Click here for "Focus on Fundamentals" Part Two, Part Three, Part Four and Part Five.
This month, we begin a five-part series called "Focus on Fundamentals." These back-to-basic tutorials explore different pump types, including various centrifugal and rotary pumps. Part One focuses on centrifugal pumps with an overhung impeller.
The heart of any centrifugal pump is the impeller. Fluid enters the impeller through the "eye" and is "centrifuged" (hence the name) to the impeller periphery, with assistance from the impeller vanes. Impeller designs can be open or closed, with one or many vanes, or no vanes at all (a disc or a set of discs-or a variation of a disc-like surface-sloping toward the higher radius).
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The impeller is enclosed into a casing. The geometry of the throat, a part of the casing, receives fluid exiting the impeller and determines the most optimum operating flow, or best efficiency point (BEP). However, a pump only operates at this BEP flow if the system in which it is installed properly matches this condition. Otherwise, a pump may operate at off-BEP condition, at the point where a pump performance curve intersects with a system curve.
Another part of the casing guides the incoming flow to the impeller eye. A straight shot-no turns and bends-is the most efficient way to bring the flow to the impeller, and such designs are called end-suction. In the back, a mechanical seal or a set of packings separates the fluid from leaking out. Following the seal, two bearings support the entire rotor (impeller, shaft, sleeves).
For end-suction designs, the impeller is cantilevered against the inner bearing. On the positive side, there is no restriction to incoming flow, which would be caused by placing one of the bearings at the front side-also called a between-bearing design (covered in Part Two). Cantilevering the impeller against the inner bearing also helps efficiency and makes the design simple and less expensive (with only one seal).
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However, a long cantilevered rotor is prone to deflections, which may overload the bearings and cause seal failures. For this reason, rotor stiffness is an important factor, and designs with bigger shafts (D) and lower overhung length (L) tend to be more reliable. The factor L3/D4, a coefficient of proportionality between force and deflection, is a good measure of comparison between similar designs. The lower the L3/D4, the more robust the rotor-which results in better resistance to deflections.
Only end-suction pumps are covered by dimensional interchangeability specification, but not all of them are covered. In the chemical industry, plants tend to have a great number of similar pumps, and interchangeability and reduction of spare parts are important considerations. ANSI B73.1, which covers such end-suction pumps, requires that all major outside dimensions are the same regardless of the manufacturer.
For refineries, API-610 covers centrifugal pumps, and includes end suction, double suction and other types. The main focus of the API-610 is reliability, not dimensional interchangeability. This does not mean that ANSI pumps are unreliable, but rather API-610 pumps have an added aspect of robustness, due to the high temperatures and critical nature of their service.
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API-610 end suction pumps are centerline-mounted (as compared to foot-mounted ANSI pumps), which has a better tendency to keep the centerline from growing with temperature (the casing expands uniformly up and down in relation to centered feet) and limit deflections and stresses on bearings and seals.
Like other pump types, end suction pumps are available in a wide variety of materials of construction, sealing (or sealless) arrangements and impeller design specifics to properly match applications.
Next month we will cover the impeller-between-bearings design.
Pumps & Systems, April 2008
