Page 253 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

11/230 Distribution Systems
Score the potential for atmospheric corrosion as shown in highly localized corrosive conditions. In a city environment,
Chapter 4. the high number of construction projects leaves open the
opportunity for many different materials to be used as fill,
Internal corrosion foundation, road base, etc. Some of these materials may pro-
mote corrosion by acting as a strong electrolyte, attacking the
Water is a pipelined product that presents special challenges in pipe coating, or harboring bacteria that add corrosion mecha-
regard to internal corrosion prevention. Most metallic water nisms. In the case of cast iron, a lower resistivity soil will pro-
pipes have internal linings (cement mortar lining is common) to mote graphitization of low ductility cast iron pipe as well as
protect them from the corrosive nature of the transported water. corrosion of carbon steel.
Raw or partially treated water systems for delivery to agricul- Points should be reduced where soil conditions are unknown,
tural and/or landscaping applications are becoming more com- known to be corrosion promoting, or where placement of non-
mon. Water corrosivity might change depending on the native material has added an unknown factor.
treatment process and the quality of the transported water. Score this item as described.
With the lower pressures normally seen in distribution sys-
tems, infiltration can be a potential problem. Infiltration occurs Coating
when an outside material migrates into the pipeline. Most com-
monly, water is the substance that enters the pipe. While more In general, the coating condition variables for subsurface
common in gravity-flow water and sewer lines, a high water metallic pipes can be scored as detailed in Chapter 4. Some dif-
table can cause enough pressure to force water into even pres- ferent coating materials might be found in distribution systems
surized pipelines including portions of gas distribution sys- compared with transmission pipelines (such as loose polyethyl-
tems. Conduit pipe for fiber optic cable or other electronic ene bags surrounding cast iron pipes), but these are still appro-
transmission cables is also susceptible to infiltration and subse- priately evaluated in terms of their suitability, application, and
quent threats to system integrity. the related maintenance practices.
When foreign material enters the pipe, product contamina-
tion and internal corrosion are possible.. Scoring the variables Cathodic protection
for internal corrosion, product corrosivity, and internal protec-
tion can be done as described and in consideration of additional Modem metallic distribution systems (steel and ductile iron,
corrosion scenarios as discussed above. mostly) are installed with coatings and/or cathodic protection
when soil conditions warrant. However, in many older metal
Subsurface corrosion systems, little or no corrosion barriers were put into design con-
siderations. Note that the absence of an anticorrosion coating,
In this section, the evaluator looks for evidence that corrosion when one is warranted, scores no points-high risk of corro-
can or is occurring in pipe buried underground and that proper sion--under this evaluation system. Full points, however, can
actions are being directed to prevent that corrosion. A distinc- be awarded in both the cathodic protection and condition of
tion is made between metal and nonmetal buried pipe. For non- coating variables when the criterion of “no corrosion possible”
metal pipe, a subsequent section offers ideas on how to assess is met, even if an engineered corrosion prevention system does
corrosion potential. Another section shows one methodology not exist. That is, if it can be demonstrated that corrosion will
for combining subsurface corrosion assessments of metal and not occur in a certain area, credit for a cathodic protection sys-
nonmetal pipe. tem may be given. The evaluator should ensure that adequate
Common industry practice is to employ a two-part defense tests of all possible corrosion-enhancing conditions at all times
against galvanic corrosion of a steel pipeline. One line of of the year have been made. In general, the cathodic protection
defense is a coating over the pipeline-the other line of defense variables for subsurface metallic pipes can be scored as detailed
is application of cathodic protection (CP). These are discussed in Chapter 4 with special attention paid to the increased poten-
in detail in Chapter 4 and can be generally assessed according tial for interferences in a more urban environment. This and
to the protocols described there. Additional considerations for some other considerations are discussed below.
Chapter 4 variables are discussed below. Distribution systems are often divided into sections to opti-
mize cathodic protection. Older, poorly coated steel sections
Subsurface environment will have quite different current requirements than will newer,
well-coated steel lines. These systems must be well isolated
Because a coating system is always considered to be an imper- (electrically) from each other to allow cathodic protection to be
fect barrier, the soil is always assumed to be in contact with the effective. Given the isolation of sections, the grid layout, and
pipe wall at some points. Soil corrosivity is primarily a measure the often smaller diameters of distribution piping, a system of
of how well the soil can act as an electrolyte to promote gal- distributed anodes-strategically placed anodes-is some-
vanic corrosion on the pipe. Additionally, aspects of the soil times more efficient than a rectifier impressed current system.
that may otherwise directly or indirectly promote corrosion
mechanisms should be considered. These include bacterial Cathodicprofection effectiveness Test leads. Where cathodic
activity and the presence of corrosive-enhancing chemicals in protection is needed but is not being used, this item should nor-
the soil. mally score 0 points. While it can be argued that pipe-to-soil
The evaluator should be alert to instances where the soil protection readings can be taken even in the absence of applied
conditions change rapidly. Certain road bed materials, past cathodic protection, this information may only provide an
waste disposal sites, imported foreign materials, etc., can cause incomplete picture of corrosion mechanisms.