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If you have struggled for a while with conventional pumps, viscosity-correction factors, and the like, you may be ready to think about recessed-impeller units. They’re often considered to be optimum for pumping duties that include free-flowing slurries, sludge, and fibrous materials. A standard centrifugal pump may clog, become vapor-bound, or wear excessively in those types of services. 

For the applications mentioned here, pumps with fully recessed impellers (see illustration below) should be given serious consideration. Recessed “gentle pumping action” impellers incorporate the vortex principle illustrated here, wherein only an estimated 15% of the total fluid throughput contacts the fully recessed impeller. Such pumps are typically available in flow capacities approaching 100 l /sec (1,580 gpm) and heads ranging up to 130 m (~430 ft).

Recessed impeller pumps certainly aren’t new: They’ve been around since the 1930s. Unfortunately for the user, several manufacturers offer recessed impeller pumps that have not advanced from their respective configurational or hydraulic performance constraints for 40 or more years.

It’s also fair to note that some legacy models require a degree of maintenance involvement that might have been considered acceptable decades ago but is no longer tolerated by today’s best-in-class (BiC) pump users. In short, while some seemingly small, yet important, design changes may have been incorporated over the years, the overall vortex-type operating principle has remained virtually the same.

Most recessed-impeller pumps rotate the liquid and solids inside the casing until the solids reach a speed at which they exit the casing. This recirculation of solids creates wear in the casing and increases damage to soft solids.

One noteworthy manufacturer has overcome that problem by designing the casing with an “axial spiral” in the casing. Visualize an automobile tire to represent the basic design of a recessed-impeller casing. Cutting the tire at the top and then twisting it yields a spiral. The axial-spiral contour helps guide solids out of the casing, which, in turn, prevents solids recirculation.

It can be said that the axial-spiral design substantially improves the true overall hydraulic efficiency of these pumps. In addition, the axial-spiral twist has greatly reduced component wear and damage to solids being pumped. As a further point of interest, the minimum flow capability of a recessed-impeller pump is much lower than that of conventional radial-spiral casing designs.

On the minus side, top centerline discharge implies a measure of vulnerability when pumping large, hard solids. Solids such as rocks might, on rare occasions, smash through the casing neck. In some rock-feed applications, tangential discharge might be viewed as an advantage.

In many cases, end-users and engineering-design contractors elect to place emphasis on pump efficiency. When asked to define efficiency, they inevitably refer to power draw. That, unfortunately, is seriously wrong.

Some pumps achieve seemingly high hydraulic efficiency by letting the impeller edge protrude into the casing. Protruding impellers, of course, limit unimpeded passage of solids through the pump. Reliability professionals are urged to re-think what is of true importance here: that is the efficiency with which both liquids and solids are being transported.

Some “old style” recessed-impeller designs have simply not progressed much since being introduced into the marketplace. Their best efficiency operating points (BEPs) are often in the range of 30% to 40%.

On the other hand, advanced designs that incorporate axial-spiral-design casing internals and fully recessed impellers can have true and effective BEPs of around 50% to 60%. Less energy goes into the liquid, and less power is consumed to forward-feed solids.TRR

Editor’s Note: Click Here To Download A Full List Of Heinz Bloch’s 24 Books

Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX. 

Tags: reliability, availability, maintenance, RAM, industrial fluid-handling, recessed-impeller pumps, centrifugal pumps