Intermittent (Bottom) Blowdown
Suspended solids in the boiler can be suspended as long as the boiler water is active (turbulent), but as soon as the agitation stops, it sinks to the bottom of the boiler. If the solids that settle to the bottom are not removed from the boiler, it will prevent heat transfer from the boiler flame-smoke pipes within a certain period of time and this may cause overheating and even deterioration (collapse) in the boiler construction.
The recommended method for removing this sludge is to blowdown from the bottom of the boiler by means of a blowdown valve. The purpose of doing it intermittently is to allow time for the sludge to redistribute itself so that more can be removed at the next blowdown. This is why a single four-second blowdown every eight hours is much more effective than a long blowdown every twelve hours.
With Vira BK 4000 Bottom Blowdown Controller, you can set the desired bottom blowdown time and how long you want to do. In addition, if the BKV 4000 Bottom Blowdown valve remains open or does not open for any reason, you can receive its information and intervene. Also, you can continuously monitor information such as the remaining time to the next blowdown and the set time via the controller.
The blowdown water is released into either a brick lined drain pit placed underground or a steel blowdown vessel above ground. The size of the blowdown tank is determined by the flow rate of the blowdown water and flash steam entering the vessel when the blowdown valve is opened.
Although the use of a blowdown tank is not compulsory in Turkey, there are regulations in many countries of the world regarding the temperature of the drained water as it may harm the environment and human health, and the use of a blowdown tank becomes mandatory due to these regulations. For example, in England, it is desirable that the drained blowdown water should not be above 43 °C and the blowdown ratio should not be more than 5% of the boiler capacity. When these conditions are met, most of the time the water in the blowdown tank is naturally cooled and drained through heat transfer and heat dissipation. However, if the boiler design and system conditions do not comply with these regulations, an extra cooling system may be needed in the blowdown tank.
The main effects on the blowdown rate are:
- Boiler Pressure
- Blowdown Line Size
- Length of the blowdown line between the boiler and the blowdown tank
In practice, a reasonable blowdown line length is 7.5 m and often sizing is done accordingly. The blowdown lines will include elbows, check valves, and the blowdown valve itself, and these parts will cause a pressure drop along the blowdown line. From the tables, the equivalent pipe length can be calculated according to the diameters of these elbows, valves, check valves, etc., and can be added to the total pipe length.