Positive displacement pumps--part 2

Home | Using Industrial Hydraulics |

Applications of Computer-Aided Manufacturing

__5.9 Vane pumps

In a rigid vane pump the rotor is eccentrically located in the pump casing and the vanes slide in and out to maintain contact with the casing wall. In a flexible vane pump the rotor is mounted eccentrically in the pump casing and the vanes flex to maintain contact with the casing wall. The pumping action is created by the variation in volume between the vanes, Figs. 81 and 82.

In both types of pump the liquid is drawn into the pump by an increasing volume between the vanes on the suction side, transported to the discharge side whilst trapped between the vanes and a proportion forced out by a decreasing volume.

The vanes in sliding vane pumps may be controlled by springs, hydraulic pressure or rely entirely upon centrifugal forces induced by rotation. Some types use a rotating cam to guide the movement of the vanes.

There are many types of vane pump where the vanes are located in the pump casing (stator), Fig. 83.

Another variant of vane pump is the sliding shoe pump where the vanes are U-shaped and operate against two separate surfaces, one against the rotating cam, whilst another surface slides against a valve plate, Fig. 84.

The pump casing and rotors for vane pumps with sliding vanes are usually made of cast iron, gunmetal or stainless steel, whilst bronze or glass fiber reinforced PTFE is used for the vanes.

Vane pumps with sliding vanes are suitable for most clean liquids and especially suited for those with entrained gases or those having a low latent heat; petrol or ammonia for example.

Maintenance costs are relatively low due to the ease of replacement of the vanes, even when being used to pump liquids which are somewhat contaminated, waste oil for example. Vane pumps can operate within a large range of viscosities, although it’s necessary to compensate for the speed, i.e. the higher the viscosity the lower the speed. This phenomenon is common to most displacement pumps. Vane pumps are capable of suction lifts of 2 to 5 m. Flexible vane pumps are suitable for discharge pressures up to 5 bar, sliding vane process pumps of over 10 bar. Sliding vane pumps for hydraulic power applications can run at over 3000 rpm at pressures to 200 bar.

In a flexible vane pump, the rotor (flexible impeller), made of a synthetic elastomer, creates a seal between suction and discharge side. The pumps are therefore dry self-priming with a suction lift of 4 to 5 m, i.e. don’t require to be vented and primed. With liquid in the pump it’s capable of a vacuum of up to 500 to 600 mm Hg.

As the rotor material depends on the liquid pumped for lubrication, it’s generally recommended that the pump should not operate without liquid for more than 30 seconds. This is more than sufficient time for self priming with properly designed pipework.

Flow capacities for flexible vane pumps range from 0.1 to over 30 m^3/h, sliding vane pumps are available for over 300 m^3/h.

The single rotor, multi-vane principle, provides largely pulsation-free flow and enables the pump to handle products susceptible to damage.

Chemical resistance information, compiled by manufacturers of flexible vane pumps, provides a guide to the compatibility of various elastomers with liquids commonly used in industry.

Broadly speaking the temperature range of some of these rotor materials is as follows:

--Neoprene 4 to 80 degr.

--Nitrile rubber 10 to 90 degr.

--Viton| 4 to 90 degr. However, as with any type of pump when pumping liquids which are not cold, great care must be taken to observe suction limitations and provide sufficient NPIP. (Viton is a registered trade mark of DuPont Performance Elastomers LLC.)

Fig. 82 Vane pump with sliding vanes in the rotor

Fig. 84 Sliding shoe pump Delivery phase; Suction phase

Fig. 85 Peristaltic hose pump

__5.10 Peristaltic pumps (including rotary peristaltic pumps)

Peristaltic hose pumps

A peristaltic hose pump transports liquid by mechanically squeezing a space enclosed by a flexible element. The most usual peristaltic pump, the flexible hose pump, operates by means of rollers or cams acting directly upon the hose containing the liquid, Fig. 85. The number of rollers or cams vary depending upon the manufacturer; industrial pumps have 2 or 3 but small laboratory scale pumps can have as many as 7. Fig. 86 shows an industrial size pump with two spring-loaded rollers. A pump may be fitted with one or more hoses, sometimes as many as thirty.

At one time the use of peristaltic pumps was confined largely to laboratory and similar specialized applications but developments in rubber technology have resulted in a new generation of heavy duty pumps and peristaltic units are now to be found on a wide range of industrial duties involving viscous and/or abrasive sludges and slurries as well as products with high solids content.

An important factor in favor of the peristaltic concept is the pump's ability to handle solids up to the full bore of the hose tube. With most pump designs, solids handling capacity is restricted to approximately 15% of the pump branch sizes.

In one modern heavy duty design the roll/squeeze peristaltic movement takes place in a sealed housing partially filled with a mixture of glycerine and glycol. This not only lubricates the cams or sliding shoes from which the roll/squeeze motion is derived; it also acts as both coolant and lubricant to the exterior of the pump hose tube. This prolongs hose life and reduces the incidence of mechanical failure. Hose life in excess of 3,000 hours is quite common.

The same design uses a hose tube with an inner core of soft natural rubber or NBR rubber with an outer covering of hard natural rubber reinforced with braided nylon. During the roll/squeeze phase of the peristaltic cycle, solids present in the pumped liquid are cushioned in the soft inner core of the tube with minimal damage or erosive effect and are released gently back into the liquid stream when the squeeze cycle is concluded. This enables delicate, shear sensitive solids to be pumped without damage and abrasive material to be handled with little or no effect on the hose interior.

The peristaltic design is inherently sealless, with the pumped liquid completely enclosed with no possibility of leakage except in the case of a hose failure. Failure from a cause other than mechanical stress is rare and, under stable operating conditions, can often be predicted with considerable accuracy. Regular hose inspection is of course advisable.

Peristaltic pumps can be used for all types of liquids and to some extent even gases. The suction capacity varies greatly for different constructions. Discharge pressures are usually below 10 bar but pumps have been used up to 40 bar; tube life is extremely short! Large pumps, with 125 mm tubes, are rated at 2.5 bar. The capacities vary from very small metering quantities of 0.001 I/h up to flows of 50 m^3/h. Special pumps are available for over 3000 m3/h.

Rotary peristaltic pumps

Another type of peristaltic pump, sometimes called an orbital lobe pump, works by means of an eccentric rotor operating within a flexible elastomeric element with the liquid trapped between the elastomeric element and the pump casing. The rotation of the eccentric rotor transfers the liquid from the inlet to the outlet, Fig. 87.

