A Flexible Impeller pump or Flexible Vane pump is a type of Positive Displacement pump that moves fluid from the suction inlet by essentially trapping a specific amount of the fluid and then moving it around and the pump head discharging it.  

Although the impeller is round on our flexible impeller pumps, the body is not.  There’s a flat spot in one section of the pump head that forces the impeller’s vanes to compress and relax as they pass over the area.  This compression / expansion cycle happens hundreds of times or more per minute.  The movement creates a seal and chamber effect between the vanes, pulling fluid from the inlet into the suction area, and forcing it towards the outlet / discharge port within the pump.

Some industries that would benefit from using a flexible impeller pump are wineries, food processing, cosmetics and pharmaceuticals.

Benefits

Higher level of suspended solids

A positive displacement pump is best used for transferring liquids with a high concentration of solids. While centrifugal pumps struggle to pump viscous liquids, positive displacement pump efficiency increases with increasing viscosity.  This is due to there being less “slip” with thicker liquids as the liquid doesn’t easily slip through the gaps and clearances near the rotors. This is particularly useful for grain or mash suspended in wort, grape stems and skins, and syrups.

Reversible

A unique feature of flexible impeller pumps is that they are reversible, meaning that the pump can control the direction that the fluid moves.  This is particularly useful for enabling multiple applications to be performed by one pump, such as tank loading and unloading, returning excess liquid sitting in the pipework to the container, and enabling the running through of cleaning fluid after use for Clean-in-Place.  This feature not only saves time and effort but also reduces the need for additional equipment or manual intervention.

Self-priming

Flexible Impeller pumps are self-priming, providing flexibility to the system’s layout. These pumps are capable of suctioning from a maximum height of 5 meters (15 feet). The pumps have small clearances that create an internal vacuum within each chamber formed between the vanes that will expel air through the pump until the liquid reaches it. This is especially useful when pumping out of barrels, totes or kettles that don’t have a bottom port. 

The tight fit of the rubber impeller allows the pump to better handle what are called “air pockets.”  Air pockets are an accumulation of air bubbles in the pump's working mechanism, which can impair the pump’s proper operation, however the FIP pump passes the air pockets right through it with little or no effect.

Again this is particularly useful for removal of fluid from sumps and the draining of tanks, drums, and IBCs.

 Low and high viscosity

When lower flow is required, a centrifugal pump can run off its best efficiency point, which increases energy usage, negatively impacts performance, and can potentially damage the pump. With a positive displacement pump, you have the ability to pump products with both low and high viscosity at different speeds. They are able to maintain their efficiency and priming ability even at low motor speeds. This means that shear sensitive fluids can be handled without any damage caused to their structure. This makes the positive displacement pumps a better choice for pressure and flow when operating a pump off its best efficiency point.

Non-pulsating

The working principle of a flexible impeller pump is that it benefits from a steady pumping mechanism, unlike some other positive displacement pumps,  that results in the flow output being more gentle and smooth on your product.  It's even flow rate makes it ideal for filling, transfer and accurate dosing.

Things to consider

There are a few things to consider before getting a flexible impeller pump:

Firstly, you cannot run a flexible impeller pump dry. The impeller is made of synthetic rubber and the head is cooled and lubricated by the liquid being pumped so leaving the pump dry running  it will burn / melt your seal and impeller.

Lastly, flexible impeller pumps produce lower pressure and flow rates than centrifugal pumps.  Since the impeller always stays in contact with the inner walls of the pump, the pumps must operate at lower speeds, typically between 900 and 1500 rpm. This results in lower volumes and pressures than a similarly sized centrifugal pump.