25 May 2005
Feedwater Quality Limits. Tables 3-8 through 3-12 show feedwater
limits. The limits are specific for the type of boiler, operating pressure, and steam
application. Since feedwater is the combination of makeup water and condensate
return, recovering as much uncontaminated condensate as possible is advisable,
considering the cost of boiler water treatment and the benefits derived from both energy
and water conservation. The high purity of uncontaminated condensate minimizes the
requirement for treatment of makeup water and boiler water. The feedwater parameters
shown in Tables 3-8 through 3-12 do not specify levels for alkalinity and silica because it
is assumed that these levels in the boiler water will be limited through blowdown. In
higher pressure boilers using demineralized makeup water, alkalinity and silica will be
very low or even absent. Although not specified, it can make sense to reduce feedwater
alkalinity and silica in boilers less than 6205 kilopascals (900 pounds per square inch
gauge) to allow for an increase in COC and, consequently, to reduce chemical demand.
The tables show limits for hardness, iron, and copper. Hardness and iron can be
removed by softening the makeup water. Iron and copper may still be present in the
feedwater if they are present in the condensate due to corrosion. In many instances,
these limits cannot be met for small heating boilers (e.g., oxygen content in feedwater
softeners). The only alternative is to chemically treat boiler water and control COC, as
Table 3-8. Suggested Feedwater Limits for Industrial Water Tube Boiler
02068 KPa (0300 psig)
Dissolved oxygen ppm O2 before
chemical oxygen scavenger addition
Total iron ppm Fe (as Fe)
Total copper ppm Cu (as Cu)
Total hardness ppm (as CaCO3)
pH at 25 C (77 F)
Non-volatile TOC ppm (as O2)
Oily matter ppm
1. Makeup water percentage: Up to 100% of feedwater.
2. Assumes existence of deaerator.
3. Conditions: no superheater, turbine drives, or process restriction on steam
4. Steam purity: 1.0 ppm TDS maximum.
5. NS = not specified.
6. See ABMA Standard Boiler 401