Steam Generation System: Consequence of reduction in continuous blowdown (CBD) flow rate

Steam generating systems namely utility fuel fired boilers and heat recovery steam generators (HRSG or waste heat boiler, WHB) resort to continuous blowdown (CBD) of water from steam drum in order to maintain within limits the total dissolved solids (TDS) concentration in the boiler drums circulating water. TDS concentration in mg/l is empirically measured by measuring specific conductivity, μS/cm of CBD water and certain range of Specific conductivity is maintained, typically 40-80 μS/cm for high purity combined BFW quality. Typically, CBD valve is an FCV controlling the CBD flowrate as a proportion of BFW. CBD control philosophy is: 1.?CBD flow rate is 1 – 1.25% of BFW flow rate, controlled by CBD FCV. 2. Online conductivity meter input to CBD FCV to override FCV setpoint in case of high conductivity mainly due to: a. Malfunctioning FCV and or b. Contaminants in BFW. ???

CBD regime though essential for boiler operation is a source of energy loss and hence needs to be maintained at most optimum flow rates as determined by the standard guidelines for boiler water chemistry control. Often there is a scope for reduction of CBD when the following parameters permit:???????????1. Steam purity measured by steam sample specific conductivity, typically <5 μS/cm. 2. Boiler drum water specific conductivity close to lower control limit(LCL). In such a case CBD can be progressively reduced by throttling the FCV to achieve a CBD specific conductivity closer to the upper control limit (UCL). Note that the override specific conductivity value will be typically 20-35% higher than the UCL. ??

The consequence of reduction in CBD flow rate directly results in increase in steam production when the steam generating is operating at steady state at fixed load conditions. This gain in steam production is the biggest energy saving benefit for optimizing CDB from time to time. CBD reduction must be considered when there is an increase in steam condensate return and recycle from steam users. The following sample calculation illustrates the beneficial consequence of CBD flow rate?reduction. A steam production gain of ~6.6MT/H is estimated by reducing excessive CBD flow rate to normal values. At 20$ per MT of steam for 8000 hours of annual operation, the value of energy gain is approximately 1.1Million US$ per year.