Tag Archive for: slurry pump impeller

I was recently asked to investigate a vibration in a vertical cantilever pump. The client had recently rebuilt the pump and was wondering what he had missed or done wrong. In the absence of any vibration monitoring equipment, we resorted to physical checks.

With the pump out of the sump, we carefully rotated the shaft and there was no evidence of any damage so we proceeded to remove the wet end. With the whole wet end off we again rotated the shaft assembly and it seemed solid and a check of the impeller journal confirmed only 2 thou run out, not the problem.

We checked the impeller and wear plates for witness marks but there was no indication of anything rubbing. During the inspection of the impeller, I did however notice an absence of any balance marks. As we were looking it also became clear it was not an OEM part. A local shop agreed to do a static check on the impeller for us and “bingo”, it was way out.

I suggested a “proper dynamic balancing” be performed. The client was unfamiliar with what I was suggesting so after an explanation and a few days with the impeller at a balancing shop the pump was reassembled and ran perfectly.

Today I thought I would share with you a summary of what I discussed with the client regarding the different types of balancing.

An imbalance in a rotating component is often the cause of excessive vibration within machinery.

This vibration can cause unacceptable levels of noise and more importantly will substantially reduce the life of shaft bearings. It would be ideal to remove all causes of vibration and for the assembly to run totally “smooth”. Unfortunately, in practice, the ideal cannot be achieved and whatever one does some inherent cause of vibration or unbalance will remain.

Hence, the best one can do is to reduce this unbalance to a level that will not adversely affect the bearing life and will reduce noise levels to an acceptable level. Some pump specifications call for all rotating components to be balanced. However due to the mass of an impeller, and the fact that it is a cast component, all specs should as a minimum require this component to have some type of balance performed.

Static Balancing

The simplest form of static balance involves placing the unit on low friction bearings and allowing it to rotate and “settle” with the “heaviest” point falling to the bottom. Material is then removed from this point (or added at the top point) and the unit gently rotated until, when stopping, the new “heavy” point again falls to the bottom. This process is then repeated until no obvious “heavy” point seems to exist.

The equipment required for a static balance is minimal and can be performed by staff with little to no training.

A simple variant of the static balancer was the bubble balancer, used in small tire shops up until about 1970.

Dynamic Balancing

If a component with an offset mass (out of balance) is rotated or spun, centrifugal force increases the effect of the offset mass. Therefore, when we spin a component that has an offset mass we can in effect amplify the readings and therefore identify very small amounts of imbalance. This allows for a finer balance.

A machine that has a bearing connected to sensors (displacement or acceleration sensors) can detect the “heavy” point, in relation to a datum on the unit, while it is being rotated.

With two sensors on a single bearing, a computer can calculate the location and magnitude of the imbalance.

It should be noted that it is not necessary to rotate the shaft at the final operating speed. It just has to be rotated fast enough to “exaggerate the imbalance”.

Single Plane Dynamic Balancing

The basic type of dynamic balancer can only measure the out-of-balance in a single plane. It, like the static balancer, measures the overall balance of the assembly. However, it cannot detect separate imbalances at each end that cancel each other out.

Objects such as pump shafts where there is a significant length to deal with may have an overall balance, but offsetting imbalances could still create loads that will be detrimental to bearings.

The figure to the left illustrates how an imbalance at one end of the shaft could be offset /balanced by an equal and opposite imbalance at the other end of the shaft.

The assembly has an overall balance, but the offsetting imbalances still create loads that will be detrimental to bearings.

Dual Plane Dynamic Balancing

A “Dual Plane” or “Multi-Plane“ dynamic balancer overcomes the shortcomings of the single plane balancer. These machines have two sets of sensors, one on each of the bearing pedestals. This allows the unbalance at each end (the two planes) to be independently identified, and thus corrected independently.

Electronics within the balancers control box can calculate the magnitude of the weights required at any two convenient positions along the length of the unit ( relative to the running bearings ).  For many types of rotating assemblies, these locations would be at the far ends.

In the case of very long units, more than two planes can be considered, although this is rarely an advantage unless the unit is to run at speeds above its first critical speed (“Super-Critical”). Similarly, the single plane is usually sufficient for very short components such as impellers.

ISO (International Standards Organization) has established various levels of balance and is often used to describe or specify a particular grade of balance. That however is a subject for another day.



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;

  1. Compare pumps with regard to expected wear
  2. 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

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