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

119

Integrated Publishing, Inc. |