What is the tip speed of a slurry pump?

In this article, we will learn about the Centrifugal Pump Impeller Tip Speed ' Calculation, Significant, Limitation.

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The impeller tip speed plays a vital role in the centrifugal pump handling liquid containing solid particles. Engineers to take extreme care to check the impeller tip speed during the engineering phase. The improper selection may lead to the continuous problem and premature failure of the pump during the normal running condition.

What is impeller Tip Speed?

Impeller Tip speed is used to define as the velocity produce at the impeller tip. It is due to the rotation and diameter of the impeller.

For multi-stage pumps tip speed will be a measure only for the first stage impeller.

Calculation of Tip Speed

The equation of impeller tip speed is

Where

u' Impeller tip speed (ft/s)

D '  Impeller outside diameter (ft)

N ' Revolution of the impeller (rpm)

In the SI unit, the above equation will be written as

Where

u' Impeller tip speed (m/s)

D '  Impeller outside diameter (m)

N ' Revolution of the impeller (rpm)

Significant of Tip Speed

Impeller tip speed is an essential parameter when the pumping liquid contains solid particles. When the tip speed increases the rate of erosion will be high and it will lead to premature failure of the pump.

But in the case of Inducer, the tip speed plays a vital role in occurring cavitation damage. At lower tip speeds, cavitation will not produce any damage regardless of Specific speed value. Refer below equation of suction specific speed of inducer.

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Where

K ' Constant

u' Impeller tip speed (ft/s)

SG ' Specific Gravity

From the above equation, lowering of liquid specific gravity also leads to an increase in the tip speed of the impeller. One more point we need to remember is that the allowable tip speed is also a function of inducer materials. If we using higher erosion resistance material, then the tip speed can increase. For example, the titanium alloy is 20% higher erosion resistance than the stainless steel. Due to this mechanical property titanium alloy will replace the stainless steel for some particular cases. Stainless steel is widely used as an inducer material.

Tip Speed Limitation

As per hydraulic institution, Impeller tip speed as distinct from rotational speed is always using as a guide for wear in the selection of slurry pumps:

• For dirty water-type applications, limited to 40 m/s (130 ft/sec)
• For medium slurries, up to 25% solids concentration by weight and mean solids size of 200 microns, limited to 35 m/s (115 ft/sec)
• Slurries with higher concentrations of solids and much larger solids size restricted to 30 m/s (100 ft/sec)
• Pumps with elastomeric impellers are commonly limited to 26 m/s (85 ft/sec).

In the case of Impeller and casing using elastomers such as rubber, neoprene, and urethane will tend to limit the impeller tip speeds less than about 85 ft/s (26m/s). Although this can rise with stiffer elastomers, at some expense to wear life. But for the impellers employing elastomers require higher available NPSH because of the thicker impeller vane sections needed, and this condition may limit their use, for example in pumping flue-gas desulphurization slurries.

It's no easy task to size and select a slurry pump. Along with considering pump hydraulic and mechanical characteristics (which we won't cover in this post, but would be happy to help you with), it has become vital to consider the impeller tip (or peripheral) speed, the impellers revolutions per minute (RPM) and how these elements have an effect on the wear of your slurry pump.  We offer a few suggestions here on how to gauge a pump's wear rate, based on tip speed, RPM and impeller size.

Slurry Pump Impeller Tip Speed and Pump Wear

One criterion that is commonly used to indicate you've selected the correct slurry pump is if the impeller tip (or peripheral) speed is below the maximum allowable tip speed, which is based on the class of slurry being pumped, and the impeller material.

It is definitely important to note that a pump's wear rate is associated with the impeller tip speed, but that it's more accurately connected with the impeller revolutions per minute (rpm).

Here's a basic formula to help remember the relationship between a pumps wear rate and the impeller rpm:  

Pump Wear Rate ' rpm3

Case Study: Different Sized Impellers with Identical Tip Speeds with Different Resistance to Wear

To illustrate this a little further, we might look at two different sized impellers: one is Ø12' and the other is Ø24'. Let's say they have identical tip speeds, but as you will discover drastically different resistance to wear when pumping abrasive slurry.

Here's how it works (again using a simple formula):

Impeller Tip Speed (ft/s or m/s)=πDN/60

D=Impeller Ø (ft or m)                  N = RPM

Looking at this example, you can hopefully see that'according to the Pump Wear Rate formula I included above'the Ø24' impeller that's operating at rpm will last about eight times longer when compared to the Ø12 impeller operating at rpm, even though the two impellers have identical tip speeds.

Clearly, from this example, the second (Ø24') impeller is one to go with.  So we can come to the following conclusion:

Higher RPM = Lower Slurry Pump Life

If you would like to learn more about Tip Speed and Slurry Pump Selection you can download our Impeller Pumps Reference Guide and our free iPhone App.

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