Carryover is any contaminant that leaves the boiler with steam. It can be in a solid, liquid, or vapor form. The most common form of carryover is boiler water droplets that contain dissolved and suspended solids.
Boiler water carryover can cause deposits to form in valves, heat exchangers, turbines, and superheaters. If deposits are significant, heat transfer and/or turbine efficiency may be reduced. Additionally, carryover can strip away the protective magnetite layer on steam lines, remove the film formed by filming amine programs, cause erosion-induced corrosion, and result in product quality problems in processes using live steam.
An often overlooked consequence of boiler water carryover is efficiency loss due to wet steam. Since water at a given pressure has a lower heat content than steam, wet steam has a lower heat content than dry steam.
The causes of carryover are generally classified as either mechanical or chemical. Mechanical causes of carryover include boiler design, high water level, firing method, and load characteristics. Chemical causes of carryover include high solids concentration (dissolved or suspended), excessive alkalinity, grease, oil, or other contamination.
Boiler design directly influences steam purity. Design factors affecting carryover include design pressure, steam drum size, design steam generating rate, circulation rate, and type of mechanical separating equipment used. Operating characteristics also directly influence steam purity. If a boiler is operated at loads in excess of its design rating or is subject to sudden increases in load, the potential for carryover dramatically increases.
Foaming and selective vaporous carryover are the two main mechanisms of chemical carryover. High suspended solids, high dissolved solids, high alkalinity, and process contamination from the condensate system can all cause foaming. In some plants, dissolved solids level is the primary factor controlling whether carryover occurs while in others it is alkalinity level.
Oil, grease, or other organic contaminants can also cause carryover due to foaming. The alkalinity in boiler water can react with these contaminants to produce a crude soap that causes foaming and carryover. Selective vaporous carryover occurs due to the solvent properties of steam. At high pressures, steam actually dissolves a portion of the mineral salts in boiler water. While this is not a concern with most common minerals at pressures less than 1,000 psi, vaporous carryover of silica can occur at pressures as low as 400 psig. Consequently, boiler water silica level must be carefully limited in higher-pressure systems, especially if turbines are present.
Prevention of carryover can also be divided into mechanical and chemical means.
Low-capacity and low-pressure boilers (typically firetube boilers) primarily rely on simple gravity separation of water and steam. At higher pressures and/or high steaming rates (typically water tube boilers), it is necessary to install internal mechanical steam separating devices to provide acceptable purity steam.
These devices may consist of baffles, screens, chevron separators, or centrifugal separators. It is important that each element of steam separation equipment be kept clean and tight. Even a ¼” gap between sections of cover baffles over generating tubes can negate their operation. Likewise, the presence of deposits on screens or mesh demisters can prevent them from functioning properly. If steam load is consistently at or above the rated capacity, a steam flow regulator (orifice) can be installed on the outlet of the steam drum, usually between the non-return valve and steam drum.
As discussed earlier, the principle chemical factors causing carryover are the dissolved solids, suspended solids, alkalinity, silica (vaporous carryover), and oil contents of boiler water. In many cases, limiting the concentration of these components will satisfactorily control carryover. The actual levels that can be maintained without carryover problems can only be determined based on operating experience with each boiler system.
It is often possible to add a treatment chemical to boiler water that will counteract foaming. These chemicals, called antifoams, can control carryover to a tolerable level without increased blowdown or additional external treatment. Their use can even reduce fuel consumption by permitting lower blowdown rates while maintaining high-purity steam. While antifoams will not control all types of boiler water foaming, they should be considered whenever foaming is a problem.
The occurrence of boiler water carryover can usually be verified by testing the condensate. As a general rule, low conductivity condensate (less than 25 µmhos) indicates high-purity steam is being produced while high conductivity condensate (greater than 25 µmhos) indicates the presence of carryover.
If boiler water carryover is suspected, it can be verified by testing condensate for such parameters as sodium, phosphate, alkalinity, silica, or sulfite. If significant levels of these contaminants are detected, boiler water is being carried over into the steam.
Note that high conductivity alone does not necessarily indicate the presence of boiler water solids in steam. In systems using highly alkaline makeup water, carbon dioxide gas generated by the breakdown of natural alkalinity in makeup water can cause large amounts of carbonic acid to form in the condensate. When high levels of carbonic acid are neutralized with amine, dissociation reactions occur that can dramatically increase condensate conductivity although no boiler water solids are present. If high alkalinity makeup water is used and condensate conductivity is consistently high, but no other evidence of carryover is detected, then carryover is probably not occurring. In this case, site-specific conductivity limits should be developed for routine monitoring purposes.
Even the most well-designed and operated boiler with a well-controlled water treatment program can produce at least minor amounts of carryover. The key is to know how much carryover can be tolerated and then operate the boiler system as required to provide the desired steam purity. Chem-Aqua can help you determine the appropriate boiler control parameters for your unique system. Contact Chem-Aqua today!