Selecting and applying positive displacement pumps (Part 4)

The PIA’s Australian Pump Technical Handbook is a cornerstone text for the Australian pump industry and, in our opinion, a must have for anyone who deals with pumps on a regular basis. In this ongoing series, we feature abridged chapters from the classic book to showcase the various areas covered and to reacquaint readers with the technical aspects of pumps. In this issue, we continue our detailed look at selecting and applying different types of positive displacement pumps.

Lobe pumps

Lobe pumps operate similar to external gears pumps in that fluid flows around the interior of the casing. However, unlike external gear pumps, the lobes don’t make contact. Lobe contact is prevented by external timing gears located in the gearbox, and since the bearings are out of the pumped liquid, pressure is limited by bearing location and shaft deflection.

A shaft seal is required for each shaft which are usually special short designs to minimise shaft length and overhang.

Lobe pumps are manufactured with a variety of lobe operations including single, bi-wing, tri-lobe and multi-lobe.

Rotary lobe pumps range from industrial designs to sanitary designs. The sanitary designs break down further depending on the service and specific sanitary requirements, including 3-A, EHEDG and USDA.

Characteristics of lobe pumps

Rotary lobe pumps are non-contacting and have large pumping chambers, allowing them to handle large solids such as cherries or olives without damage. They are also used to handle slurries, pastes and a wide variety of other liquids. If wetted, they offer self-priming performance. A gentle pumping action minimises product degradation.

They also offer reversible flows and can operate dry for long periods of time. Flow is relatively independent of changes in process pressure so output is constant and continuous.

Since the lobes don’t make contact and clearances are not as close as in other positive displacement pumps, internal leakage is increased and volumetric efficiency decreased when pumping low viscosity liquids.

High-viscosity liquids require considerably reduced speeds to achieve satisfactory performance. Reductions of 25 per cent of rated speed and lower are common with high-viscosity liquids.

Cleaning is important when the product cannot remain in the pump for sanitary reasons or when products of different colours or properties are batched. Lobe pumps are easily cleaned by circulating a cleaning fluid through them, a process generally described as “clean in place” (CIP).

Lobe pumps have a number of advantages, including:

  • Will pass medium sized soft solids with minimal damage to the solids, which is especially useful with fluids containing food
  • No metal-to-metal contact between rotors and casings
  • Very good CIP/sterilisation in place (SIP) capabilities
  • Non-pulsating discharge
  • Bi-directional flow

There are, however, also a number of disadvantages, including:

  • Requires external timing gears and two shaft seals
  • Reduced suction lift with thin liquids
  • Reduced capacity as pressure increases (due to slip through rotor and casing clearances)
  • Abrasive solids can cause high wear rates, however this can be reduced with optional materials such as rubber or urethane coated rotors and end covers

Common applications for lobe pumps

Lobe pumps are frequently used in food applications because they can handle solids without damaging the pump. They can pump larger particle sizes than other positive displacement pumps.

They are also commonly used for other industries deemed to be ‘clean industries’ such as:

  • Beverage
  • Pharmaceutical
  • Personal care products

Lobe pumps may also be used in other industrial applications listed below:

  • Polymers
  • Paper coatings
  • Soaps and surfactants
  • Paints and dyes
  • Rubber and adhesives

Specific care needs to be taken in these applications and it is best to seek manufacturer or supplier recommendation.

Circumferential piston pumps

Circumferential piston pumps are similar to lobe pumps in both design and application. However, the rotor and casing design is very different. Circumferential piston pump rotors are arranged as arc-shaped “pistons” (rotor wings) that are cantilevered from the rotor hub and travel in annular-shaped cylinders formed between the outer casing and the inner cantilevered stationary pillar.

The resulting long circumferential sealing path between the rotor and casing reduces slippage on low viscosity products and produces a smooth flow of product with minimal pulses.

The rotors are made of a special non-galling stainless steel alloy which allows very close clearances to further improve efficiency on low viscosity products.

