Asking the right questions

10 September 2023

When it comes to pumps for food processing, the handling of solids is by no means an unusual request. But with one solid being very different to another, it means that many factors need to be involved in specification beyond the usual flow rate, pressure, viscosity, and temperature requirements. Castle Pumps offers some thoughts.

Put simply, the same pump should not be recommended to transfer stones in olive oil as for cooked vegetables in soups. So, what questions should your pump supplier be asking when you require a new process pump?

Size of solids: There are certain pumping technologies that are not suitable for handling even the smallest of particles. Due to the tight tolerances between the casing and the moving parts, vane, gear and side channel pumps will jam when attempting to pass solids, causing immediate wear. Magnetic drive pumps are also generally unsuitable due to the tolerances between the magnets and casing.

Peristaltic and diaphragm pumps on the other hand are able to handle solids up to the size of their inlet/outlet. For that reason, sometimes a larger pump than is needed duty-wise is specified to handle the solid size but run at a lower speed. At higher speeds and greater differential pressure, wear from abrasive particles can be considerably higher. Therefore, whilst the initial outlay may be more expensive, running an oversized pump at a lower rpm can also help reduce maintenance costs over the pump's lifespan.

Type of solid: Are they hard and abrasive or are they soft and malleable? Are they thin and stringy, susceptible to clogging?

Peristaltic pumps have no moving parts that encounter the fluid and therefore have no valves, seals or impellers to clog or erode. As a result, their solid handling capabilities are only limited by the size of the internal hose, making them another a good choice for both highly abrasive and clogging solids. Whereas other pumping technologies could find that stringy solids, such as fruit juice pulp, wrap around their impellers.

For softer solids, flexible impeller pumps are often lower cost solution, thanks to their supple, rubber impellers being able to bend around present particles.

Should solids pass in their current state? If you are pumping a yoghurt that has fruit pieces inside, those fruit pieces need to pass through the pump whole. Certain pump technologies, such as centrifugal pumps, apply high shear due to their close tolerances, meaning that hose fruit pieces will end up crushed and turned into slurry. Low shear pumps, however, such as peristaltic, diaphragm or flexible impeller pumps, can handle solids without changing their structure due to their low internal velocity.

At the other end of the spectrum is food waste, where it may actually be beneficial for the pump to chop up the solid content to make it easier to deal with further down the line. In such applications, pumps can be supplied with grinder or cutter impellers, that chop up the solid as it passes through the pump into smaller, more manageable pieces.

Alternatively, there may have be a scenario where you don’t want the solids to remain in the pumped product such as seeds in fruit juice. Where this is the case, it may make more sense to take the responsibility away from the pump and instead add a strainer onto the suction side, ensuring large particles are caught in this before entering the pump’s inlet.

Type of fluid: The presence of solids within a thin viscous liquid, such as sugar granules in water, is much more abrasive than the same size or even larger solids, in a higher viscosity, lubricating fluid such as oil.

The continuous cavities that exist between the rotor and stator of the progressive cavity design and the slight flexibility of the stator means that large solids and thick liquids such as mince beef in Bolognese sauce can be handled with ease. However, when an abrasive, thin, water-based liquid is pumped, wear can occur much more prematurely than a peristaltic pump with limited internal components.