Focus on Harsh Environments: How to tackle corrosion in submersible pumps
By Steve Minett, Ph.D.
Submersible pumps--probably more than any other technology--have corrosion problems that must be dealt with regularly. Here are a few tips for end-users about what to watch out for, plus a survey of m...
Submersible pumps–probably more than any other technology–have corrosion problems that must be dealt with regularly. Here are a few tips for end-users about what to watch out for, plus a survey of materials and techniques to counter corrosion risks.
A submersible pump’s corrosion risk is, of course, determined by the environment it is exposed to and, specifically, by the medium it is required to pump. The following liquids can be expected to pose a corrosion risk: sea water, hydrochloric acid, some types of solvents, hydrogen sulphide, liquids with a high copper content, bases with a high pH value, and some liquids containing a mixture of acids. In addition to this general list of potentially corrosive liquids, a further “rule of thumb” should also be borne in mind. In the case of metals, it’s normally the case that the higher the concentration of corrosive substances in a liquid, the worse the corrosion will be. For rubber and plastics, however, long-term exposure to lower concentrations can also result in quite severe corrosion.
Corrosion problems in submersible pumps can be counteracted in two basic ways. Firstly, the entire pump can be manufactured in a corrosion resistant material, such as stainless steel. Secondly, various other measures can be taken, such as coatings, anodes and/or the use of resistant materials for particular components which are especially exposed to corrosion risks.
If we look first at the main material of manufacture, it can be stated that the vast majority of submersible pumping operations are carried out by cast iron pumps. This normally presents no corrosion problems when pumping liquids such as surface water and domestic sewage. The low oxygen content in raw sewage, in particular, lowers its corrosion effect to almost nothing.
In mining and construction applications, aluminum submersibles are usually preferred. This is, however, for reasons of weight, given that the majority of these pumps are portables, rather than from considerations about corrosion. The large, main pumps in mines, on the other hand, are usually made of cast iron. This material is robust, inexpensive and, as these pumps are stationary, weight considerations are not significant. (In fact, in alkaline liquids cast iron is a more resistant material, being able to tolerate pH values of up to 14, whereas aluminum is resistant only up to a pH value of around 8.)
Stainless steel is used as the main material in submersible pumps for two reasons; firstly, for use in acidic liquids and, secondly, where the purity of the liquid being pumped is a prime consideration, as in certain process industries, such as paper and paint production, where colour purity is important.
Coatings and anodes
However, a less expensive and more flexible alternative against salt water corrosion is to use a coating on a conventional cast iron pump. This is more flexible because an entire range of pumps from a manufacturer can be coated. The most widely used coating is epoxy (a polymer material).
Normally, zinc anodes are used in conjunction with an epoxy coating because of the inevitability of post-production scratches occurring in the coating. The use of anodes significantly extends the life of a coated pump. Between five and 10 anodes are implanted at various points around the cast iron structure of the pump. Because zinc has a lower electric potential than cast iron, the contact between the two materials spontaneously generates a micro-current. The presence of the current protects areas of cast iron exposed by the scratches, while the zinc anodes themselves are “sacrificially” corroded.
An alternative to using sacrificial anodes is to supply the micro current by cable from an external power source. This is known as an “impressed current.” In this method a non-sacrificial anode is suspended in the liquid beside the pump. The anode is attached by cable to the pump and the micro current is introduced into this cable.
The impressed current method, however, is complicated, expensive and requires a lot of monitoring to be effective. In addition, the extra cables required constitute obstructions in the pump well and often get damaged. Consequently, the implanted (sacrificial) anode method, being simpler and less expensive, is normally preferred.
Rotating shaft seals, rubber O rings, cables and cable bushings are crucial components of submersible pumps. They are often manufactured from resistant materials for use in corrosive environments. In conventional submersible pumps, Nitrile rubber is used for the O rings, but for pumps designed for use in acidic liquids, or liquids containing solvents, the rings are made from fluoro-carbon rubber, known as Viton
The standard materials for the outer rotating shaft seals are cemented carbides, WCCR. For use in acidic liquids, the shaft seals can be made from two types of ceramic; aluminum oxide or silicon carbide. The latter is more expensive but has the advantage of also being more abrasion resistant.
Finally, the cable for a submersible pump also needs protection in corrosive liquids. Chlorinated rubber is used as cable sheathing for conventional pumps but other types are available. Flygt, for example, has developed a special “HCR” (High Corrosion Resistance) cable, with sheathing made from a fluorinated ethylene plastic.
These corrosion resistance measures are used in various combinations, depending on the exact nature of the corrosion potential to which the pump is likely to be exposed. For example, in liquids containing solvents, it’s possible to use standard submersible pumps and simply insert corrosion-resistant O rings and fit an HCR cable.
These tips were compiled from information gathered from Flygt, a unit of ITT Industries, which claims a longer experience with submersible pump technology than any other manufacturer and, consequently, has accumulated a great deal of knowledge about how to deal with corrosion problems in this area.