This pump is leak-free, conditional on the flexible elastomeric element not failing as a result of the mechanical stresses. The elastomeric element should therefore be replaced regularly to avoid unexpected failures. In order to select the correct elastomer and the correct pump casing material it’s important to know the chemical properties and temperature of the liquid to be handled. Capacities range from 0.06 to over 50 m3/h. Suction capacity 1 to 3 m. The pump creates a strongly pulsating flow making it advisable to use flexible hoses in the suction and delivery lines. If fixed pipes are used some type of pulsation damper should be fitted between the pump and the pipe. Rotary peristaltic pumps are suitable for pressures over 10 bar.

__5.11 Rotary eccentric piston pumps

The rotary eccentric piston pump can be considered as a type of single wing circumferential piston pump or a peristaltic pump without a tube. The casing forms a circular cylinder. The piston is circular but of a smaller diameter than the cylinder. The piston does not rotate but is forced to precess eccentrically around the cylinder with a minimum clearance. A viscous seal forms in the minimum clearance to reduce internal slip. Incoming liquid is swept around the circumference of the cylinder to the discharge port. The pumping action produces very little liquid shear and is extremely gentle.

There are two slightly different versions of the pump in common use. One version uses a bearing bracket very similar to an end-suction centrifugal pump. Piston eccentricity is maintained by spring loading. These pumps have a U-ring seal, mechanical seal or stuffing box to match the liquid characteristics. These pumps cannot run dry without special sealing arrangements.

The other version uses a special non-rotating eccentric drive arrangement which does not require a rotating seal. Sealing is accomplished by rubber or metallic bellows which allows the pump to run dry. Both pump styles are self-priming and reversible. Fig. 88 shows an exploded view and Fig. 89 a longitudinal cross-section of an eccentric drive pump.

The rotary eccentric piston pump is manufactured in cast iron, steel, bronze and stainless steel and can be built to hygienic and aseptic requirements. Test have shown that CIP with 120~ water is completely effective. The pump is used extensively in the dairy industry for handling milk and milk products.

Other pumped products include; alcoholic drinks, cake mixes, chocolate, ink, jam and marmalade, mineral oil, paint, resin, soap, toothpaste and varnish.

Pumps can handle flows up to 85 m3/h at pressures up to 12 bar.

Fig. 86 Industrial size peristaltic pump

Fig. 87 Rotary peristaltic pump

Fig. 88 Exploded view of an eccentric drive pump

Fig. 89 Longitudinal cross-section of an eccentric drive pump

Fig. 90 Crank drive piston pump

__5.12 Axial and radial piston pumps

These are piston pumps for hydraulic power applications and water pumps as well!! They are characterized by their lack of external lubrication and not having any seals on the piston. All moving parts are lubricated by the product. Early systems all used mineral oil as the motive liquid and this may have been a standard industrial lubricating oil. Installations in hazardous areas, where the problems caused by any fire were great, looked for replacement liquids to reduce fire risks. Mixtures such as water/glycerine and water/glycol were tried but equipment needed derating to cope with the poor lubrication properties.

Soluble oils were developed which led eventually to 95/5 and 97/3 water/oil emulsions.

Modern oils are biodegradable and can be disposed of in normal sewers. However, they are also costly and so leaks are kept to a minimum. Piston leakage is controlled by the clearance between the piston and the cylinder bore. Any leakage returns to suction. Piston pumps for hydraulic applications fall into two broad categories; axial piston and radial piston. Inline piston pumps, the forerunner of modern axial and radial pumps, are still used for a few applications.

Axial piston pumps consist of a number of cylinder bores in a rotating block. The pistons can be anchored in various ways to produce the stroke as the cylinder block rotates. Most pumps have pistons which are driven sinusoidally. The stroke can be variable. Variable flow can be produced from fixed speed pumps. Some pumps can adjust the stroke so that the pump runs in reverse, i.e. the suction becomes the discharge. Suction and discharge valve functions are performed by ports in the cylinder block support plate which connects cylinders to the suction or discharge at the correct part of the stroke.

Radial piston pumps have multiple cylinders spaced radially around a central cam shaft. Most pumps are fixed stroke.

Valves can be poppet, like engine valves, or ball; both spring loaded and automatic.

Inline pumps are similar in construction to inline car engines.

Several cylinders are mounted above a cam shaft. Pumps tend to be of fixed capacity.

Pumps are available for pressures up to 670 bar for flows up to 8 l/s and operate at speeds up to 5000 rpm. Applications for these pumps are the hydraulic systems found in all parts of industry. The list below is not exhaustive:

--Agricultural machinery

--Coal mining

--Crude oil production

--Direct-acting process pumps

--Earth moving equipment

--Machine tools

--Power transmission

--Raw steel products

--Steelmaking

Pumps are now available with axial piston designs in ceramics for use on water and seawater. Low cost offshore developments are in seabed mounted wellheads with pipelines linking to centralized facilities. Wellhead hydraulic requirements serviced by seawater would have no leakage or liquid shortage problems. Optimizing proportions and material combinations to reduce wear and extend operational life is a continuing priority.

Current designs are limited to 160 bar.

__5.13 Inline piston pumps

Piston pumps are similar to plunger pumps but have a moving seal on the piston. Piston pumps can be single acting or double acting. Double acting pumps have a stuffing box similar to a plunger pump stuffing box. The piston is driven by means of a rotating crank or eccentric, connecting rod and crosshead, see Fig. 90. This figure shows the use of Mission valves, the most popular type, and has an internal single reduction gear set built into the crankcase. (See Fig. 96 for other valves for plunger pumps.) Applications for piston pumps are wide ranging but duties on drilling rigs handling drill mud and crude oil production far exceed all others. Piston pumps are ideal for viscous liquids and handling solids which are not abrasive. Special pumps have been developed for pumping coal. Coal can be transported as a suspension in water or can be blended with oil to form a fuel for direct injection into boilers. Most applications are below 140 bar.

The comments regarding self-priming and cavitation in Part. 5.15 may be helpful and apply equally to piston pumps.

Piston pumps are manufactured mainly in cast iron, cast steel and abrasion resistant chrome iron.

High pressure water is used for descaling and cleaning. The high pressure produced by the plunger pump is converted to kinetic energy in a nozzle. The high velocity water jet impacts on any surface converting the kinetic energy to a destructive force.

