Impeller Basics - Part 1

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by Esteban Araza

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(Above photo is from social media and is used under fair use doctrine for educational purpose.)

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This article explains some terms and functions associated with an impeller.

The photo shows a radial flow impeller - its discharges flow is in radial direction from the shaft. If the discharge flow were at an angle from the vertical centerline it would be a mixed flow impeller, and if it were in axial direction it would be an axial flow. In the specific speed (Ns) range, radial impellers are in the low end, mixed flow are in the mid-range, and axial flow are in the high end of the Ns range.

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A radial impeller develops its head by means of centrifugal force. A mixed flow develops its head by a combination of centrifugal force and the lifting action of the impeller vanes (or blades). An axial flow develops its head solely by the lifting action of the vanes. Only radial and mixed flow impellers are of centrifugal type, axial flow impellers are not.

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Right side photo

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The impeller is an enclosed type - it has shrouds on both its front and back shrouds, as shown in the photo. If it had no front shroud, it would be a semi-open. If it had no front and back shrouds it would be an open impeller.

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Impellers are normally enclosed for high efficiency and structural strength. Semi-open are used when the liquid has small solid particles such as in slurry service to prevent clogging the impeller. Open impellers are used when the liquid has bigger solid particles such as in sewage service; they are used in low head, low HP services because they lack structural strength in the absence of shrouds. In the efficiency range, enclosed impellers have the highest efficiency, semi-open are in midpoint, and open-type are in the lowest range - for pumps in similar Ns range.

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The impeller shown has one suction (or inlet) area located on its left side - it is single a suction (SS) impeller. If it had suction areas on both sides, it would be a double suction (DS) impeller. DS impeller requires less NPSHR because of its bigger eye area, but it is sensitive to suction piping and unbalanced flow rates on its two sides. On the other hand, SS impeller has the disadvantage of higher axial thrust when used in overhang pumps with high suction pressure.

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The photo shows the suction eye diameter - the area of this diameter, minus that of any blockage, is called the annular eye area. In addition, there are the true eye area (Ae) and eye area between impeller inlet vanes (Aev) – these latter eye areas determine the impeller NPSHR. The ratio of Aev/Ae is an indicator if an impeller were of good design. (This is a topic for advanced impeller hydraulic design.)

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In the impeller nomenclature, the suction eye diameter is referred to as De or D1. The maximum diameter is referred to as D2. The D1/D2 ratio is an indicator of the impeller Ns, and also if the impeller were of radial flow, mixed flow, or axial flow design. A D1/D2 ratio of 1 indicates the impeller is axial flow. A ratio below 0.5 indicates it is a radial flow impeller with low Ns, and a ratio above 0.5 indicates it is a radial flow or a mixed flow impeller with high Ns.

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The impeller exit width is referred to as BA. The impeller design flow is largely a function of its BA, although other impeller parameters such as the number of vanes and their discharge angles also affect the design flow to a lesser degree.

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The photo shows an outer hub on the front shroud. This is also called the impeller ring land because it is the landing area for the impeller wear ring, IWR. (IWR is not shown in the photo). A standard impeller typically has ring lands and IWRs in both the front and back shrouds. The IWRs diameter may be the same, or different, for the front and back shrouds, based on what diameter is required to minimize the pump axial thrust balance.

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Note the impeller has no outer hub on its back shroud, instead it has back vanes (or back ribs) – this is an? indication that it is a special purpose impeller. Back vanes are added for specific purpose such as (1) in slurry pumps to prevent the settling of slurry particles, (2) to keep solid particles in suspension and prevent them from entering the stuffing box, (3) to reduce the pressure at the face of the mechanical seal box, (4) for balancing the hydraulic axial thrust in an open-type impeller, etc.

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Left side photo

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The impeller on the left shows a typical radial-type impeller; it has six vanes and six drilled holes located near the tip of the inlet vanes. The holes are called balance holes and are provided to reduce, or to equalize, the hydraulic axial thrust on the front and back sides of the impeller. The number of balance holes are always the same as the number of vanes. The size of the balance holes, and its base circle diameter, depend on the hydraulic axial thrust acting on the impeller.

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An impeller with balance holes always has an outer hub on the back shroud; the diameter of the outer hubs is usually the same, except if some unbalanced (or unequal) diameter is required to fine-tune the hydraulic axial thrust. Omitting both the balance holes and the outer hub on the back shroud will increase the pressure on the back shroud the impeller – this arrangement is usually used to counter the high thrust on the front shroud when the suction pressure is too high.

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In centrifugal pumps, the number of impeller vanes (Vn) used is typically 3, 5, 6, 7, 8 - but in rare instances impellers with 2 or 9 Vn are used. Impellers with odd Vn (3, 5, 7) are preferred to use in double volute pumps to reduce the vibration at impeller vane pass frequency. On the other hand, impellers with even Vn (4, 6, 8) may be used in single volute pumps, or in instances where it is needed to detune potential mechanical, structural, or acoustic resonance.

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Tags: #radial flow, #radialflowimpeller, #mixedlfow, #mixedflowimpeller, #radialflow, #axialflowimpeller, #enclosedimpeller, #semiopenimpeller, #openimpeller, #doublesuction, #singlesuction, #axialthrust, #radialthrust, #balanceholes, #NPSHR, #impellereye, #impellereyediameter, #impellereyearea, #impellershroud, #specificspeed, #efficiency

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Disclosure: The author spent his entire professional career with multiple companies in the pump industry. He is now retired and wrote this article in his personal capacity. This article is preliminary for further discussions as each situation is uniquely different. For more information, please email the author at [email protected].

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