Nace CP 1, Course Manual

Page: 403 / 535

RP0575-2001
NACE International 5
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Section 6: Reference Electrode Entrance
6.1 Methods
6.1.1 Entrance ports for insertion of reference
electrodes should be installed in the vessel at the same
time as the anode entrance ports.
6.1.2 In compartmented vessels, reference electrode
ports must be installed as far from the anodes as
possible in order to obtain potentials representative of
the steel surface. These ports are usually 2.5-cm (1-
in.) diameter connections welded into the vessel wall in
each compartment and equipped with a 2.5-cm (1-in.)
full-opening valve so that the reference electrode may
be inserted into the vessel.
6.1.3 In vertical cylindrical vessels equipped with roof-
type hatches, the reference electrode may be inserted
through the hatches. If this is anticipated, the anodes
should be installed at the maximum distance from the
hatch.
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Section 7: Criteria for Protection
7.1 Steel-to-Water Potential
7.1.1 The presence of a protective potential should be
verified after the vessel is polarized.
Polarization
normally occurs within two weeks in bare vessels and
within a few minutes in coated vessels.
7.1.2 The steel surface is protected if the surface is
more negative than -0.85 V vs a CSE (-0.80 V vs a
Ag/AgCl electrode).
However, if the fluid contains
sulfides/H
2
S, a protection potential of -0.95 V vs a CSE
(-0.90 V vs a Ag/AgCl electrode) is required to achieve
protection. Polarization and protection of the vessel
are generally assured if the potential immediately after
switching the cathodic protection current off is equal to
or more negative than these values.
7.2 Coupon Tests
7.2.1 Coupons may be installed to monitor the
effectiveness of cathodic protection. Coupons must be
electrically connected to the vessel wall.
Coupon
weight-loss measurements are used to determine the
effectiveness of protection.
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Section 8: Monitoring, Records, and Maintenance
8.1 Monitoring
8.1.1 Operating personnel should visually inspect
surfaces under cathodic protection whenever vessels
are opened. In addition, thickness measurements of
vessel walls should be taken to determine cathodic
protection effectiveness.
8.1.2 Potential and/or anode current output surveys
should be made monthly after initial installation of
cathodic
protection equipment until current
requirements are established. After current
requirements are established, quarterly surveys are
usually adequate.
8.1.2.1 Care must be taken in placing the
reference electrode into the treating vessel. For
potential measurements, the electrode must be as
far from the anodes as possible.
In pressure
vessels, the electrode is “inserted” (introduced into
the vessel against existing vessel pressure)
through a full-opening valve installed in the vessel
for that purpose (see Paragraph 6.1.2). Reference
electrodes manufactured to withstand pressure
and temperature can also be permanently
installed.
8.1.2.2 Contamination of the reference electrode
with oil or sediments such as iron sulfide must be
avoided.
A salt bridge may be used to prevent
contamination of the reference electrode.
1
8.1.2.3 Location of the reference cell near an
anode may indicate a higher potential than
elsewhere in the vessel.
8.1.2.4 Water levels lower than normal may result
in higher potentials because of increased current
density.
8.1.3 Galvanic anodes are sometimes installed without
shunts.
However, if the cathodic protection system
cannot be monitored by potential measurements,
shunts of known resistance (normally 0.01 ohm) should
be installed. Current measurements can be obtained
by measurement of the potential across the shunt of
known resistance.