Data Sheet: The need for Boiler Blowdown Controllers

Boiler feed water contains dissolved solids, both from raw water and water treatment chemicals. While a water softening plant reduces the “hardness” (i.e. the presence of scale forming salts), it does not reduce the TDS of the feed water. As steam is raised from a boiler, the level of concentration of Total Dissolved Solids (TDS) in the boiler water increases. If uncontrolled, the TDS level rises above the set point for a particular boiler, leading to the following problems:

Foaming & Carryover
Pure water does not foam when it boils. However, as the amount of impurities rise, a foam layer is formed at the steam separation surface. The amount of foaming is directly proportional to the TDS level in the boiler. Foaming (or “priming”) causes carryover of water, or wet contaminated steam, which may be carried over into the steam system. The products of carryover would be deposited on heat transfer surfaces and ancillary equipment, reducing steam system efficiency and plant productivity. This is what causes fouling of heat exchangers, malfunctioning of control valves and steam traps etc.

Scale Deposition
If the TDS is too high, scale will deposit on the boiler tubes and furnace (water side). This has the effect of reducing heat transfer with its subsequent effect on fuel consumption and safe operation of the boiler.

A scale deposit 1mm thick on the water side could increase fuel consumption by 5 to 8%
(Source: PCRA Handbook No. 3 - Efficient Generation of Steam)
Tube surfaces underneath the scale may become overheated leading to tube damage or tube failure. Figure 1 shows the effect of varying thickness of common scales on tube temperature.
In order to prevent these problems, the TDS needs to be controlled within a certain specified maximum limit. The chart below shows the recommended water characteristics for shell boilers in accordance with IS: 10392-1982 and BS: 2486-1964, for pressures up to 25 bar g:

Blowdown of the boiler can keep TDS within the required limits. Blowdown is achieved either by manual or automatic methods. In the manual method, blowdown is achieved by opening a large bore valve at the bottom of the drum (or on the side of the drum in case of continuous blowdown). However, this practice can be highly wasteful. As the period of blowdown is not related with either boiler steam load or feedwater purity, the TDS level in manual methods can vary greatly, causing an average TDS level much lower than the allowable limit, and leading to excess blowdown.

On the other hand, an automatic blowdown control system, based on TDS measurement and subsequent corrective action, can maintain a TDS level much closer to the set point, resulting in considerable fuel savings.

As seen in the graphs above, the automatic control of TDS results in an average TDS level much closer to the set point. This means that the actual quantity of blowdown over a period of time gets reduced compared to the manual method. Blowdown water is water that has been heated to the saturation temperature of the boiler, so it contains a lot of heat. At a boiler pressure of 10.5 Kg/cm2 g, each kg of blowdown water contains almost 190 kcal of heat energy. If an automatic boiler controller can reduce the blowdown of a 10 TPH boiler from 6% to 3%, i.e. a saving of 3%, the blowdown quantity would reduce by 300 Kg/hr, or 7200 kg/day. This would mean a saving of 1368000 kcal/day. This would mean a fuel saving of approximately 180 litres of oil, if the boiler was fired with furnace oil. The cost benefit of preventing corrosion in the boiler and the steam system, though it cannot be quantified exactly, would be in addition.