Positive displacement pumps--part 1

Positive displacement (pd) pumps operate on completely different principles to rotodynamic pumps. The most important difference is speed. Positive displacement pumps don’t rely on speed to develop pressure. And now we talk of pressure, not head, because the small velocity head is not converted to static head. Velocities tend to be very low compared to rotodynamic pumps. Also differential head is replaced by discharge pressure. Positive displacement pumps don’t try to increase the suction head by a fixed differential head. Positive displacement pumps develop sufficient discharge pressure to force the liquid into the discharge system.

In broad terms, positive displacement pumps compress the liquid from suction conditions to discharge conditions and can achieve this at any speed. As already stated, velocities tend to be low so there are no problems in ignoring the kinetic energies in the suction and discharge pipes. The pressures developed tend to be very large in comparison to the physical dimensions of the pumps; so there are no problems in ignoring potential energy due to elevation in the suction and discharge systems.

Positive displacement pumps can produce very high pressures, over 10000 barg is possible but uncommon. Standard pumps used for high pressure cleaning and descaling operate at 1000 barg to 2000 barg. Pumps used for water jet cutting and cold pasteurization operate up to 4000 barg. Liquids are compressible. The liquid volume at discharge conditions may be considerably smaller than at suction conditions; less than 85%. If the discharge flow rate is important, rather than the mass flow, compressibility cannot be ignored and must be included in pump and system design calculations. Power is required to compress liquids. The standard power approximation may under estimate the power required to drive a pump when the liquid is very compressible.

NOTE: A review of the first paragraph of Part. 1, before reading the following Sections is recommended.

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Fig. 63 Gear pump with external gears

Oil reservoir with filter; Hot coolant return; Hydraulic gear pump; Thermostatic valve; Hydraulic gear motor with switching valve drives cooling fan; Cold coolant supply

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Fig. 64 External gear pump hydrostatic fan cooling drive

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Fig. 65 External gear pump with four stuffing boxes, Albany Engineering Company Ltd

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__5.1 External gear pumps

Gear pumps generally have two gears. Some designs are available with three, one gear being driven by the motor, the others by meshing with the driven gear. The driven gear usually runs in plain bearings. The bearings and shaft journals are located in the pump casing and surrounded by the pumped liquid. These bearings are thus dependent upon the lubricating qualities of the pumped liquid. Gear pumps are not therefore suitable for handling non-lubricating liquids. For reasonably acceptable pump life, gear pumps should not be used for such dry liquids as water or petrol. Paraffin and diesel oil are examples of liquids whose lubricating properties, whilst not being good, are perfectly acceptable for gear pumps.

The simplest gear pump has two external gears, Fig. 63.

This type is used for relatively free-flowing liquids and can produce quite high pressures. Pumps for process applications start at 7 bar, hydraulic power applications have a standard rating of 170 bar but pumps are available for 300 barg. Gear pumps are self-priming with a suction capacity of 4 to 8 meters.

Pumps are built in cast iron, steel, stainless steel, titanium, Hastelloy and non-metallic materials.

Simple gear pumps are used for hydraulic power applications, where the motor can be a gear pump running in reverse. Fig. 64 shows a low power application for driving a radiator fan on an engine. The thermostatic control valve provides speed control for energy saving and faster engine warm up. This type of drive is useful in hazardous areas where electrical equipment is bulky and costly.

Fig. 65 shows an external gear pump with four stuffing boxes, external bearings and external timing gears. This style of pump is used for viscous liquids with solids, such as asphalt.

External gear pumps can be equipped with heating or cooling jackets and complex sealing systems.

__5.2 Internal gear pumps

A far greater range of application is possible with gear pumps having a small rotor mounted eccentrically within a larger external gear. A crescent shaped partition separates the two gears, Fig. 66. When the rotor is driven, the gear also rotates. The difference between the diameters of the rotor and gear and the eccentric location means that the gear teeth engage only at one point. The tooth of the gear successively rotates out of the gear pocket in the rotor during the first half of rotation. This induces a vacuum and the gear pocket is filled with liquid from the suction line. The liquid is forced out into the discharge line by the gear teeth during the second half of rotation.

The number of teeth is kept to a minimum in order to make maximum use of material to obtain deep pockets between the teeth and achieve large displacement. By using sophisticated tooth profiles the number of teeth for external gear pumps can be as low as 10 to 15. Internal gear pumps can utilize the advantages of more favorable gear tooth engagement and therefore have even less teeth, see Fig. 67.

