Article Summary
- Low water fuel cut off (LWFCO) devices.
- Float and probe type LWFCO devices.
- Components of LWFCO devices.
- Testing of LWFCO devices.
- Operation of LWFCO devices (how low water fuel cut off devices work).
- Boiler gauge glasses.
Introduction
For industrial boilers, accurate measurement of the boiler steam drum water level is vital for a boiler’s safe operation. It is also important that the fuel supply to the boiler can be interrupted automatically in case, should the event occur, the operator fails to realise that the water level has fallen below a defined critical level. Low water fuel cut off (LWFCO) devices are used to measure the water level in a boiler drum and to interrupt the fuel supply should the water level be too low.
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Low Water Level Cut Off (LWFCO) Device
In most boilers, the water that is heated by the boiler tubes to create steam is also the water that cools the tubes. Loss of this water can thus cause the tubes to overheat or even melt, leading to expensive and lengthy plant outages.
What is a fuel cut off device?
It is important to install fuel cut off devices that interrupt a boiler’s fuel supply in order to stop the heating process if an emergency scenario should occur. A LWFCO device interrupts the fuel supply if the water level in the steam drum is too low, but the fuel supply may also be interrupted if the drum steam pressure is too high, drum steam temperature is too high, upon loss of flame, or for other reasons specified by the manufacturer or governing body (ASME, BS etc.).
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Low Water Fuel Cut Off Device Functions
A boiler drum contains the water reservoir that is circulated within a steam generator. Watertube boilers have a steam drum above the boiler tubes, whereas firetube boilers have tubes within the boiler drum. In both boiler designs, water heated by the tube walls is also cooling the tube walls; thus a low water level scenario will lead to overheating of the tubes and potential boiler failure.
Watertube and Firetube Boilers
To maximise efficiency in watertube boilers, combustion takes place within a chamber whose walls are made up of water carrying tubes and/or tubes located directly within the flame or hot flue gases. It is vital that the tubes remain filled with water when the boiler is in operation; accurate and reliable water level measurement equipment is installed for this reason.
For firetube boilers, the water within the drum surrounds the tubes and it is the water level within this drum that is measured and connected to a low water fuel cut off device. It is also possible to install LWFCO devices on water heaters although these operate at much lower pressures and temperatures.
Watertube and Firetube Boiler Heat Transfer
The phenomenon of drum swell and shrinkage primarily affects larger boilers, requiring the use of advanced water level measurement and fuel cut off devices. This article addresses fuel cut off devices for smaller units only.
It is important to note that fuel cut off devices are not only installed for the purpose of isolating the fuel supply in the event of a critically low water level, but also in case of other events that may otherwise render the boiler unsafe or risk major plant damage (high steam pressure, high steam temperature, loss of flame, abnormal furnace pressure etc.).
Alarms as well as shutdowns are usually connected to the LWFCO device. For example, an alarm will sound upon a low-water level scenario and for a high-water level scenario.
Construction
The below image shows a typical low water fuel cut off device; the device shown is of the float design. A boiler gauge glass may be integrated into the LWFCO along with try cocks (small red valves used to prove the water level in the water column). The LWFCO device may be incorporated into a boiler water column depending upon the boiler design.
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Low Water Level Cut Off (LWFCO) Device
The above LWFCO float design has:
- A large float connected to an arm/lever, which then connects to the electrical circuit.
- Three try cocks for proving the water level in the water column.
- A boiler gauge glass with a steam, water, and blowdown valve.
Boiler Gauge Glass Measuring Boiler Drum Water Level
The LWFCO device has connections to the steam and water side of the boiler. The steam line from the LWFCO device connects to the top of the boiler drum. The water line from the LWFCO device connects to the drum at a level much below that of the boiler’s normal operating water level (NOWL).
LWFCO – Float Type
Float type LWFCO devices are connected to an electrical circuit which controls the operation of the burner and fuel supply. As LWFCO floats move proportionally with the boiler drum water level, any low water level scenario will cause the float to drop and the burner(s) to shut down.
LWFCO devices can also be designed so that when they shut down the burner, they also automatically start the feedwater pump, monitor the water level until it reaches the normal operating water level (NOWL) again, then start the burners once a safe working level is again reached.
Float Controlling Boiler Water Level
The boiler gauge glass is installed at the same level as that of the fuel cut off device, and so any reduction in the water level detected by the fuel cut off device will cause a water level reduction to be shown by the gauge. At the time when fuel is interrupted, the water level should still be seen at the bottom of the boiler gauge glass.
LWFCO - Probe Type
The probe type LWFCO device uses conductivity probes to detect the water level, cutting off the fuel supply if the water level falls below the minimum requirement. Probe type LWFCO devices are generally robust and require minimal maintenance. Probe type LWFCO devices are housed within a sleeve or chamber to increase their reliability.
Boiler Probe Type Water Level Measurement
Float Type Low Water Fuel Cut-Off Testing
The float type LWFCO design shown previously is fitted with its own blowdown valve which is maintained according to the boiler manufacturer’s instructions and the applicable standards. The LWFCO device is tested by means of opening the blowdown valve and observing whether the burner is shut down when the water level in the water column falls below the defined low water level; another procedure used to prove the working of the LWFCO device is the evaporation test.
For both LWFCO device types (float and probe), it is essential that blowdown testing be conducted at regular intervals to ensure that the devices are operating correctly. This is especially important with reference to probe type devices, which are more prone to fouling by sediment and scale in the boiler water.
Additional Resources
https://www.stheatingservices.co.uk/industrial-boiler-systems-work
https://inspectapedia.com/heat/LWCO_Low_Water_Cutoff_Control.php