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Useful information on pumping abrasive liquid

What is an Abrasive Liquid?

An abrasive liquid is one that causes increased wear to a pump either by eroding surfaces through physical force or by chemical reaction.

Physical erosion arises from the action of entrained solids carried by and within the pumped fluid. Many pumped liquids contain solid matter either as contaminants (such as lime-scale), as components of a slurry (sewage treatment) or in a suspension for transfer or transportation purposes (mining and paint applications).

Chemical effects are the result of reactions between the pumped medium and the components of the pump. The simplest example is that of water, which will cause iron or steel components to rust. In many cases, both physical and chemical processes will occur simultaneously. For example, solids carried in water will abrade corroded metallic surfaces within a pump, exposing metal surfaces for further corrosion.

Although chemical processes must be taken into account, an abrasive liquid is considered to be one carrying a finely divided, refractory material such as lime-scale, clay, paint pigments, carbon, or metal fines. The main concern regarding pump selection is identifying a system suitable for transporting the medium efficiently while minimizing pump wear, maintenance costs and downtime.

The wear experienced by a pump is determined by the characteristics of the solids, their concentration, the viscosity of the liquid, the materials used in the construction of the pump, and the operating conditions – in particular the pump speed and pressure.

Which Solids Cause Abrasion?

Clearly, the harder the solid, the greater the risk of pump wear. The hardness or scratch resistance of solids is measured using the Mohs scale. This ranges from 1 (talc) to 10 (diamond). For example, aluminium has a rating of 2.5, iron 4.5 and hardened steel 8. Abrasion and pump wear becomes a serious problem with solids greater than 6 on the Mohs hardness scale.

In addition to hardness, the shape and size of entrained solids are important factors in determining wear. When the diameter of the solids exceeds the clearances within the pump, extreme wear can occur even at low concentrations.

If the fluid contains a high percentage of solids, a pump will generally experience greater wear. However, particle size and hardness play a more important role. For example, iron oxide slurries often have a solids content of more than 50% but wear from abrasion is low because the particle sizes are small (generally less than a 10-3mm).

How can Abrasiveness be Measured?

One method of measuring the abrasiveness of slurries is the Miller Test (standardised in ASTM G75-15). This ranks the abrasiveness of slurries against a standard reference. Slurries with a Miller Number less than 50 can be pumped with minor abrasive damage to the system. Above this, greater damage from abrasion is to be expected. The scale is also linear, so a mixture with a Miller Number of 200 can be expected to produce twice as much wear as one with a value of 100.

A Miller Number for a liquid-solid mixture can be useful in selecting a suitable pump. However, it is not advisable to extrapolate values from one mixture to another because of the effects of variable composition, particle size and shape. For example, sand can have a Miller Number anywhere between 50 and 250.

How does Viscosity Affect Wear?

The viscosity of the liquid carrying the solids plays a role in determining the degree of abrasive wear. If the viscosity is high, an efficient fluid film is maintained between the pump’s moving and static components. This cushions the impacts of solid particles, reducing wear. For example, sand in oil is much less abrasive than sand in water.

How do Operating Conditions Affect Abrasion?

The operating conditions applied to a pump carrying an abrasive liquid have a large effect on service life. At higher pump speed or greater differential pressure, the impacts between solid particles and pump components are more energetic and damaging. In fact, wear rate is an exponential function of differential pressure. By halving the pressure, wear may be reduced by a factor of four or more.

For pumping abrasive liquids, manufacturers will often recommend larger pumps so these can be operated at lower speeds and differential pressures. Although the initial investment may be more costly, it will be cheaper in the long run when maintenance and replacement costs are taken into account.

What Types of Pump are Suitable for Handling Abrasives?

The following table illustrates the tolerances and suitability of various pump designs for transporting solid-containing liquids.

Pump Type Typical Maximum Particle Size (mm)
Gear 0.1
Vane 0.2 (soft solids only)
Screw 2
Single Stage Centrifugal  6
Piston / Plunger / AODD 6
Progressive Cavity 40
Rotary Piston 40
Mechanical Diaphragm 60
Lobe 100
Hydraulically Actuated Piston  100

 

Gear pumps operate with tight tolerances and are susceptible to wear or even damage from sub-millimetre solid particles. External gear pumps have four bearings in the pumped medium so are less suited than internal gear designs. These are more robust having only one bearing (sometimes two) running in the fluid. A gear pump should always have a strainer installed on the suction side to protect it from potentially damaging solids.

Centrifugal pump designs are commonly used to pump abrasive liquids although impellers are susceptible to wear because of their high operating speeds. The choice of seals and bearings can be an issue, particularly if the fluid is hazardous. Solids tend to become embedded in packing materials or seals which can accelerate shaft wear and cause leaks. Mechanical seals are often preferred in applications with abrasive liquids. The gap between the faces of a mechanical seal can be as small as 10-3mm and solid particles that could damage the seal faces are unable to gain access.

Standard magnetic drives are not suited to applications with abrasive liquids because of the close tolerances between the internal magnets and the containment shell. This is especially true if the fluid contains magnetic solids such as iron fines. These can collect around the magnet elements causing wear and potentially seizing the motor. If magnetic couplings are required because the liquid is corrosive or hazardous, a barrier fluid can be used to prevent the abrasive liquid from entering this area.

For carrying fluids containing large particulate matter, diaphragm, lobe or piston pumps are the preferred systems. Diaphragm pumps are sealless – ideal for carrying corrosive or hazardous liquids containing solids. Lobe pumps, although similar in design and action to gear pumps, have lower internal tolerances and do not compress the pumped medium. Rotary or piston pumps provide a gentler pumping action, reducing the abrasive impacts of solid particles. However, wear occurs on packing seals and the piston itself and these may need frequent replacement (a disadvantage when compared with diaphragm or lobe pump alternatives).

Summary

An abrasive liquid is one that causes increased wear to a pump, generally by eroding surfaces through physical force but also potentially by chemical reaction. The wear experienced by a pump is determined by the characteristics of the entrained solids and their concentration. This can be measured by assessing the Miller Number of the mixture. Wear is greater when the liquid component has a low viscosity and when the pump is operated at high speeds and particularly at high pressures.

The problems of wear can be reduced by using larger pumps running at lower speeds. In all applications involving liquid-solid mixtures it is important to discuss the suitability of particular pumps with the manufacturers.