Maintaining an accurate and stable boiler drum water level is one of the most critical safety and efficiency requirements in power plants and industrial boilers. Even a small deviation in drum level can cause carryover, water hammer, turbine damage, or dry-firing, leading to severe equipment failures. To protect the boiler from such risks, many systems use a highly reliable device known as the Morbey Switch.
In this article, we will explain what a Morbey switch is, how it works, and how it controls boiler drum level.
What Is a Morbey Switch?
A Morbey Switch is a mechanical, float-operated level switch commonly used in boilers to sense low water level or very low water level (LL, LLL). It is also known as a Marshalling or Morbey switch, depending on the manufacturer.
It provides critical safety interlock signals for boiler trip or feedwater control based on drum water level.
Key Features
- Simple mechanical design
- High reliability in harsh boiler environments
- Provides ON/OFF type electrical contact
- Widely used for low water level protection
- Suitable for high-pressure boiler drums
Why Drum Level Control Is Critical
Boiler drum level must be kept within a narrow and safe range.
If the level goes too low, boiler tubes may overheat and fail.
If the level goes too high, water may enter the steam line and damage the turbine.
Drum Level Issues
- Low Level (LL): Tube overheating
- Very Low Level (LLL): Boiler trip condition
- High Level (HL): Steam purity affected
- Very High Level (HLL): Possible water carryover
The Morbey switch primarily focuses on low-level protection.
Construction of Morbey Switch
A typical Morbey switch consists of:
- Stainless steel float ball
Morbey Switch internal parts - Float arm mechanism
- Magnetic coupling
- Micro-switch (SPDT contacts)
- Tough SS chamber mounted to the drum
- Electrical termination box
The switch is mounted on the boiler drum level piping or water column.
Working Principle of Morbey Switch
The working principle is simple but extremely effective:
1. Float Senses Water Level
The float inside the chamber rises and falls with the boiler drum water level.
2. Magnetic Movement Transfers Motion
A magnetic coupling transfers this movement to the switch assembly—keeping electrical parts isolated from boiler pressure.
3. Micro-Switch Operation
When the water level drops to a preset point:
- The float moves downward
- Magnet actuates the micro switch
- Electrical contact changes state (NO to NC or vice-versa)
4. Signal Sent to Boiler Control System
The output signal goes to:
- DCS/PLC
- Safety interlock system
- Feedwater control system
This signal initiates alarms, pumps, or trip actions, depending on the design.
How Morbey Switch Controls Boiler Drum Level
Here’s the step-by-step operation:
1. Monitoring the Drum Water Level
The switch continuously tracks actual water level in the drum.
2. Low Level Condition (LL)
When water falls below safe level:
- Morbey switch activates LL contact
- DCS raises Low Level Alarm
- Feedwater control system increases pump flow
3. Very Low Level (LLL) – Critical Condition
If level continues to drop:
- LLL contact operates
- Boiler trips automatically to prevent tube failure
4. Preventing False Signals
The switch has a robust float mechanism to avoid false trips caused by:
- Pressure fluctuations
- Water turbulence
- Steam bubbles
5. Works as a Redundant Protection
Even if electronic transmitters fail, Morbey switch provides mechanical backup protection.
This is why it is widely used in critical and high-pressure boilers.
Applications of Morbey Level Switch in Power Plants
- Boiler drum low water level protection
- Feedwater pump startup logic
- Burner cut-off in critical low level
- Steam drum safety interlocks
- Heavy-duty industrial boilers
- Redundant backup to level transmitters
Boiler Drum Level Control – Morbey Switch
Advantages of Using Morbey Switch
- Highly reliable under boiler conditions
- No drift in calibration
- Simple design with long service life
- Provides direct mechanical protection
- Suitable for high-pressure, high-temperature systems
Conclusion
A Morbey switch is a crucial device for safe boiler operation. By continuously monitoring drum water level and initiating alarms or trip actions during low-level conditions, it ensures that the boiler operates within safe limits.
This simple mechanical device plays a major role in protecting boiler tubes, maintaining steam quality, and preventing catastrophic failures.
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