Boiler Water Treatment Primer

catastrophic. I am really not an ‘expert’ in the field of
boiler water treatment but one who found it necessary to gain an
understanding of the subject. Let me share that with you.

Impurities in water which form scale inside a boiler are
hardness, silica and corrosion products. Scale is formed by
impurities precipitating from solution directly on boiler plates
and tubes or the suspended solids settling out and then being baked
on these heat transfer surfaces. Scale causes failures of metal due
to local overheating and also contributes to boiler corrosion.

The parameters of boiler design involved here are pressure and
steam temperature i. e., saturated or superheated with pressure
being the primary consideration. Since our interest is in traction
engine and portable power boilers we will limit ourselves to the
problems associated with 125 pound saturated steam operations.

By way of some background in the subject of water treatment,
most waters available to Iron-Men will contain constituents that
can be placed into three categories: temporary hardness, permanent
hardness, and suspended solids. If the water source happens to be
from a municipal supply and not some nearby creek then suspended
solids can probably be neglected thus avoiding a costly and
difficult coagulation type treatment step or, at least, a slow sand
filter bed operation.

At a risk of over simplification we can define the two types of
hardness by saying that temporary hardness elements in the water
supply, and which cause excessive scale formation, are of a
chemical nature that can be converted to compounds that will remain
in solution in the boiler water through rather simple treatment
steps to later be removed by hot blow-down on a regular basis. On
the other hand, permanent hardness elements do respond to such
simple treatment steps but require more difficult to achieve
alkalinity control. If they are of sufficient amounts in the
supply, they too can be a source of potential damage to the boiler.
In general, the carbonates of calcium and magnesium constitute the
temporary category while the sulphate form of calcium and magnesium
constitute permanent hardness.

Those using large quantities of water and having need for
extremely pure water either for process needs or very high pressure
(2400 psig) boilers would need elaborate and costly treating
plants. More modest needs can often be served by use of zeolite
softners Hey, Culligan Man followed by storage tanks. Since this
type of service can often be rented it would be an ideal answer
except for the traveling habits of old traction engines and the
temporary nature of many show grounds. This being the case, the
most practical solution to our water treatment problem will
undoubtly be in the form of a system of internal treatment within
the boiler itself or in the engine mounted water tank. That will be
the approach taken here.

In its simplest form this will consist of mixing the proper
amount and type of chemicals in a 55 gallon drum and then adding
this concentrated solution to the engine’s water tank in
measured amounts as the tank is filled from time to time. These
amounts of solution are going to be in the neighborhood of two or
three gallons of solution per 100 gallons of raw feed water.

Let us begin at the beginning which is to find out just what is
in the water that will be fed to the boiler. Look up in the Yellow
Pages of your telephone directory under ‘water’ and find
the nearest place that provides water treatment service. Generally
these will be dealers that will want to sell you water softeners
and service, however, they do water analysis for a nominal charge,
usually less than $20 per sample.

Using a CLEAN glass one quart container (they may provide you
with one liter sample bottles) get a sample of the water to be
tested. Fill the bottle completely and seal it to keep out air and
contaminants. These people don’t often do tests for boiler
treatment and need to be told what it is you want, The report will
have two parts; mineral content and bacteria logical. Under mineral
content be sure to ask the following: alkalinity, hardness, and pH.
These are the values that you will use in determining the treatment
program.

A word about pH and ppm and grains at this point may be in
order. That strange symbol, pH, stands for ‘hydrogen ion
concentration’. Zero pH is pure acid while 14 pH is totally
basic or caustic. A pH of 7 is neutral. Parts per million is what
ppm stands for. Sea water is about 35,000 ppm salt which is 3%,
just to give you a feel for the numbers. You may find that hardness
is given in grains or it may be reported in ppm. Again, a feel for
the numbers; 10 grains hardness corresponds to 170 ppm and is
usually reported as calcium carbonate. And, for those of us that
haven’t converted our feel for numbers to the metric system, a
liter is near enough to a quart to give us perspective there.

Let us now take a specific example in which the report shows the
following: alkalinity 130 ppm, hardness 120 ppm and pH at 7. We
will take care of the temporary hardness first.

Calculations will show that this level of hardness can be
controlled by adding 0.44 pounds of tri-sodium phosphate in the
crystaline form per 1000 gallons of raw untreated feed water. (This
is Na3 PO4 .12 H20 for the chemists among us.) Phosphate in this
form has a solubility of 0.215 pounds per gallon. Mix 10 pounds of
the phosphate in 50 gallons of water in a separate container such
as an open top 55 gallon drum. This will be a saturated solution.
Each time that the engine’s water tank is filled add some of
this concentrated solution at the rate of 2 gallons per 100 gallons
of raw untreated water. The quantities have been rounded off for
convenience and there is some surplus in order to maintain a
phosphate residual in the boiler. We’ll talk about testing for
the proper level later.

Keep in mind that you will be increasing the amount of dissolved
solids in the water in the boiler. These will become concentrated
as the boiler steams and the control of both the proper residual
and total dissolved solids in the boiler is by blowing down the
boiler and adjusting the phosphate feed, i.e., gallons of mix per
100 gallons water.

Your water may have a greater or lesser hardness but your case
can be proportioned to the numbers in the example. For instance,
suppose that your hardness is 10% greater, then use 2.2 gallons of
solution per 100 gallons of raw water.

