Page 233 - Pipeline Rules of Thumb Handbook

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cathodic protection current to the wells. This may not even Two key considerations from a corrosion standpoint are:
be necessary if distributed ground beds are used. (1) avoid the use of dissimilar metals; (2) match the material
Some companies insulate compressor stations from the to its environment. Examples of these would include the use
lines entering and leaving the station. Other companies do of a brass valve in a buried steel water line; or use of mecha-
not. This is a matter of individual company preference. nically strong bolts in the wrong electrolyte, resulting in sub-
sequent failure from stress corrosion cracking.
Water systems Select materials carefully, with corrosion in mind as well as
the more obvious considerations. If design engineers do this,
Each water system (jacket water, piston water, boiler water,
many subsequent failures can be eliminated.
etc.) should have corrosion monitoring provisions designed
into the system, such as coupons or corrosion rate probes.
Retirements
If there is a gas treating plant in conjunction with the com-
pressor station, corrosion monitoring provisions should be
Many compressor station designs involve retiring some-
designed into that system also.
thing that is already there.
Water treatment for corrosion control would have to be
If something must be retired, especially underground, take
considered, depending on individual circumstances. Some
it out. Many times a company pays unnecessarily to continue
treatment facilities may be designed into the systems.
protection of facilities retired in place.
Provisions should be considered for cathodic protection of
Even if the retired facilities can be completely discon-
the internal surfaces of storage tanks and water softeners. One nected from the protective systems, shielding is still a
problem often encountered on smaller tanks is adequate problem. Shielding is a problem by foundations also, so if they
openings. Enough openings of sufficient size should be are retired they should be removed.
allowed on any tank for possible future work. Thus, attacking the corrosion problems requires a truly
Water should be kept out of some systems. For instance,
use air dryers on control lines. interdisciplinary effort. In fact, corrosion offers one of the
richest fields for interdisciplinary activity.
(Based on a paper, “Construction Design to Facilitate
Materials selection
Corrosion Control—Compressor Stations,” presented by the
Proper selection of materials is important throughout author at AGA Distribution Conference, May 8–10, 1972,
design. Atlanta, Ga.)

Project leak growth

The number of leaks in older pipeline systems tend to pipe replaced, added, abandoned or installed should be tallied.
increase exponentially. Exponential growth of leaks will fit the If this is not done, errors will result. For example, if leak data
following equation: werecollectedona townshipbasis, and during the study period
considered, 10% of the township’s pipe was renewed, leak fre-
N = N 1 e nA quency in the old area will continue to rise while in the
renewed area there should be none, possibly giving an overall
where N = number of leaks for the study area in any year
number of leaks less than that of the year prior to the replace-
considered
ment. For an accurate picture and a determination of the leak
N 1 = number of leaks in the first year for which leak
growth-rate coefficient, the number of leaks in the years fol-
data are available
lowing the replacement should be accounted for as follows:
e = 2.718
n = nth year for which the number of leaks is to be
Let x = percentage of replacement completed.
estimated
Let y = actual number of leaks for the remainder of the
A = leak growth-rate coefficient
township for the year considered
The coefficient A can be calculated from the graph drawn of
Adjusted number N = y (1 - x)
the leak data for the study period. The study period should
preferably be in the range of 5 to 10 years. Any data less than
this will not give enough points to accurately determine the The leak growth rate for some pipelines may not follow the
coefficient A. Before a graph is drawn, leak report quantities exponential curve. For example, in a particular case, a linear
mustbeadjustedforany changes to the system during the study increase with time may be predicted. In that case, use a linear
period. In other words, if a township was the study area, then equation instead of the exponential form.