Double Suction Impellers - Hollow core or solid core?
CENTRIFUGAL PUMPS - Modern Design and Practices
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Double Suction Impellers
Hollow core or solid core - what you need to know
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Many pump standards no longer allow the use of double suction impellers with hollow core (or hollow hub) for safety and environmental reasons. Should hollow hub impellers be continued to use or should they be upgraded to solid core design?
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By Esteban M. Araza
Email: [email protected]
Many years ago, double suction impellers with hollow core (or hollow hub) were widely used. But in recent years their use is disallowed by many pump standards, like API 610. What are the reasons for this practice? Should impellers with hollow core be continued to use, or should they be upgraded to solid core design?
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A double suction impeller is one with suction eye (or entrance) on both sides of the impeller. The purpose is to increase its eye area to reduce its NPSHR, compared to an equivalent impeller with single suction. One can imagine a double suction impeller as similar to two single suction impellers arranged back-to-back. This design leads to the double suction impeller having much thicker metal at its core (or hub) compared to the metal thickness at its shrouds and vanes (see figure 1). This big disparity in thicknesses oftentimes results in the distortion of the impeller casting because of the uneven rate of cooling during the casting process. That, in turn, adversely affects the impeller operation and performance.
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To avoid casting distortion, double suction impellers were designed with hollow core to avoid drastic disparity in the casting thickness of its hub compared to that of the impeller vanes and shrouds (see figures 2 and 3). Some secondary advantages of this design are that it results in lighter impeller weight, lower radial thrust load, and reduced shaft deflection.
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However, hollow core impellers have disadvantages that, in recent years, overwhelm their advantages, more so in light with greater attention and priorities given to safety and environmental concerns that have become of paramount importance.
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For one, the impeller traps liquid in its hollow core. When heat is applied to remove the impeller with a tight-fit, the trapped liquid becomes hot and can cause serious injury to the shop personnel when released from the hollow core.
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And whereas the decontamination of pumps, and pump parts, is now almost a universal practice, if not a legal requirement, the decontamination of the core in a hollow core impeller is more difficult, especially those used in toxic, flammable, or hazardous service.
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Performance-wise, hollow core impellers have other disadvantages. They cannot be fully vented; the trapped air or vapor inside the core can impair the pump performance. Dirt and scale may accumulate inside the hollow core and make the impeller out of balance.
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On the other hand, recent advances in casting technology and techniques now minimize, if not totally eliminated, the risk of distortion in castings with large thickness disparity in various areas of the impeller casting, thus removing the prime mover for going with the hollow core design.
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And although a hollow core impeller is lighter and thus results in less radial thrust load, these are, in reality, not much of a benefit. The hollow core will be filled with liquid and, since it has no freedom of movement in the core, the trapped liquid will add weight to the impeller instead of providing it with partial buoyancy.
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Given the foregoing considerations, double suction impellers with hollow core should be upgraded to impellers with solid core design.
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Disclosure: Author spent his entire career in the pump industry. He is now retired and wrote this article on his personal capacity. The article is preliminary and is intended to encourage further discussions on this topic. To request permission to reprint, excerpt, or republish this article, in full or in part, please email the author at [email protected].