Former National Board Field Staff Representative and
Governmental Affairs Representative
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (2 printed pages)
The boiler inspector is always recommending solutions to boiler plant problems discovered during inspections. A common discovery is the formation of scale on the waterside heat transfer surfaces of the boiler.
The boiler inspector usually is not a water treatment specialist. It certainly wouldn't be proper to recommend a specific treatment to correct the scaling problem, since it may not be correct and could cause more problems. Besides, the inspector cannot remain in the plant to monitor the effects of the recommended treatment.
In this regard, the proper recommendation is that the services of a reputable local water treatment firm be obtained to advise the boiler owner on the proper treatment of the scaling problem.
The most common cause of overheating and failure of boiler tubes is the formation of hard scale on the boiler tube surfaces. This is caused by calcium and magnesium in the boiler water. When untreated boiler water is heated, this calcium and magnesium will precipitate from the solution to form hard scale on the tube surfaces.
In addition to the overheating and eventual failure of the boiler tubes due to scale, efficiency is also decreased in the short-term because of the scale's insulating effect on the heat transfer surfaces. A layer of scale just 1/8 inch thick can cause as much as 20-25 percent loss in efficiency - heat lost up the boiler stack.
Besides making the above recommendations, the inspector may render further assistance to the boiler owner/operator. The inspector should inspect the boiler's piping systems for leakage, because any leakage is unacceptable and should be a cause for concern. All water lost from the system must be replaced by the addition of untreated fresh water.
This can create more problems, because fresh water brings a new supply of those scale-causing minerals. The concentration of minerals in the water is referred to as the water hardness.
Another reason that the loss of hot boiler water is serious is because it increases the humidity in the boiler room, and will contribute to the malfunction and failure of electrical controls, safety devices and other electrical equipment. In addition, the loss of hot boiler water may contribute to external corrosion of metal surfaces on which the hot water is dripping. Any leakage discovered should be corrected immediately.
A good suggestion to the boiler owner/operator is the installation of a water meter in the boiler's make-up water system. These small meters are fairly inexpensive and well worth their cost to monitor water usage, since there may be instances in which the piping is not accessible for inspection, such as underground piping. Any abnormal water usage should be thoroughly investigated.
The recommendation to install a water meter in the boiler's make-up water system is a good one, even for a boiler not experiencing leakage and scaling. Constant monitoring of water usage may detect otherwise unnoticeable leaking early, so corrective action may be undertaken before any damage occurs.
Another reason to monitor water usage is that high water usage will hasten scaling when the water has a high mineral concentration (hard water). It's useful to note that the level of water hardness varies throughout the United States and Canada, and depending on a boiler's location, high water usage has been known to result in scaling within a matter of weeks (see graph below).
Remember: Small leaks never get smaller, they only get bigger.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.