The pressure of fluid causes side forces on the gear which are taken up by the bearings. The axial force is usually relatively small, although the variations in side clearance and resulting pressure variations can cause wear. Wear which causes increased gear clearance does not affect the internal leakage as much as wear at the teeth tips and sides.

It’s important to know the temperature range within which a gear pump is going to operate. Working components must take up greater clearances than normal if the pump is to operate with liquids at high temperatures. Some manufacturers supply pumps suitable for operating at temperatures of up to 300 degree. Gear pumps should be used with care for liquids containing solid particles and for abrasive liquids. Pumps are available for hard solids up to 15 mm and soft solids up to 100 mm. Slow speeds are recommended. Wear can be reduced, when handling abrasive liquids, by selecting a pump having a somewhat larger capacity enabling it to operate at lower speeds. Gear pumps are not suitable for use within the food industry where compliance with hygiene regulations is mandatory. They should not be used for handling shear-sensitive liquids.

The respective uses and applications for external and internal gear pumps varies somewhat. Greater pressure increases and increased flow is obtained with external gears, whereas internal gears are more suitable for high viscosity liquids and have better suction capabilities. To obtain a good volumetric efficiency it’s always necessary to adjust pump speed or suction pressure to suit the viscosity of the liquid. A high viscosity requires a lower speed or higher suction pressure.

Shaft seals for pumps handling lube oil can be as simple as a U ring. Stuffing boxes or mechanical seals are available to cope with many liquids. To increase pump life when handling non-lubricating liquids, special precautions such as shaft journals, self-lubricating plain bearings and special gear tooth surface treatment may be necessary.

Fig. 66 Internal gear pump showing the 3 working phases

Transfer Delivery Filling

Fig. 67 An internal gear pump with heating jacket and double mechanical seal

There are many models suitable for small flows designed for laboratory use, whilst larger industrial pumps have maximum flows over 3000 m^3/h.

External gear pumps offer an excellent middle-of-the-road choice between performance and cost. Gear pumps are extremely compact for the power they develop. Although unable to match the high pressure capability of piston pumps, gear pumps develop higher pressures than vane or lobe types. By the use of precision-cut gears and close tolerance assembly, particularly between the tips of the teeth and the casing, volumetric efficiencies in excess of 90% are normal.

Gear pumps must be one of the most popular pump types, since every automobile engine has at least one for lube oil.

Some gearboxes and differentials have separate pumps for their lube systems. A major area of application for external gear pumps is on mobile plant and machinery. They provide a power source for a variety of lifting services, as well as acting as power-assisted steering pumps. When assembled as a tandem unit the pump becomes a compact, economic solution serving a number of circuits. In the agricultural industry, tractors often depend upon engine driven external gear pumps for powering on-board services. These pumps are also popular for use with PTO shafts, with or without assistance from a gearbox.

A number of magnetically driven sealed gear pump designs are available. These are especially useful for hazardous viscous liquids or liquids which must not come into contact with air. A disadvantage with some designs is that bearing wear can be quickly followed by damage to the magnetic coupling with the result that the complete unit is unfit for further service. With other models, should wear reach the point at which the gears come into contact with the pump casing, the magnetic coupling, by far the most expensive component, will immediately spin free without damage.

Fig. 68 Rigid screw pump construction and efficiency

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Helical blades Reinforcement tube I Flange for Belt drive disassembling , Support bearing

Fig. 69 Rigid screw pump with rotating channel

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Fig. 70 A longitudinal section showing three screws meshing . Allweiler A G

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Fig. 71 Viscous effects on triple screw pumps

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__5.3 Archimedes screw pumps

The Archimedean screw is the oldest known type of rotating pump. The rotor is in the form of a thin helical screw. The thread form being made by plate wound around the shaft. The number of threads varying from one to three. The rotor usually rotates in an open channel having a circular cross-section, enclosing about three-quarters of the rotor periphery.

The rigid screw shaft is usually inclined at an angle of 30 ~ to the horizontal, the length of the screw being approximately twice the lift obtained. The lift heads are rarely in excess of 10 m. Increased nominal flow is obtained by increasing the rotor diameter and reducing speed. Speeds range between 20 and 80 rpm.

Delivery heads are virtually independent of flow up to a maximum flow, maximum capacity loss factor or filling coefficient.

The efficiency curve is flat as shown in Fig. 68. The efficiency is only reduced by about 5% from maximum flow to 50% flow.

There are special types of rigid screw pumps, like that shown in Fig. 69, where the channel is totally enclosed and rotates with the screw. Provided that the drive and shaft bearings are completely isolated from the liquid it’s possible to transport a wide variety of liquids and mixtures.

