps (steam bypass)
- Steam generator sources
Figure 1 shows the percentage of water induction incidents attributed to the most common sources of water in conventional steam cycles. Although water induction into the high and intermediate pressure turbines has historically been recognized as the most damaging, experience has shown that water induction in low pressure turbines also can cause significant damage and should be taken seriously.
Water induction can happen at any time; however the most common situations are during transients such as start up, shut down and load changes. Figure 2 illustrates the percentage of times various events contribute to water induction for a conventional steam cycle. It is interesting that only 18 percent of water induction incidents occur when the unit is at load.
TDP-1 offers guidance on how to identify systems that have the potential to allow water to enter the turbine and to design, control, maintain, test and operate these systems in a manner that prevents any significant accumulation of water. This is the first line of defense in preventing turbine water damage.
However, it is recognized that malfunctions do occur, so TDP-1 offers recommendations for preventing turbine damage that include: detection of the presence of water either in the turbine or, preferably, external to the turbine before the water has caused damage; isolation of the water by manual or, preferably, automatic means after it has been detected; and disposal of the water by either manual or, preferably, automatic means after it has been detected. The philosophy of TDP-1 has been, and will continue to be, that "no single failure of equipment, device, signal or loss of electrical power should result in water or cold steam entering the turbine."
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