Selecting and applying positive displacement pumps (Part 3)

 

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 explore the defining characteristics of positive displacement pumps.

Progressive cavity pumps

Progressive cavity pumps are also known as single screw or helical pumps. They consist of a resilient stator in the form of a double internal helix and a single helical rotor, with later designs consisting of a triple internal helix and a double helix rotor.

The pump is helically shaped and follows a helical path from suction to discharge as the pump rotates.

The rotor, usually a sliding interface fit in the stator, maintains a constant seal across the stator and the seal travels continuously through the pump giving uniform positive displacement.

The single helical rotor rolls in the stator with an eccentric motion so that a universally jointed drive shaft, or equivalent, is required to drive the motor.

A pump with a single pitch stator is called “single stage” and is usually designed for a maximum rated pumping pressure of 6 bar, where a pump that uses a two pitch stator is called “two stage” and is designed for 12 bar.

Similarly an eight pitch stator is 8 stage and designed for 48 bar.

Progressive cavity pump rotors use a coupling rod or flexible drive shaft to transmit the rotary drive whilst allowing the rotor to orbit within the pump stator.

The coupling rod uses simple single pin universal joints, two pin universal joints, or gear joints which operate within the pumped fluid and are usually lubricated and sealed with flexible boots to prevent contamination of the lubricant by the pumped fluid, or vice versa.

Flexible drive shafts do not have joints that require lubrication, and are significantly longer than coupling rods.

The coupling rod is driven by a drive shaft that is supported by a separate foot mounted bearing house, or is fitted directly to a gearbox output shaft.

The most unusual drive arrangements are electric motors either flexibly coupled, or flanged, to a gearbox.

However, many other drives arrangements are also used.

Progressive cavity pumps come in a variety of sizes, ranging from small chemical dosing pumps suitable for flows as low as 20 l/h and pressures to 12 bar, to medium capacity, medium pressure (100 m3/h and 72 bar), and to high capacity, low pressure (420 m3/h and 6 bar) (300, discharge pipe size).

Wide or open throat variations are available with large rectangular inlet connections for very high viscosity liquids, and liquids containing very high solids content.

Characteristics of progressive cavity pumps

The interference fit between the hard rotor and soft resilient stator results in low slippage, good volume efficiency and very good self-priming.

Since small abrasive solids are enveloped in the stator as the rotor passes them, abrasive wear is relatively low and subsequently the range of suitable applications is large.

Non pulsing flow, low shear pumping action, and bidirectional flow possibility also increase the pump versatility. The elastomeric stator should be selected for good chemical compatibility and temperature capability.

Clean in place (CIP) can be used provided the pump selection takes into account cleaning fluid chemical compatibility and temperature.

Progressive cavity pumps should not be run dry. If the process and piping system cannot be designed to avoid this possibility, some type of dry running shutdown should be considered.

Dry running warning or shutdown systems often use flow sensors, but the most popular system uses a temperature sensor embedded in the pump stator to detect frictional heating of the stator prior to high temperature damaging the stator.

Common applications of progressive cavity pumps 

Common applications for progressive cavity pumps include, but are not limited to:

  • Sauces, confectionary, yogurt, and shortening in the food industry
  • Chicken offal, meat purees, potato waste, thick non-flowing pastes
  • Domestic and industrial sewage, primary and secondary sludges, dewatered sludge cake
  • Shampoo, cosmetics, light creams
  • Lime, bentonite and clay slurries
  • Dosing of flocculants and various chemicals
  • Wine must (grape juice with stalks and skins) using rectangular inlet versions
  • High head mine dewatering usually containing abrasive rock particles and dust
  • Shaft and submersible motor drive borehole pumps
  • Low flow, high head, agricultural water pumping, particularly where long suction lines are required

Progressive cavity pumps are commonly used in sewage application

Selection of progressive cavity pumps

Some key points in selecting progressive cavity pumps are:

  • Elastomer material – Progressive cavity pumps are available in a wide range of elastomers for the resilient stator and the universal joint boots. Selection of stator material is important and should take into account chemical and abrasion resistance, as well as strength. Chemical resistance is important as swelling of the elastomeric stator may cause an increase in interference between the rotor and stator, a loss of efficiency, and starting problems due to an increase in starting torque
  • Self-priming – Provided piping is designed to ensure some liquid is trapped in the pump after shutdown and before initial start, the self-priming capability of these pumps is very good
  • Rubbing velocity – A conservative approach should be taken when selecting the rubbing velocity between the rotor and stator, to ensure that an economical life can be achieved, even when pumping fluids with abrasive solids. Pump specifications frequently specify a maximum rotation speed (rpm) to avoid excessive rubbing velocity (m.s). However, since different size pumps  have different rotor diameters and eccentricities, a maximum rubbing speed velocity should be specified instead of rotation speed. The most common progressive cavity pump geometries are designed for maximum rubbing velocity of 3.0 m/s. This usually provides a good life on clean water, but speed derating to 1.5 m/s or less is often desirable for liquids containing abrasive solids and/or high viscosity liquids
  • The pump starting torque can be relatively high – The drive should be selected with an adequate starting torque factor for safety. This is particularly important when variable speed drives are used for either speed control or to reduce starting currents

Internal gear pumps

Crescent internal gear pumps have an outer or rotor gear that is generally used to drive the inner or idler gear.

