Page 254 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Assigning risk scores 11/231
Pipe-to-soil protection readings can also be taken at other In this item and also in the cathodic protection item. the
aboveground locations, such as meter risers. Credit may be evaluator should be alert to situations where piping of different
given for these locations where meaningful information on cor- ages and/or coating conditions is joined. Dissimilar metals, or
rosion control is regularly obtained and properly analyzed. even minor differences in chemistry along the same piece of
To assess this item for distribution systems, pages 80-82 steel pipe, can cause galvanic cells to operate and promote
provides background information. A scale can be set up to corrosion.
assess the effectiveness of the test leads based on an estimation Because distribution systems are often located in areas con-
of how much piping is being monitored by test lead readings. gested with other buried utilities, the evaluator should look for
As with transmission pipelines, we can assume that each test operator methods by which interference could be detected and
lead provides a reasonable measure of the pipe-to-soil potential prevented. Examples include strict construction control, strong
for some distance along the pipe on either side of the test lead. programs to document locations of all buried utilities, close
As the distance from the test lead increases, uncertainty as to interval surveys, extensive use of test leads and interference
the actual pipe-to-soil potential increases. How quickly the bonds.
uncertainty increases with distance from the test lead depends Score as described on pages 82-85.
on soil conditions (electrolyte) and the presence of other buried
metals (interference sources). Rather than a linear scale in AC-induced current AC induction presents a potential prob-
miles of pipe between test leads, a percentage of pipe moni- lem in distribution systems as it does in transmission pipelines.
tored might be more appropriate for a distribution piping grid. Anytime high voltages are present, there exists a risk of a
A distance can be assumed (perhaps a few hundred feet in nearby buried metal conduit becoming charged. In a distribu-
relatively uncongested areas) and an approximation as to how tion system, the grid-type layout, increased sources of AC
much pipe is being protected can be made as follows: power, and the often-extensive presence of other buried utilities
might complicate the analysis of this variable.
Less than 30% yf'piping monitored-a high incidence of Score as shown on pages 83-84.
other unmonitored buried metals with potential interfer-
ences Mechanical corrosion
Thir[iapercent to 70% ofpiping monitored-moderate inci-
dence of other unmonitored buried metals Score as shown on pages 77-78
Greater than 70% of piping monitored-few incidences of
unmonitored other buried metals. Subsurface corrosion qf'nonmetallic pipes
The interval of monitoring at the test leads is critical, as is the An alternate methodology is needed to score the risk of buried
interpretation ofthose readings. pipe corrosion for nonmetallic materials. since coatings and
cathodic protection are not normally corrosion control meth-
Close interval strrveys Although not as common as in trans- ods. For nonmetallic pipe materials, the corrosion mechanisms
mission systems, the close interval or close-spaced survey may be more commonly described as degradation mecha-
(CIS) technique can be very important in a metallic-pipe distri- nisms. Under the term corrosion, all such mechanisms that can
bution system. Many potential sources of interferences can reduce the structural integrity ofthe nonmetallic pipe should be
often be detected by a CIS. A major obstacle is the prevalence examined. Because this section of the evaluation applies to all
of pavement over the pipelines preventing access to the elec- nonmetallic pipe materials, some generalized relationships
trolyte. Score as detailed on pages 80-82. between likelihood of corrosion and preventive measures exist
and can be used to evaluate the threat.
Cathodic protection interference Interferences are situations Corrosion mechanisms such as chemical degradation, ultra-
where shorting (unwanted electrical connectivity) occurs with violet degradation, temperature degradation, attack by soil
other metals or shielding prevents adequate protective currents organisms, attack by wildlife (such as rodents gnawing on pipe
from reaching the pipe. Interference will hinder the proper walls--considered here rather than as an external force), corro-
functioning of cathodic protection currents and may lead to sion of a part of a composite material (such as the steel in rein-
accelerated corrosion. forced concrete pipe), dissolution by water (some clay or wood
A problem sometimes encountered in distribution systems pipes are susceptible), and general aging effects. Where
is the use of the pipe as an electrical ground for a building's cementing agents or adhesives are used (usually in the joining
electric system. Although normally a violation of building process), corrosive effects on these materials must also be con-
codes (and other regulations), this situation is nevertheless sidered.
seen. Unintentional shorting can occur across the electrical In the case of plastics, resistance to inorganic chemicals is
isolators normally placed near customer meters. This occurs if usually high. Only very strong oxidizing or reducing agents
items such as a bicycle chain lock, garden tool, or metallic will damage most modern plastic pipe materials. Organic
paint are placed in a way such that an electrical connection is chemicals, however, can damage plastics by solvation-the
made across the isolator. Some companies perform regular sur- absorption of a foreign liquid such as a solvent, possibly result-
veys to detect all such shorting situations. The evaluator should ing in swelling, softening, reduction in physical properties, or
he alert to the problem and seek evidence that the operator is even dissolution of the material. Organic chemicals can also
sensitive to such scenarios and their possible impact on aggravate environmental stress corrosion cracking [2]. Aging
cathodic protection, corrosion, spark generation, and other of plastics is theoretically possible because chemical and phys-
possible effects. ical changes result from oxidation, hydrolysis, absorption, or