Boiler Water Level Controller – Float Operated Type
A simple float-operated water level controller consists of:
- A float chamber connected to the boiler steam drum by two lines — one for steam and one for water.
- A float inside the chamber, which rises and falls with changes in water level.
- A mechanical linkage or rod attached to the float, which extends to an electric sensor unit mounted above the chamber.
Working Principle:
- As the float moves up or down, it shifts a contactor along a variable resistance track or magnetic switches.
- This movement changes the electrical output signal, which is sent to a square-root converter.
- The converter transforms the electrical signal into a proportional pneumatic signal.
- The pneumatic signal acts on the diaphragm of the feed water control valve actuator, modulating feed flow to maintain the set water level.
Reasons for Using a Float-Operated Type
- Reliability: Unlike constant/variable head leg systems, there is no need to maintain a filled reference column.
- Simplified Installation: Electrical sensing eliminates the need for long impulse tubes for remote indication.
- Ease of Maintenance: The electric sensor unit can be easily replaced without dismantling the float chamber.
- Lower Cost: Fewer mechanical parts and no head leg piping reduce installation and maintenance expenses.
Explanation of Terms (Analogy with Controller and Boiler)
: In this system, the float is the detecting element. It directly senses the water level, which is the controlled variable, and its movement provides a signal that represents the current state of the system.
: The square root converter and the feedwater controller collectively act as the servo motor. They are the mechanisms that receive the signal from the detecting element and perform the physical action (opening or closing the feedwater valve) to correct the water level.
: The set point is the desired value. This is a fixed input to the square root converter (or a comparator) that represents the ideal water level that the system aims to maintain. The controller continuously works to match the actual water level to this desired value.