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Boiler Water Carryover and its Causes

Carryover is the term applied to the presence of contaminants in steam leaving a boiler. These contaminants can be in the form of a solid, liquid or vapor. Carryover can create problems in plant equipment such as turbines or may represent a potential violation where steam purity is a requirement, such as in food plants. Below we have listed the most common causes of carryover broken into two categories – chemical and mechanical.

Chemical Causes of Carryover

Excessive dissolved and suspended solids in the boiler water. Additionally, high alkalinity levels or the presence of a surfactant can lead to foaming which results in carryover. The immediate solution for this type of carryover is additional blowdown. Additionally, any contamination should be located and the application of a boiler antifoam considered.

Selective vaporous carryover. In boilers operating above 400 psig, silica can be present in the steam as a result of vaporization. This condition is usually only a problem when turbines are present and can be prevented by restricting silica concentrations to the maximum allowable limit for the operating pressure. 

ASME Guidelines

Feedwater - ASME Consensus on Operating Practices

Drum Operating Pressure (psig)
Feedwater 0-300 301-450 451-600 601-750 751-900 901-1000 1001-1500 1501-2000
Diss. Oxygen <40ppb <40ppb <7ppb <7ppb <7ppb <7ppb <7ppb <7ppb
Total Iron <100ppb <50ppb <30ppb <25ppb <20ppb <20ppb <10ppb <10ppb
Total Copper <50ppb <25ppb <20ppb <20ppb <15ppb <15ppb <10ppb <10ppb
Hardness <300ppb <300ppb <200ppb <200ppb <100ppb <50ppb - Not Detectable-
pH 7.5-10.0 7.5-10.0 7.5-10.0 7.5-10.0 7.5-10.0 8.5-9.5 9.0-9.6 9.0-9.6

Note: These feedwater guidelines are based on achieving steam purity required for superheaters, turbines, or process restrictions. Many plants operate outside of these criteria. For specific recommendations ask our water treatment experts online.

Boiler Water - ASME Consensus on Operating Practices

Drum Operating Pressure (psig)
Boiler Water 0-300 301-450 451-600 601-750 751-900 901-1000 1001-1500 1501-2000
Silica <150ppm <90ppm <40ppm <30ppm <20ppm <8ppm <2ppm <1ppm
Total Alkalinity <350ppm <300ppm <250ppm <200ppm <150ppm <100ppm --Not Specified--
Conductivity <3500 <3000 <2500 <2000 <1500 <1000 <150 <100
Free Hydroxide -----------------Not Specified----------------- ---Not Detectable---

Note: These Boiler water guidelines are based on achieving steam purity required for super heaters, turbines, or process restrictions. Many plants operate outside of these criteria. 

Mechanical Causes of Carryover

In today’s steam systems the most common factor in carryover is load changes. These usually involve very sudden and excessive increases in steam demand. In such cases, the drop in pressure in the steam header is of such magnitude that boiler water is "pulled" through the steam purifying equipment.

Water level can also have an impact on carryover and steam purity. The higher the level of water in the steam drum, the less disengaging space, and the greater the chance that water level will be a mechanical factor in increased carryover.

Carryover can also occur due to damaged or defective steam separation equipment.

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