f. Use Scrapers. A scraper will remove buildup on the interior of the pipeline. Corrosion can be
controlled but the pipeline over a period of time can have the inside diameter enlarged if the scraper is run too
frequently. Additionally, the wall thickness is reduced and external corrosion can weaken the pipeline causing a
break.
NOTE: A scraper program should be setup based on need, such as a decrease in throughput and a increase in
pressures to maintain maximum flow and reduce product contamination.
4.
Measurement of Internal Corrosion.
a. Internal corrosion decreases the capacity of the line by decreasing the effective area of the line. If the
velocity of the line is held constant, there will be a decrease in throughput.
b. Several equations have been used to compute a factor to describe the condition of the pipe in service.
This is called the C-Factor, and it is computed from an observed head loss between two points in the line under
conditions of flow.
(1) We use an equation from "Darcy-Weisbach" to compute head loss due to friction when the
friction factor for a new pipe is known. The friction factor derived through the Darcy-Weisbach equation is
affected by internal corrosion. The effects of internal corrosion on the relationships expressed by the Darcy-
Weisbach equation is to:
(a) Increase the friction factor which results in increased head loss.
(b) Decrease the effective diameter of the pipe which results in increased head loss. Actual
reduction in the diameter may be small, but the relative effect is great.
NOTE: For a given flow rate (without considering increased roughness), a 2-percent decrease in diameter
increases pressure drop by 10-percent; a 50-percent decrease in diameter increases the pressure drop 27-percent.
(2) The "Hazen Williams" equation is probably the simplest and handiest of several equations which
described the relationship among the factors governing the rate of fluid flow through the pipeline.
(a) The C-Factor equation we will use is based on a pressure loss per 1,000 feet of pipe.
(b) The C-Factor is based on an empirical scale which varies from 60 to 160, depending upon the
smoothness of the inside wall of the pipe. The smoother the pipe wall, the higher the C-Factor, and the less
resistance there is to flow.
(c) A coefficient of 140 is set for new steel pipe, however, this factor may be increased to 150 or
160 through the use of scrapers and inhibitors which have a polishing effect on the inside of the pipe.
(d) A drop in the C-Factor due to build-up of corrosion products in the pipe results in a drop in
throughput.
NOTE: An 18-inch line C-Factor drop from 160 to 150, there would be an annual loss in throughput of
approximately 4,600,000 barrels.
(3) The results derived from the equation if less then 140 for the C-Factor indicates a POTENTIAL
need to schedule a scraper run, or could be for some other reason; such as a partly closed valve or other type of
blockage in the pipeline.
15-3
QM5200
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