Background
The accidental introduction of water in any part of a steam turbine can cause serious damage, requiring extended outages to make costly repairs. A proliferation of such incidents in the electric power generation industry prompted the American Society of Mechanical Engineers (ASME) Standards Committee to develop a uniform set of design criteria to alleviate the problem. With the recent update of ASME Turbine Water Induction Protection (TWIP) standards and the continued focus on fossil plant reliability, TWIP system upgrades have gained increased focus as capital improvement projects in the industry. TG Advisers has identified many clients that have experienced major water induction turbine outages, resulting in millions of dollars of damage. In the extreme case, full blade replacements and rotor straightening technologies have been employed to return the unit to service. Forced outage durations can extend to 6 months or more. The following photo was taken after an LP heater level control failure on a 1960s vintage General Electric LP rotor design. Extensive
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In more severe water induction incidents, turbine rotors have been permanently bowed. Repairs in these cases could be as extreme as full rotor replacement, or a combination of weld repair or heat treatment of the bow location in the rotor.
The proliferation of combined-cycle units adds to the water induction potential due to the increase in cycling operation. During a “hot” start, even a short period of accidently admitting saturated steam to the hot steam turbine can quench the hot rotor, damaging components and embrittling stationary blade root seals, radial tip seals, admission end shaft seals and balancing piston seals (where applicable). These might not result in the immediate need to open the turbine, but can result in greatly increased heat rate and operating cost. Likewise, units that cycle frequently have had steam seal issues when auxiliary boilers fail to provide sufficient superheat in the seal steam, or when a slug of saturated liquid is injected into the hot steam seals.
Causes of water induction
There are many sources of water induction in a steam turbine. The following is a list of the most common sources that are addressed in the ASME TDP-1 standard:
- Motive steam systems
- Steam attemperator systems
- Turbine extraction/admission systems
- Feedwater heaters
- Turbine drain systems
- Turbine steam seal systems
- Start-up systems
- Condenser steam and water dumps (steam bypass)
- Steam generator sources
Generating units that were designed and built prior to the development of the design criteria are consequently at risk. To properly assess the status of steam cycle piping systems with regard to the current design criteria, a Steam Turbine Water Induction Protection Unit Checklist is used. Input for the program can be completed by plant and engineering personnel that are familiar with the plants configuration and operation. The program then compares plant equipment to the ASME standards and identifies where additional protection schemes are required.
For example, typical shortfalls that have been identified on vintage steam turbine generator feedwater systems include the following:
- Upgraded level transmitters and associated modifications of the digital control system to include inputs and new alarm outputs. Many units use single alarm conditions to manage water induction protection devices; however the ASME standard recommends using 2-out-of-3 alarm logic to minimize false alarm operation of these protection devices.
- Installation of alternate feedwater heater drains to the condenser with power operated block valves. Normally feedwater heater systems are equipped only with the normal cascading drain lines to the next lower pressure heater. In some cases there might be an alternate drain from the first high pressure feedwater heater to the last low pressure heater when bypassing the deaerator is required. In many cases, alternate drain lines from the feedwater heaters to the condenser are required to meet ASME code requirements.
- Installation of extraction steam line power operated block valves. Most units have power assisted non-return valves installed in the steam extraction lines; however these valves are not leak tight and were originally installed for overspeed protection, not water induction protection. To meet the ASME standard, a power operated extraction steam line block valve is required. This also requires drain pots ahead of and following the power operated block valves, as well as power operated drains from these drain pots to the condenser or an atmospheric drain tank.
- Installation of feedwater block and bypass power operated valves where extraction steam line power operated block valves are not practical. Many low pressure feedwater heaters were equipped only with manual block and bypass valves. If the level in these low pressure heaters increases rapidly due to a tube leak, the normal drain system might not be able to maintain the heater level and water can be introduced to the LP turbine. In this case, the only way to protect the turbine is to rapidly bypass the condensate side of the feedwater heater.
- Modifications of steam system drain connection to manifolds at the condenser are often required. Some older units have steam systems that drain to the condenser at manifolds that are installed incorrectly. These drains often enter the manifold at 90 degrees. If one drain system discharges a high volume of steam in the manifold, it might result in back flowing condensate into the drains connected to the manifold further from the condenser wall. All drain connection to the manifold should be introduced at a 45 degree angle directed toward the condenser to prevent this.
