ARTICLE #45: Focus on Fundamentals: Centrifugal Pumps – Overhung Impeller
Dr. Lev Nelik, P.E.
A heart of any centrifugal pump is impeller. Fluid enters the impeller through the “eye” and is “centrifuged” (hence the name) to the impeller periphery, while also assisted/guided by the impeller vanes. Impeller designs can be open, closed, with one or many vanes, or with no vanes at all (a disc or a set of discs, or a variation of a disc-like surface, sloping to ward the higher radius).
Open, 6 vanes closed two discs
Impeller is enclosed into a casing. Geometry of the part of a casing (throat) that receives fluid exiting the impeller determines a most optimum operating flow, a so-called BEP (best efficiency point). However, a pump would only operate at this BEP flow if a 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 a system curve:
a. Pump performance curve b. System curve c. Pump operating points depends on a system
Another part of the casing guides the incoming flow to the impeller eye. A straight shot, with 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 impeller is cantilevered against the inner bearing, which is has pros and cons. On a positive side, there is no restriction to the incoming flow, which otherwise would be caused by placing one of the bearings at the front side, making such design a between-bearing design (we will cover it at another time). That helps efficiency. It also makes the design simple and less expensive (only one seal). 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. A factor L3 / D4 happens to be a coefficient of proportionality between force and deflection, and thus a good measure of comparison between similar designs. The lower the L3 / D4 the more robust is the rotor, resisting deflections better.
End-suction pumps are the only type that is covered by dimensional interchangeability specification, although not all of them. In the chemical industry, plants tend to have a great number of similar pumps, and interchangeability, reduction of spare parts is important considerations. ANSI B73.1 covers such end suction pumps, and basically requires that all major outside dimensions are the same regardless of who makes the pump:
For refineries, another specification (API-610) covers centrifugal pumps, and includes end suction, double suction, and other types. The main focus of the API-610 is not dimensional interchangeability, but rather reliability. That is not to say that ANSI pumps are not reliable, but rather that API-610 have an added aspect of robustness, due to high temperatures, and critical nature of service they typically see:
An API-610 end suction pumps are centerline-mounted (as compared to foot-mounted ANSI pumps), which would have a better tendency to keep centerline from growing with temperature (casing expands uniformly up and down in relation to centered feet), and limiting such growth limits deflections and stresses on bearings and seals.
Like other pump types, end suction pumps come in wide variety of materials of construction, sealing (or seal-less) arrangements, and impeller design specifics, to properly match applications.
To learn more, sign up for one of the upcoming Pump School 2-day sessions, schedule of which is posted at Pumps & Systems News section, or at www.pumpingmachinery.com/pump_school/pump_school.htm.
Dr. Lev Nelik, P.E., APICS
Pumping Machinery, LLC
Pump School Training Services