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Low Water Cut-Off Technology

Print Date: 12/11/2017 5:23:43 AM

Ian D. Oldfield
Executive Account Manager, Gestra, USA

Winter 1999  

Category:  Design/Fab

 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)

 


 

New technological advances in the field of low water cutoff devices have been few and far between in the last 30 to 40 years throughout the United States. The National Board Incident Report shows, on average, that a high percentage of incidents involving deaths or injuries is due to low water conditions. Those in the industry should be aware of the potential problems caused by low water cutoff devices and should adopt a policy to educate operators to the importance of maintenance and inspections.

 

During the 1970s and early 1980s, very similar statistics were reported both in England and in Germany - where low water incidents were a major problem. At that time, the approach was to identify the basic causes for failures by looking at the operation and failure modes of different types of low water cutoff devices. A review of cutoff devices and possible failure modes follows:

Float Type - A float type device is most widely used in residential, institutional, and commercial heating boilers, but the devices may also be found in process steam boilers.

The principle of the device is a mechanical linkage that connects a float to a switch (or switches) that, in turn, is tied into the burner circuit. The float is in an external chamber and it rises and falls with the boiler water level. If the float drops too low, the linkage opens the switch and trips the burner control circuit.

Potential Failure Modes - What to Watch For

Level Electrodes - There are several different types of electrode designs currently in use. Some designs are internal to the boiler while others are installed in external mounting pots. When water is present in an electrode design, an electrical circuit is made between an electrode tip and the body of the electrode. When the water level in the drum drops and the tip is no longer immersed in water, the electrical circuit is tripped and a low water shutoff occurs. All of the electrode type devices operate on this basic method.

 

Potential Failure Modes - What to Watch For

There are different types of electrical designs that may cause the following problems. It is important to be aware of the large number of differences and what may occur.

Technological Advancements

As the result of failure mode studies, it was suggested the causes of failures in low water cutoff devices be addressed. To concentrate on improving the reliability of electrode technology, the concept of a self-monitoring device followed by automatic routine testing was developed and is practiced throughout Europe. The concept of self-monitoring allows two independent circuits to verify whether the measuring electrode is functioning properly (see figure at left). The first circuit, called the measuring circuit, has a measuring tip that acts as the primary alarm when low water is identified. The second circuit (known as the compensating circuit) is used to compare with the measuring circuit. The circuits are designed to sense if there is any imbalance in the boiler (improper installation, scaling, etc.) and if so, the sensors will trip the low water cutoff.

Automatic routine testing was developed to check the integrity of the wiring in the low water cutoff circuit. In a system with this design, an electronic pulse is sent through the circuit that checks for any damage to the wiring, insulation or other junctions. The low water cutoff will alarm if damage is found.

More than 30,000 functioning boilers in Europe have low water cutoff devices in place that use the developments mentioned above. Low water level limiters are yet another monitoring device.

With the increased use of more sophisticated electronics, especially the technology used for controls on the fire side of the boiler, it is worthwhile to consider improvements of the water side of the low water cutoff system as well. Reducing the number of boiler incidents all over the world caused by low water is an issue that should be continually addressed.

As referenced in ASME CSD-1 Contols and Safety Devices for Automatically Fired Boilers, each control and safety device covered by this code should be accepted for intended service by a nationally recognized testing agency such as UL, FM, or International Approval Services-US, Inc. (formerly AGA).

 


 

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.