What exactly is an impeller’s “tip speed” and how do you calculate it?
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 (3.14159) 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 3.14159 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;
- Compare pumps with regard to expected wear
- Ensure that the pump’s rotational speed does not exceed the limits of the type of material used in the impeller.
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.
- Spheroidal graphite cast iron EN-JSlO3O EN.GJS-4OO-15…. 50 m/s ( 9840 ft/min)
- Stainless steel CA 15 1.4008 GXTCrNiMol2-l…. 95 m/s (18696 ft/min)
- Stainless steels A743 Grade CA-6NM 1.4317 GX4CrNil3-4…. 110m/s (21648 ft/min)
- Bronze and brass 2.1050 G-CuSn l0….50m/s ( 9840 ft/min)
- Gray cast iron EN-JLlO4O EN-GJL-250…40m/s (7872 ft/min)
- White iron ASTM 532 class 3 Type A… 42 m/s(8200 ft/min)
- Wear resistant soft natural rubber: 25.0 m/sec (4920 ft/min)
- Typical natural rubber: 27.5 m/sec (5410 ft/min)
- Anti-thermal breakdown rubber 30.0 m/sec (5900 ft/min)
- Nitrile 27.0 m/sec (5310 ft/min)
- Neoprene 27.5 m/sec (5410 ft/min)
- Butyl and Hypalon 30.0 m/sec (5900 ft/min)
- Polyurethane 30.0 m/sec (5900 ft/min)
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.)
Suggested maximum operating values for acceptable wear
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 604-298-1213 or email at solutions@hevvypumps.com.
Bye for now
RJ
