Pumping solids

Pumping fluids containing substantial solids, slurries and pastes are problems that many industries encounter.

In the food manufacturing sector, the solids are most commonly soft and need to retain their form and consistency, whilst slurries and pastes can have high levels of viscosity that can lead to blockages. In other industries, slurries and pastes can also provide the additional challenge of being abrasive.

Pump blockages resulting from large quantities of grit, debris and fibrous matter is a problem that operators of wastewater and effluent treatment plants face almost daily. For many it is a problem that is on the increase. The fouling of pump impellers is a longstanding problem that costs companies a lot of money in terms of plant downtime, pump repair and even complete pump replacement.

There is no single type of pump that can accommodate all these demands, although there are some that do come close. The question that end-users face is: what is the best pump type and technology for the application? Getting the correct answer can, and will involve a degree of research and detailed discussions with manufacturers and distributors. The consequences of not getting the correct pump for the job will impact on production and can be very costly.

The range of pumps that can be considered for handling solids laden liquids, slurries and  pastes can be narrowed down to rotary lobe, rotary piston, reciprocating positive displacement, progressing cavity, hose, centrifugal and air-operated double diaphragm (AODD) designs. Making a decision as to which type to select is dependent on the nature of the media being pumped. Where low shear forces are desirable and maintaining the structure of the solids content is essential, then a pump with a gentle action is going to provide the answer. If the solids are large and potentially damaging then a robust centrifugal pump with an open impeller is more likely to fit the bill.

Hose pumps
The transfer of solids-laden water encountered particularly in  mining and  quarrying demands a type of pump that is extremely robust, low maintenance and capable of standing up to the aggressive nature of the pumped liquid.  Whilst rotating pumps containing open impellers can perform outstanding service, there is always going to be the problem of impeller wear and damage to the pump housing, making maintenance a constant requirement along with frequent parts replacement. An alternative pump type that should be considered is the, hose pump. Suitable for applications ranging from precision dosing through to transferring sludges and slurries the hose pump delivers leak-free operation, is ideally suited to challenging open environments and most significantly, is easy to maintain.

The main features of hose pumps include:

  • Sealless design
  • Valve-free
  • Dry self-priming
  • Gentle pumping action
  • Dry running
  • High suction lift
  • Abrasion resistant
  • Low maintenance
  • No slip
  • Reversible

The hose pump is a positive displacement pump where the fluid is contained within a hose or tube inside the casing. The fluid is moved into and out of the tube/hose by the action of rotating rollers or shoes which alternatively compress and relax the tube/hose. This ‘peristaltic’ process ensures that the fluid is transported accurately and without any variation in the volume. What’s more, the dosing pump is economic with the liquid pumped because at the end of the processing cycle or operation, the pumping motion can be reversed and any fluid remaining in the hose/tube can be returned to its source for future use and the tube/hose is drained.

The decision to use a shoe or roller hose pump is largely dependent on the application and the operating pressures required for the application.  Rollers are more suited to lower pressure applications, typically up to 4 bar, and usually limited to a maximum of 8 bar. Therefore, smaller drives are required which results in less power being consumed. Roller technology is easier to maintain, with simple hose replacement and less waste fluids due to the elimination of hose lubricant. Shoe compression pumps are more suited for applications demanding operating pressures up to 16 bar and higher flowrates. This pump is a robust, workhorse built for the most arduous of operating conditions and fluids.

Centrifugal pumps
Where grit and sand are encountered, a pump can literally grind itself to pieces. However, it is not beyond the capabilities of the centrifugal pump to withstand abrasive materials and deliver trouble-free performance over long periods. One such example is the radial impeller design pump where, grit-laden water is discharged at high velocity against the pump casing or the wear ring.

Unlike radial impeller vortex pumps, the cupped impeller action of the pump deflects material away from the case and back into the flow stream towards the extra-thick sacrificial suction piece.  Behind the impeller is a robust wear plate which can withstand the grinding action of grit. To further maximise performance and minimise wear, the large diameter impeller design is the key to pump’s considerable durability and non-clogging performance.

The Wemco Model C torque-flow pump has the ability to withstand the harshest of conditions due to its hydraulic vortex pump technique which is based on the principle of a whirlpool. The vortex is created by the rotating recessed cup-shaped impeller extending into the suction line, drawing the liquid / solids into the pump and then quickly through the discharge, whilst minimising the fluid / solids contact with the impeller and volute. This simple design allows solids to pass through the pump without choking the impeller, so considerably reducing pump wear and the potential for blockages.

The fully recessed cup-shaped impeller and free passageways within the pump casing offer almost total freedom from blockages when handling rags and fibrous solids typically encountered in waste water treatment plants. The standard option of High Chrome Iron for the wet end, the hydraulic design and thickened wear sections provide a more than adequate combination to deal with grit and silica sands carried into the treatment works.

