Even though we add chemicals to the feedwater to protect the boiler from scale and corrosion, they do add to the ‘Total Dissolved Solids’ (TDS) content. As steam is produced, the dissolved solids remain in the boiler and concentrate up.
Excessive concentrations of dissolved and suspended solids must be avoided if foaming at the interface of water and steam is to be controlled and carryover and instability avoided.
Too high a level and problems with foaming and carry over are prevalent, too low and excessive energy is wasted.
Given that in protecting the boiler from corrosion and scale we adversely affect the dissolved and suspended solids, it is in the interest of the operator to ensure that water quality is correct throughout the system; from the incoming cold feed to the returning condensate. In so doing we ensure the cleanliness of the heat transfer surfaces and the overall efficiency of the boiler.
Total Dissolved Solids (TDS)
As steam is evaporated the concentration of total dissolved solids (TDS) increases in the boiler water. If the TDS concentration is allowed to get too high then carryover of boiler water will occur. This carryover can cause serious damage to the steam and condensate systems through corrosion and deposition on heat transfer surfaces.
In order to limit the TDS concentration it is normal to drain off, (or “blowdown”) boiler water and replace it with relatively low TDS feedwater. Excessive blowdown is very costly in terms of lost energy and water treatment chemicals.
Manual control of boiler TDS is labour intensive, inefficient and costly in lost energy. Automatic TDS control will ensure that the boiler water is maintained at the optimum level. This will reduce thermal losses due to excessive blowdown, reduce the risk of priming and carryover and enable up to 90% of the energy contained in the blowdown to be recovered.
For every 1% reduction in boiler blowdown 0.19% reduction in fuel is achievable when operating the boilers at 7BarG.
The conductivity of water is temperature dependent: every 1°C temperature variation introduces a 2% error in the measured value. With a boiler on ‘Hot Standby’ the error in measured value could be 35% to 40%.
For accurate control temperature compensated measurements are imperative.