Have a look inside your kettle or hot water jug at home, and take note of all the staining and deposits. Then consider that this comes from water that is actually boiling into steam for only a few seconds every day, and imagine how much muck and filth you would get if the water was boiling all the time, as much as 24/7, like many industrial steam boilers.
The most under-appreciated fact about boiling water is that what boils off into steam is only the water. In other words, it is just the liquid water that turns into the gas steam, so everything that is in the water, and which is not water, stays behind in the water that is left in the boiler. So as the boiling continues, the water in the boiler becomes more and more ‘concentrated’, with dissolved and undissolved (suspended) solids, much of which become deposited on the internal surfaces of the boiler, especially the heating surfaces. Without a ‘boiler blowdown’, the water in the boiler would soon become a thick muddy soup.
What boiler blowdown does is discharge a certain volume of this ‘concentrated’ water. This is then immediately and automatically replaced by the same volume of fresh boiler feedwater, which will be much lower in dissolved solids than the blown-down water. The overall concentration of the water in the boiler is thus reduced, and the ultimate objective is to maintain the average concentration of the boiler water within predetermined maximum and minimum limits.
Different boilers and operating conditions will require different maximum and minimum limits. Too high a maximum, and the boiler is likely to ‘carryover’, for example some of the concentrated water will get carried out with the steam (wet dirty steam), and the amount of deposits on the internal surfaces will be too great. Too low a minimum limit results in a waste of water and energy, for example increased operating costs, because all the blowdown water has to be discharged to waste (although if the blowdown rate is big enough, heat recovery from it is possible).
How should a boiler be blown-down? The procedure is simple: you just need to open a drain valve on the boiler for a few seconds or minutes, every so often. All boilers are fitted with a ‘bottom blowdown valve’, which will literally be right at the very bottom of the boiler, but the primary function of this valve should be to discharge the sludgy loose deposits that build up. These are deliberately precipitated as a result of the water treatment regime, instead of hard scaly deposits building up on the heating surfaces. To best withstand the harsh operating conditions, a purpose-made ball valve is best. Usually, it will be manually opened, for just a few seconds, once a day, but it can be automated with an actuator and timer.
All boilers should also be equipped with a secondary blowdown valve, which should be opened and closed frequently so as to maintain a reasonably stable level of dissolved solids (TDS) in the boiler. This is difficult and time-consuming to do manually, so automatic operation is much preferable. A solenoid valve and timer are commonly used. However, this still requires manual monitoring, by frequently testing a sample of boiler water, to ensure that
the required TDS level is being maintained. This is especially so when operating conditions and water quality change over time, as they can and usually do, and in practice, this time-consuming chore is rarely done as well as it should be.
The best way of controlling secondary (or TDS) blowdown, is to fully automate it, and this is easily and simply done by replacing the timer with a TDS controller, which constantly monitors the actual TDS level in the boiler and only opens the blowdown valve as and when required, and only enough to drop the TDS level to the predetermined minimum. Needless to say, Spirax Sarco provides advise and supply on this equipment and more.