ASME TDP-1-2006 pdf download

ASME TDP-1-2006 pdf download.Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation.
3.1.3 It is recommended that start-up systems on other than once-through units (such as drum type) be designed so that no single failure of equipment can result in water entering the motive steam line. The design method to best accomplish this objective should be determined by the designer.
3.1.4 Special consideration should be given to drum level control, flash tank level control, and/or leveling vessel control design to realize a high degree of reliability such that no failure of instrumentation or equipment results in a release of water into the motive steam lines.
3.1.5 Heat recovery steam generator (HRSG) system configurations typically include as many as three steam drums, each with level controlled by feedwater valve modulation and condensate or feed pump recirculation or similar method of controlling inflows. The same plant design requirements that apply to other steam generators apply to HRSGs.
3.2 Steam Attemperators
This paragraph applies to all steam attemperators in or connected to motive steam and steam generator systems.
3.2.1 Spray water introduced into the steam generator ahead of the final superheat section is a means to control steam temperature at the superheater outlet.
These sprays are generally not effective in controlling final superheat steam temperature at low loads or during turbine roll. The opportunity exists for water to accumulate in the pendant elements of the superheater during low load operation from either condensation or overspraying. Units that operate for extended periods of time with the spray header system charged to full
pump discharge pressure (e.g., during start-up and shut-down conditions) are subject to possible leakage of the spray valves. Such leakage can result in water accumulating in the pendant superheater sections and may even flow over into the main steam system. When steam flow is increased rapidly, this accumulation of water can be injected into the turbine.
3.2.2 Spray water injection in the cold reheat line or between primary and final reheat sections is used as a means to control steam temperature at the reheater outlet. These sprays are not effective or normally required for reducing final reheat steam temperature when used at low loads or during turbine rolling. Most incidents of turbine water damage caused by attemperators have occurred during these periods as a result of overspraying. The water accumulates and flows back to the turbine, in most cases because of low steam velocity and the arrangement of the piping. Another potential source of water results when water accumulates from condensation in pendant elements of the reheater during a low load operation. This water can be injected into the turbine if flow is increased rapidly.
3.2.3 The use of attemperators external to the steam generator, downstream of the last superheating (or reheating) element, is discouraged; however, it is recognized that under some conditions it cannot be avoided. When this type of attemperator is required in the motive steam line to control the temperature of the steam entering a steam turbine, the following features should be provided in addition to the other features recommended in paras. 3.2.5 through 3.2.14:
(a) The attemperators should not be allowed to operate when the steam exiting the attemperator will contain less than 50°F (28°C) of superheat unless a higher temperature is required by the turbine manufacturer.
(b) The attemperator should be located as far from the steam turbine inlet as possible, but in no case closer than 50 pipe diameters.
(c) The attemperator should be located such that any associated water accumulation will drain in the direction of steam flow to a drain pot located at an elevation lower than the connection to the steam turbine. Thus, the piping should be sloped toward the drain pot, and the drain pot should be located between the attemperator and the steam turbine.