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