Page 163 - Boiler plant and distribution system optimization manual

148 Boiler Plant and Distribution System Optimization Manual

the temperature of the tube metal. At sufficiently was quite possibly plugging an existing hole. At
high temperatures, in the neighborhood of 1,000°F some point these holes would start to leak, possi-
the tubes begin to lose strength and rupture. bly causing an expensive shutdown.
The chemical cleaning process is straight for-
ward for a Fire Tube boiler about 10-12 hours is
6. CHEMICAL SCALE REMOVAL required; cleaning a water tube boiler may take 18
to 24 hours.
Cost
Moderate (essential maintenance)
7. BLOWDOWN HEAT RECOVERY
Potential Savings
Moderate (2%-3%) Cost
Low to Moderate
Even a thin layer of scale lowers boiler ef-
ficiency. On water-tube boilers the water side Potential Savings
scale blocks heat transfer so the tubes may over- Moderate (2% to 5%)
heat, lose strength and rupture. Scale also creates
a breeding ground for under-deposit corrosion Description
which can cause expensive equipment outages Blowdown water temperature is usual-
and repairs. If you do not acid clean your boiler ly over 300°F depending on the pressure and it
you may have to replace tubes at some point [ex- contains a lot of energy that can be wasted if it
pensive repairs]. is not put back to work in the boiler somehow.
Once the scale is removed an inspection will About 15% of the blowdown water will flash to
reveal the true condition of your boiler, 90% of the low pressure steam so it is a very good source
time leaks do not occur because of chemical clean- of low pressure steam and is usually used in the
ing. In those cases where leaks develop, the scale deareator/feedwater heater. This steam can be re-

Figure 10.42—Shows how scale build up reduces heat transfer efficiency 2%
to 8%. Scaling is most prevalent in the hotter zones in a direct line of sight
from the flame.