Boiler Blowdown: Don’t Blow It! | Rasmussen Mechanical

Boiler blowdown is the act of removing water from a boiler in operation in an effort to remove sediment, chemical concentrations and solids. This helps to prevent carryover, corrosion and ensures high quality steam.

Types Of Boiler Blowdown

Manual blowdown — This is your bottom blowdown and needs to be done periodically. It is done to get rid of all the sludge and sediment that is at the bottom of the boiler. Sometimes this is done every shift or every day, but it depends on how much the boiler is being used and water quality.

If an RO system is present, chances are you won’t have to blowdown as much.

Continuous Surface Blowdown — This is generally a dip tube that goes into the boiler, right above the water level (2 to 3 inches). Sometimes these dip tubes will run the entire length of the boiler with holes punched in them the entire way. Continuous surface blowdown involves the slow extraction of water along the surface of the water. This is done to remove any of the scum that forms on top of the water.

So why do we care about removing scum? Well scum increases surface tension in the water. When surface tension is high, it takes more BTUs (or more fuel) to get those team bubbles to pop through. This results in a less efficient system.

Probes or electrically actuated blowdown valves are generally used to measure the Total Dissolved Solids (TDS) or conductivity of the water. When this conductivity starts to get to high above the set point it will open a valve and begin dumping.

VALUE SEQUENCING

This is one of the most common mistakes we see facility managers make.

When you are performing a bottom blowdown, it is important to open your values in the correct order. The quick opening value should be opened first and closed last. The slow opening valve is opened second, and then is closed once blowdown is complete.

In higher pressure boilers you may have two slow opening valves. In this case you should open the inside valve first and then outside. By being consistent in this way when you open the valves, you only allow one valve to be the “throttling valve”. This ensures the valve closest to the boiler maintains a tight seal.

Note: If a blowdown valve is open, NEVER leave the area. You should keep you hand on the valve until you are ready to close it.

Blowdown Separators

These are generally on every boiler that is over 15 psi. These are in place to drop lower the pressure and cool the water before it goes down the drain. As the blowdown drains into a blowdown separator, the pressure of the water drops. This causes some water to flash to steam which gets exhausted via an exhaust pipe.

After flashing the water has cooled down to closer to 210 degrees but that is still too hot to enter the sewer system. An aftercooler is then used to drop the water temperature down to an acceptable temperature to then enter the sewer system.

While PVC pipes should not be used as a drain line in a boiler room, sometimes that is the case. In situations like that, separators become even more important. This is because the hot blowdown can melt the drain in a very short amount of time.

Boiler blowdown separators are now an ASME code vessel. That means if they need a repair, then an ASME stamped company needs to perform the repair.

Blowdown Tanks

These perform the same function as a blowdown separator but are much larger. The main advantage of a blowdown tank is that they don’t require water cooling because of the mass. The heat dissipates into surrounding area over time as the blowdown sits in the tank. As the next blowdown occurs, water (that has been cooling) from the previous blowdown leaves the tank.

Blowdown tanks can also help in situations where sludge from a boiler is clogging a sewer line. Because of the larger size, sediment can settle the bottom of the tank. It can then be cleaned out periodically to prevent it from entering a sewer line.

The main disadvantage of a blowdown tank is that the material costs are higher, and it takes up more floor space.

Blowdown Heat Recovery Systems

These are generally used when a boiler is performing continuous surface blowdown. Because the blowdown is continuous there is a significant amount of energy being wasted if it were to just be dumped down the drain.

Boiler heat recovery systems take this blowdown and use the heat to preheat the make-up water. They are generally not overly expensive and can show a pretty quick return on investment.

Boiler Blowdown Water Tests

Boiler carryover is what we are trying to prevent with blowdown. There are a number of ways to test for boiler carryover, including condensate conductivity testing, Alkalinity testing, or chloride testing.

Comparing boiler water conductivity to condensate conductivity is the simplest way to gauge carry-over and where the water is going. However, this method isn’t as accurate.

Chlorides testing is often the most accurate way to determine if you have boiler carryover. This is because boiler conditions do not chemically affected chlorides. According to Viking Water Technology : “Chloride ions…are extremely soluble and do not precipitate or decompose when subjected to boiler conditions. Therefore, chlorides are used as a measure of boiler water concentrations.”

Questions?

For 50 years we have been the Midwest’s Boiler experts. Call us at 1–800–237–314, chat with a support agent , email sales@rasmech.com, or simply contact us online. Whatever your boiler room needs, we’re here to help!

Originally published at https://www.rasmech.com on June 15, 2021.