ASME’s latest revision of its Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation: Fossil-Fuel Plants, ASME TDP-1-2006, contains much important design and operating advice that is proven to protect steam turbines. However, many in the industry are not as familiar with the update as they should be. This article provides a concise overview of this critical design standard.
ASME TDP-1, Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation: Fossil-Fuel Plants, was initially developed in response to a rash of water induction incidents in the 1960s as power plant ratings were scaled up above 150 MW. TDP-1 was originally published in 1972; revisions were made in 1979, 1985, 1998, and 2006. The latest revision to TDP-1 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 therefore not a mandatory code; if you want the features described in TDP-1 to be included in your plant design specifications, it must be specifically called out in your contract. Merely writing into your plant specifications the requirement to "comply with all ASME standards" will not automatically include this recommended practice.
Sources of Water Induction
Water can be inducted into a steam turbine from several sources, including these:
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Motive steam systems
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Steam attemperation systems
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Turbine extraction/admission systems
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Feedwater heaters
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Turbine drain systems
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Turbine steam seal systems
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Start-up systems
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Condenser steam and water dumps (steam bypass)
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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 high-pressure (HP) and intermediate-pressure (IP) turbines has historically been recognized as the most damaging, experience has shown that water induction into low-pressure (LP) turbines can also cause significant damage and should be taken seriously.
1. Sources of water induction. Source: Serge P. Barton et al., “A Water Induction Monitor for Steam Turbines” (ASME)
Water induction can happen at any time; however, it is most common during transients such as those that occur during start-up, shutdown, 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% of water induction incidents occur when the unit is at load.
2. When water induction occurs. Source: Serge P. Barton et al., “A Water Induction Monitor for Steam Turbines” (ASME)
When water induction does occur, it can damage steam turbines in several ways. The damage is usually 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:
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Thrust bearing failure
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Damaged blades/buckets
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Thermal cracking
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Rub damage
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Permanent warping distortion
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Secondary effects, including seal packing ring damage, pipe hangar and support damage, and damage to instrumentation and controls
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