The idler gear is smaller than the rotor gear, and rotates on a stationary pin and operates inside the rotor gear.

The gears create cavities as they come out of mesh and liquid flows into the pump.

As the gears come back into mesh, the cavity volumes are reduced and liquid is forced out of the discharge port. Liquid can enter the expanding cavities through the rotor teeth or recessed areas
on the head, alongside the teeth.

The stationary crescent is integral with the pump head, and prevents liquids from flowing to the suction port from the discharge port.

The rotor gear is driven by a shaft supported by journal or anti-friction bearings. The idler gear contains a journal bearing rotating on a stationary pin in the pumped liquid.

Depending on shaft sealing arrangements, the rotor shaft support bearings may run in pumped liquid, which is an important consideration when handling abrasive liquid as it can wear out a support bearing.

Typically these pumps are made in a range of sizes, from one to 10 inches, flows to 360 m3/h, pressures to 14 bar, and temperatures to 230°C.

Selection of internal gear pumps

Gear pump type and size are usually chosen from selection charts and then performance is found from individual performance curves. Individual curves are produced for a range of viscosities, and the curve for the viscosity nearest to that to be pumped is used.

Performance can be checked by reference to the manufacturer or distributor.

Characteristics of internal gear pumps

Internal gear pumps are exceptionally versatile.  While they are often used on thin liquids such as solvents and fuel oil, they excel at efficiently pumping highly viscous liquids such as asphalt, chocolate, and adhesives. The useful viscosity range of an internal gear pump is from 1cPs to over 1,000,000cPs.

In addition to their wide viscosity range, internal gear pumps also have a wide temperature range, handling liquids up to 250°C and in some cases, 400°C.  This is due to the single point of end clearance (the distance between the ends of the rotor gear teeth and the head of the pump).

This clearance is adjustable to accommodate high temperatures, maximize efficiency for handling high viscosity liquids, and to accommodate for wear. Internal gear pumps are non-pulsing, have some self-priming capability, and can run dry for short periods.

They’re also usually bi-rotational, meaning that the same pump can be used to load and unload vessels. Because internal gear pumps have only two moving parts, they are reliable, simple to operate, and easy to maintain.

Common applications of internal gear pumps

Common applications for internal gear pumps include, but are not limited to:

  • All varieties of fuel oil and lube oil
  • Resins and polymers
  • Alcohols and solvents
  • Asphalt, bitumen and tar Polyurethane foam (isocyanate and polyol)
  • Food products such as corn syrup, chocolate and peanut butter
  • Paint, ink and pigments
  • Soaps and surfactants
  • Glyco

External gear pumps

External gear pumps come in single or double (two sets of gears) pump configurations with spur, helical and herringbone gears.  Helical and herringbone gears typically offer a smoother flow than spur gears, although all gear types are relatively smooth.

Large capacity external gear pumps typically use helical or herringbone gears. Small external gear pumps usually operate at speeds up to 3000 rpm and larger models operate at speeds of up to 640 rpm.

External gear pumps have close tolerances and shaft support on both sides of the gears, allowing them to run to pressures in order of 150 to 200 bar, making them well suited for use in hydraulics.

With four bearings in the liquid and tight tolerances, they are not well suited to handling abrasive or extreme high temperature applications.

Tighter internal clearances provide for a more reliable measure of liquid passing through a pump and for greater flow control, making them popular for precise transfer and metering applications involving polymers, fuels and chemical additives.

Typically the pumps are made with a size range from 3⁄8 to four inches, flows to 45 m3/h, pressures to 150 or 200 bar, and temperatures to 230°C.

Characteristics of external gear pumps

The design of external gear pumps allows them to be made to closer tolerances than internal gear pumps. The pump is not very forgiving of particulate in the pumped liquid.

Since there are clearances at both ends of the gears, there is no end clearance adjustment for wear. When an external gear pump wears, it must be rebuilt or replaced.

External gear pumps handle viscous and watery-type liquids, but speed must be properly set for thick liquids. Gear teeth come out of mesh a short time, and viscous liquids need time to fill the spaces between the gear teeth.

As a result, pump speed must be slowed down considerably when pumping viscous liquids. The pump does not perform well under critical suction conditions.

Volatile liquids tend to vaporise locally as gear teeth spaces expand rapidly. When the viscosity of pumped liquids rises, torque requirements also rise, and pump shaft strength may not be adequate. Pump manufacturers supply torque limit information when it is a factor.

External gear pumps are popular style and are often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines.

Common applications for external gear pumps

Common applications for external gear pumps include, but are not limited to:

  • Lubrication pumps in machine tools, in fluid transfer units, and as oil pumps in engines
  • Various fuel oils and lube oils
  • Chemical additive and polymer metering
  • Chemical mixing and blending (double pump)
  • Industrial and mobile hydraulic applications (log splitters, lifts, etc)
  • Acids and caustic (stainless steel or composite construction)
  • Low volume transfer or application

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 continue our explorations into the selection and application of different types of positive displacement pumps.

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