Rigid screw pumps are used for transporting liquids containing large solid contaminants, untreated sewage for example. They are also ideal for low delivery heads and large flows, up to 21000 m^3/h. As a sewage pump the rigid screw pump is self-regulating according to the supply flow. When the level in the pump sinks then the capacity loss factor, or filling coefficient, automatically reduces causing the pump flow to decrease.

The advantages of the rigid screw pump are:

--High efficiency

--Simple robust construction

--No seals

--Can run dry

--Flat efficiency curve

--Self-regulating for varying output

--Not sensitive to contaminants

__5.4 Twin-rotor screw pumps

All screw pumps are grouped together in this section even though the operating principles of two screw pumps is different to three and five screw pumps.

Two screw pumps consist of two identical profiles meshing together and sometimes with external timing gears. Without external timing, the idler screw is driven by liquid pressure between the profiles. The two screws don’t touch and a minute flow path from discharge to suction always exists.

The most common type of screw pump has three rotating screws of two different profiles, Fig. 70. The central screw is the driver whilst the basic function of the two idler screws is to provide sealing by hydrodynamic bearing films. The screw helixes form a number of sealing elements with the pump casing which transports the liquid axially when the screw rotates. The liquid flow is smooth without disturbing flow variations. The screw pump is quiet running and can be run at speeds up to 4500 rpm. By constructing pumps with longer screws more sealed elements can be obtained resulting in higher discharge pressures.

Screw pumps rely on viscosity for sealing, and in triple screw pumps for driving the idle screws. Fig. 71 shows the effect of viscosity on flow and power at constant speed and differential pressure.

Fig. 71 Viscous effects on triple screw pumps

__5.5 Triple-rotor and 5-rotor screw pumps

Triple screw pumps are used primarily for oil, and other lubricating liquids like ethylene glycol, which should be free from contaminants. They are self-priming but must not be run dry as this can cause damage to the screws and casing. Screws and casings coated with ceramic can handle water but differential pressure is limited to about 100 bar.

Triple screw pumps are available for flows ranging from 300 l/h to over 800 l/h. Low pressure ranges cover 7 to 10 bar discharge pressure. High pressure pumps are available for 250 bar. Viscosity should not be too low and should be in excess of 1.5 cSt. The maximum permissible viscosity is about 5000 cSt; factory approval should be sort for viscous applications over 400 cSt on higher pressure applications. Screw pumps can be used for liquid temperatures ranging from -20 degr. to +155 ~ Mechanical efficiency can be over 85%. Figs. 72 and 73 show vertical and horizontal versions.

__5.6 Twin-rotor geared-screw pumps

Screw pumps with two screws and an external synchronizing gear can handle liquids containing small quantities of solid particles, 2 mm may be possible. Pump users have been looking for pumps which can handle mixtures of liquids and gases, two phase flow. Crude oil production, from some wells, varies considerably with water and CO2/H2S content. Centrifugal pumps can only accept approximately 15% gas percentage by volume before losing prime. Twin screw pumps have been developed to handle continuously variable mixtures from 100% liquid to 97% gas.

Screw pumps are made from grey cast iron, nodular cast iron and carbon steel. Special surface treatment of components can improve wear resistance. Shaft sealing is by means of stuffing boxes or mechanical seals.

Axial thrust is taken up partly by hydraulic balancing and partly by thrust bearings. Large pumps are constructed with two opposed screws on each shaft. Liquid enters at the center of the shaft and flows outward to the discharge. Equal and opposite flows creating a high degree of hydraulic balance. With regard to the construction of thrust bearings, screw pumps are normally designed for one direction of rotation and flow.

Twin synchronized screw pumps are available up to approximately 2200 m^3/h at pressures of 240 bar. These pumps can handle water and there is no maximum limit on viscosity. Gas entrained in viscous liquid tends to reduce the effective viscosity. Slugs of 100% gas and entrained abrasive solids, such as sand, can be handled.

__5.7 Progressive cavity pumps

Progressive cavity pumps may seem to be a new pump type.

This is the modern name for an old pump. In the past, these pumps have been called, eccentric screw pumps, eccentric gear pumps and helical gear pumps. The new name is better as it describes the action of the pump. The principle of operation was first established in the 1930s by Rene Moineau.

Progressive cavity pumps have only one rotor working within a rubber stator. The rotor, which looks something like an elongated corkscrew, rotates in a flexible stator having double internal helixes. The pitch of the stator helix is twice that of the rotor's. The difference in pitch forms sealed cavities between the rotor and the stator which, with rotation of the rotor, are caused to travel axially along the stator resulting in a smooth axial flow.

