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Proper Marine Boiler Blowdown

Boiler water has a certain amount of dissolved or suspended solids and as the water boils and steam isgenerated, there is a gradual concentration of remaining solids that are left behind. Boilers have limitationsas to how concentrated boiler water can become until there are problems such as scale, foaming orcarryover. Therefore, in order to regulate the amount of concentrated boiler water, a portion of the watermust be removed by blowdown.

Blowdown is the purge of a small portion of concentrated boiler water from the boiler in order to maintain theoptimum level of dissolved and suspended solids in the system. It is an important part of normal boileroperations and is essential in preventing waterside deposits and/or boiler carryover.

Typically there are two types of boiler blowdown: bottom and surface or top. Bottom blowdown is a manualoperation that removes both dissolved and suspended solids such as sludge that has settled out of the boilerwater. It is also referred to as flash blowdown. Surface or top blowdown is typically done on a continuousbasis to remove dissolved solids. Although we use the term continuous blowdown, it is not continuouslyperformed uninterrupted, but rather on a periodic basis.

For most low pressure boilers,

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Our ULTRAMARINE SM Coordinated Phosphate-pH high pressure boiler water treatment program isspecifically designed to minimize free caustic conditions by adjusting and controlling the chemical balanceof pH and phosphate in boiler water. In addition, boiler water conductivity is kept very low. This ensures thatthere is minimal chance of dissolved solids carryover which can cause damage to critical components in thesteam cycle. Bottom blowdown in high-pressure systems provides a safety net to sweep away any sludgethat may have developed. The frequency of bottom blowdown depends on the amount of suspended solidsor sludge being generated. Because high quality makeup water is critical to a high-pressure system, theamount of sludge generated is normally less than would be generated in a low-pressure system which thenallows for less frequent bottom blowdowns. When conditions are compromised, bottom blowdown schedulesshould be adjusted accordingly.

Baked-on sludge can be found on high heat transfer areas or in small tube diameters. High iron and copperlevels from condensate, that are returned in the boiler though the feedwater, may bind sludges making themdifficult to remove. In addition, organic contamination such as oil can also bind sludges to the internal boilertube surfaces. Oil that has been conditioned with LIQUID COAGULANT boiler sludge conditioner in low-pressure boiler water systems can sometimes be in the form of small balls that fall to the lowest parts of theboiler which are easily removed by bottom blowdown. These problems can be minimized by proper bottomblowdown.

Surface or top blowdown controls dissolved solids where they are most concentrated, just below the waterlevel in the steam separation chamber. This extraction point is also where it imposes the least interferenceto the boiler circulation flow patterns.

Conductivity, total dissolved solids, and chlorides should be controlled by using the surface or top blowdownconnection. This will limit the impurities and decrease the potential for deposit formations, foaming, andcarryover.

Proper boiler blowdown is an integral part of the total boiler water treatment program. Follow the boilermanufacturers instructions regarding how long to curtail or suspend the boiler firing while bottom blowingand the frequency and duration of the continuous surface or top blowdown.