Pump packages are produced in several configurations:

--Electric motor driven

--Engine driven

--Skid-mounted

--Two or four wheel site trailer

--Two or four wheel road-going trailer with brakes and lights

--Vehicle-mounted

Packages may be complete with a water tank or have suction hoses for connection to a water supply. Facilities may be incorporated for hose reels and lance storage. Units may be capable of supplying more than one lance simultaneously.

These packages are produced to standard designs by pump manufacturers and by specialist firms who purchase all the components, including the pump. Horizontal and vertical pumps are used for these applications. If a standard package is not available to fulfill a particular requirement it’s possible to buy a purpose-built unit.

The following equipment should be fitted, as a minimum, to all systems:

--Suction strainer with Dp indicator

--Pressure control valve for each operator

--Relief valve

--Low suction pressure cut-out and indicator

--Driver high temperature cut-out and indicator

--Pump high temperature cut-out and indicator

Pumps for high pressure descaling and cleaning usually run at 690 bar to 1050 bar, but manufacturers have offered pumps for 1900 bar. Portable electric motor driven packages are available from 7.5 kW. Skid-mounted engine units are available to over 1000 kW. These packages are used for many applications:

--Concrete cleaning

--Drain cleaning

--Graffiti removal

--Heat exchanger tube cleaning

--Insulation removal

--Paint stripping

--Ship hull cleaning

--Vessel cleaning

__5.15 Plunger pumps (includes horizontal and vertical)

Plunger pumps are available from single cylinder hand pumps, capable of pressures over 300 bar, to nine-cylinder vertical pumps absorbing over 3000 kW. Pumps are built with 1,3, 4, 5, 6, 7 or 9 cylinders. Horizontal pumps have 1,3, 5 or 7 cylinders.

Vertical pumps have 3 or more cylinders. Increasing the number of cylinders increases the pump capacity if speed and stroke remain constant. Also increasing the number of cylinders tends to smooth the flow variations, reduces pressure pulsations and torque variations at the crankshaft. Pulsation 1.5.14 Descaling pumps

These include portable plunger pump packages for high pressure descaling/cleaning dampers can be fitted to both suction and discharge pipework to further attenuate pressure pulsations.

It must be stressed that flow variations and pressure pulsations are not just a function of the number of cylinders in the pump.

Pump speed and the pipework system design have significant effects which cannot be ignored. Changes in pipework can eliminate the most serious problems. Acceleration head losses in the suction pipework are always considered when assessing NPIP available.

The horizontal pump shown in Fig. 91 has suction valve unloaders fitted and would be driven through an external gear box or V-belt drive. Liquid ends can be built in different configurations for various applications. The monoblock liquid end shown is typical for standard pumps on non-corrosive applications. Other designs are available with separate manifolds and externally clamped valves.

Vertical pumps, Fig. 92, are used for the most arduous applications. More space is available for special liquid end designs, bigger valves and longer stuffing boxes. The vertical pump shown is a modern pump design. The original vertical pump designs had the liquid end at ground level with the crankshaft above, see Fig. 93.

Fig. 91 Horizontal plunger pump with monoblock liquid end and wing guided valves

Fig. 92 Vertical plunger pump

Fig. 94 shows a good horizontal pump installation. Notice the close proximity of the pulsation dampers to the pump connections; remote dampers are less effective. Also notice the suction pipework is larger than the pump connection.

Fig. 95 shows a vertical plunger pump under construction.

At the time it was believed to be the largest reciprocating pump operating in the North Sea offshore industry.

Plunger pumps can be fitted with a variety of valves to suit the application, see Fig. 96.

--Plate valves

General purpose valves for clean liquids with excellent dynamics, can have low NPIPr, suitable for speeds over 300 rpm. Mass produced by proprietary manufacturers as well as individual pump manufacturers. Generally limited to 400 barg. Standard horizontal pumps, with monoblock liquid ends usually have plate valves.

Plug valves Heavy duty valves for clean liquids, medium NPIPr characteristics, usually operated below 350 rpm.

Specially made by pump manufacturers for applications up to 550 barg.

--Wing-guided valves Can be designed as light or heavy duty, has good dynamics, low NPIPr and suitable for all speeds. Available as a proprietary design and also as specials by pump manufacturers. Suitable for pressures up to 550 barg, some designs can have an elastomeric seal for solids handling, 5% by volume approximately up to 500 mm.

--Plug-guided ball valves Suitable for clean liquids at the highest pressures. Dynamics are poor and should operate below 300 rpm.

-- Mission valves Proprietary valves manufactured by TRW Mission specifically for oil well mud pumps. Poor dynamics, high NPIPr and should operate below 200 rpm. Can handle solids, up to 1 mm in the larger valves, at concentrations up to 65% by weight. Designed for low abrasion solid mixtures, less than Miller Number 50.

--Ball valves for solids Special valves made to order. Almost the ideal solids handling valve. Suitable for the most abrasive solids at concentrations of 65% by weight. Solids up to 6 mm in the larger valves. Dynamics can be poor, can be very low, largest valves suitable for speeds below 150 rpm. Long parts life, but requires lots of space and is costly.

Also very good for high viscosity liquids.

Fig. 93 Low pressure vertical sewage plunger pump, Dawson Downie Lamont Ltd

Fig. 94 Horizontal pump installation

Fig. 95 A vertical plunger pump under construction, Flowserve Corporation

Fig. 96 Valves for plunger pumps: Outside guided plate valve; Center guided plate valve; Outside guided plug valve; Center guided plug valve; Wing guided valve; Plug guided ball valve; Mission valve; Ball valve for solids

The choice of material for the valve and the valve seat is critical.

The valves are opened by a liquid differential pressure and held open by drag forces. Closure is spring-assisted. Allowances must be made in the valve design for shock loading due to rapid closure at higher speeds and pressure pulsation loading.

Sometimes valves can be actuated hydraulically for special applications.

Modern pumps are designed to operate at mean plunger speeds of 1.5 to 2 m/s. Pumps may operate a lower speeds to achieve better NPIPr or to suit operating conditions. The pressure developed by any plunger is related to the plunger load; the force applied by the crankshaft/connecting rod/crosshead to the plunger. Current designs include plunger loads over 400kN. The liquid is prevented from escaping from the liquid end by the stuffing box. Various seal designs, from simple to complex, can be fitted to suit the liquid and the environment. Pumps handling abrasive solids in suspension can have the front of the stuffing box flushed with clean liquid to protect the packing. Pumps with very low suction pressures can have a quench across the back of the packing to prevent air being induced. A quench can also be used to prevent hot liquid vaporizing. Liquid can be circulated between seals to cool, lubricate and/or remove vapors.