AODD pumps
When it comes to moving solids that can be either bulky or need a gentle handling action the air operated double diaphragm (AODD) positive displacement pump is a hugely flexible and versatile pump.  It is portable, straightforward to use, easy to maintain and will perform in the most difficult conditions as well as clean room environments. The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics. The AODD pump offers the ability to handle shear-sensitive fluids, keep thick and viscous fluids constantly in motion and provide leak-free operation. In addition, this pump can accommodate dead-heading during filling applications, a task that motor driven pumps are not able to handle effectively.

Air-operated double diaphragm pumps (AODD) use compressed air to act upon the diaphragm and draw the liquid media into the pumping chamber and then eject it, so there are no rotary parts involved. This pump is ideally suited to applications in the food processing industries where a gentle action is required for solids, such as fruits and meats that must retain their form. Equally, they are at home shifting waste matter resulting from food processing.

The deep liquid chamber design results in superior handling of delicate particulates and solids that tend to settle because there is a large gap between the clamped area of the diaphragm and the liquid chamber wall. Unlike other pumps, the product particles do not get wedged in this area, thus eliminating a factor that can cause product contamination.

When running, the dynamic components that come into contact with the fluid are the two diaphragms which are connected by a common shaft, the two inlet valve balls and the two discharge valve balls. The diaphragms act as a separation membrane between the compressed air supply and the fluid.

Driving the diaphragms with compressed air rather than the shaft, balances the load on the diaphragm which removes mechanical stress and therefore extends diaphragm life. In addition, reducing the stress to which the diaphragm is subjected will contribute to long diaphragm life. A diaphragm that is moulded in an ‘operational’ shape will ensure that stress concentration is minimised throughout its entire stroke length. The valve balls open and close on the valve seats to direct the flow. The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics.

Rotary piston and lobe pumps
Rotary pumps use an operating principle known as the external circumferential piston (ECP). In this design the arc-shaped rotary pistons, or rotor wings, travel in annular-shaped cylinders machined in the pump body. The resulting long sealing path reduces slippage and produces a smooth product flow without destructive pulses or pressure peaks, and without the need for valves or complex parts. However, unlike progressing cavity and rotary lobe pumps, they are not adversely affected by slurries which have a propensity to settle in the pump.

Where high viscosity fluids and solids are involved, the large fluid cavities of the rotors, together with the large easy-entry anti-cavitation ports enable efficient pumping. Maximum service life even under severe operating conditions where fluids are non-lubricating or abrasive can be achieved, because there is no contact between the bearings and the pumped fluid, nor is there sliding or rolling contact and rotor-to-rotor contact.

The food processing industry provides an excellent illustration of the capabilities of the rotary piston pump. Taking the Waukesha Universal ECP as an example, this pump can handle both thick and thin mixes without causing any damage to the pumped product. It combines a very gentle, pulse-free pumping action with the high suction capacity necessary for allowing the thick mix to be drawn into the pump with any separation of the ingredients.

Where the product contains fruits, meat vegetables or other solids it is essential to the finished quality of the product that these ingredients are moved to the depositing line without being damaged. The low shear action of the pump guarantees that all ingredients arrive at the depositing line in pristine condition every time.

The rotary lobe pump provides an alternative to the rotary piston pump, employing timing gears which eliminate contact between the rotors and enables them to handle non-lubricating fluids.  There are various designs of rotor, including bi-wing and multi-lobe options. Providing low shear and gentle handling of the liquid, thereby minimising product degradation, rotary lobe pumps are also easy to clean in place (CIP) or strip clean between operations for batching applications.

Rotary Lobe Pumps are self-priming, valve-less, positive displacement pumps, whereby the even rotation of the rotor pair creates a vacuum on the priming side of the pump. This is defined by the direction of rotation of the drive. This vacuum draws the liquid into the pump chamber. With further rotation, the pumped medium is conveyed past the pump wall into the pressure area.

For duties where solids-laden liquids and slurries/pastes need to be moved at low velocities, progressing cavity pumps also provide low shear characteristics. Employing a single rotor to move fluids in a continuous flow, the performance of progressing cavity pumps is largely down to the geometry of the rotor and the stator in which it turns. The materials from which these two elements are manufactured need to be matched to the pumped media in which they come into contact. In order to achieve life cycle costing goals the rotor and stator need to offer high levels resistance to aggressive fluids, which brings into play rotor geometry and material specification.

Summary
When it comes to pumping liquids containing solids, whether they be large, small hard or soft, there is a wide range of technologies that can meet many of the demands that arise. Some are more specialist than others, but that is because they address the needs of specific applications or industry sectors. Where several pump technologies may appear to be suitable for the application, an exacting review of the nature of the media being pumped and how it needs to be handled has to be undertaken and the results discussed with the manufacturers or suppliers of the pumps under consideration.

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