Nace CP 1, Course Manual

Page: 172 / 535

Field Measurements 5:25
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
In heat exchangers where the tube sheet is perhaps Monel metal and the
water box is cast iron, it is often desirable to protect only the water box.
This can be achieved by electrically isolating the water box from the shell.
There are times when it may not be desirable or even practical to isolate
protected from unprotected structures. Examples are refineries, industrial
plants, large tank farms, and similar complex facilities. There are many
connections to the underground structures in such places and each would
require a dielectric fitting. Maintenance of such a large number of fittings
becomes prohibitive; also, one short circuit could cause loss of protection
on a large number of structures.
Occasionally it is desirable to provide joint cathodic protection for a
number of pipelines belonging to different companies, usually all running
in the same right of way. In cases like this, the pipelines can be connected
together and protected by a series of rectifiers, with each company
assuming responsibility for a portion of the system.
Isolation (Insulating) Joints
Commercial fittings available for providing electrical isolation include:
•
flanges
•
couplings
•
unions
•
monolithic isolating pipe joints
•
nonmetallic pipe and structural members.
Accidental Contacts
Some of the locations where electrical isolation can be compromised are:
•
Crossing structures. Underground contacts made accidentally when one
line crosses another. A good example is a copper water service line in
contact with a steel gas line. Even though the gas line is coated,
movement of the piping may eventually cause the copper line to wear
through the coating and contact the steel.
•
Attachments. Many attachments may be made to a protected structure.
These include telemetering and other remote monitoring equipment,
electrical valve operators, gauge lines, and the like. If these attachments
bypass an isolating fitting, a short circuit will be created. You need to be