Click here for "Focus on Fundamentals" Part One, Part Three, Part Four and Part Five.
In the April issue, we discussed fundamentals of the overhung impeller centrifugal pump design. Its advantages are a single bearing housing and a single seal or packing, but an overhung impeller load is a disadvantage. By placing bearings on both sides of the impeller, a better support is achieved.
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Axially Split Design
This impeller has two "eyes," each receiving half of the flow. This design uses two sets of bearings, one in each bearing housing, which provide better rotor support. To reduce hydraulic radial load, a dual volute is typically used, especially on larger sizes.
The casing can be either axially or radially split, with radially split designs found in tougher applications, such as API-610, where a large gasket area of the axially split design could be a concern for leakage. In fact, axial load is nearly completely eliminated due to impeller symmetry.
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Radially Split Design
Axially split casing designs typically have both suction and discharge nozzles integrally cast with the lower casing half. This allows easy disassembly-the upper half is unbolted and the entire rotor (shaft, impeller, sleeves and bearing housings) are removed for repair or replacement. However, this can also lead to ("cutting-corners") problems, because the split line eventually wears out or deforms, and the main gasket no longer provides proper sealing, which causes leaks.
To remedy, both casing halves would need to be removed, milled flat and then line-bored to restore concentricities. This may significantly add to repair cost, and if competitive bids do not have a clearly specified requirement for such a procedure, a low repair bid gets the job and can produce a poor quality, non-repaired split line, causing a leak. Usually, a need for main flange re-milling becomes obvious when the two halves are removed. Unfortunately, by that time, a repair award decision is made, which typically may not include this important step. Some pump end users can perform a strip-and-advise (or similar term) inspection separate from the repair procedure itself to evaluate the pump's true condition, and a spec is prepared based on the known condition of the pump internals. The repair requirements then apply to all bidders consistently and fairly.
Removal of the bearing housings is easy, but has its nuances. Centering the rotor within the casing requires concentricity between impeller rings and casing rings, and casing rings within their bores-otherwise they often get squashed and damaged, and rotors seize. This is accomplished by moving both bearing housings up and down, and from side to side, to establish center. Then, housings need to be doweled in position, or re-doweled during repair, if the casing halves had been machined to restore main flange split line.
Performance of the split case pumps is similar to the end suction pumps, with similar limitation on minimum flow, to ensure absence of damaging suction recirculation. Conservative specs, such as API-610, limit minimum flow to 60 percent of BEP as allowable, and 70 percent preferred, and on a right side of the BEP to 120 percent as allowable, or 110 percent preferred. A more exact value for the MCSF (minimum continuous stable flow) can be calculated with more precision, but-in practice-a simplified rule-of-thumb is applied, often picking a percent as flow below which the pump should not operate for an extended time to achieve good reliability and long life.
Suction approach to the double suction split case pumps with between-bearing rotors can be finicky. A 90-deg bend in suction pipe/channel can cause uneven flow, with one eye of the impeller receiving much more flow than the other, for a given overall flow. This could cause cavitation damage on one side, and recirculation damage on another, plus vibrations, instabilities and failures. The Hydraulic Institute, for that reason, requires sufficient distance between the turn of the elbow and the inlet to the pump, typically five diameters, although pump manufacturers often reduce this requirement to two, in which case special anti-vortex devices can be (and often should be) used to rectify the flow.
If possible, however, avoid a need for such rectifying, after-the-fact methods during the system design stage since application of flow rectifiers, as an after-measure, requires review of the specifics of the installation, and each solution to such problematic installation may be somewhat different. The illustrations show how distorted flow can be as it enters the end of the sharp bend, and flows into the pump impeller eye:
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Inlet channel cross-section: highly distorted inlet flow
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(a) Cross-section of the original inlet to the impeller eye (separation on the short radium of the turn)
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(b) With special flow rectifier separating is reduced, flow more uniform within the cross-section entering the eye
A parting quiz: Does a larger impeller eye typically help pumps fight suction side problems? The answer may surprise you. (As usual, the first three people to correctly respond receive a free admission ticket to the next Pump School).
Next month we'll cover the fundamentals of rotary lobe pumps.
Pumps & Systems, May 2008