Boiler Water Treatment A brief overview
Boiler Water Treatment – Introduction
The following is designed to give a brief overview of water treatment for boilers. The problems identified in this document are not, by any means, an exhaustive list; however, we hope that it will give you an idea of the types of issues that may be encountered and common water treatment solutions to those problems. Further information is available from B&V Water Treatment’s technical specialists.
Boilers– The Basics
Inside a boiler, water is converted into steam with feed water added to the boiler to make up for losses from a range of causes, including condensation. Often steam is condensed and returned to the boiler as part of the feed water. Any additional water needed to supplement this returned condensate is known as “make up water”.
If boilers are to operate effectively, their design and water treatment should ensure that they are safe and reliable, that they minimise the use of energy and that they produce pure steam. Water treatment programmes aid these objectives by protecting the structural integrity of the system and by maintaining efficient heat transfer.
Scale and corrosion are the causes of many problems encountered in the operation of boilers. However, sludge deposition and foaming resulting from dissolved solids in the boiler water entering the steam are also frequent issues. The result of these problems may be seen in a reduction in boiler efficiency, a reduction in plant life and in the worst cases, a catastrophic failure.
Best practice includes both the external and internal treatment of boiler water. Techniques such as water softening, ion exchange and reverse osmosis are all used for the external treatment of make- up water to reduce or remove impurities, which is particularly crucial for high pressure boilers.
The internal treatment of boiler water involves the addition of chemicals to the feed water, boiler water and steam. Corrosion and dispersants are generally used although other products such as sludge conditioners, alkalinity builders and anti foam agents may also be required. B&V Water Treatment offer a wide range of products which are optimised for different systems.
A major factor leading to corrosion is dissolved oxygen particularly when combined with a low pH. The solubility of oxygen decreases as the temperature increases which means high feed water temperatures and allowing the oxygen to escape will help to reduce corrosion. The oxygen content can be reduced further by the use of chemical oxygen scavengers. There are many types of oxygen scavengers, but the ones most commonly used in boilers are:
- Sulphites – The most widely used oxygen scavengers in low pressure boilers
- Tannins – An organic material that does not contribute significantly to the total dissolved solids.
- Erythorbates - Faster reaction than the sulphites; also a passivator of ferrous metals
Corrosion in condensate return lines is generally due to oxygen in the steam or carbon dioxide which leads to the production of carbonic acid. Ideally treatment of the feedwater should have removed the oxygen but any remaining oxygen can be removed by a steam volatile oxygen scavenger; e.g.:
- Hydrazine/ carbohydrazine - Volatile in steam therefore giving condensate line protection Often used in high pressure boiler systems
- DEHA (N,N-diethylhydroxylamine) – Similar activity to the hydrazines but lower toxicity
Steam volatile neutralising and filming amines are also used to combat corrosion. The neutralising amines (such as morpholine, cyclohexylamine and diethylaminoethanol) neutralise the carbonic acid, therefore raising the pH). Filming amines such as octadecylamine work in a different manner to the oxygen scavengers, in that they adsorb onto exposed metals to form a protective film, a physical barrier to corrosion. These amines are particularly useful in the protection of condensate lines.
The formation of insoluble calcium and magnesium salts results in scale, a rock like coating on the inside of boiler tubes. Dispersant chemicals are used which have two functions, namely to react with any residual hardness present in the incoming feed water and to prevent scale precipitating onto the boiler surface. Phosphate and chelate treatments are typically used.
Sludge conditioners, which are often organic polymers, are used to minimise the adherence of suspended solids onto heat transfer surfaces. Antifoams are sometimes used to allow a reasonable concentration of dissolved and suspended solids in the boiler water without foaming. Alkalinity builders may also be used.
Testing should be conducted to ensure that any water treatment programme is working effectively. In general, the concentration of oxygen scavengers, dispersants and the pH should all be measured.
B & V Water Treatment Products
B & V Water Treatment have many products which have been optimised for different systems. We employ all of the technologies described above and many more. Our most popular boiler products can be found here but please speak to us to see which product we recommend for your needs. We are also able to offer products in a solid form.