Page 276 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 261

the water and eventually reach a concentration where, with other ions to form scale. Keeping the scale forming
despite treatment, the sludge sticks to a heating surface ions in suspension allows their removal in the continu-
and becomes scale. ous blowdown where the energy and some water are
If your boiler contains scale and tests of it indicate recovered thereby reducing the losses associated with
a high percentage of phosphate that’s an indication you bottom blowoff. Chelants also attack scale that’s already
have that problem. Sludge handling problems include formed, returning it to solution so it can be removed
plugging of blowoff piping and valves, usually resolved with the blowdown. Used properly a chelant treatment
by more frequent bottom blows. Problems with sludge program can remove scale formed on a boiler as the re-
remaining in suspension in the water is attacked with sult of an upset condition.
other chemicals called “sludge conditioners” that are There are two hazards associated with the use of
designed to reduce the tendency of the sludge to stick chelant treatment. First, if used to remove existing scale
and increase the density of the sludge so it will settle it has to be performed in a manner that doesn’t result in
out. fast removal of the scale. The chelant tends to break the
The conventional system for treating boiler wa- bond of the scale to the iron first so any rapid attack on
ter is called “soda-phosphate” and now you know the the scale will result in large pieces of scale releasing into
derivation of the words. Caustic soda is added to raise the boiler water and transporting to points of restriction
pH and alkalinity and phosphate is added to remove where it can plug tubes resulting in overheating and
scale forming salts by combining with them to produce failure. Even when that extreme isn’t reached it can pro-
removable sludge. The performance of the phosphates is duce so much loose scale accumulating in the bottom of
dependent on the maintenance of alkalinity and to work the boiler that you’ll have problems with blowoff valves
best the pH should be maintained between 10.5 and 11.5. and piping plugging up. The second hazard has to do
To be certain that there’s phosphate laying in wait with the fact that iron is related to magnesium and cal-
for any calcium or magnesium ions that manage to find cium and chelant insists on having something to seques-
their way into a boiler we maintain a residual of 60 ppm ter; if it runs out of calcium and magnesium then it will
of phosphate. Sometimes that is a little tricky to do. I re- grab iron, the stuff the boiler is made of. That requires
call one ship where the method of treatment was sodium careful and closely controlled use of chelant.
hexa-meta-phosphate. I actually liked the treatment To ensure the scale forming ions are sequestered
because the water was clear (many of the treatments before they get near the boiler heating surface chelant is
produce a muddy looking water) but it had a bad habit normally introduced into the boiler feedwater. The typi-
of changing concentrations depending on boiler load. cal means is to introduce it using a “quill” which is best
I don’t to this day know if it was the chemical or described as a thermometer well with a hole drilled in
the boiler but the residual values would shoot up into the side at the tip. By injecting the chelant into the water
the hundreds when we were in port (boiler loads were through the quill into the center of the feed piping it
low) then drop to almost nothing when we got under- will encounter the scale forming ions in the water before
way (full boiler load). The water treatment consultant it reaches the iron in the pipe. The quill should always
the shipping company used told me it was “hidden be installed upstream of a long straight run where it
phosphate” but never came up with a good explanation can uniformly mix. Any elbows, valves, or pipe fittings
for why it did that. I learned to live with the high values downstream of the injection point should be inspected
in port and always checked it the minute we were under one year after beginning treatment and at five year in-
way. tervals thereafter to ensue they aren’t corroded away by
In instances of other scale treatments phosphate is the chelant.
also used as an “indicator.” By maintaining a residual To ensure the chelant doesn’t attack the iron it
level of phosphate in the boiler any failure of the other must be fed at the same rate that the scale forming
program is indicated by a reduction in the phosphate salts enter the system. A chemical feed pump capable
residual. of varying the feed rate automatically is required to
A better, and more complicated, method of con- feed proportional to feedwater flow and testing of the
trolling scale emerged in the late 1960’s. The treatment hardness of feedwater before the chelant feed to make
is generally called “chelant” and it comes in many adjustments of the proportions of chemical feed to water
patented forms. Phosphates are used to remove scale flow must be made regularly. Testing of the water for
forming salts from the water but chelants simply build any residual should be frequent when boiler loads or
a barrier around them that prevents their combining feedwater blends vary to ensure a residual doesn’t build