Boiler maintenance fundamentals
It’s winter in the Northeast and we are running into the coldest months of the year. If you operate a commercial property, now is not a good time to have a boiler failure. If you operate an industrial or manufacturing facility, there is NEVER a good time to lose a boiler! Unfortunately, this a common occurrence throughout the winter months and culprit is very often cold feed water. Monitoring your boiler feed water temperature should be an important part of your daily boiler inspection and should be worked into your overall boiler maintenance program. As you probably learned in high school science class, metal expands when it is hot and contract when it is cooled. Remember when the teacher filled the metal gas can with hot water, put the cap back on, and then ran the can under cold water? What happened? That’s right; the can “crushed itself” almost as if by magic. This same principle can be catastrophic to a boiler. Although modern boilers are manufactured to withstand some temperature fluctuation, they are not designed to withstand severe fluctuations or even constant “mildly” severe fluctuations. In a boiler system is manufactured to precise specifications and running at them at their intended optimal operating temperatures are vital for overall system longevity. A common and underestimated issue that plague many systems during this time of year are the effects of allowing cold feed water into a hot boiler system. Most boiler systems are designed and installed to address this issue, however, sometime hasty planning or improper operation can cause cold water related issues to pop up. Low feedwater temperatures during boiler operation have three major negative impacts: Greatly increased fuel costs due to loss in efficiency. (No surprise here - it costs more money to heat cold water.) Increased occurrences of corrosion due to increased dissolved gases in feedwater. Oxygen pitting is common in systems with low temperature feed water. This quickly lead to premature system failure. Increased boiler water treatment Boiler chemicals are used to remove excess oxygen from feed water. The colder the feed water, the more dissolved oxygen it can hold. The more dissolved oxygen in the feed water, the more boiler chemicals needed to remove it. Increased chance of thermal shock (also known as boiler shock) which could lead to sudden pressure vessel failure, a potentially dangerous and catastrophic event. (Remember the high school professor’s gas can “crushing” demonstration?) Cold feedwater is usually less of an issue in system that return a healthy amount of condensate (75% or more); unless that condensate travels over a very long distance. In steam systems that return very little condensate, it is necessary to make up the lost water with fresh incoming water. This “make up” water is much cooler than the steam system needs to perform at optimal levels, and therefore needs to be pre-heated before entering the boiler. 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. ASME guidelines often calls for water to be introduced into a steam boiler at no less than 200 degrees Fahrenheit . 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. What is Boiler Shock? A hot boiler can undergo significant damage if cold water is introduced without first being tempered. This phenomenon is commonly referred to as thermal shock or boiler shock. In extreme situations of an extremely hot boiler system and extremely cold feed water, catastrophic failure can happen almost immediately, however this is typically not the case. Even though the term boiler "shock" suggests that a boiler could be effected instantaneously or suddenly, the reality is that most failures happen over time. The more typical problem is stress cycling caused by the continuous heating and cooling of the internal metal components of the boiler. Thermally induced stress cycling could be causing significant damage to a boiler system over the course of a few weeks, but could happen over months or years; and is particularly common in boilers installed in hot-water heating systems. When failures eventually do occur, they are typically due to metal fatigue. Eventually the metal breaks down due to the consistent and extreme expansion and contraction. Failures caused by boiler shock are usually not an indication of poor boiler design or manufacturing deficiencies, but instead are due to overall plant design (cutting corners) or the manner in which the heating system is being operated or maintained. In the following video, you can see the effect of thermal boiler shock on a boiler. In this case, cold water introduced into the boiler cause micro fissures in the tube sheet and unseated many of the boiler tubes. Depending on the size and type of boiler, this type of failure could cost tens of thousands of dollars to repair. The Use of a Steam Sparger Avoiding boiler shock is a mechanical operation. A boiler system’s own steam is sometimes used for preheating its own feedwater. The direct steam injection into the feedwater tank referred to as steam sparging. A steam sparger is often engineered into the condensate return tank of a fire tube boiler system to maximize efficiency by making sure that the boiler does not have to work harder to transform cold makeup water into steam. Sometimes a condensate return tank may be retrofitted with a steam sparger to fix a newly developed issue with low temperature condensate; however, this must be done by an experienced engineer. Some tanks were not designed to handle the increased heat that a steam sparger will cause in the feedwater. Also, the sparge design and location affect the efficiency of the process. Sparge Pipe vs Direct Steam Injection While installing a sparge pipe into a condensate return take is a relatively low cost and easy installation to have performed, it could have some drawbacks depending the size and overall design of your system. While a job of steam sparger in a condensate return tank is to simply maintain a feed water temperature of 200 degrees or better, the steam sparge pipe design is not always the most efficient way to do it. Sparge pipes (simple pipes with drilled steam distribution holes) tend to produce uneven water temperature control; they can also cause water hammer and heavy vibration due to collapsing steam bubbles. Another method of heating the water in a holding tank is through a direct steam injector or steam injection heater. This type of direct steam contact offers greater control and has the capability of addressing some of the potential downfalls and inefficiencies of the sparge pipe method. Water Treatment Alone Cannot Do The Job of Removing Dissolved Oxygen from Feedwater Proper boiler maintenance and operation require multiple fields of expertise. While boiler water treatment is an important part of a sound boiler maintenance program, it is equally important to have system that is mechanically and operationally running at peak efficiency. Oxygen is present in all make up water. Oxygen causes red iron oxide to form on a mild steel surfaces immersed in water. This rust is active corrosion and it will continue until the metal is completely corroded away. If the amount of oxygen in the water is limited, the iron oxide film cannot form as easily; instead, the surface of the steel tarnishes with a very thin film of iron oxide which is not so fully oxidized as the red iron oxide caused by full oxygen exposure. This thinner film is also more dense and it tends to resist further corrosive attack. In water of with higher alkalinity, this oxide film becomes more stable and gives more protection to the steel. This is why your water treatment professional may have said to you that a little bit of rust color in your opened boiler is necessarily a bad thing. Maintaining a higher temperature feedwater supply also reduces the work the boiler has to do to produce steam. Increasing the feedwater temperature to 200 degrees or greater will save money in fuel costs and reducing the chances of oxygen pitting in the boiler. It will also help reduce the costs of boiler water treatment chemicals. To better understand your boiler system's chemical requirements, it is best to consult your water treatment company.
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