How Electronic Level Control Works

Engineers must ensure that the boiler level is kept under control.

Boilers are devices designed specifically to heat water. Most homes have boilers that provide hot water for showering, cleaning and making food. The boilers risk overflowing at times, so these systems need an electronic level control to ensure the boiler does not fill up with too much water. Also, the boilers must always have enough water inside so the heat does not damage the boiler.

Conductivity Probes

Level control systems have an on-and-off signal responsible for level control. Normally, instruments control water level by starting a feedpump at a low level and running it until it reaches a higher level within the boiler system. Magnetic switches have a built-in hysteresis or dead-band, which is a system that can predict the boiler's flowrate. Conductivity probes have on-and-off probes for fixed switching levels. A capacitance probe gives on-and-off switching levels. Those boilers that have under 5,000 kilogram-an-hour steam generation usually use the on/off control system because this system is the least expensive. However, the high flowrate of cold feedwater -- water supplied to the boiler -- can reduce the boiler pressure. This results in an inconsistent boiler firing rate. The firing rate might need to be higher when the feedpump is turned on as opposed to when the feedpump is turned off. When the boiler pump has such variation, the boiler's burner controls tend to wear down, the temperature cycles and wastes energy and the steam flow rate develops a sawtooth. Multi-boiler systems can develop additional problems as the water levels become more unstable, which can lead to low water level knockout.

Modulating Boilers

Modulating boilers control the feedwater flowrate. A modulating level control floats in the boiler water and continually sends a signal to the rest of the system. This sends a signal to the level controller, which reduces the amount of water the level control valve allows into the boiler.

Recirculation Systems

When the boiler does not have enough water in it, the boiler can overheat and cause damage. Therefore, boilers must have recirculation systems that can ensure the boiler has a minimum flowrate. A valve or an orfice plate controls the water recirculation.

Speed Variation

Some boilers vary the speed of the boiler feedwater pump. A controller moderates how much AC voltage travels to the pump motor, which changes the speed of the boiler. The pump runs at a higher speed when the boiler needs more water, while the pump runs at a lower speed when the boiler does not need as much water.

References

• Spirax Sarco: Automatic Level Control Systems
• TC Electronic: Level Pilot