Single shot lubrication, where the lube oil mixes with the product and is lost, is the most popular stuffing box system.

Pumps are selected by adjusting all the variables; speed, stroke, plunger diameter; to suit the application. Knowledge of the liquid compressibility is essential for proper selection when the differential pressure exceeds 40 bar.

The plunger diameter is calculated from the plunger load. This diameter is rounded down to a standard seal diameter. The volumetric efficiency is calculated based on the pump proportions, the differential pressure and the liquid compressibility. The flow per revolution is then calculated based on the plunger diameter, stroke, volumetric efficiency and number of plungers. The pump speed can then be calculated for the rated flow. If the pump speed is too fast, more cylinders can be considered or a larger pump used with a higher plunger load and consequently larger plunger diameter. For pumps to run at full speed adequate NPIP must be available.

Plunger pumps can operate with high suction pressure; over 135 bar in some applications. High suction pressure is defined as a suction pressure which produces a plunger load higher than a prescribed limit. Some pump designs use 5% as the benchmark, others 20%. High suction pressure must be considered in the selection process because it affects volumetric and mechanical efficiency. Mechanical efficiency is reduce because of increased friction losses during the suction stroke.

Plunger pumps should not be used in self-priming applications unless the manufacturer has fully approved the operating conditions. Pumping air or vapor corresponds to 100% cavitation and will do no damage. The damage will occur during the transition from 100% cavitation to zero cavitation. Very high internal pressures may be created, far higher than the normal discharge pressure. This will lead to increased wear and component failure due to fatigue.

The most common failures are:

--High bearing wear and failure of bearing shells or bail/ rollers

--Crankshaft failure

--Crosshead failure

--Very short packing life

--Liquid end stud/bolt fracture

--Suction valve failure, rapid seat wear

--Cylinder cracking

Plunger pumps are capable of working at the highest pressures, with the most difficult liquids and with abrasive solids if necessary. Pressures to 500 bar are common in process applications. Discharge pressures of 1000 bar are common in descaling and high pressure water jetting. Pumps for 3500 bar are available to order. Flows up to 500 m3/h are possible at pressures up to 100 bar.

In addition, plunger pumps are used for low pressure, difficult applications where good suction capabilities are required and low speed is essential to reduce wear. Tank bottom residues, sediment, industrial and municipal wastes and sewage are typical mixtures handled by the original vertical pump design, shown in Fig. 92.

The crankcase assemblies are standard using mostly cast iron.

Liquid ends, stuffing boxes and valves can be made in almost any material. Standard materials include cast iron, bronze, steel, stainless steel, duplex stainless steel. High pressure pumps require materials with high fatigue endurance stress levels, such as 1% CrMo, 15-5PH and 22Cr 13Ni 5Mo.

Fig. 97 shows one of the latest developments in plunger pump technology. Most plunger pumps are 'slider-crank', this is cam-operated. Even more unusually is that this is an axial pump. The pump shown is effectively a much larger version of the axial piston pumps used for hydraulic fluid power. The cutaway view in the Figure shows the construction. The pump is a 'vertical plunger' almost in the style of the original crankshaft pumps. Cam-driven pumps have been tried many times; not always with much success.

Fig. 97 A cam-operated vertical plunger pump. National Oilweli Varco.

Currently this is the largest cam pump in production. Having six cylinders is very unusual and defies the simple (useless) theory used by most engineers. The stuffing boxes and valve assemblies are built as 'cartridges' for easy maintenance. The two drive motors, are integrated in the pump and drive through a single reduction gear. The pump is capable of about 1800 hydraulic kilowatts. Of course, there are flow and pressure limitations, but a useful pump for clean liquid applications.

Plunger pumps applications are diverse. The following list highlights some of the important areas:

--Boiler feed pumps

--Carbon dioxide injection for crude oil recover

--Chemical processing

--Crude oil/water emulsion recovery

--Domestic high pressure cleaners

--Ethylene glycol injection at wellheads

--Garage carwashes

--Gas drying

--Gas sweetening

--High pressure water jetting

--Industrial waste

--LNG re-injection

--Methanol injection at wellheads

--Municipal waste

--Oil/water emulsion hydraulics

--Reverse osmosis

--Sewage

--Soap powder manufacture

--Tank residue disposal

--Viscous crude oil pipelines

--Water injection for crude oil recovery

__5.16 Syringe pumps

Syringe pumps are different to most of the other pump types considered. Syringe pumps cannot operate continuously, only intermittently. Syringe pumps can only be used for batch processes. One of the biggest problems encountered with pump applications is ensuring an adequate supply of liquid to the pump. This problem increases as the liquid handled becomes more viscous. Syringe pumps overcome this problem by removing it from the normal pump operation completely; syringes must be filled before the pumping operation commences. As filling is divorced from discharging filling can take as long as necessary.

Syringe pumps consist of one to ten syringes and a mechanical/electric drive unit, see Fig. 98. Normal glass and plastic medical syringes can be used, up to 21 bar, and stainless steel syringes can be used for high pressure applications, 100 bar.

Fig. 98 Syringe pump system

K D Scientific Inc

Fig. 99 Mechanical diaphragm pump

Fig. 100 Portable engine driven pump

Gilbert Gilkes & Gordon Ltd

__5.17 Diaphragm pumps (includes mechanical and hydraulic actuation)

Plunger pumps and piston pumps have stuffing boxes which are potential areas of leakage. Seals, such as mechanical seals, are not yet available for reciprocating pumps. The problem of normal leakage can be completely eliminated by using a diaphragm pump, see Fig. 99. Reciprocating motion is produced in the conventional manner (the Figure shows an eccentric shaft rather than a crankshaft). The motion from the crosshead is transmitted directly to a diaphragm.

