Roscoe Moss Company Memo - Well Screen Open Area   -   3

Ground Water - It's Development, Use and Conservation

Entrance Velocity As Related To Corrosion

Bennison in his chapter, Design, Construction, and Use of Well Screens, he discusses corrosion and encrustation. He develops a theory whereby much encrustation of well screens occurs as a result of reduction of pressure in the water in the formation in the well while pumping. According to Bennison,

"Because the pressure head is reduced carbon dioxide gas in the water is released, and the water is unable to carry or rather hold in solution its load of mineral salts. Consequently these mineral salts are precipitated or dropped from the water and deposited where the reduction of pressure has been the greatest. This is in and around the lower end of the well or the well screen."

He argues that a method of mitigating such encrustation would be to use the screen that will permit water to enter the well with the least resistance at the screen openings. In other words, a high area of opening continuous slot screen. Bennison's theory regarding carbon dioxide is totally incorrect as discussed in the publication Fundamentals of Metallic Corrosion in Fresh Water, by J.R. Rossum. No other scientist or engineer has made this claim. If water is originally super saturated with a gas at a given pressure (atmospheric pressure in the case of ground water) it would fizz like soda water when exposed to air. It is frequently true that encrustation occurs near the lower end of the well or the well screen, but this is because water velocity is lower at these points. In pipelines, corrosion is usually least at normal velocities (one to five ft/second).

Bennison also states,

"...the rate of corrosion tends to increase with an increase in velocity of the water over the surface of the metal being corroded."

While the word "tends" is ambiguous, it can be emphatically stated that his statement is incorrect, as also explained in the publication authored by Rossum and many other publications. It is true that many tests have shown that corrosion rates increase with velocity where the environment is very corrosive, for example, iron in dilute sulfuric acid. Under moderately corrosive conditions, velocity may have the reverse effect. Water system operators have known for years that reasonable velocities were essential in mitigating corrosion. Where velocity is low, particularly in dead ends, corrosion is severe. Merril & Sanks (AWWA Handbook on Corrosion Control) state that one of the attributes of a well conditioned water is a velocity of at least 2 ft/sec.

Romeo, Skrinde & Elliassen (ASCE SAN Div., July 1958), in their study on the Effects of the Mechanics of Flow on Corrosion, found that corrosion follows an exponential curve and that the corrosion rates for the first two months were greater at velocities of 1 ft/sec. and 2 ft/sec. than the corrosion rate at 0.125 ft/sec. When their curves are extended to long times, it is found that corrosion rates decrease as velocity increases over the range of their investigation of from .125 ft/sec. to 4 ft/sec. Thus the results of the investigation are in accord with general water works experience.

In the Illinois State Water Survey Bulletin No. 59 Corrosion by Domestic Waters by T. E. Larson, it is shown that initial corrosion rates are lower at 0.05 ft/second than at 0.34 ft/sec. But after 100 days the effect is reversed in conformance with the results of Romeo et al.

The reasons for these results are easily understood. As the velocity increases the thickness of the laminar layer next to the metal decreases so that oxygen can diffuse to the metal surface more readily. However, high flow rates produce hard durable coatings (McCauly and Abdulah - JAWWA, Vol. 50, page 11419, Nov. 1958) so that after the coatings have formed, corrosion rate decreases at higher velocities.

Bennison's 1966 Book offered much valuable information to the industry. However, as an employee of the Johnson Screen Company, his expositions regarding the advantages of high area of opening continues wire wrap screen were fallacious and self serving. Unfortunately these statements have been repeated through the decades, and their acceptance has frequently led to unnecessary costs and poor results.

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