25 May 2005
8. Prepare this mild, acid-rinse solution: Add 7.57 liters (2 gallons) of
hydrochloric acid (ASTM E 1146) for each 3785 liters (1000 gallons) of
water. Also add corrosion inhibitor, Military Specification MIL-1-17433, in
the amount recommended by the manufacturer.
9. Fill the boiler with the preheated (71 to 77 oC [160 to 170 oF]) mild acid-
rinse solution and soak for 30 minutes.
10. Drain the mild acid-rinse solution under nitrogen pressure at 276 to 345
kilopascals (40 to 50 pounds per square inch gauge). Maintain a positive
pressure of nitrogen in the boiler to prevent outside air from leaking inside.
11. Prepare this passivating solution: To each 3785 liters (1000 gallons) of
distilled water (or other water with less than 50 ppm total hardness [as
CaCO3]), add 36 kilograms (80 pounds) of passivation compound 0.5% by
weight sodium nitrite and 0.25% by weight monosodium phosphate.
12. Fill the boiler with the passivating solution preheated to 65 to 71 oC (150 to
160 oF), circulate for 10 minutes, and hold in the boiler at 65 to 71 oC for
an additional 30 minutes.
13. Drain and rinse boiler until the pH of the rinse water is pH 8 to 10.
Circulating Method Without Heat. The steps below describe a typical
process for descaling smaller equipment, such as enclosed vessels or hot water heater
coils, without heating the inhibited acid solution:
1. Note that an acid cleaning assembly may consist of a small cart on which
is mounted a pump and an 18.9- to 189-liter (5- to 50-gallon) steel or
polyethylene tank with a bottom outlet to the pump.
2. Install sill cocks at the bottom of the water inlet of the heat exchanger and
the top of the water outlet so that a return line can be connected directly
from the acid pump and from the heat exchanger to the acid tank.
3. Prepare an inhibited acid cleaning solution (see paragraphs 9-2.2 and 9-
4. Pump the acid solution into the heat exchanger through the hose
connection. Continue circulation until the reaction is complete, as indicated
by foam subsidence or acid depletion.
5. If the scale is not completely removed, check the acid strength in the
system (see paragraph 9-2.6). If the acid strength is less than 3%, add
fresh acid solution and continue circulation until the remaining scale is
removed. Usually an hour of circulation is adequate.