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T.U.T.O.R.
Does a Closed System Need Chemical=20
Treatment?
By Special Topics Subcommittee =
of the=20
AWT Technical Committee
Good reasons=20
why the answer is yes!
There are several common =
misconceptions about closed systems. One is that closed systems =
are sealed=20
off from the atmosphere and circulate the same body of fluid all =
the time.=20
Therefore, they are quite stable and never need any form of =
chemical=20
treatment to protect them. Well, of course we all know this is not =
quite=20
true. Modern automobile cooling systems are essentially closed =
loops, but=20
would you ever consider operating the engine cooling system with =
just=20
plain water? We know what the result would be - rusty water and =
pretty=20
soon, an invoice for a new water =
pump.
The=20
effects on a closed loop system used for any kind of heating or =
cooling=20
are the same - corrosion, deterioration of components and loss of =
heat=20
transfer efficiency. What causes these problems to=20
occur?
Some waters used to fill a =
closed loop=20
can be acidic and contain dissolved oxygen. Low carbon steel and =
iron=20
corrode rapidly under these conditions releasing small flakes of =
rust into=20
the circulating stream. These particles are abrasive and tend to =
erode the=20
components of the system. This can be particularly harmful in the =
area of=20
pump shaft seals.
What happens=20
after all the oxygen and acidity has been used up? Would the =
system become=20
stable and corrosion cease?
Perhaps in an ideal =
situation, but if we discount for a moment the damage that can =
occur to=20
our brand new system, can we guarantee that the water in this =
system will=20
ever reach a stable condition? There are several reasons why we =
cannot.=20
Consider this:
Most so-called closed loops are not truly =
closed.=20
Provisions must be made to automatically compensate for any =
pressure=20
changes or water losses. To accomplish this, the system will =
normally=20
utilize an expansion tank and relief valve, at the same time being =
connected to the water main via a pressure-reducing valve=20
(PRV).
Expansion tanks usually have a =
volume of=20
air trapped above the water. The oxygen dissolves in the water and =
is then=20
circulated throughout the system. Makeup water enters the system =
as a=20
result of air bleeding from the loop or because of water leaks, =
bringing=20
with it a fresh supply of dissolved oxygen. Sometimes substantial =
leaks or=20
losses can go undetected for a long time since the PRV =
automatically makes=20
up for them.
The net result of this is =
the=20
introduction of a small but continuous supply of acidic water =
containing=20
corrosive oxygen. The problem is aggravated considerably if the =
system is=20
opened for any kind of routine maintenance or component =
replacement. As=20
corrosion in the loop continues, heat-transfer surfaces become =
coated and=20
lose efficiency and tubes become plugged. Layers of debris lead to =
an=20
effect known as under-deposit corrosion, which can cause pitting, =
a form=20
of concentrated, localized corrosion. Pitting should be avoided at =
all=20
costs because it leads to rapid perforation or component =
failure.
How can =
you tell if=20
system has problems?
Some simple tests on the =
circulating water=20
will give a very good indication of whether things are under =
control.
Appearance=E2=80=93 Look at the color =
and clarity of=20
the water. It should be almost clear and colorless. Dark brown or =
black=20
water indicates a serious corrosion problem. There should be very =
few=20
solid particles collecting at the bottom of the sample =
container.
Examine the filter=E2=80=93 If there =
is a filter on=20
the system - and there should be - look at the element to =
determine what=20
kind of solid material, and how much, is being removed from the=20
circulating stream.
Dissolved iron and total iron =
tests=E2=80=93 If the=20
sample contains more than 1 mg/L iron in either of these tests, =
the system=20
is corroding and needs to be brought under control as soon as=20
possible.
How can=20
you restore control using chemical treatment? =
A closed=20
loop chemical treatment program should provide the following:
pH control=E2=80=93 Overcoming the =
effects of an=20
acidic raw water supply is accomplished by a suitable pH =
adjustment=20
additive.
Oxygen scavenger=E2=80=93 Dissolved, =
corrosive oxygen=20
is continuously removed from the water before it can cause any =
damage.
Film-forming corrosion =
inhibitor=E2=80=93 Coating all=20
internal surfaces of piping and equipment with a protective, =
monomolecular=20
film will ensure that no further attack is possible. Use a product =
that is=20
recommended for all closed systems.
What else can=20
be done to protect a closed loop system?
Start with a clean system. If the loop =
contains dirty=20
water and large amounts of rust and mill scale, clean and flush =
the system=20
using an appropriate product, such as a purging compound designed =
for the=20
purpose. An inhibitor product will then provide protection to all =
parts of=20
the system and pumps, and heat-transfer equipment will operate at =
design=20
efficiency.
Measure the corrosion inhibitor concentration=20
regularly. The inhibitor cannot function if it is being=20
continually diluted as a result of continuous water leaks or =
draining=20
parts of the system for maintenance.
Examine the system for leaks. If leaks are =
suspected,=20
installation of an inexpensive totalizing water meter in the =
makeup line=20
will provide early detection.
Inspect equipment. Any defective components,=20
especially near the expansion tank area, could admit oxygen to the =
system=20
and should be repaired or replaced as soon as possible.
Examine the filter element. Regularly inspect =
the=20
filter element. If solid material is circulating because of under=20
treatment or contaminated raw water resulting from reservoir or =
water main=20
repairs, it will become obvious that appropriate action is =
required.=20
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