Characteristics of circumferential piston pumps

Circumferential piston pumps have very similar characteristics to lobe pumps. However, the long circumferential sealing path reduces slip and improves volumetric efficiency on low viscosity liquids at medium to high pressures. Shearing of shear sensitive liquids is also lower, handling of slurries is better and NPSHR is usually lower.

Circumferential piston pumps are good for handling slurries because if the pump stops and the slurry settles to the bottom of the casing, when restarted the rotor wings will act like a shovel and scoop the settled slurry out of the discharge port.

The closer clearances, made possible by the non-galling alloy rotors and the long leakage paths inherent in the rotor design, improve volumetric efficiency on low viscosity liquids compared to typical lobe pumps.

Common applications for circumferential piston pumps

Circumferential piston pumps can be used in the same applications as lobe pumps, with selection preference often being decided upon standardisation and/or experience with existing plants in the user’s plant.

Lobe and circumferential piston pumps are commonly used in the pharmaceutical industry.

Twin screw pumps

Twin screw pumps use a pair of specially profiled, opposite rotation screws that mesh in an interchangeable casing liner.

On most designs, small but positive clearance between the screws is maintained by external timing gears. Most twin screw pumps are double suction designs which are inherently in hydraulic balance in the axial direction due to their symmetry.

Most designs use mechanical shaft seals and external roller bearings, although some use pumped liquid lubricated bearing brushes which reduce the number of seals required.

Each wrap of screw thread forms a cavity that moves axially from suction to discharge. The wrap, or cavity, acts as a pressure stage. Low pressure pumps have only one or two wraps (stages), while high pressure pumps may have 12 or more wraps.

The staging effect allows each stage to handle a moderate pressure rise, resulting in low stress levels within the pump even at high pressure operation.

Characteristics of twin screw pumps

Designs have external timing gears and bearings to keep the screws from contacting each other or their casing bores.

These pumps can successfully handle low viscosity, low lubricity, liquids — including liquids with light abrasives — in addition to more viscous fluids. They can also run dry without damage and are self-priming.

Twin screw pumps are usually available in low (16 bar), medium (80 bar) and high bar (200 bar) ranges with capacities up to approximately 5000 m3/hr. Efficiencies are usually very high and noise levels low.

Common applications of twin screw pumps

Twin screw pumps are generally used on hydrocarbons ranging from crude oil to bitumens and lubricating oils, vegetable oils and water.

Applications include:

  • Tanker unloading
  • Fluid transfer
  • Fuel feed
  • Hydraulic power
  • Heavy machinery lube oil supply
  • Multi-phase oil and gas flow in oil well production pumping
  • Industries include oil and gas refineries, shipbuilding, power generation and fuel oil systems, and chemical manufacturing
    and supply.

Lobe pumps can handle large solids such as cherries without damage.

Triple screw pumps

Triple screw pumps have one driven screw and two idler screws running in three precision bores in the housing or liner.

The power rotor drives the rolling idler rotors which are separated by a thin film of pumped liquid. The screw profiles form a seal between the liquid filled cavities that convey the liquid from suction to discharge.

Triple screw designs are usually single suction and are fitted with one seal whereas twin screw pumps usually require four seals; one at each end of the two shafts, although single seal designs with product lubricated bearings can be used if no abrasives are present.

Characteristics of triple screw pumps

Triple screw pumps are suitable for use with non-abrasive, lubricating fluids with low to medium viscosities. They are not suitable for dry running.

Operating speeds can be quite high, with high efficiencies and low noise levels. Flow rates are low to medium (to about 700 m3/hr) with pressures to approximately 300 bar.

Common applications of triple screw pumps

Triple screw pumps are used on many of the same fluids, applications and industries as twin screw pumps, within their smaller capacity range, provided that the fluids are lubricating and non-abrasive.

Further information and detailed diagrams, equations and schematics can be found in the Australian Pump Technical Handbook, available from the PIA website. In the next edition of Pump Industry, we will explore the selection and application of different types of slurry pump.

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