What is Boiler Blowdown? Everything You Need to Know

Routine maintenance is a crucial part of owning a boiler, and if avoided, it can lead to boiler failure. In fact, lack of boiler maintenance is one of the biggest contributors to failing boilers.

Boiler blowdown is a routine maintenance procedure that can be performed daily to ensure your boiler operates optimally. One of the main reasons boiler blowdown is important is because it prevents staining of the water gauge glass. When this happens, it can be tricky to know the actual amount of water inside the boiler. The gauge may look as though there is sufficient water, when in reality, the boiler is very low on water. The safety of and boiler’s performance and longevity is directly related to operating with the proper water levels. Additional benefits of adhering to a proper boiler blowdown schedule and procedures are discussed below.

If you are unfamiliar with the boiler blowdown process, this post will guide you through everything you need to know, including the types of boiler blowdown and what causes problems in boilers.

The Lowdown on Boiler Blowdown

Boiler Blowdown is a water treatment process where water is forced out of the boiler vessel to help prevent a high concentration of impurities (such as chemical concentrations, sediments, and solids).

If left unchecked, these impurities tend to lower the boiler’s operational capacity by insulating the heating surfaces on the boiler’s waterside. Once the metal overheats, it begins to reduce the lifespan of the boiler and can cause the pressure vessels to fail. The impurities in boilers are referred to as total dissolved solids, or TDS.

TDS includes both Suspended and Dissolved Solids. Dissolved solids are tiny substances that are too small for humans to notice, but remain in water such as calcium, magnesium or sodium. You may be familiar with these elements creating “hard” water. Suspended solids on the other hand do not dissolve fully in water and are usually minerals.

With regular boiler blowdown, your boiler will produce high-quality steam and have a normal level of TDS. The boiler blowdown procedure requires a commitment to reading and logging vital information about your boiler. With this data, you can actively prevent scaling, corrosion, and carryover of boiler water into steam which can cause further damage to piping and other equipment.

The Boiler Blowdown Process

When using a boiler, the water turns to steam, and if there are any impurities or solids, they will be left inside the boiler tank. Over time, the impurities will accumulate, as described above. The boiler blowdown process involves forcefully draining the boiler, while under pressure, to get rid of sludge and adding “makeup” water back into the boiler to compensate for what is being blown out. This process also helps control the concentration of dissolved solids in the boiler water.The frequency and duration of the blowdown procedure is affected by factors such as type of boiler, amount and quality of makeup water, operating pressure, and water treatment. The blowdown rate of a boiler differs for each boiler application, and you can acquire more information about your specific boiler from a water treatment consultant. Through testing of the makeup water and learning more about your specific operation, they can give guidance as to how often and how long blowdowns should last.  While the process might differ from boiler to boiler, here are some basic steps in most boiler blowdown processes.

• Step one: Open the quick opening valves, the valves closest to the boiler, which allows water to flow to the slow opening valve. The quick opening valve goes from fully opened to fully closed in one complete motion.
• Step two: Open the slow opening valve. The slow opening valve has five complete 360-degree turns to go from fully closed to fully open. 
• Step three: Pay attention to the boiler’s water level throughout, if you allow it to go too low, the boiler will cycle off on its level control.  Multiple shorter blowdown cycles can be used to achieve the desired total blowdown time.  After the valve is left open for the recommended (total) time (consult with a water treatment specialist), you are ready to close end the blowdown.
• Step four: Close the slow-opening valve,  then close the quick-opening valves.
• Step five: Reopen the slow-opening valve to drain the piping between the two quick and slow valves.  After cooling off, confirm that slow-opening valve shut offs tight and no water is flowing from the bottom blowdown piping.

The order of operating the valves is important.  The quick-opening valve should always be opened first and closed last. This prevents water hammers which can damage pipes and valves. It is best practice to avoid leaving an open valve unattended since it could lead to personnel injuries.. Additionally, you can perform repairs and maintenance on the slow-opening valves that are farthest from the boiler without draining the boiler.

Types of Boiler Blowdown

Boiler blowdown is divided into three main categories: bottom blowdown, surface/ continuous blowdown, and water column blowdown.

Surface Skimmer/ Continuous Blowdown

Surface blowdown ensures a slow, continuous removal off the top of the boiler water level. This process can be controlled manually by setting a needle valve appropriately to control the flow of surface blowdown. This process can also be automated by use of a Sensor (conductivity), motorized valve, and controller.  This method will monitor the boiler’s water quality and open surface blowdown valve as needed to maintain the conductivity setpoint.  

Regular skimmer blowdown is effective in dealing with impurities with a minimal amount of boiler water lost to drain, hence controlling cycles of concentration. Your boiler manufacturer’s literature, American Boiler Manufacturers Association, and water treatment specialist are all good resources for determining the correct boiler water quality targets.

Bottom Blowdown

Bottom blowdown is the most commonly used method due to its effectiveness in preventing the build-up of dissolved solids. The procedure utilizes the slow and quick opening valves, as outlined above, and is what most people are referring to when talking about boiler blowdown. The valves are connected at the bottom of the boiler and are designed to remove sludge concentration that precipitates during boiling.

Water Column Blowdown

Water column blowdown helps to control the buildup of sludge in the water level controls and sight glass. It removes impurities from the water column and gauge glass, ensuring operators can clearly see water levels in the boiler. In this process, the water gauge glass, water column, and the associated blowdown piping system and valves are cleaned out. Float type low water cutoffs have a bowl assembly, if not routinely blown down, sludge can build up and interfere with the proper movement of the float assembly.

There are some additional boiler blowdown systems/components, including blowdown heat recovery systems, separators, and tanks.

• Blowdown heat recovery systems refer to any sort of system used to recover wasted energy. It uses the heat from the blowdown to preheat feed water before it enters the boiler, recapturing otherwise wasted energy, reducing costs and improve boiler plant efficiency. 
• Blowdown separators are vessels designed to perform two important functions;  lower the pressurized blowdown from the boiler to ~atmospheric through steam flashing, they also cool the blowdown before it enters a drain by use of an aftercooler
• Blowdown tanks are vessels that also reduce the high pressure blowdown to atmospheric pressure, but instead of immediately cooling and sending the blowdown to drain, they collect and store the blowdown water, allowing it to cool before going to drain.

Conclusion

If you want to get the most out of your boiler’s operational capacity, boiler blowdown should be part of your routine boiler room maintenance program. . For blowdowns and all your preventative boiler maintenance, contact the W.C. Rouse Team.