1.2 Boiler Blowdown

 Boiler Blowdown

When water is boiled and steam is generated, any dissolved solids 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 exceeded and they deposit from the solution. Above a certain level of concentration, these solids encourage foaming and cause carryover of water into the steam. 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 ‘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 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. The maximum amount of total dissolved solids (TDS) concentration permissible in various types of boilers is given in Table.

 

RECOMMENDED TDS LEVELS FOR VARIOUS BOILERS
	Boiler Type	Maximum TDS (ppm)*
1.	Lancashire	10,000 ppm
2.	Smoke and water tube boilers (12 kg/cm2)	5,000 ppm
3.	Low pressure Water tube boiler	2000–3000
4.	High Pressure Water tube boiler with super heater etc.	3,000–3,500 ppm
5.	Package and economic boilers	3,000 ppm
6.	Coil boilers and steam generators	2000 (in the feed water

Conductivity as indicator of Boiler water quality

Since it is tedious and time consuming to measure total dissolved solids (TDS) in boiler water system, 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.

Conventional methods for blowing down the boiler depend on two kinds of blow down intermittent and continuous

Intermittent Blow down

The intermittent blown down is given by manually operating a valve fitted to discharge pipe at the lowest point of boiler shell to reduce parameters (TDS or conductivity, pH, Silica and Phosphates concentration) within prescribed limits so that steam quality is not likely to be affected. In intermittent blow down, 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 blow down 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 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.

Continuous Blow down

There is a steady and constant dispatch of small stream of concentrated boiler water, and replacement by steady and constant inflow 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 steam can be used for pre- heating boiler feed water or for any other purpose (see Figure 2.9 for blow down heat recovery system). This type of blow down is common in high-pressure boilers.

Blow down calculations

The quantity of  blow down required to control boiler water solids concentration is calculated by using the following formula:

Blow down (%)     =         Feed water TDS × % 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

 

Benefits of Blowdown

Good boiler blow down control can significantly reduce treatment and operational costs that include:

–                      Lower pretreatment costs

–                      Less make-up water consumption

–                      Reduced maintenance downtime

–                      Increased boiler life

–                      Lower consumption of treatment chemicals