AU647475B2 - Pressure balancing for rotary ram pump - Google Patents

Description

4 7475

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIF ICATION FOR A STANDARD PATENT

ORIGINAL

6 Name of Applicant: ABB MINERAL SLURRY TRANSPORTATION PTY LTD Actual Inventor: Bengt Eskil Torbjom Ode Address for Service: Invention Title: SHELSTON WATERS Clarence Street SYDNEY NSW 2000 "PRESSURE BALANCING FOR ROTARY RAM PUMP" Details of Associated Provisional Application No: PK 0315 dated May, 1990.

The following statement is a full description of this invention, including the best method of performing it known to us:- 1 The present invention relates to hydraulic pumps and motors.

The invention has been developed primarily for pumping suspensions of particulate solids in slurry form for pipeline transportation and will be described hereinafter with reference to this application.

However, it will be appreciated that the invention is not limited to this particular field of use.

The costs associated with transportation of important minerals such as coal often comprise a substantial proportion of the total costs of material 6* S. handling. Conventional transportation systems such as S. belt conveyors, road and rail are generally inefficient and make considerable demands on space and resources.

In many circumstances, transportation costs are often higher than the actual mining costs and usually represent the principal cost at the point of consumption.

O o o As an alternative to these conventional methods, attempts have been made to transport minerals in .20 suspension in slurry form along a pipe line. However, such slurry systems have not generally gained acceptance as a commercially viable alternative means of mineral transportation, largely due to a lack of development in a number of associated areas of technology. In particular, it has been found that pump pulsations, inadequate valve performance in handling larger particles, problems associated with slurry preparation, and inadequate theoretical understanding of the dynamic 2 behavioural characteristics of the slurry have resulted in slurry transportation systems not yielding the economic motivation for widespread commercialisation.

In an attempt to ameliorate these problems, a so called rotary ram pump was developed and this is described in detail in Australian patent specification 461204. The object of that invention was to provide the advantages of a positive displacement pump while at the same time attempting to provide substantially uniform delivery characteristics. However, in practice this *pump has been found to be inadequate in several respects.

Firstly, the efficient performance of the pump is dependent upon accurate control of critical clearances Sbetween the rotary cylinder barrel and the respective stationary port blocks and thrust bearings at either end of the pump. Accurate control within precise tolerances is essential in order to minimise leakage .:o.o flow and control wear to within acceptable limits. In the past, these critical clearances have been mechanically adjusted by means of shims and/or a threaded adjustment sleeve. However, it has been found "that even if the clearances are correctly adjusted S. before start-up, pump pressurisation produces an indeterminate elongation of the housing which increases the clearances beyond acceptable limits for efficient operation. In particular, this results in leakage flow through the clearance space defined between the cylinder barrel and the port block at the delivery end of the 3 pump. In addition, differential pressure distribution on the respective end faces of the cylinder barrel produces a twisting force couple tending to wedge the barrel within the casing. This effect reduces the effective service life of the expensive hydrostatic radial bearings, makes accurate mechanical control of the critical valve clearances impossible thereby exacerbating the leakage problem, and can even result in jamming of the pump. These problems become particularly significant in larger pumps of the type needed for the transportation of mineral slurry on a commercial scale.

In addition, further research has now shown that for some coals, stable pumping and the ability to re-start the line can only be achieved if the concentration of solids is maintained between relatively narrow critical limits. Furthermore, these limits become narrower if the coal is deficient in "fines". It has been found that there is a relatively strong correlation between the rate of leakage flow and the valve clearance at the delivery end of the pump. The leakage flow in turn directly affects the concentration of the remaining solids so that an increase in leakage eeeo flow through the valve clearances effects a proportionate increase in the relative concentration of solids delivered to the pipe line. If the clearance is constant, the effect of consequential leakage can be off-set. However, any transient variation in the critical valve clearances arising as a result of \7F)S Cu-4 Si non-uniform or fluctuating pressure distribution over the valve surfaces and inadequate control over the clearances causes a corresponding variation in the leakage flow rate and hence the concentration of solids, thereby causing instabilities in the hydraulic transport line which prevent stable and efficient pumping of the slurry.

Further to this, it has also now been observed that a significant proportion of fine solids is suspended in the leakage flow, and is thereby removed from the system. Such a loss of fines causes a reduction in the .6 .6 S6 critical range of concentration in which pumping is stable and in which the line can be easily re-started.

This loss of fines is even more undesirable if a coking coal is being pumped and the macerals essential for developing coking properties are concentrated in the fine particles. These problems, arising largely due to 6 the inability in the past to accurately mechanically control the critical pump valve clearances, have tended to prevent the commercial acceptance of the known rotary ram pump as a commercially viable means of slurry transportation.

It is therefore an object of the present invention to provide an improved pump which overcomes or substantially ameliorates at least some of these disadavantages of the prior art.

