What Is Boiler Blowdown?
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 scaling. In short, boiler blowdown ensures high quality steam and a reliable, long-lasting boiler.
Types Of Boiler Blowdown
Sometimes called blowoff, bottom blowdown is typically performed every shift on each blowoff connection. Bottom blowdown rids the boiler of sludge and sediment at the bottom of the boiler. This makes it imperative that each boiler blowdown connection is used to prevent accumulation of sludge and solids in that part of the boiler. Frequency and duration of bottom blowdown depends on how much the boiler is being used. Take into account the boiler load profile, water quality, and the chemical treatment program when making this decision. Your chemical treatment provider can provide guidance for frequency and duration of bottom blowdown. Periodical internal inspection is crucial to diminish boiler sludge and solid levels. If solids are found, frequency or duration should be increased to compensate. Always blowdown the bottom of a boiler before taking it offline.
Known as the boiler skimmer, surface blowdown is another type of blowdown Sometimes the continuous blowdown is as simple as a dip tube in the boiler right below the water level. In larger boilers, the continuous blowdown skimmer is a tube that runs the length of the boiler. It sits just below the water surface with holes space along the entire length. Continuous surface blowdown involves the slow extraction of water along the surface of the water. This removes scum that forms on top of the water. It also prevents the buildup of dissolved solids, often referred to as Total Dissolved Solids (TDS).
So why do we care about reducing TDS? Well, high TDS levels increase the surface tension of the boiler water. When surface tension is high, steam bubbles won’t separate cleanly from the water surface, forming foam and causing boiler water carry-over. Carry-over can wreak havoc on the steam distribution system.
Utilizing water softeners and Reverse Osmosis systems to treat incoming boiler water can reduce the usage of chemicals and reduce the rate of blowdown. Adding water softeners and reverse osmosis will very likely decrease operating costs by saving boiler water chemistry, wastewater, and blowdown.
This is one of the most common mistakes we see boiler operators make.
When you are performing a bottom blowdown, it is important to open your values in the correct order. The quick opening valve (if equipped), or inside valve should be opened first, due to its close proximity to the boiler. This will prevent high velocity flow from cutting the seats of this valve, maintaining a tight seal. Next, the slow opening valve is furthest from the boiler and is fully opened second. This valve is designed to take the higher velocity flow and is undamaged. Valves close in reverse order after blowdown occurs. Outside valve closed first, then the inside valve.
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.
SAFETY NOTE: If a blowdown valve is open, NEVER leave the area. You should keep your hand on the valve until you are ready to close it. Always have visual contact with the gage glass during a blowdown to ensure you do not cause a low water scenario.
Experts encourage blowdown separator installation for every boiler over 15 psi.. They allow the hot boiler water to drop safely to a lower pressure, then cool the water before it goes down the drain. As the blowdown drains into a blowdown separator, the pressure of the water drops. A vent pipe provides a helping hand to exhaust the product, steam.
After flashing, the water has cooled down to 212 degrees, but is still too hot to enter the sewer system. An aftercooler is then used to drop the water temperature to an acceptable temperature to 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 these situations, 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.
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 volume. 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. Periodic cleaning prevents it from entering the sewage line.
The main disadvantage of a blowdown tank is that the material costs are higher, and it takes up more floor space. Also, as the heat leaves the blowdown water, it is heating up an already hot and uncomfortable space!
Energy Savings Measures
Blowdown Heat Recovery Systems
Boiler water is very hot (boiling hot!), so blowdown wastes valuable heat. Blowdown heat recovery systems capture this heat and put it to use. These systems provide aid during processes like 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 cold make-up water. They are generally not overly expensive and can show a pretty quick return on investment.
Conductivity Blowdown Controllers
Because continuous blowdown can waste a lot of water and heat, reducing blowdown can provide big savings. Conductivity probes and actuated blowdown valves, controllers, are present in larger systems to control the conductivity of the water without wasting energy. Boiler water conductivity relates to the Total Dissolved Solids (TDS) in the water. Dissolved solids increase water conductivity, so measuring this conductivity gives an indication of TDS. When this boiler water conductivity exceeds the set point, the blowdown controller will open a valve and begin a blowdown cycle. This valve will stay open until the boiler is below set point again.
Conductivity blowdown systems can save massive amounts of blowdown, especially in boilers that run at a lower rate at night or on weekends. Further, the boiler water will maintain a more even conductivity, allowing more accurate chemistry control, saving on chemical usage. Lastly, in the event of a water treatment issue, the conductivity system would adjust blowdown accordingly, preventing a major scale buildup in the boiler!
Boiler Blowdown Water Tests
Boiler carryover, boiler sludge build-up and boiler scale are what we are trying to prevent with blowdown.
There are a number of ways to test for boiler carryover, including condensate conductivity testing, or chloride testing. Comparing boiler water conductivity to condensate conductivity is the simplest way to gauge carry-over and where the boiler water is going in the system. However, this method isn’t as accurate as chloride testing. Chloride testing is often the most accurate way to determine if you have boiler carryover. This is because boiler conditions do not chemically affect chlorides. According to Viking Water Technology: “Chloride ions…are extremely soluble and do not precipitate or decompose when subjected to boiler conditions. Therefore, you can use chlorides as a measure of boiler water concentrations.”
Along with uncovering boiler scale, internal inspection also helps locate sludge. Boiler sludge and other solids signal the need for changes in bottom blowdown procedure.
Internal inspection is the only way to measure boiler scale. However, by carefully monitoring boiler water chemistry and following good water treatment recommendations, boiler scale should not be a problem.
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