The maximum impeller tip speed is based on tests that were done by the Hydraulic Institute www. pumps.org. You can find more information in the Hydraulic Institute's Centrifugal Pumps for Design and Application ANSI/HI 1.3 standard.
The recommendation on the maximum tip speed for dirty water:
-- 130 ft/s
Higher tip speeds are likely to cause excessive erosion.
For an enclosed impeller, that is a realistic limit.
For a semi-enclosed (1 shroud) or open (no shroud) impeller, the limit is based on the material strength, and the geometry: height and thickness of the vanes. To reach 130 ft/s for a semi-enclosed impeller, you will need a material stronger than carbon steel in most cases.
I will add, 85 ft/s EYE tip speed is accepted limit for water. Any higher can cause suction problems, and is generally a good indication that the pump has not been selected correctly (running faster than it was ever intended because "the selection software said it was ok.")
Your reference has no context as far as the basis for the material limits listed.
Please note that for erosion of the impeller due to fluid velocity, the valid comparison is impeller tip speed to fluid velocity, this varies with flowrate. It is related to, but not the same as, the impeller tip speed, and while the impeller tip speed is a good guideline, it is not the only basis for the 130 ft/s rule.
I stated that for certain impeller configurations, allowable material stress will be reached at tip speeds less than 130 ft/s. You appear to disagree.
The problem here, is you are implying that erosion is the one and only mechanical design requirement for selection of impeller material, which is naive at best. If you ignore the various dynamic forces that an impeller experiences, I'm afraid you will not do very well designing (or commenting on the design of) centrifugal pumps.
If you were to produce a propeller type (axial flow) pump with a cast iron impeller, and operate it at vane tip speeds of 130 ft/s, I guarantee you that the vanes would fly off well before you had to worry about erosion.
Normally I fix RPM taking into acount hydraulic cgaracteristics like good suction perfonmance (low NPShR) but normaly I do not consider outlet maximum tip velocity when submergence requirement is good enough.
I mean. For all your notes, maximum allowable tip velocity is related more with pump durability in terms of selected materials etc...
Am I correct?
To properly select a pump, the pump supplier should place his pump curve on top of your system curve and the required operating window should fall within the pump's operating window on either side of the best efficiency point.
Whenever possible the lowest centrifugal pump speed should be selected, as this will save wear and tear on the rotating parts. The advantages of a higher speed pump are 1) a reduction in pump casing and motor frame size, resulting in reduced capital cost; and 2) an increase in pump efficiency, reducing annual electrical operating costs. The increase in pump speed however must be balanced with consideration of the potential abrasive wear caused by solids content. However, over the entire service life, the lower speed pump should have the lowest cost.
NPSH is not normally a problem with radial/semiaxial sewage pumps because of the arrangement of the pump installation. The pump is installed such that the suction lift is generally minimal. However, NPSHa and NPSHr should always be checked in design. Axial flow impellers (high specific speed numbers) run at the highest efficiency and have the lowest NPSH requirements.
For sewage pumps, maximum allowable tip velocity is associated with pump durability because of the potential for abrasive wear caused by the solids in the sewage.
However if pumping solids and a 2 pole speed is selected you could still end up with two higher tip speed. In addition slurry pumps are oftern vee belt driven so tip speed again is variable.
Sewage could contain gravel and other wearing solids. Most authorities specify pumps should be 4 or 6 pole speed. This results in a larger impeller but lower tip speed.
Wear is dependent upon solids Cw%, Cv%, density of slurry, density of solids, particle size distribution, pH, temperature, particle angularity, hardness and sharpness. Now this may not be applicable to sewage but needs to be considered when pumping solids.
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Impeller tip speed or peripheral speed is the product of rotational speed (RPM) and its diameter which decide the head generated by the impeller.
If you reduce the RPM and increase the impeller diameter to achieve the same head, you need the same tip speed.
Slurry pumps are selected generally to have a full size impeller and the speed is changed by Vee belt or VSD to get the derated duty point to meet the service.
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His Holiness the Dalai Lama.
Slurry pump impeller are made of wear resistant hard metal which are very difficult to machine. Hence generally supply in full cast diameter and using pulley or VSD to get the required head and flow.
