What exactly is an impeller&#;s &#;tip speed&#; and how do you calculate it?
If you are looking for more details, kindly visit tsp vertical slurry pumps solutions.
Establishing an impeller&#;s &#;Tip Speed&#; is very useful when selecting the best pump for your application, but what exactly is tip speed and how do you calculate it?
The tip speed is simply the distance that any selected point on the peripheral of the impeller travels in a set time. In other words the &#;speed&#; of that point. To calculate tip speed you simply multiply the diameter of the impeller by pi (3.) which gives you the circumference of the impeller at the outermost tip. You then multiply by the rotational speed of the impeller (usually rpm, or rps) and this result is tip speed. The units for tip speed depend on the type of linear units and time units used, but when applied to impellers the common units are feet/min or meters/ sec.
Let&#;s use an example of a pump with a 24-inch impeller that was running at 800 rpm. First, convert the 24 inches to 2 ft by dividing by 12 inches per foot. Then multiply by 3. to give you ft per revolution and then by a speed of 800 revolutions per minute.
Now that we know how to calculate tip speed, the question becomes what good is it?? Well, it can be used to;
How tip speed relates to wear is fairly obvious. The faster the tip of the impeller travels through a liquid or slurry, the greater the impact on the impeller and the faster it will wear. In short, when selecting a pump for longevity opt for a low tip speed.
The second item above is based on the premise that the strength of any given material can be exceeded if rotated too quickly. Just like a child may be thrown off a merry-go-round spinning too fast an impeller can suffer a similar fate. In the case of an impeller, if the strength of the material or coating is exceeded then the item thrown off may be an impeller vane, just as the impeller disintegrates. Below is a chart of suggested max tip speeds for some common materials used in pumps.
The speeds listed above are guidelines for maximum tip speeds and may need to be de-rated if applied to a service containing large solids. ( An impeller rotating at relatively high speed when struck by a solid, such as rock, may fail at lower than the indicated speeds due to the additional stress created by the impact of said solid.)
When comparing pumps for a particular application, tip speed is clearly not the only factor to consider. It is however an important factor, especially when dealing with slurry. For more information or help when selecting pumps, please feel free to contact Toyo&#;s application team at 604-298- or at .
Bye for now
RJ
(Mechanical)
(OP)
23 May 12 11:42Which is the maximum allowable tip speed for a radial/semiaxial pump?
Fluid: Water or Sewage
(Civil/Environmental)
23 May 12 12:04
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.
(Mechanical)
23 May 12 13:26For 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.")
(Civil/Environmental)
23 May 12 14:01Grey cast iron is good for 130 ft/s. The maximum tip speed is limited by erosion from solids not materials of construction.Why are you asking about tip speed?
(Mechanical)
23 May 12 15:33Ok... By that logic, what pressure is grey cast iron good for?
(Civil/Environmental)
23 May 12 16:06Here is the reference
Contact us to discuss your requirements of ahr rubber slurry pump replacement factory. Our experienced sales team can help you identify the options that best suit your needs.
(Mechanical)
23 May 12 18:15Your 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.
(Mechanical)
23 May 12 19:57Generally for solids bearing fluids I would not go over 25m/s.BHRA give wear rates as wear = velocity^2.5-4.5 for slurries.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
(Civil/Environmental)
24 May 12 00:37Enough with the shaggy dog story.The posted question was "Which is the maximum allowable tip speed for a radial/semiaxial pump?"Here is a reference for 112 ft/sec tip speed:Centrifugal Pumps by Johann Gulich
(Mechanical)
(OP)
24 May 12 03:17Normally 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?
(Civil/Environmental)
24 May 12 08:14To 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.
(Mechanical)
24 May 12 19:11bimr,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.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
(Mechanical)
26 May 12 06:11stainer
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.
(Mechanical)
27 May 12 20:50Pumpsonly,Must have been having a senior moment there.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.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
(Mechanical)
28 May 12 11:53Slurry 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.
Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.
Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.
Click Here to join Eng-Tips and talk with other members! Already a Member? Login
If you want to learn more, please visit our website ahr rubber slurry pump equivalent factory.