What is the Low Water Level in Steam Boilers?

A low water level in a steam boiler occurs when the water level falls below the lowest water level mark or even below the top of the tubes, consequently leading the tubes to overheat. If the water reaches a lower level than the main firing tube, it may fail the steel itself. However, it can be prevented with proper steam boiler maintenance.

As the furnace temperatures generally exceed 1,800°F (980°C), the strength of the steel falls significantly at temperatures above 800°F (480°C). Therefore, low-water levels may cause the melting of the steel tubes. As water enters an exceedingly overheated boiler, the water flashes to steam. The impacted metal parts rupture due to the sudden increase in pressure, causing a steam explosion.

Effects of Low Water Level in Steam Boiler:

One of the most hazardous cases of low water takes place when the water is added to the steam boiler, but the system has already passed the point of no return as the boiler vessel is extremely overheated. It causes the water to quickly turn into steam, resulting in tremendous pressure and the explosion of the steam boiler system. It is one of the most dangerous results for any boiler system and must be prevented.

Commonly found damage caused by low water levels includes:

• Loose tubes
• Crack tubes
• Melting metal
• Leaking water
• 
  

Controlling Water Drum Level:

Controlling the boiler drum level is challenging as the water level in a steam drum is, in fact, an unstable combination of water and steam bubbles that shrinks and swells with the changing pressure. The increasing steam demand reduces the pressure in the boiler, causing the water level to rise. Additionally, as the freshwater enters from the feed tank and cools the drum water, it leads to the collapse of the steam bubbles and a drop in the water level. The solution for the shrink and swell effect in the steam boiler is opting for a 3 level drum control.

Low water conditions result from both operator and equipment issues. Some of the common causes include:

• 
  
• Failure of feedwater pump
• Failure of control valve
• Water loss to the deaerator or make-up water system
• Failure of drum level controller
• Drum level controller in the manual position
• Plant air pressure loss to the control valve actuator
• Safety valve lifting
• Large and sudden change in steam load
• 
  

Low Water Cutoff Device:

Steam boilers are installed with low water cutoffs to prevent any significant damage. The devices operate to identify the low water condition and shut down the boiler system. The burner remains off till the water returns for the burner to fire normally.

Some low water cutoff devices serve the dual function of starting and stopping feedwater flow and stopping the burner. Such a device is known as the Primary Low Water Cutoff Device. A Secondary Low Water Cutoff Device serves another purpose of initiating a safety shutdown and lockout.

Failures in Low Water Cutoff Device:

Low Water Cutoff Devices are usually very reliable and well-functioning. However, the following factors can result in a breakdown.

Deposits: Condensate water and new make-up water enters a steam boiler and deposits minerals and solids.

Outdated models: Most boiler manufacturers recommend periodic replacement of the mechanical components of the steam boiler system. If overlooked, outdated boilers can lead to leaks and parts failure.

Loss of Water: Steam leaks and faulty steam traps requiring a high amount of makeup water cause issues in steam boilers and Low Water Cutoff Devices from loss of water. In such situations, the entire system requires proper maintenance.

Jumpers: At times, jumper wires are used temporarily for diagnosing a problem or bypassing a circuit for testing. If ignored, the Low Water Cutoff Device fails to shut down the burner.

Maintenance: Proper boiler maintenance needs the purging of the Low Water Cutoff Device system. It requires attention as improper flushing can also cause damage.

Conclusion:

As one of the world’s leading combustion experts and boiler manufacturers, Rakhoh specializes in thermal solutions. We have more than 39 years of experience in designing and manufacturing steam boilers and boiler accessories. Rakhoh offers a range of high-quality steam boilers that are compatible with a variety of fuels and applications.

For more details on our products and services, visit www.rakhoh.com