Boiler blowdown is an integral part of proper boiler maintenance.
Carrying out a blowdown procedure at regular intervals helps you ensure you are keeping your boilers well maintained and running efficiently. However, blowdown is often carried out manually by a boiler technician. This is where many blowdown procedures can be improved. Whereas manual blowdown can tie up skilled staff and result in energy wastage automatic systems can provide energy and financial savings.
Why Is Boiler Blowdown Needed?
Boiler blowdown is used to keep a check on water contamination. As a boiler generates steam any impurities in the system feedwater that fail to boil off concentrate in its water. As the concentration of these chemicals increases steam bubbles don’t burst. When this happens the bubbles concentrate in the void above the boiler water and move across into the steam system.
This is bad for two reasons. One, it can make the steam leaving the boiler extremely wet. Two, damaging or clogging contaminants are passed into the control valves, heat exchangers and other components. To counter this, blowdown is carried out. This involves drawing off boiler water which is then replaced with clean water from the feed tank. This should lower the concentration of contaminants.
This results in energy bleed from your boilers system as the hot water is expelled. Here’s an example of the potential energy losses that can be incurred during blowdown. When blowing down a 10,000 kg/hr boiler that runs at 10 bar g can result in an energy flowrate of up to 240kw. This is enough energy to heat 19 houses with an average domestic central heating system.
With these potential energy losses in mind it is important that a boiler isn’t subjected to over frequent blowdown.
How Is Boiler Blowdown Carried Out?
Boiler blowdown can take two forms. Side blowdown and bottom blowdown.
• Side blowdown – This is used to control the Total Dissolved Solids (TDS) level by drawing water off through a small bore valve fitted just below the minimum water level line. This can be carried out continuously or at regular intervals. This water is then moved into a heat recovery system to ensure energy efficiency.
• Bottom blowdown – This is a slightly more heavy duty process and uses a large bore valve that is fitted to the bottom of the boiler. This valve is opened fully at regular intervals to expel the sludge that gathers at the bottom of the tank. If this process isn’t carried out the sludge will build up and can lead to damage. The waste water is transferred into a blowdown pressure vessel where the water can be safely cooled and removed.
So why should this be done automatically?
You will quite often find that operators are overzealous when it comes to boiler blowdown techniques. Rather than risk venting off too little water and exposing the boiler to possible damage they would rather vent off too much water and ensure the contaminants are properly expelled.
Automatic blowdown systems remove the guesswork from the blowdown procedure. They always ensure the minimum blowdown rate is achieved which means energy loss is perfectly minimized. When it comes to side blowdown automatic TDS control measures detect the actual TDS level of the water.
Water is drawn off continuously and at varying rates to ensure a constant and correct TDS level. Automatic bottom blowdown works in a similar manner and opens the bottom blowdown valve at regular intervals. This also means skilled staff are freed up to attend to other matters.
Why Utilise Heat Recovery?
If you don’t use heat recovery all the energy locked in the expelled water will be lost. However, by implementing a heat recovery system you can capture 80% of this. This can result in a considerable financial saving that will cover the cost of automatic system implementation within months.
The heat recovery system works by flashing the blowdown water to low pressure steam which is captured by a flash vessel. This vessel then separates the flash steam from the condensate. The flash steam is then fed to the boiler feedtank and then to the feedwater. While this is happening the condensate is passed through a heat exchanger. The extracted heat is then used to pre-heat the makeup water.
However you decide to do it blowdown should remain an integral part of your boiler operation process. Without it contaminants can reach critical levels in your boilers system and eventually lead to complete failure.
Written by Ryan Hill