James R. Stringer
From steam boilers constructed during World War II to today’s high efficiency hot water heating condensing boilers, the needs of boilers vary with their applications. Below is a brief overview of some common methods of treating boiler water. Of course, always seek out the expertise of chemical treatment professionals that can tailor a chemical treatment program for your facility.
Molybdate based corrosion inhibitors for hot water heating boilers include a pH buffer that coats the inside of the system’s pipes that provides a protective barrier. This protective barrier is formed from a chemical reaction between the molybdate and iron. Molybdate corrosion inhibitors are economical, provide corrosion protection regardless of amount of air in the system, and easy to test. However, it has a high environmental impact and if excessive water make up occurs, the molybdate can be precipitated out of system potentially leading to corrosion.
Nitrite based corrosion inhibitors for hot water heating boilers also include a pH buffer and is an oxygen scavenger that reacts with the oxygen within a system, absorbing the oxygen and changing to nitrate. Nitrite corrosion inhibitors will also provide a protective barrier when it reacts with the iron. Nitrite corrosion inhibitors provide great corrosion protection, economical, and easy to test but also have a high environmental impact, it is a nutrient for bacteria, and continuous air infiltration into the system will make it ineffective. Nitrite must also have a high concentration in the system which will mean higher total dissolved solids that can cause extra wear on pump/valve seals and cause deposits in areas of leaks.
Neither the molybdate or nitrite corrosion inhibitors can be used in systems for potable water.
The primary way to prevent scale in steam boilers is to remove the scale forming minerals before they enter the boiler:
The first method is precipitating the scale forming solids from the water by chemical means from the boiler using phosphates. This keeps the minerals suspended in the water and settles where they can be blown down. Orthophosphates are the most common chemicals (such as trisodium phosphates). The pH of this program should be 11-12 for high pressure boilers; below this pH and calcium will not precipitate efficiently. Adding caustic soda to the system is used to raise the pH.
The second method uses a chelate that prevents the minerals from leaving the water but this method is for steam boilers operating below 4000 kPa. Chelate use is the opposite of a phosphate program and uses the principal of solubilization. Chelates are weak organic acids and can be used to rid the system of existing scale without removing the boiler from service. Overfeeding of chelate however can remove the protective layer of magnetite. Chelates are expensive.
Water softening for steam boilers or humidifiers with salt (NaCL) make use of ionic exchange which takes harmful calcium (Ca+) and magnesium (Mg+) scale forming salts and exchanges them with sodium (Na+) from salt compounds to make non-scale forming salts. A sodium zeolite softener, for example, will have a brine (salt water) absorb all the Ca+ and Mg+ from the zeolite while also exchanging the Na+ from the brine. The brine with the Ca/Mg is flushed to sewer while the Zeolite with the fresh Na+ can now soften water and the cycle continues.
Conductivity is the ability of water to conduct electricity. For an exact amount, it is called specific conductivity and an indication of total dissolved solids (TDS). Dissolved solids should be kept at the required minimums to prevent carryover, which are contaminants that leave the boiler with steam, in steam boilers.
Continuous blowdown removes concentrated water (TDS) from the steam drum.
Intermittent blowdown removes suspended solids from the mud drum.
Micron filters are placed inside canisters that have a portion of a system cycle through them picking up particulates and suspended matter. Micron filters are changed out periodically but it is recommended to check them at least monthly for level of cleanliness and replace as needed.
A Deaerator is a piece of equipment that can be added to a steam boiler system that is designed to aid in the removal of dissolved gases, mainly oxygen. Typically, feedwater enters at the top of the deaerator tank as a fine spray while steam from the boiler enters below the feed water intake. The steam, along with any trays between, will scrub the feedwater as it enters and dissolved gases that are heated will exit through the top of the deaerator through a vent. The deaerator should be kept at a low-pressure, high-temperature condition which is fundamental in allowing the oxygen to break free from the water.
An oxygen scavenger (commonly sodium sulphite which reacts with the oxygen to produce sodium sulphate) is used to remove dissolved oxygen from the deaerator system. A residual amount of the unreacted oxygen scavenger is desirable in the system as this suggests all oxygen should have been removed. The residual scavenger will then carry into the boiler and continue working.
Reverse osmosis is applying pressure to a highly concentrated solution that pushes water through a semi-permeable membrane creating a high purity on the low-concentration, low-pressure side while having the dissolved solids left behind; this can remove 99% of dissolved salts from water. The membrane over time may become a haven for suspended solids and micro-organisms, so it will have to be cleaned periodically with cleaners designed for the membrane.
Power Engineering Third Class Edition 2.5; Part B1; Chapter 10: Internal Water Treatment for Boilers and Chapter 11: Boiler Water Pretreatment
Proprietary Scale and Corrosion Inhibitors by G.F. Yuzwa: http://www.infrastructure.alberta.ca/content/doctype306/production/techpres29.pdf