The Equipment Design Process
365
1. It can act as an electron acceptor, allowing corrosion to continue.
2. It reacts with hydrogen to depolarize the cathode and thus speed up
corrosive reaction rates.
3. It reacts with iron ions to form tubercles and leads to pitting of
copper.
If oxygen could be removed from water economically, the chances of corrosion starting, as well as the corrosion rate once started, would be reduced.
Unfortunately, oxygen removal is too expensive for municipal water systems
and is not a practical method of control. However, there are ways to minimize the addition of oxygen to the raw water, particularly groundwaters.
Often, aeration is the first step in treating groundwaters having high
iron, hydrogen sulfide (H 2 S), or CO 2 content. Although aeration helps
remove these substances from raw water, it can also cause more serious
corrosion problems by saturating the water with oxygen. In lime-soda softening plants for the treatment of groundwater, the water is often aerated
first to save on the cost of lime by eliminating free CO 2 . Iron is oxidized
and precipitated in this step, but this is incidental because the iron would
be removed in the subsequent softening process even if the water were not
aerated. The actual result is that the dissolved oxygen (DO) increases to
near-saturation, and corrosion problems are increased. Thus, the attempt
to save on lime addition may actually end up costing a great deal in corrosion damage.
Measures that help keep the DO as low as possible include:
Sizing well pumps and distribution pumps so as to avoid air
•
entrainment
Using as little aeration as possible when aerating for H
•
2 S and CO 2
removal
This can be achieved by bypassing the aerators with part of the raw water. It
has even been possible to eliminate completely the use of aerators if enough
detention time is available in the reservoir so that enough oxygen can be
absorbed at the surface to oxidize the H 2 S or to let the CO 2 escape. DO levels
can be kept as low as 0.5 to 2.0 mg/L by this method. This is low enough in
many cases to reduce corrosion rates considerably.
Linings, coatings, and paints. Another way to keep corrosive water away
from the pipe wall is to line the wall with a protective coating. These linings are usually mechanically applied, either when the pipe is manufactured or in the field before it is installed. Some linings can even be applied
after the pipe is in service, although this method is much more expensive.
The most common pipe linings are coal-tar enamels, epoxy paint, cement
mortar, and polyethylene.
365
1. It can act as an electron acceptor, allowing corrosion to continue.
2. It reacts with hydrogen to depolarize the cathode and thus speed up
corrosive reaction rates.
3. It reacts with iron ions to form tubercles and leads to pitting of
copper.
If oxygen could be removed from water economically, the chances of corrosion starting, as well as the corrosion rate once started, would be reduced.
Unfortunately, oxygen removal is too expensive for municipal water systems
and is not a practical method of control. However, there are ways to minimize the addition of oxygen to the raw water, particularly groundwaters.
Often, aeration is the first step in treating groundwaters having high
iron, hydrogen sulfide (H 2 S), or CO 2 content. Although aeration helps
remove these substances from raw water, it can also cause more serious
corrosion problems by saturating the water with oxygen. In lime-soda softening plants for the treatment of groundwater, the water is often aerated
first to save on the cost of lime by eliminating free CO 2 . Iron is oxidized
and precipitated in this step, but this is incidental because the iron would
be removed in the subsequent softening process even if the water were not
aerated. The actual result is that the dissolved oxygen (DO) increases to
near-saturation, and corrosion problems are increased. Thus, the attempt
to save on lime addition may actually end up costing a great deal in corrosion damage.
Measures that help keep the DO as low as possible include:
Sizing well pumps and distribution pumps so as to avoid air
•
entrainment
Using as little aeration as possible when aerating for H
•
2 S and CO 2
removal
This can be achieved by bypassing the aerators with part of the raw water. It
has even been possible to eliminate completely the use of aerators if enough
detention time is available in the reservoir so that enough oxygen can be
absorbed at the surface to oxidize the H 2 S or to let the CO 2 escape. DO levels
can be kept as low as 0.5 to 2.0 mg/L by this method. This is low enough in
many cases to reduce corrosion rates considerably.
Linings, coatings, and paints. Another way to keep corrosive water away
from the pipe wall is to line the wall with a protective coating. These linings are usually mechanically applied, either when the pipe is manufactured or in the field before it is installed. Some linings can even be applied
after the pipe is in service, although this method is much more expensive.
The most common pipe linings are coal-tar enamels, epoxy paint, cement
mortar, and polyethylene.