Viewpoint: pick the right pumps to handle tricky liquids

There are all kinds of issues to consider when ensuring a product is pumped safely and efficiently without destroying the fluid or the pump.

A liquid that is either viscous, volatile, corrosive or aggressive/abrasive – or a combination of any of these factors – presents the user with some challenges.

In order to understand the pumping challenges, it is necessary look more closely at the rheology of the fluid:

Viscous fluids fall into five main groups:

Newtonian: water, alcohol, oil – liquids that don’t change by shearing

Dilatant: butter, cream – liquids that thicken by shearing

Thixotropic: paint, soaps, grease – liquids that are shear thinning

Bingham plastic: mayonnaise, toothpaste, ink – liquids with a yield point prior to thinning

Super viscous: pitch, tar, asphalt, bitumen – excessive shear or heat is applied to move the liquids.

Pumping viscous fluids

Fluids with a kinematic viscosity of up to 200 cSt can be pumped with a centrifugal pump. However, as the viscosity increases, positive displacement (PD) pumps are required.

The flow rate in a centrifugal pump reduces as the viscosity increases, whereas in a PD pump it increases through higher volumetric efficiency.

There are many different types of PD pumps, depending on what liquids are being handled:

Internal gear pumps: oils, resins, chocolate, bitumen with fillers (materials that are high viscosity or shear sensitive)

External gear pumps: thin or viscous clean liquids (good for metering)

Lobe pumps: viscous, shear-sensitive or fragile solids (food grade)

Diaphragm: wide range of liquids but not the panacea

Peristaltic hose pumps: wide range of liquids; will handle slurries, viscous, shear-sensitive and aggressive fluids

Flexible impeller pumps: mainly thin liquids or liquids containing some solids

Vane pumps: good for both thin and high viscosity clean fluids – no solids

Progressive cavity pumps: wide variety of thin or thick fluids containing solids.

Volatile fluids

These are fluids that generally evaporate at room temperature and are therefore highly flammable. As a rule, containment is the major issue when pumping such liquids, and this is where the ATEX directive (from the French ATmosphères EXplosibles) covers the type of products used in potentially explosive environments.

The selection of construction material,motor type (flameproof ) and sealing system form the basis of any pump selection.

Corrosive fluids

Corrosive or hazardous fluids and acids or bases (caustics or alkalis) directly attack pump materials with the rate of corrosion depending on the concentration and temperature of the fluid. To counter these effects, material compatibility is vital.

Materials as simple as stainless steel may suit one product but could be totally unsuitable for another that requires a non-metallic or thermoplastic solution.

Aggressive fluids

Handling aggressive and abrasive fluids is a difficult application for any pump because the particles promote material wear.

As a general rule of thumb, larger pumps running slowly give longer lifecycles and prove to be the cheapest option in the long term. There are two ways of handling abrasion from a material point of view; either by utilising a hard material that resists wear or using a soft lining that is sacrificial and can be renewed.

So what are the basic rules?

Give your supplier as much detail about the fluids that you intend to pump: flow, head and suction conditions, the location of the pump, details of the pipe run and material, filters, meters, valves, and the chemical name of the fluid or mix of products.

A competent pump supplier should be able to offer more than one solution and be able to explain the benefits linked to them. And always ask about previous experience in handling similar fluids.

* Peter Staddon is the managing director of The Pump Company