MIL-HDBK-1003/6
Continuous Blowdown. The boiler water is continuously discharged
7.3.5
in an amount which maintains a constant boiler water concentration ratio of
solids. For a typical diagram, see Figure 25. Continuous Blowoff Piping
Diagram.
Boiler Blowdown Piping and Valve Standards. See the ASME Boiler
7.3.6
and Pressure Vessel Code, Section 1 Para. PG-59.3.3 and para. A-56 to A-60,
and ANSI/ASME B31.1, for minimum requirements.
Calculations.
Calculate percent blowdown as follows.
7.3.7
EQUATION:
Percent Blowdown = 100 /CR
(10)
where:
CR = concentration ratio
Calculate continuous blowdown rate as follows:
EQUATION:
Z = B x C/(A - B)
(11)
where:
Z = Blowdown rate in pounds per hour (pph).
A = Predetermined boiler water concentration as total
solids in parts/million (ppm) or mg/L.
B = Total solids in feedwater as fed to boiler in
parts per million (ppm) or mg/L.
C = Steam output in pounds per hour (pph).
For an example of design calculations for continuous blowdown equipment,
assume the following values:
Total solids in makeup
170
ppm
(170 mg/L)
Total solids in condensate return
25
ppm
(25 mg/L)
Total solids in boiler water (A)
2,500
ppm
(2 500 mg/L)
Steaming rate (C)
15,000
pph
Losses in system
6,000
pph
Blowdown (by trial and error)
530
pph
Makeup = blowdown
plus losses =
6,530 pph
Condensate = steam rate
minus losses =
9,000 pph
B = (6,530 pph x 170 ppm) + (9,000 pph x 25 ppm)
(6,530 + 9,000) pph
= 86 ppm = 86 mg/L
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