The pump shown has ball valves. Liquid would enter through the bottom valve and be forced out of the top valve; the liquid end would be self-venting. Pumps of this horizontal style are built with one, two or three cylinders although there is no practical limit on the number of possible cylinders. Horizontal pumps are also built with a Scotch Yoke mechanism to produce the reciprocating motion. No crosshead is required if two cylinders are mounted back to back, the diaphragm motion rods can support the yoke. Scotch Yoke mechanisms tend to have more friction than crankshafts or eccentric shafts. Mechanical diaphragm pumps are also constructed as vertical pumps; these are nearly always single cylinder pumps but twin cylinder pumps are built. Single cylinder vertical style pumps are also called Single Disc pumps. Most versions of the vertical pump don’t have a crosshead; the connecting rod is coupled directly to the diaphragm. This style of construction imposes extra loads on the diaphragm.

Some versions of the vertical pump have a spring-loaded coupling incorporated in the connecting rod, see Fig. 100 which shows a portable engine driven pump. The spring loading allows the diaphragm to short stroke if large solids restrict the diaphragm motion. The vertical style is popular as a site pump and can be engine driven. Pumps can be fitted with flap, ball or duck-billed valves as appropriate for the operating conditions.

The mechanical diaphragm pump is usually a low pressure pump, discharge pressures up to 6 bar are possible. Site pumps may be only capable of 1.5 bar. As a low pressure reciprocating pump it has some useful advantages over both piston and plunger pumps. Pumps are dry self-priming and when the valves are maintained in good condition suction lift capabilities of 6 m are achievable. Continuous dry running is acceptable.

The mechanical diaphragm pump can snore. (Snoring describes a pumps ability to run dry when the liquid supply disappears and then to self-prime and pump when the liquid supply returns.) Depending upon the type of valves fitted the pump can handle large solids and high viscosity liquids.

Mechanical diaphragm pumps can be constructed from a variety of materials, to suit many applications, see Tbl. 3. Metal diaphragms can be used for high temperature applications.

===

Tbl. 3 Popular mechanical diaphragm pump materials

Liquid ends Aluminum Cast iron SG iron Bronze Stainless steel Polypropylene PVDF PTFE Diaphragms Buna 'N' Neoprene Viton EPDM PTFE Valves Buna 'N' Neoprene Polyurethane PTFE Borosilicate glass

===

Mechanical diaphragm pumps can handle flows up to about 80 m3/h.

Flow variations and associated pressure pulsations may be a problem with single and twin cylinder pumps. Some pumps can be supplied with integral discharge air chambers. The manufacturers advice should be sort if cyclic flow variations could create operating problems. A section through a diaphragm pump with a pneumatic unloader valve option is shown in Fig. 101.

A special derivative of the mechanical diaphragm pump has been developed specifically for viscous liquids and stringy solids handling. The Double Disc pump utilizes two rubber diaphragms which act as pumping element and valves. The first diaphragm opens and allows product into the pump. On closing the diaphragm forces the product into the second chamber where the diaphragm is open. When the second diaphragm closes the product is pumped into the discharge system. These pumps are only capable of generating 3 barg but have found favor in the sewage and waste water treatment industries. Flow rates up to 25m^3/h are possible.

Fig. 101 Diaphragm pump, Uraca Pumpenfabrik GmbH & Co KG

Fig. 102 Compressed air-driven diaphragm pump

__5.18 Air-operated double-diaphragm pumps

Air-driven diaphragm pumps are a style of direct-acting pump which utilize compressed air to deflect the diaphragm. This type of actuation imposes much lower stresses on the diaphragm because of the high degree of pressure balancing. The basic pump construction is very similar to the pumps in Part. 5.17, suction valves at the bottom and discharge at the top, but two diaphragms are mounted back-to-back, see Fig. 102.

Compressed air is applied to the inside face of the diaphragms, alternatively, to create the stroke cycling. Three styles of valves are popular; flap, plate or cone, or ball. The choice of valve is dictated by the type of product to be handled and the size of solids. Flap valves can handle the largest solids.

Diaphragm pumps are leak-free since there are no plungers or piston rods which pass through the process pressure boundary.

Leakage can occur however if the diaphragm fails. It’s therefore necessary to replace the diaphragms regularly, as part of a preventative maintenance program. Some models are available with a double-diaphragm incorporating a rupture detection system. The material used for the diaphragm is usually an elastomer such as nitrile rubber, Neoprene or fluorocarbon.

Pumps suitable for hygienic applications are available. Fig. 103 shows a non-metallic pump. This particular design is manufactured in polypropylene, Acetal or PVDF and suitable for pressures up to 8.3 barg.

The air-driven diaphragm pump is not a fixed speed pump like an electric motor driven pump. Because the power supply is compressed air the pump is not fixed to an electricity supply frequency. Diaphragm pump speed is controlled by the differential pressure between the compressed air and the liquid. Increasing the air pressure increases pump speed. Also, reducing the liquid pressure increases the pump speed. Compressed air flow control, as well as pressure control, may be required. Using compressed air in place of electricity means the pump is inherently safe and can be installed in any hazardous area. Also the pump can be submerged without creating a hazard.

Air-driven diaphragm pumps have good self-priming and suction lift capabilities. Suction lift can be up to 8m depending upon the pump size, speed and type and condition of the valves. Flap valves can allow the passage of solids which are 90% of the suction connection size. Industrial pumps are manufactured in a wide range of metallic and non-metallic materials; aluminum, cast iron, stainless steel, polypropylene, PVC, PVDF, PTFE. Industrial pumps can handle 60 m^3/h at pressures up to 7 barg when powered by factory air at 8.3 barg. Hygienic quality pumps can handle flows up to 28 m3/h. Air-driven diaphragm pumps always produce a pulsating flow because the pump design is twin cylinder. Problems caused by pulsating flow can be alleviated by fitting suction and discharge pulsation dampers.

Fig. 103 A non-metallic air-operated double-diaphragm pump

__5.19 Metering pumps (flow within _+ 0.1% to __ 3% with substantial dp changes)

Metering pumps are also called dosing pumps and proportioning pumps. Metering pumps are used to add small quantities of liquid to other liquid streams or vessels. Tbl. 4 shows examples of processes which use metering pumps.

======

Tbl. 4 Processes in which metering pumps are used

Pressure Flow Temp bar

Liquid Process Problem

hydrochloric acid water preparation corrosion mercury chemical high density lead metallurgical high temperature sodium chemical explosive nuclear fuel plutonium salt radioactive recovery soft drink sugar solution manufacture hygienic tungsten carbide spray drying abrasion suspensions chlorine chlorination toxic oxygen chemical low temperature explosion cleanliness vinyl acetate toxic high pressure polythene catalyst manufacture high pressure

====

Other typical applications include the injection of biocide into water and seawater pipelines to prevent the growth of organisms. Injection of methanol or ethylene glycol into wellheads to prevent the formation of hydrates. The injection of chemicals into seawater prior to reverse osmosis treatment.

