Taking A Look At The Specificities Of A Cantilever Agitator Pump

Our last discussion centered around the definition of a cantilever pump and how shaft deflection issues can determine maximum setting length. Today, I would like to focus on agitator pumps and leave non-agitator pumps for another day. 

Agitator wet end ​
(Drive assembly not shown)​

For those readers who are unfamiliar with the term “agitator pump”, it is simply a pump design where the main pump shaft extends past the impeller hub so an agitator can be mounted just in front of the suction inlet. The agitators is there to resuspend solids so they can be pumped along with the surrounding liquid.

Agitator pumps are generally associated with solids suspension and pumping slurry. Within that definition, cantilever agitator pumps are used anywhere a vertical pump, with a setting length of approximately. 7 ft or shorter is required. This covers a wide range of applications, but by far and away the most numerous is a collection sump. Prior to the advent of cantilever pumps, vertical column pumps or submersible pumps were the only choice for sumps. 

Submerged Bearings and Seals​

Conventional grease or oil-lubricated bearings must be isolated from water and solid contamination. This means that the bearings used to support the rotating elements in conventionalvertical or submersible pumps must have seals to protect them from contamination. Submergedseals have a finite operating life, especially in slurry applications. When seals fail users are subject to downtime and expensive repairs. Cantilever pumps require no submerged bearings and therefore require no submerged seals. This is a key feature of cantilever pumps!!!

Short Vertical Column Pumps​​

With cantilever agitator pumps being limited to settings in the area or 7 ft, it is only reasonable to compare them with vertical column pumps of similar setting length. The much smaller shafts and support structures of the column pump make them lighter. The cost savings in steel are somewhat offset by the extra cost of the seals in their wet end. However, any slight advantage either pump may have in capital cost is a non-issue, as capital cost is greatly overshadowed by the reliability and associated cost saving of the cantilever design. This, of course, stems from the cantilever design eliminating all submerged seals or bearings. As suc,h I will, for the balance of today, drop the vertical column pump as an alternative to a cantilever pump.​

​Motor Availability ​

Cantilever pumps have separate detachable motors. Generally speaking, the motor is an off-the-shelf standard “T” or “TC” frame. This allows for quick and easy re and re of the motor. It also may allow the user to increase the motor size if operating conditions call for more power. Any increase in motor size would, of course, have to be done within the original design restraints of the pump assembly. Submersibles have no such ability. ​

Variable Speed​

​Both submersible and direct drive cantilever pumps require a variable frequency drive to modify pump speed to adjust for any change in operating conditions. Cantilever pumps are often purposely built as a V-belt driven pump, knowing that speeds may easily be altered with a quick and inexpensive change of sheaves and belt. Advantage cantilever!

Repair Shops​

​As soon as the motor is separated from a cantilever, it becomes a purely mechanical device. As such, a standard maintenance department has no issues accepting it for repair. Submersibles, on the other hand, are often thought of as an electric motor and sent to the electric shop. The electric shop, however, sees it as a pump and refuses to work on it. This problem is generally resolved by sending the submersible off-site for repairs, both costly and consuming. Another point for cantilevers!​

​Repair Quality

Assuming the windings on a submersible are okay, it is possible for any maintenance shop to facilitate a repair. The personnel working on the pump, however, need to be the type of individual who takes pride and care with the repair. Mistakes as simple as a pinched o-ring will result in a complete failure when the pump is reinstalled. A cantilever pump, on the other hand, is a simple, robust piece of equipment that anyone can repair should the need arise.

Down time

Submersible pumps all have submerged seals, and often the first sign of seal failure in a submersible is the pump’s electrics tripping out as water reaches the motor windings. This failure, happening with no warning, can lead to a facility scrambling to counter rising water levels. Conversely, most cantilever failures/problems occur over time and, as such, can be scheduled for maintenance as time allows ​

​Capital Investment​

​Submersible pumps are clearly an alternative to cantilever pumps, and in small horsepower applications, the low initial cost can make them quite attractive. This advantage, howeverdisappears as the size of the unit increases. Somewhere around 10 hp, a crossover happensand a cantilever unit requires less initial investment than an equivalent submersible agitator slurry pump.​

​Adaptability

The ease with which cantilever pumps can be adapted to meet different conditions (with belt drives) allows one pump size to be used in different applications requiring different flows and heads. This standardization minimizes spare pumps and or parts inventory, and allows the pump manufacturer to batch manufacture and pass on the savings in the form of lower pump prices.

Summary

Today I think we have shown a pretty strong case for opting to a vertical cantilever pump over a submersible. That is not to say that all short setting sump applications must be cantilevers, but it is probably fair to say that cantilevers would be a good choice in many cases. Before leaving today… I am sure someone will ask, “Well what about deep sumps??” Stay tuned, next time we will answer that question.​​​​

​Cheers

RJ