A boiler needs lots of things to run properly: metal, fuel, insulation, air, electricity, and water. While they’re all critical to the steam equation, we’re here to talk about water. Specifically, making sure you have enough of it in your boiler. Because if you don’t, you’re looking at lost efficiency, or way worse.
A steam system is basically a closed circuit through which water circulates in different forms. Along its journey, it changes phases, does its work, and comes back to do it all again. But at the beginning, it’s just water in its liquid form. Hopefully, that water has been softened and treated to prevent boiler damage, but that’s a conversation for another time.
As the water in the boiler gets heated and converted to steam, the water level in the vessel will naturally drop. While a lot of that volume will be returned in the form of condensate from the deaerator or condensate receivers, your boiler system will gradually lose overall water volume over time due to system leaks and blowdowns. Your boiler can also lose water volume if one of your end work processes involves actually releasing steam, for example in a laboratory sterilization unit. If you’re using your boiler to generate hot water with direct injection, you’re also going to lose volume as the water is consumed. No matter what the situation, though, it’s important to stay on top of that water level.
During normal boiler operation, it’s important to check the sight glass at regular intervals to make sure the level is normal. However, there is also a safety system in place that controls the level, and shuts down combustion if the water level gets too low. Commonly, the low-water cutoff is a float switch, meaning it’s connected to a hollow bulb that floats within the water column. If the water and float fall too low, it will trigger the low-water cutoff switch and shut down combustion.
Float switches are used for low-water cutoff switches because they’re extremely simple, and reliable when maintained correctly. They also may have manual reset buttons built into or around them, making it easy to reset the system after the water level returns to normal.
Float cutoff switches aren’t the only kind used in the boiler industry. Many low-water cutoff devices use electronic probes to detect the presence of water, creating a feedback loop with the boiler’s control system to alert it immediately if the water level drops below spec. Since they’re electronic, sensor-driven low-water cutoff switches won’t have a mechanical manual reset button like a float switch. Instead, they are put back into use by resetting a relay. Depending on how your boiler system is designed, that relay is usually installed in the boiler’s electronics area, along with the relays and controls for other parts of the system.
While a float-driven switch is extremely reliable and durable, there are a few advantages that electronic low-water cutoff switches offer over their float-driven counterparts. First of all, because they use very little space, a probe-driven system can be mounted in more places. They’re also easier to repair, because the probe element itself simply unscrews from the housing for easy swap-out.
A tip from Jude Wolf: Never forget the crush washer when replacing a probe. It’s a tiny part that makes a huge difference in the integrity of your boiler system.
Boiler water level is such an important parameter to monitor, most large boiler systems will have a redundant water cutoff switch in place in case something happens to the primary switch. That’s a pretty good plan, because the results of too little water can be great big disasters.
Getting Hot In Here
If your boiler vessel’s water level gets too low, the first thing that will happen is that the metal surface will start to overheat. Water needs heat to make that phase change from liquid to gas, which is why the water inside the vessel is constantly absorbing heat on its way to becoming steam. If there isn’t enough water, all that extra heat will just hang around and build up, making the metal hotter and hotter. That will stress the metal, causing it to expand and warp. That can lead to cracks, and drastically shorten your boiler’s safe operation and longevity.
On The Surface
The water in your boiler vessel should include chemicals that serve the purpose of keeping dissolved solids in suspension, so they can settle at the bottom and exit through the blowdown valve. As the water level recedes, however, it could leave behind some small amounts of those dissolved solids. Over time, those dried-out, dissolved solids will just bake onto the vessel and tubes’ surfaces and form a layer of scale. Scale’s bad because it adds an extra layer of insulation between the water and the heat exchanger. That causes uneven heat transfer, which will not only stress the metal of your boiler, it’ll require burning more fuel to generate the same amount of steam. But that’s not all those dissolved solids are doing. Remember that extra heat that’s building up in your low-water boiler? That heat reacts with the dissolved solids and the oxygen hanging around if the water is not treated correctly, causing chemical reactions that accelerate pitting and corrosion.
Another important reason to avoid a low-water situation has to do with rectifying it. Once you’ve reached a critical low-water situation, adding water back into your boiler isn’t as simple as turning a valve. If you just dump a bunch of water in, you’re going to thermally shock the boiler. This will halt or greatly reduce steam production, requiring extra time to get back up to rate. Depending on how hot the boiler has gotten, some of the new water may also flash to steam, which can lead to an explosion. So if the water level can’t be verified in the sight glass, you should allow the boiler to cool and verify a normal stack temperature before considering adding water.
Staying on top of your boiler’s water level is a crucial thing. That’s why the experts at WARE are standing by to help you make sure your low-water cutoffs are working properly. WARE’s online parts store Boilerwarehouse.com also stocks a wide range of water level monitoring equipment. If you’d like to know even more about the water in your boiler, or any other aspect of how a boiler works, consider taking a few in-person or online classes at WARE’s Boiler University. Whatever we can do to help, just let us know.