The basic requirements of a metering pump are to deliver measured volumes of liquids accurately without significant changes in volume as discharge pressure varies. Typical accuracies would be +0.25% to + 3% of the rated flow. Positive displacement pumps are ideally qualified and specific variants of reciprocating pumps have evolved to fulfill the functions. Metering pumps are usually small plunger pumps, diaphragm pumps or peristaltic pumps for low pressure applications.

Plunger pumps used as metering pumps are modified versions of the pumps described in Part. 5.15. The main modification involves providing a variable stroke mechanism so that the flow can be adjusted while the pump runs at constant speed. In theory, pumps can operate from zero to maximum flow. In practice, accuracy can only be maintained from 10% to 100% flow.

Fig. 104 shows a plunger pump type metering pump fitted with ball valves on suction and discharge.

Raising or lowering the Z-shaped crankshaft changes the length of the stroke. Metering pumps are single cylinder pumps.

However, pumps can be ganged together on one drive shaft to form multiple units. This type of construction is useful when more than one liquid is metered and when several metering rates must be adjusted, in the same proportions, simultaneously. Stroke lengths and plunger diameters of each cylinder can be different. The stroke of each cylinder is adjusted individually to deliver the correct volume; the speed of all cylinders is varied simultaneously for collective flow adjustment.

Diaphragm metering pumps are of two different designs. Small pumps can have the diaphragm driven directly by an electric solenoid. Stroke length is adjusted mechanically and stroke speed electrically. Larger diaphragm metering pumps have the diaphragm actuated hydraulically, the hydraulic power being provided by a plunger pump. Fig. 105 illustrates the modifications necessary to add a diaphragm.

The movement of the diaphragm is limited by supporting walls with slots on each side of the diaphragm or by profiled cylinder walls. Hydraulically actuated diaphragm pumps can become complicated. Provision must be made to maintain the oil volume behind the diaphragm. Leakage through the stuffing box must be replenished to preserve accuracy. Also overpressure of the hydraulic oil may present problems.

Ball valves are the most popular choice of suction and discharge valve for metering pumps. Ball valves have the longest possible life while preserving good sealing capabilities. Metering pumps always run slowly and ball dynamics are not a problem. Some designs incorporate soft seals within the valve seat to improve accuracy. Pumps are often fitted with double spring-loaded ball valves in both the suction and discharge in order to reduce the risk of backflow through the valve after each respective suction and delivery stroke.

Fig. 104 Plunger type metering pump

Fig. 105 Hydraulic diaphragm type metering pump

Plunger style pumps are used for all pressure ranges when the liquid is not toxic and there is no risk of corrosion of the pump external components. Diaphragm pumps are used when any leakage from the pump is unacceptable. Because metering pumps run slowly there are no real problems with viscous liquids. The ability to handle solids in suspension is dependent upon the pump size and should be confirmed with the manufacturer.

Metering pumps can cause pressure pulsations. Pulsation dampers may be required on the suction to preserve the NPIPa. Dampers may be required on the discharge to eliminate pipework vibration and smooth out the dosing rate.

Metering pumps are available in a wide range of materials including complete construction in PTFE. Flow rates can be as low as 0.1 I/h and over 10 m3/h per head.

__5.20 Direct-acting reciprocating pumps (includes pneumatic, hydraulic and steam actuation)

Direct-acting, d-a pumps are one of the oldest styles of positive displacement pumps. Direct-acting pumps are fluid powered reciprocating pumps which have no rotary mechanisms; a reciprocating motor provides the power to drive a reciprocating pump. The air-driven diaphragm pump, in Part. 5.18, is technically a direct-acting pump but all the pumps within Part. 5.20 have piston motors. Pumps within this category can be divided into three subgroups which are listed in order of popularity:

--Air-driven pumps

--Hydraulically-driven pumps

--Steam-driven pumps

The steam-driven pump is the oldest style of d-a pump and became popular as a boiler feed pump around 1841 when Henry Rossiter Worthington of New York patented the direct-acting simplex steam piston pump. The duplex pump followed in 1859.

The motion of d-a pumps differs from slider-crank pumps because it does not tend to be sinusoidal. The motion of d-a pumps tends to be trapezoidal. The acceleration and deceleration of the pumping element(s) can be controlled independently of the (fairly) constant velocity which is maintained for most of the stroke. The effects of acceleration and deceleration can be alleviated by multiplexing or by pulsation dampers.

The air-driven pump is currently the most popular style of d-a pump because of its primary area of application which is pressure testing components and assemblies. All the d-a pumps have the ability to stall and maintain discharge pressure. This means the pumps can produce zero flow but maintain a predetermined pressure. A double-acting air cylinder drives one or two single-acting plunger pumps.

Fig. 106 shows an air-driven pump with two plunger pumps, one at either end of the double-acting air cylinder and has the air filter/regulator fitted. This is an old-fashioned design; about 35 years old; but is very useful to show the basic construction of the pump. Modern pumps are unitized; it can be difficult to distinguish the air and liquid ends. When air is applied to one side of the piston the liquid plunger is withdrawn from the cylinder; suction stroke. At the end of the stroke the air valve diverts the air supply to the other side of the piston and exhausts the pressurized side. The piston moves in the other direction and the liquid plunger forces liquid into the discharge system.

The pump continues to operate until the air and liquid forces are balanced. The air-hydro produces a discharge pressure which is a function of the air supply pressure and the pump ratio. The pump ratio is the ratio of the air piston area to the liquid piston/plunger area. Most pumps are designed to work with factory air at 7 barg. Air-hydros can produce pressures from 2.7 to 6900 barg.

Fig. 106 Typical high pressure air-driven pump

Some modern designs incorporate a two-stage effect. The pump can operate with either a large or small diameter plunger.

Under low liquid pressure conditions the large diameter plunger is used for fast filling. At a predetermined pressure the large diameter plunger stalls and the small diameter plunger continues pressurization. The d-a air-driven pump is capable of water flows of 100 I/min at low pressure and 0.351/min at 2000 barg.

