In addition to other harmful ingredients, freshwater also contains alkaline earths, often referred to as “hardness”. We are also familiar with this from the deposits in the kettle or the coffee machine. A limescale layer of only 1mm can significantly reduce heat transfer on the water side. This can lead to significant damage, particularly to components that are subject to high thermal stresses such as the flame tube or the tube plate of the internal reversing chamber.
In this case, the existing softening system has been consistently overloaded by additional in-plant consumers. There was no hardness monitoring and the overloading was therefore not detected.
As a result, residual hardness also ended up in the boiler in which layers had formed on the water side. These layers reduced the heat transfer which in turn caused local overheating and cracks in the tube plate between the holes.
Did you know?
With a water hardness of 10° dH (medium hardness), up to 70g of limescale is transported per m3 of water.
If this value is extrapolated for a boiler with a steam output of 10 t/h operating continuously for 10 days, the results is nearly 500kg of limescale in the boiler.
Info on Water quality monitoring
Technical information: requirements for operation of high-pressure boiler systems without constant supervision
From an energy and efficiency standpoint it makes sense to return condensate that accumulates during operation to the water/steam circuit. It can however become contaminated during the production process. In this example, the condensate has been contaminated with grease due to a leaking production heat exchanger. The grease was deposited in the boiler on the heating surfaces and also on the low-water indicator which caused the heating surfaces to overheat.
Layer of grease and contamination in the boiler resulting in deformation of the flame tube and internal reversing chamber due to overheating
The dosing must generally fulfil two tasks. On the one hand, any components of residual oxygen or residual hardness must be bonded, and on the other the pH value in the feed water and in the boiler must be maintained within the allowable limits. The dosing pumps are frequently switched in parallel with the boiler feed pumps or make-up water control system which means that dosing is effectively quantity based. The dosing output is adapted to the values measured in the boiler water to ensure the concentration of dosing agent remains within the range of guide values. A setting within the range of 30 – 100% of the dosing pump output is advisable. If this is not sufficient, the concentration of dosing agent must be changed.
Both underdosing and overdosing can disrupt smooth boiler operation or can lead to serious damage.
Frequently recurring problems in the water quality are however not normally attributable to the dosing itself and instead can often be traced back to the upstream water treatment and monitoring. In these cases, normal fluctuations can no longer be compensated for by dosing.