The permanent hardness is next and for this treatment the boiler
water alkalinity needs to be high enough to also precipitate the
magnesium hardness. If the alkalinity shown in the report is higher
number than the hardness number there is likely enough alkalinity
to do the job. We need to have boiler water pH values in the 9.5 to
10.5 range and the best way to know the value is to run a test on a
cold sample when the boiler is shut down If the figure is too low
it may be necessary to add some soda ash to the mixture given
above. We will discuss testing later. Keep in mind, however, that
high pH values are the cause of foaming and, in the extreme, can
lead to caustic embrittlement of the boiler steel.

There is one more bad actor that we need to contend with and
that is oxygen. All natural water supplies contain more or less
oxygen. Large boiler installations in, say, a power plant are
equipped with de-aerating feed water heaters that will remove most
of the oxygen. Here again, however, we will need to do our
treatment on the engine with the equipment usually available. For
this there are two chemicals available to do the job. One, and the
cheapest, is sodium sulphite and the other is hydrazene
(N²H⁴). The sulphite reacts with oxygen in
the water to form sodium sulphate and the hydrazine yields nitrogen
and water. Unfortunately, sodium sulphite is slow to react and
ample time must be allowed. For example, adding the proper amount
to the tank on the engine will require ten minutes for a 30%
reduction and up to 30 minutes to do the job. It is possible to
purchase sodium sulphite with a copper catalyst that greatly speeds
up the reaction but it would have to be pumped into the boiler
separately for it can not be mixed in with the phosphate solution
due to the catalyst. So, use the tanks on the engine but allow
enough time for the reaction to take place that gets rid of the
oxygen that can cause corrosion of the boiler metal.

This time we are going to approach the determination of the
amount of chemical to be used in a back handed way. That is we will
test to see that we are maintaining the proper sulphite residual by
tests on boiler water samples. If we keep the sulphite residual in
the range of from 30 to 66 ppm we can be assured that we have
reacted all of the oxygen in the boiler water which will have
formed sodium sulphate. The reason for this approach is that oxygen
values in the raw feed vary with time and our test report will not
have given us the figures we need. Oxygen is reported as cc/liter
or as ppm. One cubic centimeter (cc) of dissolved oxygen per liter
is equal to 1.43 ppm and it takes 0.047 pounds of sulphite per 1000
gallons of water for 0.5 cc/liter (0.72 ppm) of dissolved oxygen.
So, you see, we are not talking about very much sulphite something
like a half a pound per 1000 gallons for a typical water whereas we
have been talking about 4 pounds of phosphate per 1000. Let’s
see, 1000 gallons equals 8330 pounds of steam or about enough water
to run a 25 horsepower engine at full load for a day’s work.
Add a little sulphite when mixing the phosphate, we’ll check it
later.

Now is the time to talk about testing to see if we are treating
water properly. All the tests that we need can be done with simple
equipment right out in the field. There are several supply houses
that sell field test kits. I will mention only one since it is the
one that I have always found to be most helpful to the small
operator or hobbyist. You may wish to contact Hach Company, Sales
Department, Post Office Box 389, Loveland, Colorado 80539. They
have a toll free number 800-525-5940. Here are some of their kits
just for your understanding of the subject.

Phosphate Test Kit: Model PO-19, catalog number 2248-00,
shipping weight 4 pounds, cost $48.25. This kit is capable of
handling two ranges of phosphate: 0-5 and 0-50. Sufficient
materials are included to run 100 tests.

pH Test Kit: Model 17, catalog number 1470-10, shipping weight 1
pound, cost $59.50. This is a narrow range type kit in that it is
specifically built for running tests in the 9.7 to 11.6 range which
is the range of control we need. Sufficient materials are included
for 100 tests.

Sulphite Test Kit: Model SU-5, catalog number 1480-02, shipping
weight 3 pounds, cost $43.50. This kit is a dual range kit (0-20
and 0-200 ppm) with sufficient materials for about 100 tests.

Going back now to the temporary and permanent hardness treatment
there is some reluctance to get too specific about residual
quantities suitable for operation. Water treatment is somewhat of
an art, in my view, though the science is well known. The reason
for this statement is that, in the final analysis, there have to be
some compromises. As long as you understand this then here are some
suggestions. Run with a phosphate residual in the boiler of 20 ppm
for if it is too high it can have an effect on pH and interfere
with the permanent hardness reaction.

Since these engine operations are intermittent and generally not
at full load, daily blow down is probably going to be often enough.
Take a sample from the boiler blow down connection when the water
is cold and use this for your tests. When the boiler comes up to
pressure you can blow down and if there is a lot of water use then
again during the day. You will find your own routine that suits
your needs best after having done a few tests.

A word of caution for those with machines equipped with exhaust
steam feed water heaters. The phosphate treatment requires
temperatures above room temperatures for the reaction to take
place. That means that some sludge formation could take place in
the feed heater and, therefore, it should be drained daily to
remove the sludge.

This is written simply as an introduction to the subject of
water treatment, a primer, with the thought that those that want to
delve deeper into the subject will do so if this sparks their
interest. That is where I found myself some years ago when it was
necessary to have an appreciation of the subject if only to
understand what it was that the experts in the field were telling
me. That is what I have just shared with you.