Since the end of the rotor describes a circle when rotating, drive is usually by means of a cardan shaft with standard universal joints (Hooke's joints) at both ends which lie in the liquid flow, see Fig. 74.

Alternative methods of flexible power transmission have been developed by different manufacturers to cope with the wide range of operating conditions, see Fig. 75. (a) shows a simple single pin coupling. This style of coupling is suitable for small pumps when the torque requirement is low. The O ring is used to provide a flexible seal; it keeps the lubricant inside the coupling and the product outside. (b)is an open version of (a). (b) is used for small hygienic pumps when it’s important to be able to clean the pump internals thoroughly. (c) and (d) show two different approaches for Hooke's joints. A crossed pin provides four bearing surfaces; two for each side of the coupling.

These are the most popular types of coupling used and are suitable for any pump size and most applications. The pin bearings can be plain or the needle roller type. Lubricant is sealed in the coupling by the flexible outer cover. All of the pin joints suffer a common drawback; the possibility of cyclic angular velocity changes. If a single Hooke's joint is used to transmit rotary power between two shafts which are not parallel, the driven shaft won’t rotate at a constant angular velocity. The shaft speed continuously changes as the shaft rotates. The driven shaft can transmit the effects of the cyclic velocity changes to the drive shaft as torsional oscillations. The cyclic angular velocity effects on the pumping element can be largely eliminated by using two Hooke's joints in series, one at each end of the drive shaft.

Fig. 72 A typical vertical pump with end-suction connection, Allweiler AG

Fig. 73 A typical horizontal monoblock "tie-bolt" pump, Allweiler AG

Fig. 74 A progressive cavity pump

Fig. 75 Variations of flexible couplings for progressive cavity pumps

However this approach is only completely effective when the two joint angles are identical. The rotor is housed within a flexible stator and does have a degree of freedom regarding positioning. The elimination of the cyclic angular velocity effects on the pumping process, can not be guaranteed. They can be completely eliminated by using a different style of coupling, the constant velocity coupling, see (e) and (f). Both couplings use a crowned gear to mesh with a toothed annulus. The load is distributed over many tooth flanks and the angular motion is guided by a spherical bearing. (e) is a double-sealed version for higher operating pressures.

All coupling problems such as bearing wear, lubrication and cyclic angular velocity, can be managed by using a relatively new driving shaft style, the flexible shaft. The flexible shaft is just that. A drive shaft with proportions which are adequate to support the torsional and axial loads but thin enough to be able to bend to follow the end of the rotor. The flexible shaft is bolted or clamped between the bearing housing shaft and the rotor. In some cases the useful life of the flexible shaft may be limited by fatigue and routine replacement may be necessary. Given the full operating condition the pump manufacturer can select the most appropriate drive shaft option.

Increasing the length of the rotor and stator makes possible greater differential pressures. Pressure differential for a low pressure pump is 6 bar. Differential pressures up to 40 bar can be obtained by increasing the length. Pumps are available for flows over 300 m^3/h.

Progressive cavity pumps are used for practically all types of liquid from very fluid to very viscous. They can handle liquids containing abrasive contaminants and are relatively insensitive to solid particles. Larger pumps can allow the passage of random hard particles of 30 mm and soft particles over 100 mm.

Special adaptations can be made to feed very viscous product in to the pump suction. Progressive cavity pumps have good suction capacity up to 8 m but their extreme sensitivity to dry running requires venting and priming prior to starting for long stator life.

Many different materials are used in the manufacture of the pump casings and rotors ranging from cast iron to titanium. The stator can be made from a wide variety of elastomers, for example natural rubber, nitrile rubber, Neoprene and Viton&. In order to select the correct elastomer it’s necessary to know the chemical properties of the liquid to be pumped, the nature of any solids and the temperature at which the pump is to operate.

The choice of material is often a very difficult and complex matter due to the wide range of materials available. When in doubt, consult a manufacturer who has experience of the duty.

__5.8 Lobe pumps (including circumferential piston pumps)

Lobe pumps have two rotors, which, unlike gear pumps, operate without metallic contact with each other. Both rotors are driven by synchronizing gears which are completely separated from the pump chamber, see Fig. 76. The pump shaft bearings are also situated in the gear case. The pumped liquid does not therefore come into contact with the bearings. Synchronizing drive gears cause the rotors to rotate in opposite directions.

