STEAM BOILER WATER SYSTEMS
Steam boiler water systems generate steam for a variety of applications: comfort heating, humidification, sterilization, cooking and many various industrial processes. Steam boilers use gas, oil or electricity to heat water to the point of steam generation. Steam is generated at pressures ranging from 15 psi to 1000 psi or greater. As the steam is used it condenses and is either returned to the boiler via a steam condensate system or is lost as part of the process (ex. humidification).
When heat is applied to water containing calcium and bicarbonate ions, the chemical reaction that takes place results in the formation of carbonate ions that have a strong affinity for calcium. Scale formation generally consists of calcium carbonate and must be controlled to prevent loss of heat transfer and localized overheating that can result in boiler tube failure. The greater the steam pressure, the more critical scale prevention becomes.
The most common cause of corrosion in steam boilers is dissolved oxygen that enters the boiler via boiler feedwater (returning steam condensate plus fresh make-up water. The lower the feedwater temperature, the greater the level of dissolved oxygen present. Dissolved oxygen can be removed through mechanical deaeration. When a deaerator is not practical, simple pre-heating and venting of the feedwater can remove some oxygen before it enters the boiler.
Steam Condensate Line Corrosion
As heat is applied to water containing bicarbonate ion and carbonate is formed, carbon dioxide gas is also released and is carried out with the steam. When the steam condenses the carbon dioxide dissolves in water to form carbonic acid, which lowers the pH of the returning condensate and can corrode the steam condensate piping system. This carbonic acid needs to be neutralized to prevent costly failures due to corrosion and leaks in the condensate piping system.
Excessive Dissolved Solids Concentration
As steam is generated, the minerals that were dissolved in the boiler water (including sodium, calcium, magnesium, bicarbonate, chlorides, sulfate and chemical additives like phosphate) concentrate. If allowed to concentrate unchecked, they will eventually cause foaming, priming and boiler water carryover into the steam, causing boiler water levels to bounce erratically (possibly causing the boiler to shut off on low-water cutout) and introducing boiler water into a condensate system not designed to handle it. The greater the steam pressure (and the smaller the boiler water volume) the more critical the need to prevent excessive dissolved solids concentration.
The most efficient way to prevent calcium carbonate scale in the steam boiler is to remove the calcium through the use of a water softener. Softeners are always recommended for steam boilers regardless of pressure and the amount of condensate return.
It is also possible to remove the bicarbonate ion that ultimately leads to carbonic acid formation in the steam condensate through the use of dealkalization or reverse osmosis. Reverse osmosis is generally preferred because all dissolved solids are reduced, not just bicarbonate, allowing boilers to dramatically reduce blowdown and realize the energy savings achieved by not discharging heated, treated boiler water to the sewer.
An important part of any steam boiler water treatment program is the addition of scale control chemicals to prevent the precipitation of calcium carbonate. Factors to be considered when selecting a scale control chemical are:
- Steam Pressure (The rate of scale formation increases as temperature increases. Also, as temperatures increase, hydroxide alkalinity formation increases, which helps to increase the rate of calcium carbonate precipitation.)
- Pre-treatment (softening, reverse osmosis, etc.)
- Boiler Water Treatment Chemicals
The key to controlling corrosion in steam boilers is to eliminate dissolved oxygen. This is most efficiently done with mechanical deaeration as a part of the pre-treatment equipment package. The boiler water treatment program will then most commonly include an oxygen scavenger such as sodium sulfite to react with the residual oxygen not removed by the deaerator.
In some applications where deaeration is not possible, a corrosion inhibitor such as molybdate or nitrite may be used to protect the boiler metal surfaces from pitting attack. In other applications organic filming agents may be successfully applied.
Steam Condensate Corrosion Control
Any bicarbonate alkalinity not removed by pre-treatment will lead to the formation of carbonic acid in the steam condensate piping system. Typically a neutralizing amine is fed as part of the boiler water treatment program. These amines (combinations of cyclohexylamine, diethylaminoethanol or morpholine) have the ability to leave the boiler with the steam and neutralize the carbonic acid that forms when the steam condenses. The selection of amines should include the length of condensate piping runs, steam pressure and the number of pressure reducing stations.
Chemical Feed and Blowdown Equipment
The success of the boiler water treatment program depends to a large degree on the proper application of the chemicals selected. Scale control chemicals are typically fed either continuously or in proportion to make-up or feedwater addition. Oxygen scavengers are typically fed continuously and neutralizing amines are fed either in proportion to make-up or in response to a drop in condensate pH.
Chemical pumps must be able to meet the output and pressure requirements of the application. Depending on the rate of steam production, automatic surface blowdown is recommended to avoid excessive dissolved solids concentration and to minimize the heat, water and chemical that are lost to drain.
Many equipment systems can provide important information to building automation systems, including alarms that notify operating personnel or an H-O-H Water Technology Water Quality Engineer.
Most water treatment formulations are liquids that can be pumped from on-site storage tanks with containment to minimize chemical handling and maximize safety. Qualified H-O-H delivery personnel add water treatment chemicals to these tanks.