25 May 2005
The entire cleaning procedure/process must be developed in detail before
starting chemical cleaning operations. Factors to be considered include:
the methods for controlling temperatures; the means of mixing, heating,
and circulating the chemical solution; proper venting of dangerous gases
from equipment to a safe area; and means for draining, filling, and flushing
under inert atmospheres. Sampling points, test procedures, and control
limits should also be established.
Methods for Removing Scale. Removing scale may be accomplished by
circulating the inhibited acid solution through the equipment or by soaking the
equipment in a tank of inhibited acid. Before starting any descaling process, check the
acid to make sure it is properly inhibited. You may check the acid by placing a mild steel
coupon into a beaker containing the prepared, diluted acid. You should notice no
reaction around the coupon. If you observe a reaction generating hydrogen gas bubbles
around the coupon, add more inhibitor.
Recirculating Cleaning Process for Boilers
. The following example
an appropriate procedure for cleaning small boilers or other systems using a hot
recirculating inhibited acid solution:
1. Fill the boiler or system with preheated (71 to 77 oC [160 to 170 oF]) dilute
inhibited acid solution.
2. Allow the dilute inhibited acid solution to remain in place for 8 hours.
Circulate the acid solution for approximately 15 minutes each hour at a
rate of about 3.15 liters per second (50 gallons per minute) to ensure good
3. Keep the temperature of the acid solution preheated at 71 to 77 oC (160 to
170 oF). Measure and record the temperature at least once every 30
4. Check and record the acid strength at least every hour (see paragraph 9-
5. Drain the system by forcing the acid solution out using 276 to 345
kilopascals (40 to 50 pounds per square inch gauge) nitrogen; follow
Federal Specification A-A-59503, Nitrogen, Technical, Class 1. If leaks
develop when the system is under nitrogen pressure, you must use an
alternate method for removing the acid, such as pumping.
6. Fill the boiler with preheated (65 to 71 oC [150 to 160 oF]) water and soak
at this temperature for 15 minutes.
7. Drain under nitrogen pressure of 276 to 345 kilopascals
(40 to 50 pounds per square inch gauge).