Low Water Cutoff 101

 Mcdonald miller low water cut offArguably the most important safety devices on a boiler are the primary and auxiliary low water cut off switches. Water in the boiler must be maintained at the normal operating water level (NOWL). Low water cutoff switches are generally located 2-6 inches below the NOWL. The low water cutoff typically uses an electric conductivity probe or a float to detect the water level inside the boiler. When water drops below this level, low water cutoffs prevent the boiler from firing. 

When the water level returns to normal, the burner can begin firing again, if only the primary low water cutout was tripped. The auxiliary low water cut off will always require an operator reset. Some systems however, may lock out a burner following a primary low water shut-down as well. This would also require a boiler operator to manually reset the switch or controls to get the boiler back up and running.

Low Water Cutoff Failure

Failures can result in boiler tube damage or catastrophic failure of your boiler! When hot combustion gasses are allowed to flow into the boiler during low water conditions (also known as dry firing) the following may occur:

  • Hot spots and metal discoloration
  • Warping of metal walls and or boiler tubes
  • In extreme cases your entire boiler can explode.

These are all indications that the steel (typically carbon steel) is overheating, and the properties of the metal are altered. This boiler should not be run until it can be thoroughly inspected. In some jurisdictions, this is a reportable failure of both low water devices and your state should be notified.

How To Prevent LWCO Failure

Low Water Cut off with sludge build up

Sludge build up around a LWCO float switch.

1. Maintenance

Perform a blowdown of the level controls, low water cut off bridle, and auxiliary low water cut off bridle every day. This will help to remove solids.

Daily quick drain tests on the low water cutoffs are typical on high pressure steam boilers to ensure safe operation. This blowdown cleans the column and the piping the switch is installed in. This blowdown will either shut the boiler down, or produce an audible alarm or indicator light.

Boiler controls equipped with a shunt button should allow verification the low water switches are working, even if the shunt prevents a burner trip. Without a shunt button, the boiler will need to be tripped to ensure these lower water switches are still working. Slow drain tests should be performed semi-annually or as recommended by the boiler manufacturer, your insurance provider, or the AHJ. 

2. Know The Age

These switches generally have a 10-year lifespan when maintained properly, but can fail sooner when neglected. By understanding the average age you can better plan for repairs and replacements.

3. Monitor Deposit Build Up

All low water cutoffs are susceptible to deposit build up inside the bridle piping and the float or conductivity probe chamber. When this happens, the switch may not be able to detect the correct water level. This creates all sorts of problems! By routinely blowing down these lines you can reduce the risk of such a malfunction. Monitor buildup by opening up floats, switches, and bridle piping crosses at every internal inspection.

Low Water Cutoff Shunts

If a low water cutoff is working properly, it will prevent fuel from going to the burner during testing and shut the boiler down. For some industries, having a boiler shutdown is not an option. This is often the case with ethanol plants, bio-refineries and other industrial producers. They will often lose hundreds of dollars a minute if the boiler goes down.

Having a boiler go through post purge, pre-purge and losing steam pressure following a LWCO test is not an option. In these situations we install a shunt button with an indicator light for each device. The push button wires into the LWCO and ALWCO circuit. This is to allow the operator to blowdown and test each low water safety device without shutting the boiler off. When the indicator light illuminates, the operator can confirm the switch is still functioning. This ensures safe operation while keeping steam production intact. 

Systems without an indicator light should be retrofitted with a light. Although it isn’t required by code, blowing down the column or bridle piping utilizing a shunt will ensure the piping is clean. However, it will not confirm the operation of the level switch.

Low Pressure Steam

These are systems where the pressure of a boiler is below 15 PSIG. They are able to use a different system to regulate low water. Oftentimes we see built-in cutoffs from the boiler OEM. These systems will also commonly have a feedwater cut-off combination float device. When they detect low water, they supply feedwater directly to the boiler. If the water continues to lower, the boiler will trip.

Low water cut off system provided by mcdonald miller

Picture courtesy of MacDonald Miller 

NOTE: Never use untreated water unless it is an emergency situation. This can create scale and corrosion in the boiler and the system!

Low Water Cut Off Questions?

Talk to the experts. We can help with installation, service and parts. Call us at 1-800-237-3141, email sales@rasmech.com or contact us online.

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