Don’t Let Thermal Shock Harm Your Steam Boiler!



Don't Let Thermal Shock Harm Your Steam Boiler

Let’s talk about a potentially serious problem that is sometimes overlooked… thermal shock in your steam boiler. Introducing cool water into a piping-hot, metal pressure vessel is never a good idea, but there are facility operators out there that do not know the consequences of this action or simply do not take it seriously enough. If you know about the issues that low temperature feedwater can cause in your boiler system, you will hopefully take the steps necessary to avoid this at your facility.

Here is a video that we took during an emergency call that shows how much damage cold water can cause in an operating steam boiler (and this is not the worst that could happen by a long shot):

Low feedwater temperatures during boiler operation have three major negative impacts:

  1. Increase in fuel costs due to loss in efficiency. (It costs more money to heat cold water.)
  2. Higher instances of corrosion, like oxygen pitting, due to increased dissolved gases in feedwater.
  3. Higher chances of Thermal Shock (sometimes called Boiler Shock) which could lead to sudden pressure vessel failure, a potentially dangerous and catastrophic event.

In steam systems that return very little condensate, it is common to “make up” the lost water with incoming fresh water, sometimes referred to as “city water.” The best practices for accepting this makeup water in a steam boiler is usually outlined in the boiler manufacturer guidelines and often includes some type of pre-treatment regimen. Pretreatment of boiler makeup water often includes, but is not limited to, chemical treatment, softening and heating of the makeup water through a deaerator tank.

Important note: Throughout the water treatment industry it is common to hear the terms “makeup water” and “feedwater” used interchangeably. However, at Clarity, we use terms to describe two separate situations. For our purposes, “feedwater” is water that has been altered to become acceptable to introduce into a system. It may have been treated with a chemical, or heated, or softened, etc. “Makeup water” is water that comes into the facility from an outside source and is untreated for process use; makeup water is essentially tap water.

Loss in Efficiency

Common sense tells us that colder water takes longer to heat than warmer water. Preheating makeup water is a critical step to maximizing the efficiency of many boiler designs. In the case of a conventional fire tube boiler, a steam sparger is often engineered into the condensate return tank to maximize efficiency by making sure that the boiler does not have to work harder to transform cold water into steam. The result of hot feedwater is less fuel consumption.

Dissolved Gas Induced Corrosion

Dissolved gases love cold water. Elevated levels of dissolved gases in your steam boiler system’s feedwater could easily wreak havoc on your boiler’s internal metal surfaces. The two biggest culprits in this category are oxygen and carbon dioxide. Often referred to as oxygen pitting, dissolved oxygen in boiler feedwater can cause extremely rapid, localized corrosion on boiler tubes. Dissolved carbon dioxide in boiler feedwater will result in depressed pH levels and the inevitable production of carbonic acid. Carbonic acid and low pH levels will attack the metal surfaces inside the boiler.

The low temperature of makeup water allows the gases to stay dissolved. (Think cold club soda versus hot club soda.) Heating the makeup water to its boiling point helps to liberate most of the dissolved gases. The gas that remains can be further liberated with the addition of chemicals. This process often takes place in a deaerator tank.

Thermal Shock

Thermal shock is a phenomenon that can happen when there is a rapid temperature change in the boiler or uneven temperature changes to boiler vessel. Parts of boiler expanding (or contracting) more rapidly than other parts can cause continuous “flexing” of metal components against unyielding parts. This can result in leaking tubes, cracked tube sheets or cracked sections in cast iron boilers. Anyone that has ever accidentally poured cold water into a heated glass or ceramic dish has seen this process in action when the dish exploded into fragments, except in a boiler, the result is usually much more severe… and costly.

Thermal shock is caused by the return of cold water to a hot boiler. An example of this is when system piping in building cools down overnight, yet the boiler is kept hot, or when secondary pumps over pump primary pumps. Also, failing to bring a cold boiler up to temperature slowly could lead to thermal shock. A cold boiler should stay at low fire until it is up to operating temperature for 30 minutes. It is important to note that it is also bad to introduce extremely hot water to a cold boiler; for example, in the case where a cold secondary boiler is started after being completely isolated from system flow.

Rule of Thumb

While each different type of boiler has its own optimal operating guidelines, it is a generally acceptable practice to keep incoming boiler feedwater at a temperature of 200 degrees Fahrenheit or higher. For a feedwater tank, 200 degrees and above is ideal. For a deaerator, the rule of thumb is (212 degrees) + (2 times the Tank Pressure). For example, if the pressure of a deaerator is 6psi, then the ideal temp should be 224 degrees Fahrenheit or greater.

Always consult with your boiler manufacture before making any changes to your existing boiler water treatment program.

Want to learn more?

At Clarity Water Technologies, we strive to be the best water treatment company. Our strategy to make that happen is to make sure that we do not take any half steps. Besides a deep commitment to top-notch customer service, we pride ourselves on being extremely thorough when we perform our initial evaluation of the water treatment needs of a new client. As professional water treatment experts, we come across many complex challenges in heating, cooling, process and wastewater treatment. Many times the solution is a combination of mechanical engineering and chemical water treatment. We understand that as facility owners and operators, you may not know (or want to know) about all the minute details regarding water treatment, however, many of our customers have saved tens of thousands of dollars in operating costs with just a small amount of education.

If you would like to learn more about common mistakes that you can avoid in your boiler room, please use the link below to download our free eBook: Ten Huge Mistakes Facilities Make in Boiler Operation and How to Avoid Them.

So what’s the catch? (There is no catch. Call us if you need us. Enjoy!)

About Clarity Water Technologies

Clarity Water Technologies is the leading industrial and commercial water treatment company, providing expert consulting and cutting-edge chemistry for your facility’s heating, cooling, potable and process applications.

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About the Clarity Blog

Clarity’s Water Treatment blog is written to provide tips and helpful information to anyone who is involved in managing a water treatment application at a facility. 

Our water treatment experts are here to help, no matter what your facility’s heating, cooling, potable or process application may be.  Check our blog for articles on topics like boiler and cooling water, cooling tower cleaning, legionella prevention and remediation, and environmentally-friendly water treatment options.

Clarity Water Technologies is the leading industrial and commercial water treatment company in the U.S.

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