The "Ten States Standards" and other municipal and state codes require submersible wastewater pumps to pass a minimum 3-in spherical solid. This is not a difficult task for many 4-in and larger non-clogs, but a problem arises when flow requirements drop and certain heads must be maintained. These lower flow, higher head applications can make it difficult to accommodate a 3-in solid.
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Have you ever wondered why a typical, ½-hp residential sewage pump has a flow rate well over 80-gpm? If we turned on every faucet, flushed every toilet and ran every appliance at the same time, flow would not exceed 30-gpm for the average three or four bedroom home. This 80-gpm (or more) flow rate is due to a seldom-considered impeller characteristic.
When we think about the varying flows and heads produced by a centrifugal pump, our brains tend to focus on the affinity laws, impeller diameter and motor speed. We know that flow is directly proportional to both rotational speed and impeller diameter and head varies as the square of a change in diameter and rotational speed. But, another factor contributes to flow: vane width, or the distance between the shrouds of a closed impeller. If we double the width, the impeller's internal volume doubles. If we keep diameter and rotational speed constant, flow will vary in proportion to vane width. Since that residential sewage pump is designed to pass a 2-in solid, its vane width will dictate a much higher flow than one might expect. The same holds true for a 4-in non-clog designed to pass a non-deformable, 3-in spherical solid.
Figure 1 shows the performance of a moderate flow, 4-in submersible non-clog that meets the Ten States standard for solids passage. It employs a two vane impeller operating at 1,750-rpm to produce a BEP flow of 450-gpm at 70 percent efficiency. Could we use this pump in an application that requires 225-gpm? If we trim the impeller to about 6-in, we could obtain that flow at about 22-ft, but our efficiency would drop to about 47 percent. A better alternative would be to reduce the rotational speed to 1,150-rpm as the pump would maintain about 60 percent efficiency since efficiency moves to the left with a reduction in speed.
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But, suppose we require 225-gpm at 38-ft? Couldn't we just trim the impeller to 7-in and run at a lower efficiency? We could, but if you read my June 2008 P&S article ("Two Steps to Longer Pump Life (Part One): The As Built Operating Point"), you know that we must be willing to accept a lower reliability due to the increased radial forces formed when operating that far off BEP.
Unfortunately, this puts us in that proverbial area between a rock and a hard place. We can reduce flow by reducing the impeller's diameter or rotational speed, but in doing so, head becomes unacceptable. We can also reduce flow by reducing the vane width. Head will remain acceptable, but the impeller will no longer pass a 3-in solid. Alternatively, we could run well to the left of BEP and accomplish both at a cost of reduced pump life. Fortunately there are other impeller options.
Single Vane Impellers
The single, or mono vane, impeller (Figure 2) was developed in the late 1940s by Fairbanks-Morse. Its unique geometry produced different flow characteristics and resulted in a steeper H/Q curve than that of a two vane impeller of the same size. It also passed a larger solid and was more tolerant of the radial forces that arose due to off BEP flow. They are still popular today. The only negative is that they can be difficult to balance in the field due to their inherent lack of symmetry.
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Figure 3 is the H/Q curve produced by a single vane impeller. This particular model will produce flows of 200- to 350-gpm at a wide range of heads and still pass a 3-in spherical solid. This impeller design offers a viable alternative to off BEP operation of a two vane impeller.
Vortex Impellers
In my August 2007 column, "Vortex Action: How Lower Efficiency Can Reduce Overall Cost", I described the two stage process that causes flow in the vortex pump, so I will not repeat it here. What I will reiterate is that the vortex impeller resides completely outside of the volute, and this feature yields two significant advantages. First, it is immune to radial thrust and can operate well to the left of BEP. Second, it allows lower flow pumps to pass a full 3-in spherical solid since the solid does not have to traverse the vane passages.
These advantages do come at a cost-lower hydraulic efficiency. This is usually a small price if you have a low flow application that requires the passage of large solids. Figure 4 shows the performance curve for a 4-in vortex pump that meets the Ten States standard. This particular model provides flows from 150- to 350-gpm at heads up to 45-ft. Other models will provide similar flows at significantly higher heads and still pass a 3-in spherical solid.
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If you have lower flow applications that still require the passage of large solids, I encourage you to investigate these impeller alternatives. Lower flow, solids handling pumps are less efficient than higher flow models so, reliability-not efficiency-should be your major concern.
Pumps & Systems, October 2008
