Boiler blowdown, one of the essential boiler operating practices, causes you a huge amount of energy loss either knowingly or unknowingly. For energy optimization and better efficiency of the boiler system, it is very essential to control the blowdown in your steam boiler. However, the amount of blowdown depends on many factors, like feed water quality and makeup water requirement. But if the blowdown water is less than the required, then a problem Carry-Over arises in the boiler. which leads to the degradation of steam quality. And an excess amount of BD water causes more heat losses along with water and treatment chemicals. So, controlling the blowdown is not only required for energy optimization but for better steam processing also.

Why Blowdown

Even with the best pretreatment programs, boiler feedwater often contains some degree of impurities, such as suspended and dissolved solids. The impurities can remain and accumulate inside the boiler as the boiler operation continues. The increasing concentration of dissolved solids may lead to carryover of boiler water into the steam, causing damage to piping, steam traps and even process equipment. The increasing concentration of suspended solids can form sludge, which impairs boiler efficiency and heat transfer capability. 

To avoid boiler problems, water must be periodically discharged or “blown down” from the boiler to control the concentrations of suspended and total dissolved solids in the boiler. Surface water blowdown is often done continuously to reduce the level of dissolved solids, and bottom blowdown is performed periodically to remove sludge from the bottom of the boiler.

• Intermittent Blowdown

The intermittent blown down is given by manually operating a valve fitted to the discharge pipe at the lowest point of the boiler shell to reduce parameters within prescribed limits so that steam quality is not likely to be affected. In intermittent blowdown, a large diameter line is opened for a short period of time, the time being based on a thumb rule such as “once in a shift for 2 minutes”. Intermittent blowdown requires large short-term increases in the amount of feed water put into the boiler, and hence may necessitate larger feed water pumps than if continuous blowdown is used. Also, TDS level will be varying, thereby causing fluctuations of the water level in the boiler due to changes in steam bubble size and distribution which accompany changes in concentration of solids. Also a substantial amount of heat energy is lost with intermittent blowdown. 

Intermittent manual blowdown is designed to remove suspended solids, including any sludge formed in the boiler water. The manual blowdown take-off is usually located in the bottom of the lowest boiler drum, where any sludge formed would tend to settle. Properly controlled intermittent manual blowdown removes suspended solids, allowing satisfactory boiler operation. Most industrial boiler systems contain both a manual intermittent blowdown and a continuous blowdown system. 

In practice, the manual blowdown valves are opened periodically in accordance with an operating schedule. To optimize suspended solids removal and operating economy, frequent short blows are preferred to infrequent lengthy blows. Very little sludge is formed in systems using boiler feedwater of exceptionally high quality. The manual blowdown can be less frequent in these systems than in those using feedwater that is contaminated with hardness or iron. The water treatment consultant can recommend an appropriate manual blowdown schedule. Blowdown valves on the water wall headers of a boiler should be operated in strict accordance with the manufacturer’s recommendations. Usually, due to possible circulation problems, water wall headers are not blown down while the unit is steaming. A blowdown normally takes place when the unit is taken out of service or banked. The water level should be watched closely during periods of manual blowdown.

• Continuous Blowdown

Continuous blowdown, as the term implies, is the continuous removal of water from the boiler. It offers many advantages not provided by the use of bottom blowdown alone. For instance, water may be removed from the location of the highest dissolved solids in the boiler water. As a result, proper boiler water quality can be maintained at all times. Also, a maximum of dissolved solids may be removed with minimal loss of water and heat from the boiler. 

Another major benefit of continuous blowdown is the recovery of a large amount of its heat content through the use of blowdown flash tanks and heat exchangers. Control valve settings must be adjusted regularly to increase or decrease the blowdown according to control test results and to maintain close control of boiler water concentrations at all times. When continuous blowdown is used, manual blowdown is usually limited to approximately one short blow per shift to remove suspended solids which may have settled out near the manual blowdown connection.

The blowdown water percentage required can be calculated with the help of given formula-

Blowdown % = feed water TDS % of makeup water Permissible TDS – Feed water TDS

For example, A 2 TPH boiler with 90% condensate recovery system working on feed water having 300 ppm TDS and the maximum limit of the TDS is 3000 ppm. Then,

Blowdown % = 300103000-300= 1.11% 

= 1.112000100= 22.22 kg/hr

Loss due to improper Blowdown

If the blowdown percentage is not optimised it can lead to a huge amount of energy as well as water loss. 

Considering the above example, if blowdown is 2% instead of 1.11%

The excess amount of water = 40-22.2 = 17.8 kg/hr

If boiler operates 12hr per day for 300 days in a year then 

= 17.8 * 12 * 300 = 64,080 kg of water

The boiler operating pressure is 10 kg/cm2 then energy associated with the water = 64,080 * 180 = 1,15,34,400 kcal in a year

Operating fuel of the boiler is coal of CV 5300 kcal/kg

Then equivalent fuel wasted in a year = 2176.30 kg

Cost of fuel is Rs 20 per Kg then,

equivalent money wasted in a year in the form of excess blowdown water = 2176.30 * 20 = 43,526 Only  0.9 % of excess blowdown causes you rupees 43,526loss

Boiler blowdown is the removal of water from a boiler. Its purpose is to control boiler water parameters within prescribed limits to minimize scale, corrosion, carryover, and other specific problems. Blowdown is also used to remove suspended solids present in the system. These solids are caused by feed water contamination, by internal chemical treatment precipitates, or by exceeding the solubility limits of otherwise soluble salts. The blowdown can range from less than 1% when an extremely high-quality feedwater is available to greater than 20% in a critical system with poor-quality feedwater. In plants with sodium zeolite softened makeup water, the percentage is commonly determined by means of a chloride test. In higher-pressure boilers, a soluble, inert material may be added to the boiler water as a tracer to determine the percentage of blowdown. The primary purpose of blowdown is to maintain the solid content of boiler water within certain limits. This may be required for specific reasons, such as contamination of the boiler water. In this case, a high blowdown rate is required to eliminate the contaminants as rapidly as possible. 

The blowdown rate required for a particular boiler depends on the boiler design, the operating conditions, and the feedwater contaminant levels. In many systems, the blowdown rate is determined according to total dissolved solids. In other systems, alkalinity, silica, or suspended solids levels determine the required blowdown rate. An automatic blowdown control system continuously monitors the boiler water, adjusts the rate of blowdown, and maintains the specific conductance of the boiler water at the desired level.