Nace CP 1, Course Manual

Page: 400 / 535

RP0575-2001
2 NACE International
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Section 3: Determination of Need for Cathodic Protection
3.1 General
3.1.1 Experience reveals that corrosion and metal loss
is to be expected in any oil-treating vessel where any
portion of the internal steel surface is exposed to
oilfield brines because of their aggressive corrosive
nature. The need for cathodic protection is contingent
on the severity of existing or anticipated corrosion and
the extent to which it affects equipment operation.
Consistent with the latter, cathodic protection should be
installed when it will accomplish one or more of the
following:
remove or minimize unsafe conditions
caused by failure, provide economical control over
equipment failures and losses, and remove or minimize
the possibility of vessel content loss because of leaks
or vessel collapse.
3.1.2 Internal coatings may be used in conjunction
with cathodic protection to protect oil-treating vessels.
Internal coating reduces the surface area of steel to be
protected.
3.2 Corrosion Rates
3.2.1 The corrosiveness of a fluid stream is a function
of the following:
3.2.1.1 Corrosion usually becomes more severe
as the conductivity (dissolved solids content) of
water (electrolyte) increases, but low-conductivity
water can be corrosive.
3.2.1.2 Corrosion in produced oilfield brines
usually increases as the partial pressure of acid-
forming components, such as carbon dioxide
(CO
2
) or hydrogen sulfide (H
2
S), increases.
3.2.1.3 Corrosion is accelerated by even trace
amounts of oxygen.
3.2.1.4 Corrosion usually increases with
increasing temperature unless scaling is
increased.
3.2.1.5 Corrosion usually increases with
increasing flow velocity and turbulence.
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Section 4: Design and Selection of Cathodic Protection System
4.1 Basic Design Criteria
4.1.1 The design of a vessel’s cathodic protection
system depends on the internal configuration of the
vessel. Selection of the appropriate system depends
on factors such as initial cost, maintenance, type and
condition of coatings (if any), power availability and
cost, and system life.
4.1.2 Vertical cylindrical vessels containing no baffles,
compartments, firetubes, etc., are usually protected
with anodes or strings of anodes suspended from the
deck (roof) of the vessel. This design method offers
two advantages over other designs: (1) better current
distribution because anodes are parallel to the vessel
walls, and (2) deteriorated or depleted anodes can be
replaced without lowering the water level or draining
the vessel.
4.1.3 Compartmented vessels or those containing
baffles, firetubes, spreaders, etc., should have at least
one anode installed in each compartment exposed to
the corrosive fluid.
If a vessel is not designed to
accommodate a cathodic protection system that will
provide effective protection throughout, it may be
necessary to make modifications or redesign the
vessel interior to provide access to areas needing
cathodic protection by installing fittings in the vessel for
inserting anodes through the walls. An example of
such modification is positioning of a spreader in a
heater treater below the firetubes so there is sufficient
clearance to install an anode between the firetube and
spreader.
4.1.4 Correct location and position of anodes in
vessels are essential for proper current distribution.
Cathodic protection anodes should be placed such that
their protective current can be distributed to all surfaces
exposed to the corrosive electrolyte.
Each anode
should be located as near to the center of the
compartment or center of the electrolyte as practical.
4.1.5 Factors determining the number, weight, and
shape of anodes required for cathodic protection of
vessels are:
(a) area of bare water-immersed steel to be protected;
(b) current density required;
(c) anticipated current output of the anodes;
(d) vessel configuration; and
(e) desired life of the cathodic protection system.