The most dangerous of all conditions in a boiler is firing during a low water condition. This is true for boilers of all types and sizes whether big or small. For this reason, understanding proper low water detection, alarming, and shutdown conditions is a critical part of boiler operation and burner management.

Why are low water conditions dangerous?

If a boiler’s water supply is disrupted while the burner is firing, the boiler’s drum level will drop as it continues to produce steam. If this low water condition is not caught and the burner remains firing, the boiler tubes and drum will overheat and become brittle as the heat is no longer being transferred to the water. The boiler drum will then rupture causing the steam to de-pressurize and simultaneously cause any remaining water to flash to steam. When water flashes to steam it expands to 1600 times its original volume. This sudden rapid expansion is extremely dangerous and will destroy the boiler.

The wrong response

If a boiler is found to be running with no water in the sight glass, the proper response is to shut the burner off, let the boiler cool down, and then inspect the boiler for damage. The wrong response is to re-introduce water to the boiler. This is because the relatively cold water being added to the boiler will flash to steam when contacting the overheated metal in the boiler. This flashing to steam combined with the potentially weakened metal can cause a boiler rupture.

The role of burner management in a low water condition

Thankfully, NFPA 85 and CSD-1 both require at least two methods of low water cutoffs. The two most common methods of doing this are with conductivity probes and a float switch. If either of these switches open indicating a low water condition, the burner management must shut the burner down.

The primary low water cutout, which is the higher of the two and should trip first, is allowed to be a recycling limit meaning that once the water level is back above the switch setpoint, the burner management can automatically restart the burner. The auxiliary low-low water cutout must be wired into the running interlocks circuit of the burner management system and requires the operator to reset the controller when the low water condition is clear. Both the primary and auxiliary low water cutouts must open before the water level is below the boiler drum sight glass.

Discrete switch methods of low water detection

Conductivity probes are often housed in the boiler’s water column and typically consist of 3 probes: Common, Low Water Alarm, Low Water Cutout. The common is the longest of these probes Sometimes a fourth probe is included for a high water alarm. If there is water above the lowest point of the Common and the Low Water Cut Out probes, current will conduct and the low water cutout switch will be satisfied that this is a safe condition. When the drum level drops below the Low Water Cutout probe, the conductivity will be lost and the Relay contact will open tripping the burner. The alternative method of low water detection is a lever-arm float that opens a switch when the float drops below the trip setpoint.

Analog methods of low water detection

There are also ways of detecting low water conditions with analog sensors. It is important to note that these are in addition to the low water cutout switches mentioned above, and not in place of them. The first way to do this is with a standalone drum level controller like Preferred’s JC-43D2. This controller monitors the boiler drum level via an analog 4-20mA signal, typically from a differential pressure transmitter. A relay contact on the controller can be wired into the Running Interlock circuit of a burner management system and when the drum level drops below the controllers trip setpoint, the relay contact will open tripping the burner. This controller also tracks regular water column blowdown maintenance and can detect when the drum level sensor is failing.

Another analog method of low water detection is the use of a high flue temperature monitor, such as Preferred’s JC-15D2. The idea here is that if the boiler is experiencing a low water condition, as the heat transfer to the water (or lack thereof) decreases and the temperature of the boiler metal increases, the stack temperature will also increase. The JC-15D2 can catch this increase of stack temperature shut down the burner with a high flue temp cutout.

Summary and Preferred products

As we have seen, low water detection is the most important safety interlock monitored by the burner management system. Preferred specializes in all things boiler and burner controls and offers a wide range of products with built-in burner management such as the Burnermate Universal complete boiler control system.