Steam boilers have two main safety devices: primary (LWCO) and auxiliary low water cut-off (ALWCO) switches. Water in the boiler should always remain at the normal operating water level (NWL). The low water cut-off switches are set below the NWL. The exact position below NWL varies, depending on boiler construction, but is set such that there is room for normal water level fluctuations (avoiding nuisance trips) but high enough to protect the boiler from damage.
The Primary LWCO is an automatic reset type of control; it will cut the boiler off at a set level but allow the boiler to restart without operator input once the water returns to an appropriate level. The ALWCO is a manual reset type of control; once tripped, it requires an operator’s intervention to manually reset the control before the boiler can restart. As with all manual reset controls, if the cause of the manual reset control tripping is not well understood, a competent boiler service company should be contacted – routinely manually resetting controls is not a good safety practice.
What is a Boiler Low Water Cut-Off?
A low-water cut-off refers to a mechanical device used to shut down the boiler and prevent it from running after water drops below a marked point. Modern boilers are very well made and durable – they are very tough, and they can withstand high pressures and operating cycles over decades.
But they can also be fragile; boilers rely on the cooling effect of the water they are heating to keep their metal (furnace) surfaces in a safe temperature range. Without water, the metal surfaces quickly overheat, leading to damage and danger. A boiler that is allowed to operate without water (or low water) is said to have been dry-fired.
Dry-firing can damage the boiler’s heating surfaces in a very short time frame. The leading causes of low-water device failure are diverse, but many revolve around improper maintenance: lack of routine maintenance, sediment buildup, and incorrect maintenance.
What Type of LWCO Do I Need?
Low water cut-off is arguably the most crucial safety device of a boiler. You can find it on any steam or hot water boilers to ensure safety if the water level goes down. They can be mechanical (float-type) or electrical devices (probe-type).
The type of LWCO used is determined by code. A mechanical low water cut-off device using a float mechanism is the most common for the Primary LWCO. These rely on a float with an arm connected to a switch tied to a burner circuit.
The float is in an external chamber and rises and falls with the level of water in the boiler. If the float drops below a certain level, it opens the switch and trips the burner control circuit.
On the other hand, level electrodes use conductivity to detect the level of water in the boiler. When water levels drop below a certain point, an electrode tip inside the water comes out (of water) and trips the electrical circuit, and low water shut-off occurs.
Low Water Cut-Off Failure
Over the past few years, there have been several boiler incidents, according to The National Board of Boiler and Pressure Vessel Inspectors reports. These incidents primarily occur due to catastrophic failures. When heat from the hot combustion gasses gets into the boiler during the low water conditions, it may cause:
- Hot spots and metal discoloration
- Warping of boiler tubes or metal walls
- In severe cases, if the furnace pass is exposed it can sag in on itself. (“Bagging the Furnace”)
- In extreme cases, the boiler may explode.
These are the significant indicators that the steel is overheating, and the metal properties are altered. Water should never be added to a boiler that has been operating without or with low water levels. The feedwater coming into contact with red-hot heating surfaces can lead to a powerful steam explosion – the safest course of action is to secure the boiler and once it is cooled and safe to do so, have the vessel opened and inspected before putting it back into service.
Over the years, technology has reduced boiler incidents and increased operator awareness of the function and operation of safety devices. Boiler operators must understand the local boiler codes and standards, along with the function and operation of their water level safety controls, to ensure effective maintenance that matches the boiler needs.
Potential Failure Modes of Low Water Cut Off
There are some common failure methods of LWCO’s:
- Pinholes and other leaks can develop in the float, allowing water into the float – the float is water-logged. If this happens, the float will sink, leading to a low water cut-off trip. This can lead to the ALWCO being tripped, requiring (continuous) resetting by the operator. To avoid tripping, we have observed operators jumping the level controls to prevent the nuisance trips – this is never a good practice.
- The float bowl will fill with “sludge” present in the boiler water, if the LWCO bowl is not blown down regularly and cleaned annually, this buildup can interfere with the proper movement of the float.
- Mechanical linkages and switches wear over time. Like anything else, LWCO’s have a finite lifespan as their components fail. Proper maintenance will lengthen service, but eventual repair/replacement needs to be evaluated.
- Improper installation of the low water cut-off can occur when retrofitting controls. Controls can be set at improper levels, too close together, or other issues.
Preventing Low Water Cut-Off Failure
You can prevent low-water cut-off failure by:
It is good practice to perform a blowdown on all level controls and low water crossover piping. Regular boiler maintenance helps to keep solids from collecting in the float bowls. The process involves opening and closing the lWCO piping’s drain valve, removing all trapped sediment. Such maintenance helps to remove solids that cause issues with the switches.
Daily or weekly drain tests on the low water cut-off are important to ensure safety. The blowdown cleaning removes any sediment from the column and the switch’s piping. It also shuts the boiler off, allowing any indicator lights or audible low water alarms to be tested.
Know the Age
Most switches used in LWCO’s only have about a ten-year lifespan when properly maintained. They can fail sooner if you neglect them. Understand the average age of your switches for effective planning and repair or replacement of your switches to enhance their lifespan.
Monitor Deposit Build Up
All low water cut-offs are highly susceptible to deposit buildup. Deposits can build up inside the crossover piping and the conductivity probe chamber. The buildup causes the switch to stop working and will not detect the water level, whether low or high.
A good water quality program and Routine blowdown of LWCO piping are two ways to reduce deposit buildup. LWCO’s should also be inspected and cleaned during every annual inspection.
Carry Out Slow Low Cut-Off Tests
One test that inspectors perform is the Slow Blowdown test. During this test, instead of blowing down the pipe quickly, the boiler is isolated from Feed Water, allowing the level to drop more slowly, better simulating the reaction of the LWCO to real-world water level changes.
During the test, your burner will need to be between low fire and 50% or lower. As noted above, instead of opening the blowdown valve quickly (creating a quick change in level), the boiler feed pump is turned off. The boiler will continue converting the remaining water to steam, and the water level will drop slowly.
The operator or person conducting the test will watch the water level at the sight glass. When the water level gets low, the burner should shut off. Water should still be visible in the sight glass when the burner cuts off (this is a code requirement). If during this test the water level falls below the level of the sight glass and the burner is still firing, your system has not passed the test.
You can hire experts to carry out these tests regularly and keep your boiler secure and operating effectively.
Low Water Cut-Off Tests?
Testing your low water cut-off valve is essential for preventive boiler maintenance. W.C. Rouse can provide preventative maintenance to reduce unscheduled boiler plant outages. Our services include emergency boiler repair, boiler tune-ups, preventive maintenance, efficiency optimization, boiler inspection, and boiler installations. Contact W.C. Rouse’s team of boiler experts to learn more.