Helical Rotor Pumps
With African designed and manufactured helical rotor positive displacement pumps and replacement components gaining ground in the mining arenas of Sub Saharan Africa and beyond, we examine these pumps and underscore their principal benefits and limitations.
The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid.
The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head. Positive displacement pumps move fluids, gases or mixes of fluids and solids by use of positive pressure.
They are widely used for pumping difficult materials such as sewerage sludges, contaminated with large particles. A helical rotor pump also known as a progressive cavity pump is a positive displacement pump.
A helical rotor pump consists of a spiral shaped rotor that fits inside a stator. The stator is normally made from an elastomeric material such as nitrile and is often coated with wear-resistant metal.
The elastomeric material comprising the stator is designed to be compatible with the liquid being pumped. The rotor may be made of material such as carbon steel and stainless steel. As the rotor rotates, fluid is gradually forced up the rubber sleeve.
The stator always has one more helix than the rotor to facilitate the progressing cavity pumping action. When the rotor turns within the stator, cavities are formed which progress from the suction to the discharge end of the pump, conveying the process fluid.
The continuous seal line between the rotor and the stator helices keeps the fluid moving steadily at a fixed flow rate, proportional to the pump's rotational speed.
Principal Benefits
Can draw well and are self-priming Granted their status as positive displacement pumps, helical rotor pumps develop a strong negative suction and can draw liquid to them.
With these pumps a primed suction line is not necessary; they however require sufficient liquid in them to provide lubrication of the rotor and the stator.
High Suction Lift Capabilities
The optimised sealing between the rotor and the stator enables helical rotor pumps to possess a particularly high suction lift capability. These pumps are also manufactured with a variety of stator/rotor tooth combinations.
Typically artificial lift applications use a two-tooth stator and a single tooth rotor pump referred to as single-lobe pump.
Higher stator/rotor tooth combinations, such as 3/2, are used to achieve higher volumetric and lift capacity although with higher torque requirements.
Constant Non-pulsating Flow
Cavities in helical rotor pumps are created by the geometry of the rotor and the stator where the stator has one more lobe than the rotor. The cavities are moved axially along the pump by the rotating motion of the rotor.
The motion of the rotor is a combination of a clockwise rotation of the rotor along its own axis and a counterclockwise rotation of the rotor eccentrically about the axis of the stator. Because the volume of each cavity remains constant throughout the process, the pump delivers a uniform non-pulsating flow.
Volumetric Accuracy
Flow in these pumps is directly proportional to the speed of rotation and independent of pressure because the cavity volume is known. Each turn results in a definite volume of product delivered into a system.
Non-shearing
Progressive cavity pumps minimize shearing to the extent that they employ fixed cavities. The rubber seal divides the cavities in the double helix off into separate zones.
Material taken up by the cavities is not dragged helictically but is instead pushed with a pistoning motion within the cavities.
It is this use of pistoning force as opposed to helictal drag that allows progressive cavity pumps to maintain their low shear rates; the pump will not chop up or shred the product being pumped.
Their non-shearing quality means that these pumps can be used with viscous or shear sensitive fluids such as heavy slurries.
Excellent Pressure Stability
The use of pistoning force further enables progressive cavity pumps to handle pressure.
Valve Less
alves are not used for progressive cavity pumps, because they are inherently designed as fixed-rate pumps.
In their place, burst disks are used for the potential event of an emergency, with fluid bypasses for constant regulation (burst disks are intentionally-weak and easily replaced or repaired inserts between pipes).
Progressive cavity pumps are constructed around the concept of flow rate not pressure and consequently cannot be regulated by valve.
Ability to transfer multiphase fluids
Progressive cavity pumps are able to convey large quantities of air, vapour or gas in a fluid without any running disturbance.
Simple design
A single rotating element (a rotor) within an elastomeric stator: only a single shaft requires sealing.
Hygienic
Helical rotor pumps are found to be hygienic in as far as all internal surfaces are smooth, making it difficult for bacteria to adhere. Any material sitting in the cavity is pushed through the pump and a decontamination wash can get to all parts of the pump and destroy any bacteria present.
Limitations
Wear prone
Clearances between the rotor and the stator must be tight. The rotor rubs against the stator as it turns and the contact edge seals off the cavity.
Once the sealing edge is lost, the product recirculates within the pump from the high pressure end to the low pressure end causing both loss of pressure and loss of product throughput.
This wears the rotor and stator which must then be replaced. Replacements for these are expensive because of the specialist fabrication requirements for their odd shapes.
Rotational Speed
Rotational speed must be as slow as is possible to minimize friction between the rotor and stator. The effectiveness of the pump depends on maintaining a sealing edge. As rubbing one surface against the other will cause friction, the rotor must be turned slowly and the rubber surfaces kept lubricated with product.
Stator Material Selection
Stator material selection is confined to a variety of different rubbers. The product being pumped dictates whether hard or soft rubber is used. It may be necessary to try different rubber until the most suitable is identified.
Need for a Pressure Relief Valve
The pressure relief valve is fitted on the discharge of the pump. The relief valve can relieve back into the supply tank or back to the suction pipe work to the pump.
Cannot be Dry Run
Progressive cavity pumps are intended to be lubricated by the materials they pump; allowing them to 'run dry' can cause serious mechanical harm and severely shorten the lifespan of the stator. The rubbing action of rotor over stator requires lubrication. Abrasive fluids, such as those containing sand will naturally shorten the life of a progressive cavity pump.
Temperature Limits
There are temperature limits for the rubber in the stator. Depending on the rubber, the working temperature limit before becoming too soft is between 90 and 150 degrees centigrade.
Cleanliness and Lubricating Properties of Liquid being Pumped Determine Pump Speed With clean, cool, lubricating liquids speeds can be faster. When particulate is present in the product, or the liquid viscosity is high, these need to be speed reduced through a gearbox or belt and pulley drive.
Shaft Sealing Requires a Mechanical Seal or a Packed Gland
If the product is viscous or a slurry, the seal will need a clean liquid flush to prevent product coming back up the shaft of a packed gland pump or across the faces of a mechanical seal.
List of Contributors:
Lifetime Reliability Consultants
Mike Sondalini
Email:info@lifetime-reliability.com
Website:www.lifetime-reliability.com
Moyno, Inc
Janet Bismark
Email:janet@tdh-marketing.com
Website:www.moyno.com