The inlet liquid flow is divided into two halves, trapped in the space formed between the rotor and the pump casing and transferred, without change in volume, towards the outlet where the rotors meet, thereby reducing the cavity and forcing the liquid out, Fig. 77. Lobe pumps are not usually applied to pressures over 30 bar so there are no problems with volume changes due to compressibility.

The absence of metallic contact between the surfaces of the rotors themselves or between the pump casing and the rotors means that wear of the rotating parts is insignificant. The only wear which occurs is due to erosion of the pumped liquid or entrained solids. Since the shaft bearings are normally mounted outside the pump casing, the shaft ends are relatively long and unsupported which imposes limits in terms of working pressure.

For pressures in excess of 12 bar some manufacturers fit plain bearings inside the pump casing. Others mount an extra bearing bracket on the outboard side of the pump casing. The latter case giving rise to four shaft seals.

The profile of the rotors varies from manufacturer to manufacturer. The most usual is shown in Fig. 78. The profile does not alter the principle of operation. It can be said however that rotors with one or two lobes give rise to greater pressure pulsations than rotors having three lobes. For gentle handling of liquids, rotors having one or two lobes should be chosen.

Rotary piston pumps are lobe pumps with greatly modified lobe profiles. The rotors are designed so that much more tip surface area is presented to the bore of the casing. This modification changes the pump viscosity characteristic and also the pumping action. The liquid is no longer squeezed out of the pockets between the lobes but is pushed directly out of the discharge.

Fig. 79 shows two typical rotor profiles.

Fig. 76 Exploded view of a lobe pump

Fig. 77 Lobe pump working principles

Fig. 78 Lobe pumps popular rotor profiles

Fig. 79 Rotary piston pumps rotor profiles

Fig. 80 Dense-pulp pump with feed screw

Fig. 81 Operating principles for vane pumps

Lobe and rotary piston pumps are suitable for both fluid and viscous products. Special feeding arrangements are necessary for extremely viscous liquids which cannot otherwise flow. Liquid temperatures of up to 200 degr. can be handled if the clearances between the rotors and the covers are increased. By increasing the rotor side clearances it’s possible to handle liquid at temperatures of down to -40 degr.

To maintain constant temperature, pumps may also be fitted with heating or cooling jackets.

Lobe pumps and rotary piston pumps handle the pumped liquid very gently. Examples of this phenomenon can be found within the foodstuffs industry where pumps are used for pumping cooked pea soup, preserves containing whole berries and other similar applications including handling whole fish. In these cases the pumps are specially constructed to fulfill the hygiene requirements; being easily dismantled and suitable for washing by hand or in accordance with the CIP method.

Pumps can also be equipped for completely aseptic pumping for use within the pharmaceutical industry. They are also extensively employed in the chemical industry for handling both corrosive and non-corrosive products. Special dense-pulp pumps have been developed for the cellulose industry, these being suitable for pulp concentrations ranging from 6 to 30%, Fig. 80. Special derivatives are manufactured for sewage handling. These pumps can have adjustable rubber lobes.

Pumps usually operate at relatively low speeds and they are often used for high viscosity liquids. The pump is quiet running and delivers a largely pulsation-free flow. For most pumps the components in contact with the liquid are manufactured in stainless steel. Pumps for non-corrosive applications can have a cast iron casing with rotors and shafts made of steel. Sometimes the rotor material can be varied in order to increase the pump's suction capacity, for example, by the use of nitrite rubber rotors. Shaft seals consist of various forms of mechanical seals and stuffing boxes. Since the pump has two shafts which pass through the pump casing, two sets of seals are needed for each pump.

The rotary piston pump is a successful variant of the lobe design. Also called the circumferential piston pump, it uses arc shaped pistons or rotor wings, as shown in Fig. 79. The rotor wings are manufactured in a corrosion resistant, non-galling copper free nickel alloy and are designed to operate with very close clearances. This feature, combined with the geometry of the rotor wings, produces a long sealing path between inlet and outlet resulting in minimal slip when pumping low viscosity liquids. On viscous liquids and when handling products with solids in suspension, the large liquid cavities in the rotor wings combined with low operating speeds and carefully profiled anti-cavitation ports produces a smooth, pulsation free, low shear pumping action. Twin wing rotors are fitted for the majority of duties; single wing rotors are preferred for easily damaged products.

Lobe pumps and rotary piston pumps are available in a wide variety of sizes from 0.1 to over 300 m3/h. A sewage version can handle 900 m3/h at 6 bar. Discharge pressures are normally up to 30 bar. Suction capacities vary between 1 and 5 m, depending upon internal clearances, pump size and speed. Solids handling capabilities are good with solids up to 100 mm on the largest pumps. (cont. in part 2)

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