why ? we need conduct boiler blow down
In this article we discuss about the boiler blow down, how it conduct? At what condition we required the boiler blow down? What are the advantage and disadvantage of boiler blow down? Due to continuous evaporation of boiler water, salts concentration in boiler water increase. Also due to phosphate treatment some non adherent sludge is formed. So the TDS total dissolved solids level of boiler water increase. To adjust this total dissolved solids level some quantity of boiler is removed from boiler and same quality of fresh water is added.
By doing so concentration of non desirable dissolved salts is maintained. This process is called as blow down process and the water drained out is called as blow down. Blow down may be continuous or intermittent. In continuous blow down some quantity of boiler water is continuous taken out from boiler drum and continuous fresh water make up, in case of intermittent blow down, blow down is given from the bottom mud to remove the settled sludge.
Boiler water salt concentration is monitored regularly. Blow down is given for a short duration when concentration level goes beyond a predetermined value. And blow down is stopped at recommended lower concentration level. The concentration level varies between higher and lower limits. During blow down some loss of energy take place , as the blow down water contains significant amount of sensible heat.
Significant loss during boiler blow down
The loss is significant when blow down quantity is more. Suitable arrangement is made to utilize the sensible heat of this blow down water .In most of the cases, blow down water is collected in a header placed just below the normal water level of steam Drum. Water is drained through a flash tank. In flash tank, pressure of water drops significantly. As the water contains lot of sensible heat corresponding to drum pressure, so flash steam is produced. This steam can be utilized suitably normally this steam is used in Deaerator for feed water heating. Before draining out the blow down water, heat can be removed from this water by placing heat exchanger.
Blowdown is required to control the concentration of the solids inside the steam drum , and these solid concentration is controlled by the process of ‘blowing down,’ where a certain volume of water is blown off and is automatically replaced by feed water thus maintaining the optimum level of total dissolved solids (TDS) in the boiler water. Blow down is essentially to protect the surfaces of the heat exchanger in the boiler. However, blow down can be a significant source of heat loss, if blow down is not carried out properly.
Monitoring boiler blow down
Boiler blow down is most important term in boiler operation and monitoring. The parameters of the blow down are to be maintained for particular boiler. This is due to the fact that water inside the boiler gets continuously evaporated due to steam generation. Concentration of these dissolve solids, inside the drum increases and reach beyond the permissible limit, so carryover of solids along with steam can occur. To prevent boiler tube chocking and overheating of the boiler tubes the blow down is necessary.
When we conduct boiler blow down it’s removed the water from boiler to maintain the solids level in the boiler drum. This can be calculated as follows:
E = Evaporation or steam generation rate.
S = Amount of solids (ppm)
B = Blow down (m3/ hr)
C = Maximum permissible concentration of solid inside the boiler drum
B = E*S ( m3 / hr.) / C-S
% B = S/C-S
It’s a important formula which is applied to control individual constituents in boiler. Blow down means, “Lost of heat”. Its due to any water inside the boiler drum, which is getting drained from the boiler drum, will be at the saturation temperature of steam and this has been heated starting from water of ambient temperature.
Benefits of Carrying Blow down in a boiler:
A significant boiler blow down control can reduce treatment and operational costs of a boiler that includes:
1.A significant boiler blow down Increased boiler life.
2. Less makeup water consumption due to maintaining the TDS level inside the drum.
3. Reduced maintenance downtime of a boiler.
4. Lower consumption of treatment chemicals and Lower pre-treatment costs.
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