Advice on selecting an abrasion resistant pump

06 June 2021

Castle Pumps offers advice on making the right pump choice to handle food products with solid content.

The pumping of fluids containing solids is quite a common requirement within food processing applications. The nature of the solid content however can differ widely – ranging from highly abrasive stones in olive oil; clogging solids such as fruit juice pulp; or soft and malleable solids such as cooked vegetables in soups.

When it comes to pump selection, there are two key considerations if solids are involved. The first is ensuring that the presence of solids does not damage the pump internals. The second is to ensure that the pump itself does not damage the solids as they pass through, as in many applications, the solids are an important element of the pumped product.

Making a choice

As you would expect, the hardness, the size and the quantity of solids being pumped will all impact how abrasive the fluid is. There are certain pumping technologies which are not suitable for handling even the smallest of particles. For example, due to the tight tolerances between the casing and the moving parts, vane, gear and centrifugal pumps with closed impellers 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.

So, what pumps are able to handle solids without damage? The continuous cavities that exist between the rotor and stator of the progressive cavity design, along with the slight flexibility of the stator, means that even large solids and thick liquids can be handled with ease. However, the complexity of the design means that maintaining them can be more costly due to the presence of more wearing parts. At the other end of the spectrum are peristaltic pumps. These 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 limited only by the size of the internal hose, making them another ideal choice for abrasive and clogging solids.

Diaphragm pumps offer another good option as they can pass solids almost up to the size of their inlet/outlet. For solids, in less heavy duty, lower pressure applications, flexible impeller pumps can offer a lower cost solution, thanks to their supple, rubber impellers being able to bend around the particles present in the fluid.

Shear sensitivity

Another factor to consider when selecting a transfer pump for products containing solid particles is whether it is important that the particles remain intact throughout the handling process. This is often the case in the food processing industry. For example, when pumping a yoghurt that has fruit pieces inside, it is vital that the fruit pieces pass through the pump whole. Certain pump technologies – such as centrifugal pumps – will apply high shear, due to their close tolerances, meaning that those fruit pieces are likely to end up crushed and turned into slurry. Low shear pumps on the other hand, such as peristaltic pumps 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 the handling of 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 these instances, abrasion resistant 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.

Does size matter?

As discussed above, peristaltic and diaphragm pumps, for example, 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.

Of course, when specifying a pump for handling solid content, there will be many other considerations in addition to the details of the solids – for example flow rate, pressure, viscosity and suction conditions – however, these factors can be given greater focus once the best type of pump to handle a particular product, is understood.