There are many pump sizes available and flow is infinitely adjustable from zero to maximum. The high pressure d-a air-driven pump can be used for other applications such as clamping or water cutting. Special versions are available to work with refrigerants.

NOTE: Investigate the availability of high pressure pipework and fittings before spending a lot of time designing a system for over 420 barg. Pulsation dampers, or the size of pulsation dampers, may be an additional problem.

Air-driven pumps are also popular for barrel and container emptying applications. It’s possible to construct a long, slender piston pump with a cylinder outer diameter small enough to pass through the standard connection on drums and intermediate bulk containers (IBCs). Pumps are manufactured in standard diameters and lengths as shown in Tbl. 5.

== Pump diameter mm 27 34 42 76 84 120 Standard lengths mm 490 210,460,1000,1090 230,1020 225,1170 278,335,1000,1015 330,1110 327,1100 396,1130 465,900 Flow range I/min 2, 7.5 4, 6.7 6, 10.5 7.4, 12.5 14, 24 14,24 20,33 31, 52

Tbl. 5 Standard sizes of container emptying pumps

==== Pumps normally have ball valves so viscous liquids can be handled, up to 10000 Poise. Spring-loaded chevron packing is an option to reduce routine maintenance. Pistons have PTFE rings. Most pump parts are 304 or 316 stainless steel. Pumps have ratios between 1 and 33 to allow pressures up to 150 barg to be developed. Pressure pulsations may be a problem when pumps operate at high speed and high pressure. Some pump manufacturers produce their own pulsation dampers.

Hydraulically-driven pumps are used for two vastly different types of application; high viscosity/large solid mixtures and high pressure liquids. Hydraulically-driven reciprocating pumps are ideal for very arduous applications because the pumps can operate at very low speed to reduce wear and pressure pulsation problems.

One of the most popular applications for hydraulically-driven pumps is handling cement on building sites. For viscous liquids with small solids a standard double-acting horizontal piston liquid end can be used driven by a double-acting hydraulic cylinder. Hydraulic power is provided by a power pack. An axial piston swashplate pump is normally used to provide a variable hydraulic flow from a constant speed pump. Control is via a small programmable logic controller (PLC). Process liquid valves can be flexible elastomer for low pressure pumps, plate, tapered plug or ball. Cylinder liners and piston rods are normally protected by a substantial layer of chrome plate. Other coatings can be applied for improved wear/abrasion resistance. A single-acting piston liquid end can be used for extremely viscous liquids. A plunger pump liquid end, with actuated valves, can be used for the most abrasive products or very large solids. This style of horizontal pump is very versatile because the stroke length is easily adjustable to produce larger or smaller pumps.

Hydraulically-driven pumps are produced in low pressure, 2barg, and higher pressures, up to 160 barg, versions. There are no practical limits for the design pressures of special pumps. Standard pumps can maintain flows of 480 m3/h. NPIPr can be as low as 0.1 bar. In applications where the process medium does not flow easily a screw conveyor can be attached to the pump suction to provide a positive supply. Special provision can be made for self-priming and dry running. A vertical plunger pump version is very popular in the sewage treatment industry; these pumps are built as simplexes and duplexes. Standard pumps can handle 216 m3/h at pressures up to 20 barg.

A special version of the hydraulically-driven pump uses the process liquid as the motive power to produce a higher pressure.

This type of pump is usually called an intensifier. Intensifiers are used to produce the highest possible pressures, 10300 barg.

Intensifiers are used in water cutting applications and in oil well servicing applications.

Many pump users think the steam pump is obsolete and has been replaced by modern technology; this is not completely correct. New steam pumps are still installed in refineries and on board ships. Steam pumps are very good with high viscosity liquids and for tank emptying duties. Steam pumps, like other d-a pumps, can operate extremely slowly and can have very low NPIPr. Also, at low speed the d-a pump can handle liquid/gas or liquid/ vapor mixtures if the differential pressure is low. Steam pumps are ideal for tank stripping. Large tanks would be emptied quickly by a centrifugal pump. As the tank level reduced the NPSHa for the pump would reduce. At some point the centrifugal pump would cavitate and have to stop. A steam pump is then used to strip the dregs out of the tank. Steam pumps are usually double-acting piston pumps and are built as simplexes or duplexes; horizontal and vertical styles are available. Horizontal pumps are preferred for land installations and some marine users prefer vertical pumps.

Fig. 107 shows horizontal duplex piston pump with discharge pulsation damper. The suction connection, at the bottom, and the valve access covers are easily seen. Pumps are capable of flows up to 545 m3/h and rated pressures are from 10 to 55 barg. Steam conditions don’t normally exceed 17barg and 380 ~ Fig. 108 shows a special quadruplex plunger steam pump for a high pressure application. Pumps of this style are capable of 500 barg.

__5.21 Non-metallic positive displacement pumps

Positive displacement pumps are often described as 'low-flow, high-pressure' pumps; this description completely ignores the pumps which are designed for 2.5 barg discharge pressure. In simplistic terms, high-pressure pumps cannot be non-metallic because non-metallic materials are not strong enough to support the stresses generated. However, non-metallic materials do possess exceptional corrosion resistance. Providing the strength limitations are strictly observed, low strength materials can make very useful pumps. The low strength of materials can be partially compensated by external reinforcing. This technique is used on centrifugal pumps, see Figs. 52 and 7.9 in Section 7. External reinforcing substantially increases the allowable forces and moments which can be applied by piping.

Metallic pumps can have wetted surfaces coated with non-metallic material. These pumps should be considered as a special case of metallic pumps.

The most popular non-metallic positive displacement pump is the air-operated double-diaphragm pump; this is one of the best selling pump types of all categories!! Typical casing materials include Acetal, nylon, polypropylene (PP), polyvinylidene (PVDF) and polytetrafluoroethylene (PTFE). Diaphragms are slightly more complicated. Because of the flexure, fatigue due to stressing becomes a problem. Popular materials are chloroprene, neoprene, nitrile rubber, polyurethane, ethylene propylenediene (EPDM), Santoprene| and fluorocarbon alloys. The diaphragm can be reinforced with fibers, such as nylon mesh, to increase strength. Diaphragms can be coated, or molded in layers, so that a good flexible material could have its corrosion resistance improved. Neoprene can be coated with a Viton | alloy or nitrile rubber faced with PTFE.

Fig. 107 A typical horizontal duplex steam pump

Fig. 108 A special horizontal quadruplex plunger steam pump

Direct-acting pumps, driven by compressed air, are fairly easy to manufacture without metal components. Pressure ratings would be much reduced compared to their metallic relatives.

