Continuous water level control is gaining popularity among users of steam boilers. This particular technology uses a water level transmitter (usually a capacitive probe) installed on the boiler to measure the water level. The level signal from the transmitter is used to control the speed of the feed water pump, which is driven by a frequency inverter. The speed of the feed pump is thus controlled by the water level – the lower the level drops, the faster the pump will run and the higher the rate at which water is delivered to the boiler will be. And as the water level rises, the pump will slow down until it may eventually stop.
There are several benefits of this technology over the established practice of stopping and starting the feed pump from a level switch:
- Reduced wear of mechanical components and electrical switch gear due to pumps starting and stopping.
- Elimination of steam pressure drop and thermal stresses while the feed pump is filling the boiler with relatively cold water at maximum delivery rate, repeating the process many times over per day.
- Water is replenished to the boiler at the rate at which it is being evaporated. No detectable pressure drop or cyclical thermal stressors.
- The continuous water level control system adds a back-up safety device for the protection of the boiler. The signal from the level transmitter can be used to sound a siren and to trip the boiler when the boiler water level approaches dangerously low levels. It can also warn against excessively high water levels and can even stop the feed pump under these conditions.
- In terms of power consumption it is not easy to come to a conclusion as to the best pumping configuration – there are just too many variables determining how the feed pump is required to operate. However, my gut feel is that the continuous water level control configuration may consume more power than its on-off counterpart. The reason for this is that the variable speed pump ideally has to operate continuously and maintain a relatively high speed, even at minimum delivery, to overcome the resistance of the steam pressure acting on the boiler water.
Setting up the water level control algorithm is however rather tricky and may even be baffling to the uninitiated. Let me explain: under normal operating conditions the water in the boiler is boiling off at a certain rate to satisfy the steam demand. The water mass thus consists of a mixture of saturated steam (vapour) bubbles rising to the water surface, and liquid water. This mixture of water and steam bubbles fills the boiler to a safe water level.
As soon as “cold” water is being pumped into the boiler, it cools down the existing water and vapour mixture, causing the vapour bubbles to collapse (condense) and the water level to drop rapidly – a phenomenon known as shrink. So although the pump may be pumping water at maximum capacity, the water level may be seen dropping for a while, after which it stabilizes and then starts rising. As soon as the set point water level is reached, the pump has slowed down to minimum speed, but at that point the “cold” water has been heated to boiling point and it starts boiling more vigorously, vapour bubbles increasingly develops and the water level may be seen to rise steadily – a phenomenon known as swell, even though the pump may not be delivering any water.
Note that a rapid drop in steam pressure will also trigger the swell phenomenon, as a reduction in pressure will increase the rate of evaporation of the saturated water mass. On the other hand, if the steam output from the boiler is reduced suddenly, the pressure will increase and cause the water level to shrink as the rate of evaporation decreases.
Rather challenging if one tries to control the water level without knowing what is happening on the inside of the vessel.
I am not going to involve myself with the various methods of water level control here – solutions range from elementary to extremely complex, especially with water tube boilers with relatively small steam drums. Maybe this may form a topic for a future publication.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
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