Is there an ideal impeller vane number?
Randal FermanCentrifugal pump design and manufacturing has been around for over 200 years. If there were an ideal impeller vane number, it seems likely someone would have discovered it by now. However, just because an ideal vane number has not yet been established is not sufficient reason to consider it doesn’t exist.
Single stage pump and first stage, high suction performance impellers tend to have fewer vanes – often 3, 4 or 5. Multistage pump series stages, pipeline pumps and large, high efficiency pumps tend to have 7 or more impeller vanes. Vane number might be dictated by special application requirements. Such is the case with solids handling impellers which often have just 2 or 3 vanes and sometimes only 1.
I recall an anecdote I heard decades ago from a senior hydraulic engineer that only 3-vane impellers are found across the entire range of specific speeds. The problem, especially for high speed, high head pumps, is a 3-vane impeller produces significantly less head than, say, a 7-vane impeller. Head rise to shutoff and pressure pulsations are also issues.
But thinking about this quasi-factual anecdote, I’ve occasionally considered that an ideal vane number does indeed exist. If there is a magic vane number, what is it? If there could be only one impeller vane number for all pumps, what would it be? Pump engineers think these kinds of thoughts.
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37 comments
sir, can we increase the pump’s discharge head by increasing vane numbers?
Jugal, Yes, it is possible to achieve greater than 90% pump efficiency. In fact, both theoretical evaluations and actual experimental results indicate pump efficiencies well into the mid-ninety percent range are attainable. There are many different pump designs and a wide range of sizes to meet requirements for an endless variety of applications. Pumps tend to be optimized for several competing design constraints and while efficiency is important, it is not always the primary design parameter of interest. In fact, for industrial applications, reliability and safety are often top priority. The highest theoretical efficiency is actually seldom achieved given the range of design criteria and practical factors that determine the outcome in a given pump product. Best holiday wishes, Randal
how to increase centrifugal pump efficiency more than 90%? is it possible or not?
Avijit,
This is an excellent question!
A rotating impeller produces a circumferentially varying pressure field, between vanes, at its exit. The periodic pressure fluctuations interact with the stator, usually a volute or a diffuser. This rotor/stator interaction (RSI) can combine and amplify pressure pulsations and pump or piping vibrations.
When an even vane number impeller is paired with a double volute, vane pass pressure pulsations can reinforce each other. Volute type pumps, except for solids handling designs and small volute sizes, are often a double volute configuration. While even vane number impellers should generally be avoided in double volute pumps, proven designs with these do exist. Attention given to the impeller-to-volute clearance Gap ‘B’, shroud radial clearance Gap ‘A’, and the shroud sidewall gaps can help mitigate pressure pulsations and RSI troubles. There are many applications where the pump energy levels are relatively low, for instance, not greater than 200 meters head (650 feet) or 225 kW (300 Hp) per stage as per API 610, and the incidence of RSI and pressure pulsation problems is rare.
In high head-per-stage machines, even or odd vane number impellers can be paired with appropriate diffuser vane numbers with due attention given to the periodicity of the RSI as described texts such as Karassik, et al, ‘Pump Handbook Fourth Edition’ and Gülich, ‘Centrifugal Pumps Second Edition’.
In lower head machines, especially those less than 30 meters (100 feet), RSI problems due to a vane number combination are all but non-existent.
Pump hydraulic designers are aware of vane pass pressure pulsation and RSI issues. Most newer designs for double volute pumps, which comprise a substantial share of total pump production, will not have even vane number impellers. Additionally I suspect this tends to create a bias against designing with even impeller vane number impellers except when they are essential for performance purposes.
Thank you for posting this one, Randal
why no. of vane are kept in odd no of centrifugal pump in most of the case?