Some rotary pd pumps are easy to manufacture from 'plastics' which can be extruded. Pd pumps don't necessarily require complicated castings. Fig. 63 shows an external gear pump with a casing machined from a solid block; this manufacturing route can be applied to plastic bar of the appropriate size.

Other pump types which are amenable to this treatment include; internal gear pumps, triple-rotor and 5-rotor screw pumps, lobe pumps, circumferential piston pumps, vane pumps, piston pumps, plunger pumps, syringe pumps and metering pumps. The peristaltic pump is always a non-metallic pump. The only component in contact with the liquid is an elastomer tube. The pump structure is usually metallic, but not normally 'in-contact'.

When considering a non-metallic pump for a specific application, it’s important think about the piping at the same time. Flexible pipes, hoses, don’t transmit significant forces and moments to the pump connections. But, flexible pipes do try to straighten out when pressurized. Also they can 'kick' a lot when the liquid flow pulsates. Non-metallic pulsation dampers are available to alleviate these problems.

__5.22 Sealless positive displacement pumps

The term 'sealless' is being used irrationally by some pump manufacturers. The original intent of 'sealless' was to describe a pump which did not have a dynamic leak path requiring a dynamic seal of some description. Sealless meant a static seal; a stationary boundary. Most pump developments are driven by centrifugal pump requirements and new ideas are tried on them first. There are two solutions to the 'sealless' problem, canned drive motors and magnetic drives. Canned motors cannot be used on all applications because the process liquid may damage the motor. Magnetic drives can be used on a wide range of applications.

Manufacturers of peristaltic pumps are claiming them to be 'sealless'. This claim is true to the extent that the pump does not have a rotating shaft seal or a reciprocating rod seal. But, the pump does have a dynamic seal, the hose! The hose may fail due to wear or fatigue. Static seals don't usually wear and don’t suffer from fatigue problems. The peristaltic pump is a very useful pump but it isn't sealless! (One manufacturer claims his peristaltic pumps are contactless! Be very wary of pump manufacturers' literature!) Reciprocating pumps cannot be sealless. A diaphragm pump can be sealless to the same extent as the peristaltic pump; the seal is dynamic.

Fig. 109 A magnetic drive internal gear pump

Rotary positive displacement pumps can be sealless in the same ways as rotodynamic pumps. Canned motor pumps are very rare. Small sliding vane pumps powered by 12 V DC are used as petrol pumps on some vehicles; this is the only canned motor pd pump seen to date. Some rotary pd pumps are available as magnetic drive versions: internal gear pumps (see Fig. 109), external gear pumps, triple screw pumps and progressive cavity pumps. Pump materials are not affected. Just one last thought ..... the jet pump is sealless. Of course it’s a mixer as well. So why not multi-task and save time and money? It can be a heater as well ..... if you use steam! Fig. 109 raises a very interesting question. Why would an exotic shaft seal be necessary if threaded process connections were adequate?

__5.23 Hydraulic motors

Most positive displacement pumps can run in reverse and act as a motor. Reciprocating pumps with valves; diaphragm, piston, plunger; can't run backwards unless there is a major leakage problem with the valves. Both suction and discharge valves must be worn to allow this to happen. The pump may not rotate; this depends upon the speed ratio to the driver. Reciprocating pumps don't usually require a non-return valve in the discharge piping because the internal valves prevent reverse flow. Rotary pumps can usually run in both directions; not necessarily with the same effectiveness. Whether a pump can operate as a hydraulic motor, intentionally or unintentionally, is dependant upon the speed ratio to the driver and the nature of the power transmission and driver. Many rotary positive displacement pumps are installed without non-return valves and the possibility of 'turbining' exists. The level of static pressure in the discharge system is a critical factor.

Hydraulic motors are used extensively in hydraulic fluid power applications. The most popular styles are vane, gear and piston. This guide is concerned with the process applications of pumps and hydraulic fluid power is mentioned only in passing.

Process applications for hydraulic motors are relatively limited but this is likely to change as the move towards greater operating efficiency, to reduce energy consumption and energy costs, concentrates attention on process detail design. Hydraulic motors are used in applications where the inclusion produces obvious operational advantages.

One area of pump use has adopted hydraulic motors to improve operations, well drilling, most notably oil well drilling. Drilling rigs, for oil wells, use 'mud' to lubricate the drill bit and to remove cuttings/debris from the hole bottom. The mud is pressurized by pumps on the rig. The potential energy of the mud can be used to drive a motor. Progressing cavity pumps have been adapted to provide rotary power for the drill bit and other tools, reamers, re-reamers and under-reamers. These motors are usually called 'mud motors'.

Mud motors driving drill bits are used for three basic reasons: speed, reach and accuracy. Typically, a mud motor drill head will penetrate up to four times faster than a drill-string assembly.

Tests have been conducted with high pressure mud, 690 barg, so that the rotary drill action can be augmented by high pressure jets. Tests showed a speed increase up to eight times for drilling sandstone and limestone. Tests were very short-term due wear problems experienced by the high pressure nozzles.

Remember, drilling mud is abrasive; it's not like clean water.

Faster drilling speeds reduce production costs and allow a rig to drill more wells. Mud motors have been used to drill much longer angled and horizontal wells. This allows more reservoir to be coupled to a central base; cheaper installation. Mud motor assemblies have been controlled to within +1.5 meters; this might have been at the end of a 8000 meter well. Instrument packages can analyze the mud, close to the drill bit, and confirm the pressures and formation composition.

Mud motors must be designed to fit inside standard well casing bores. Typical sizes range from 1~6" (36.5 mm) to 11~" (286 mm). Motor assemblies range from about 2.2 m long to 7 m.

Stator designs follow the pump pattern, normal variable thickness elastomer and constant thickness elastomer. Power transmission from the rotor is accomplished by flexi-shaft, sometimes in titanium, or a shaft with two sealed flexible coupling. Bearings, radial and thrust, are sealed and lubricated.

Tests have been conducted with diamond-faced thrust bearings to cope with higher loads and produce smaller bearings.

Typically, motors run with a 15 to 35 bar differential pressure and rotate, when loaded, between 50 and 150 rpm. A small motor will require about 10 m^3/h.

Prev. | Next

Comprehensive Pumping Manual    Home     top of page