Ian D. Oldfield
Executive Account Manager, Gestra, USA
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
- If a float gets water-logged, it will naturally sink. When this happens the low water cutoff is tripped. This is not considered a problem because the failure is detectable. Problems occur when operators bypass the electrical connectors at the switching mechanism which eliminate low water cutoff protection.
- Depending on the chamber design, there can be a small amount of clearance between the chamber wall and the float. Dissolved solids in the boiler water can precipitate out to form scale on the inside of the chamber. As this scale increases, the clearance between the float and the chamber (now lined with scale) decreases. This causes the problem of a float "hanging up" on the scale in the chamber. As the water drops, the float remains up and the low water cutoff is rendered useless. There is a danger if this goes undetected because it leads to a low water condition.
- If a float type device is a combination feed water control (pump on/off) and low water cutoff - and the float hangs up - this can cause more than one problem. When the float hangs up, water is no longer fed into the boiler and the low water cutoff is rendered useless. The only thing to prevent a no-water situation is a second low water cutoff device and an observant operator. If a float type device is having problems with scaling up, using the same device as the second low water cutoff may also create problems at the same time. A different or back-up type of technology is probably a good idea.
- Mechanical linkages are always subject to wear over time. The number of problems this could cause is dependent on their design. It is very important to make sure that the mechanism trips at the same point and does not change over time. Preventive maintenance for float type devices is necessary to maintain proper function. (See Fall 1997 National Board BULLETIN article entitled "Steam Traps Affect Boiler Plant Efficiency.")
- Improper installation can occur when retrofitting controls. In some cases the float is unable to move up and down freely.
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.
- Scale build-up on the electrode can occur on the tip because of the large quantity of dissolved solids in a boiler. Scale bridging of the tip can complete the electrical circuit and may cause the electrode to detect water when it is not present.
- If the electrode uses a DC current, plating of scale can occur fairly rapidly. Keeping track of which and how often each electrode is cleaned is a necessity. (Manufacturer's recommendations should be followed.)
- Some manufacturers use a square wave DC current to change the polarity on the tip. This is more effective than straight DC current but is not as effective as AC current. Again, determining the frequency of cleaning is important since it varies with boiler water chemistry.
- If the electrode uses an AC current, the integrity of the electronics will affect the degree of scaling that can occur. In general, the higher the frequency and the lower the voltage, the better the resistance to scale. Some electrodes are very effective using frequencies up to 1,000 Hz to eliminate polarization on the tip.
- A short circuit in the wiring can give a false indication that water is present. When the alarm is actually tripped, the short completes the circuit and overrides the alarm. This can happen with either a float switch or an electronic switch.
- The mechanical/electronic defect of the switch will lead to a malfunction and not break the circuit. It is a good idea to have a device that has a normally open switch. In the event of a loss of power to the low water limit, the switch opens and the circuit is broken. It is extremely important to perform routine tests regularly to check the integrity of the switch. Visual examination is not reliable.
- Jumping the circuit is not recommended because it creates a potentially dangerous situation. It should only be done under authorized conditions, and the jumper should be removed immediately after use. This is not a failure mode, however, it is the cause of many low water conditions. If the low water cutoff is frequently causing a trip of the boiler it should not be overridden because it is a safety device!
- Damaged wiring or insulation should not be a cause of failure if the circuit is cut but not shorted. This should in effect break the circuit and cause the boiler to trip. If, however, the circuit is designed in such a way that the switch has to make contact (complete the circuit ) to trip the boiler, a disconnected or broken wire, or a piece of insulation in the terminal block, can prevent the circuit from being closed and a low water trip will not occur.
- Improper installation is always a problem, especially when the electrode tip may be in contact with the wall of the enclosure/pot. In this case, the circuit is always made by metal to metal contact and will never trip on low water.
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.