In a high pressure large capacity boiler, the drum acts not only as a water reservoir, but also has a major function of separating the steam water mixture coming into it from the raisers or waterwall tubes. This is done by a set of separators and dryers housed inside the drum. Dry steam with the quality required for superheating and admitting into the turbine is the output of a drum with good internal design.
Depending upon boiler operating pressure, the boiler water and superheated steam quality are decided. Water treatment plants are provided in boiler plant installations to ensure the quality of boiler water. Parameters like alkalinity of water expressed in pH levels, total dissolved solids in ppm, specific electrical conductivity of water in µS/cm, residual phosphate in ppm, and silica in ppm level are to be maintained with in specific limits. This is achieved by conditioning feed water to boiler within preset quality levels.
The typical recommended boiler water limits for a boiler with operating pressure range from 160 to 180 kg/cm2 (g) is given below.
- Total dissolved solids (Max) ppm 25
- Specific electrical conductivity at 25 deg C in µS/cm (Max) 50
- Phosphate residual ppm 5 to 10
- pH at 25 deg C 9.1 to 9.8
- Silica (Max) ppm – To be controlled on the basis of silica in boiler water and drum pressure relationship to maintain less than 0.02 ppm in steam leaving drum.
Carryover from the drum can be of two types: mechanical carryover and vaporous carryover. Mechanical carryover take place in a boiler mainly due to drum internals; drum level, foaming, and spray water. If the alignment of drum internals are not maintained as per requirement then the separation efficiency comes down and carryover occurs. Faulty screen driers also contribute to carryover. It is necessary to adhere to the dos and don’ts given by the designer during operation and maintenance of the boiler to avoid carryover. Maintaining a very high drum level reduces the separation efficiency and a high level of carryover results. High alkalinity, oil in water and high TDS (Total Dissolved Solids) in boiler water will result in foaming in drum which leads to carryover. The chart attached gives a total picture of carryover.
Carryover can be minimized by adhering to good operating and maintenance practices.
- Maintain drum water level within +/- 50 mm
- Maintain silica below the recommended value for the operating pressure
- Check and maintain the drum internals periodically and whenever carry over is noticed
- Feed water conductivity at 25 deg C to be maintained between 0.2 and 0.3 µS/cm
- Boiler water solids to be maintained during operation within limits
- Measure steam purity and maintain within the specified limits
- Ensure feed water conductivity meter online to help monitoring the same
The end result of carryover will lead to deposition in superheater tubes which will result in long term overheating of superheater tubes. Carryover will also lead to deposits in the low pressure stage blades of turbine which reduces blade efficiency and increase heat rate of the machine.