Steam Boiler: Blowdown System

Boiler blowdown system is a process of removing unwanted dissolved solids in boiler water. Dissolved solids in boiler water will be left behind during the evaporation process to generate steam. The longer the dissolved solids in boiler water will more and more until the level of solubility exceeds the allowable limit. If the concentration of dissolved solids exceeds a certain limit, then the boiler water tends to form foam that allows water carried along with steam.

In addition the concentration of dissolved solids in water or can be referred to as sediment can cause the formation of crust on the inside of tube, header or drum. This will result in heat transfer process to be disrupted, so that the metal tube or header to be over-heat and the boiler water temperature to be low resulting in less heating in evaporation process.

Therefore, the concentration level of dissolved solids in boiler water must be controlled by the way of doing boiler blowdown system. Boiler water in certain volume is discharged in the process of blowdown system and replaces it with make-up water in the feedwater system.

Boiler blowdown system consists of two types namely:

Continuous blowdown
Intermittent blowdown

Continuous blowdown

Continuous blowdown is discharge process of dissolved solid in boiler water continuously. Therefore, make-up water should be added in the feedwater system continually to compensate amount of discharged boiler water because of this process to maintain the water balance of a steam boiler. Concentration of Total Dissolved Solid (TDS) must be maintained so as not to exceed a certain level so that the purity of steam produced can be maintained.

Blowdown valve setting in this process is only done once on a particular condition, so the operator does not need to set up again at any time. Boiler blowdown system will make the boiler heat energy is wasted, but it can also be used by blowing heat of blowdown into a flash tank to produce flash steam.

Preheating feedwater can utilize the heat from the flash steam. So boiler efficiency can be increased by re-utilize heat energy from blowdown system. Continuous blowdown is generally applied at high pressure boiler.

Intermittent blowdown

Intermittent blowdown is the removal process of dissolved Solid, silica, and other unwanted particles that carried out intermittently. Intermittent blowdown is done manually by opening the valve of blowdown piping system that is usually placed at the lowest level, so that the sediment in the boiler water can be reduced and the expected quality of steam can be maintained properly.

Intermittent blowdown is very effective to remove solids that have been separated from the solution and precipitate in the tube, the header or drum boiler. For example, intermittent blowdown is done in a shift work of operator, performed blowdown within 2 minutes. Intermittent blowdown causes the make-up water must be added in the feedwater system in a short time and large quantities. Therefore its operation requires greater feed water pump than continuous blowdown.

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Boiler blowdown will be a really essential aspect of almost any water treatment application. Boiler blowdown has function to control content level of impurities in boiler water. Determination amount of boiler blowdown is crucial: very much will leads to high cost of chemical water treatment and energy loss; very little will leads to too much content level of impurities. There are not specific rules to regulate amount of boiler blowdown due to variation in quality of water may differ from one location to other location. It may range from 1% until 25% according to the flow of feedwater.

Blowdown could be performed intermittently or continuously. Boiler blowdown can be performed either from the bottom of steam drum, header, or mud drum, or it could be performed from the base of steam boiler. The following are several principles to assist determine an efficient boiler blowdown application:

For boiler with fire tube type, blowdown may be done either intermittently or continuously. It could be blown down through the base of boiler. Timeframe, rate of recurrence, and type be determined by operating conditions, design boiler, and the kind of water treatment system.
For steam boiler which has drum type, the content level of water must be regulated through blowdown via steam drum. Continuous boiler blowdown is definitely recommended.
Likewise in steam boiler with drum type, blowdown through bottom header or mud drum will perform removal of suspended solids in steam boiler. Attempting to manage the content level of impurities through blowdown from this place could result in a serious interruption of water circulation and leading to deterioration to steam boiler. If bottom blowdown is performed, it must be in quick timeframe or intermittent. This condition is established by operating conditions, design boiler, and also the deposition rate of suspended solids.