Impeller Basics - Part 2

(Photos by Geoff Magnusson. They are published in LinkedIn and are used here with his permission to explain the differences in the design of double suction impellers.)

Introduction

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The four photos above show double suction (DS) impellers of varying designs and machining styles. This article explains the reasons for the differences.

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Most DS impellers are enclosed type – they have shrouds, or wall plates, on both sides, as shown in the photos. But they may also be available in open-type design (without the shrouds) such as those used in coke-crusher or slurry services. (I had designed open-type DS impeller, in the past.)

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The impellers may have either a full cut or undercut trim diameter. A full cut (also called a standard cut) is when the vanes and shrouds are cut to the same diameter. An undercut is when the vanes are cut to the required diameter but the shrouds are kept at a bigger, or at full, diameter. The reasons for this are discussed in another article.

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Photo 1 (top row)

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This is a standard DS impeller. It has balanced wear rings - the ring OD is the same on both sides. The vanes are perpendicular to the shrouds – there is no rake angle, or displacement angle from the horizontal plane. It is also called a skew angle.

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The impeller is at full cut trim diameter. The vane tip finish is per common practice but there are other styles of finish depending on their specific use. For instance, in reverse running pumps (HPRT or PAT), the impeller vane tips are rounded.

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Note in this photo that the rectangular vane thickness along the circumferential plane of the impeller OD is much thicker than the shroud thickness. A typical thickness is around ?” to 1/2”, or about 1.5 to 2.5 times the shroud thickness, depending on the impeller cut diameter.?

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The vane thickness can be altered to change the pump performance. A vane overfile can be done to reduce the vibration amplitude at vane pass frequency (VPF), whereas a vane underfile can be done to correct an under performance, or to improve the performance.?

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An overfile is the removal (by grinding) of excess vane thickness on the overside of the vane, hence the term overfile. This process will not increase the vane discharge area and will not change the pump Q/H/E performance.

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An underfile is the removal of excess vane thickness on the underside of the vane, hence the term underfile. This process will increase the vane discharge area and will change the pump Q/H/E performance. The increase in performance is a function of the increase in vane discharge area as a result of the underfiling.

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More about this in a separate article.

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Photo 2 (left, 2nd row)

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This photo shows a DS impeller with V-cut, or V-notch. The V-cut is typically in the range of 30 to 45 degrees from the horizontal centerline, depending on its purpose.

There are two reasons for doing a V-cut: one is to reduce the vibration amplitude at impeller VPF, and the other is to increase the head rise to shut-off. A V-cut to reduce vibration is less effective if the vanes are already designed with a rake angle.

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The cut in the photo is unusual because it goes all the way down to the center rib, so it must be done for another specific reason.

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More about this in a separate article.

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Photo 3 (middle, 2nd row)

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This DS impeller is designed with a rake angle to reduce the potential for high vibration at impeller VPF. With a rake angle the impeller vane tip is not in parallel plane with the volute lip so the energy of interaction between the vane tip and the volute lip, at every vane pass, is not at full force but is somewhat diffused in a time domain.

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Photo 4 (right, 2nd row)

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This DS impeller has a full center rib and staggered vanes. A full center rib is one where the rib (also called center shroud or center hub) has the same outer diameter (OD) as the two outer shrouds. The rib is sometimes added to provide structural support to the vanes when they are too wide. But in most instances it is added to allow for the vane width to be split in halves and displace the left hand (LH) vanes from the right hand (RH) vanes.

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The purpose of staggering the vanes is to reduce the energy of interaction between the impeller vanes and the volute lips, thereby reducing the vibration magnitude at impeller vane pass frequency (VPF). There will be twice the number of vane-volute interactions but at half the energy level per interaction. (It is akin to the smoother run on a 4-cylinder engine compared to a 2-cylinder one.)

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But why not make all DS impellers with full center rib and staggered vanes? The answer is that a full center rib increases the impeller’s disk friction; it will result in some head and efficiency loss. A typical DS impeller has its center rib extended to about 60% of its maximum diameter, so there is a significant increase in disk friction when it is extended to 100% of maximum diameter. The performance of a DS impeller with full center rib is akin to the performance of two single suction impellers mounted back-to-back.

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There are two schools of thought on how to stagger the impeller vanes. The first is to stagger the vanes equally – the LH vanes are in the middle space of the RH vanes. The other one is to move the LH vanes from the RH vanes at different angles of displacement ?based on the number of impeller vanes.

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When displacing the LH from the RH vanes it does not matter if the angular spacing is in a forward or backward location.

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Note in the photo that the impeller eye (or suction) diameter is too large in proportion to the impeller diameter – probably about 80% of the impeller diameter indicating that this is a very high specific impeller. Its oversized eye diameter and very short vanes make this impeller very susceptible to flow recirculation and high vibration – it is likely the reason why the vanes are staggered.

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Many engineers have a mistaken notion that an oversized eye diameter (resulting in an oversized eye area) will result in lower NPSHR. Not true, in fact it may result in higher NPSHR because the peripheral velocity of the impeller eye diameter will increase, and so will the suction losses.?

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Tags: #doublesuction, #doublesuctionimpeller, #fullcenterrib, #staggeredvanes, ?#enclosedimpeller, #openimpeller, #NPSHR, #impellereye, #impellereyediameter, #impellereyearea, #impellershroud, #specificspeed, #efficiency

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Please like and share if you find this article helpful.

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For more technical information on centrifugal pumps, please follow the author and also join the public group Centrifugal Pumps – Modern Design and Practices, here in LinkedIn by clicking on the two links below.

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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 earaza@msn.com.

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