Successfully Combat Pipeline Corrosion

A comprehensive maintenance strategy can enhance efficiency and safety.

By Andy Santalucia, Clean Harbors

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Technologies to assess line condition and evaluate welds include visual, X-ray, magnetic flux leakage (MFL) and ultrasonics. The most common are caliper or geometric inspection, MFL and ultrasonic.

Caliper or geometric tools effectively detect and size deformations that can affect pipeline integrity or impede the passage of other inspection (or cleaning) tools.

MFL and ultrasonic "smart pigs" can detect anomalies and metal loss over the 360° circumference of the pipeline wall, differentiate between internal and external location, and produce longitudinal distance measurements that accurately pinpoint anomalies. They come with analysis software that provides hard copy reports with pipeline defect definition data as well as joint-count and anomaly tables. These detailed reports highlight out-of-specification findings based on 360° scans at a rate of up to 8,000 samples/second with resolution of ¼ inch. The reports include A, B and C scan data, as well as 3-D color-coded representations of wall thickness.

In selecting inspection technology, a plant should consider factors such as cost, pipeline bend radius, length of tools, fluid medium, pressure ratings of the pipeline and wall thickness. MFL tools are less expensive than ultrasonic ones but typically are longer (up to 8-ft long for an 8-in. line) and may require higher pressures and flow rates. Ultrasonic tools generally are smaller than MFL ones and can be bi-directional but are more expensive and must be run in water or another suitable fluid.

Effective inspections examine both the inside and outside of the pipe wall, identify corrosion, and record denting, pitting, ovality and swelling.

Inspections also provide a baseline for future evaluations and to develop more precise estimates of corrosion rate and pipeline life expectancy. Some companies with regular pipeline maintenance programs install permanent pigging launching and receiving facilities to make it easier, less costly and more efficient to conduct frequent inspections.

MAINTENANCE PROGRAM
Outside of capital investments, effective pipeline maintenance is the only way to significantly impact performance. Even incremental improvements may deliver substantial returns.

An operating company should establish a pipeline maintenance program for every site. This involves first evaluating each pipeline to gauge current throughput efficiency, performing thorough pipeline maintenance, and then rechecking efficiency to evaluate the overall upgrade. Regular pipeline flow studies over time can track declines in product flow efficiency — and prompt pigging, flow sweeping, corrosion inhibition and other maintenance activities when the efficiency drop becomes significant.

Pipeline flow efficiency test. It's relatively easy to estimate what the expected pressure drop of a particular pipeline should be by analyzing original design drawings and calculations. The difference between original design and actual pressure drop indicates lost efficiency due to buildup of scale, sediment or corrosion on the pipe wall, and enables estimating potential energy savings and throughput increases to justify the cost of a cleaning program.

Recently, a refinery estimated solids' buildup in a 22-in. raw water line was causing an additional pressure drop of 7.5 psi. It reckoned cleaning the line would increase flow to 3,400 gpm from 2,800 gpm; this would allow for single pump operation most of the year, saving $100,000/year.

A variety of details are helpful for assessing the potential improvement: the piping's manufacturer, age, outer and inner diameters, wall thickness, and whether it's piggable and there's a history of corrosion or leaks; the fluid's composition, specific gravity, temperature, pressure, flow rate and velocity; the upstream and downstream pumps' suction and discharge pressures; and the type of efficiency problem (low flow, solids, sludge, scale, etc.) if known.

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