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Boiler blow down procedure types, benefits, calculation
BOILER OPERATION
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Boiler blow down procedure types, benefits, calculation
Today we discussed about a very important article “Boiler blow down procedure types, benefits, calculation ” in this article we learn about the boiler blow down and types of blow down , benefits of boiler blowdown and what is the importance of boiler blow down and also know about the boiler calculation when water is boiled and steam is generated any dissolved solid contained in the water remain in the boiler. If more solids are put in with the feed water they will concentrate and may eventually reach a level where their solubility in the water is exceed and they deposit from the solution above a certain level of concentration these solids encourage foaming and causes carryover of water into the steam.
Also due to the phosphate treatment some non adherent sludge is formed so total dissolved solids (TDS ) level of boiler water increases. The deposits also lead to scale formation inside the boiler resulting in localized overheating and finally causing boiler tube failure. It is therefore necessary to control the level of concentration of the solids and this is achieved by the process of “blow down ” where a certain volume of water is blow 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 necessary to protect the surfaces of the heat exchanger in the boiler. However, blow down can be a significant source of heat loss , if improperly carried out.
Benefits of boiler blow down :
-Lower pretreatment cost
-Less make up water consumption
-Reduced maintenance downtime
-increased boiler life
-Lower consumption of treatment chemicals
-Conventional methods for blowing down the boiler depend on two kinds of blowdown – intermittent and continuous
IBD ( intermittent blow down ) :-
The intermittent blow down is given by manually operating valve fitted to discharge pipe at the lowest point of the boiler shell to reduce parameters ( TDS or conductivity , ph , silica and phosphate concentration ) within the prescribed limits so that the st2 quality is not likely to be affected. In intermittent blow down large diameter line is opened for a short period of time, the time begin based on a thumb rule such as “once in a shift for 2 minutes “. Intermediate blow down requires large short terms increases in the amount of feed water put into the boiler and hence may necessitate large feed water pumps than if continuous blow down 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 substantial amount of heat energy is lost with intermittent blow down.
CBD (continuous blow down)
there is a steady and constant dispatch of small stream of concentrated boiler water and replacement by steady and constant in flow of feed water. This ensures constant TDS and steam purity at given steam load . Once blow down valve is set for a given conditions there is no need for regular operator intervention. Even though large quantities of heat are wasted, opportunity exists for recovering this heat by blowing into a flash tank and generating flash steam. This flash stem can be used for preheating boiler feed water or for any other purpose. This type of blow down is common in high pressure boilers.
Conductivity as Indicator of Boiler Water Quality
Measure total dissolved solids (TDS) in boiler water system are a tedious and time consuming process. Conductivity measurement is used for monitoring the overall TDS present in the boiler. A rise in conductivity indicates a rise in the “contamination” of the boiler water.
ASME (American Society of Mechanical Engineers) Water standards parameters for water tube boilers
| Boiler Pressure (psig) | Iron Concentration (ppm) | Copper Concentration (ppm) | Hardness CaCO3 (ppm) | Silica Concentration (ppm) | Alkalinity CaCO3 (ppm) | Total Dissolved Solids – TDS – (ppm) | Specific Conductivity (μS/cm)* |
| 0 – 300 | 0.1 | 0.05 | 0.3 | 150 | 700 | 3500 | 7000 |
| 300 – 450 | 0.05 | 0.025 | 0.3 | 90 | 600 | 3000 | 6000 |
| 450 – 600 | 0.03 | 0.02 | 0.2 | 40 | 500 | 2500 | 5000 |
| 600 – 750 | 0.025 | 0.02 | 0.2 | 30 | 400 | 1000 | 4000 |
| 750 – 900 | 0.02 | 0.015 | 0.1 | 20 | 300 | 750 | 3000 |
| 900 – 1000 | 0.02 | 0.015 | 0.05 | 8 | 200 | 625 | 2000 |
Boiler Blowdown calculation
Unnecessary blow down is also considered into the boiler loss because during the blow down latent heat of water removed through the IBD tank and appear into environment as a flash steam so calculate amount of blow down is required for controlling the parameters of water. The quantity of blowdown required to control boiler water solids concentration is calculated by using the following formula
Blow down (%) =Feed water TDS x % Make up water/Maximum Permissible TDS in Boiler water
If maximum permissible limit of TDS as in a package boiler is 3000 ppm, percentage make up water is 10% and TDS in feed water is 300 ppm, then the percentage blow down is given as:
=300 x 10/3000
= 1 %
If boiler evaporation rate is 3000 kg/hr then required blow down rate is:
= 3000 x 1/100
= 30 kg/hr
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