Accordingly, the present invention provides a positive displacement pump including: 5 a housing; a barrel rotatably mounted within the housing and including a plurality of peripherally spaced motor cylinders and a corresponding number of pump cylinders in respective coaxial alignment with said motor cylinders forming a peripheral array of generally longitudinally extending spaced apart motor and pump cylinder pairs; a plurality of double acting piston assemblies each comprising a first piston disposed within a respective o** motor cylinder and a second piston interconnected with *0 and spaced apart from said first piston and disposed 0 within a corresponding pump cylinder, said piston assemblies being sealingly and slidably mounted for reciprocation within respective motor and pump cylinder pairs; interconnecting means associated with said motor

S

and pump cylinder pairs whereby axial displacement of any one of said piston assemblies effects a corresponding axial displacement of another of said piston assemblies in predetermined phase relationship; a stationary motor port block fixed with respect to

S

said housing and including a driving fluid supply port and a driving fluid exhaust port disposed respectively to overlap periodically with corresponding openings in said motor cylinders whereby driving fluid under pressure is alternately admitted into successive motor cylinders through said supply port and discharged 6 through said exhaust port in response to rotational movement of said barrel within said housing thereby to effect a corresponding phase related reciprocating movement of said piston assemblies; a stationary pump port block fixed with respect to said housing and including an inlet port and a discharge port disposed respectively to overlap periodically with corresponding openings in said pump cylinders whereby fluid to be pumped is alternately admitted into successive pump cylinders through said inlet port and discharged under pressure through said discharge port in a response to said reciprocating movement of said piston assemblies; oaf said cylinder openings and said ports being disposed such that the total cross sectional flow area exposed to each of said ports is substantially constant irrespective of the rotational position of said barrel within said housing such that a constant volumetric flow .0 rate of said driving fluid tends to effect a corresponding substantially constant volumetric flow rate of said fluid to be pumped; oleeQ •e•and at least one asymmetrically disposed

S.

hydrostatic bearing pocket recessed into an axial bearing surface associated with said barrel and adapted for connection to a supply source of fluid under pressure to provide an asymmetric hydrostatic pressure distribution on an end face of the barrel tending to balance an opposing torque resulting from pressure from 7 1 W 1 said fluid to be pumped acting on the barrel adjacent said discharge port.

Preferably, the peripherally spaced apart motor and pump cylinder pairs are substantially parallel to the rotational axis of the barrel.

Preferably also, the interconnecting means comprises a plurality of independent hydraulic fluid circuits connecting central regions of respective diametrically opposed motor and pump cylinder pairs bounded by corresponding first and second pistons of respective piston assemblies whereby axial displacement of any one of the piston assemblies effects a corresponding axial displacement of the respective diametrically opposed piston assembly in the opposite direction. In this configuration, the hydraulic circuits act in a way analogous to mechanical rockers whereby diametrically opposed piston assemblies move 180' out of phase.

Preferably, the pump includes four peripherally 20 spaced motor and pump cylinder pairs, each connected to a..

a diametrically opposed pair by means of an independent hydraulic fluid circuit.

oo The hydrostatic bearing pocket is preferably in the .oo.e: form of an eccentric annulus recessed into the motor oo 25 port block to nestably receive a complementary asymmetric floating thrust block slidably disposed intermediate the port block and the barrel. The arrangement is preferably such that the axial centre of \V2\ 8 pressure of the bearing pocket and the corresponding centre of pressure from the fluid to be pumped acting on respective end faces of the barrel are colinear.

A preferred embodiment of the invention will now be described, by of example only, with reference to the accompanying drawings in which: Figure 1 is a cut-away sectional plan view showing a positive displacement pump according to the invention; Figure 2 is a cut-away cross sectional side elevation showing the pump of figure 1; Figure 3 is an end view taken along line 3-3 of 6eO 9 6 figure 2; Figure 4 is an opposite end view taken along line S" 4-4 of figure 2; Figure 5 is a sectional view taken along line of figure 2 showing an asymmetrically disposed hydrostatic axial bearing surface according to the invention; Figure 6 is a sectional view taken along line 6-6 of figure 2 showing the hydraulic circuits interconnecting diametrically opposed motor and pump eoo.o cylinder pairs; o• "Figure 7 is a sectional figure taken along line 7-7 of figure 2.

Referring generally to the drawings, wherein corresponding features are denoted by corresponding reference numerals, a positive displacement pump 1 includes a generally cylindrical housing 2 and a 9 cylindrical barrel 3 rotatably mounted within the housing. Barrel 3 includes four uniformly peripherally spaced motor cylinders 5 and four corresponding pump cylinders 6 in respective coaxial alignment with the motor cylinders. Motor cylinders 5 and corresponding pump cylinders 6 together form a peripheral array of uniformly circumferentially spaced apart motor and pump cylinder pairs 7 parallel to the rotational axis 8 of the barrel. The barrel is rotatably supported within the housing by means of axially spaced apart hydrostatic radial bearings 10 supplied with water under pressure from a supply source through respective orifices 11 to provide relatively frictionless rotation and a continuous flushing action which minimises wear by transporting any abrasive particles in suspension away from the bearings. Leakage flow is drained from the housing through drainage pipe 12.

The pump further includes four double-acting piston assemblies 15, each comprising a first piston 16 disposed within a respective motor cylinder 5 and a second piston 17 interconnected with, and spaced apart from, the first piston 16 and disposed within a corresponding pump cylinder 6. The piston assemblies are sealingly and slidably mounted for reciprocation .25 within their respective cyclinders.

Diametrically opposed cylinder pairs 7 are interconnected by respective independent hydraulic fluid circuits 18. As best seen in figures 1 and 2, hydraulic /1 circuits 18 connect regions of opposed cylinder pairs 7 bounded by corresponding first and second pistons 16 and 17 of respective piston assemblies 15 whereby axial displacement of any one of the piston assemblies effects a corresponding displacement of the diametrically opposed piston assembly in the opposite direction. In this way, it will be seen that the interconnecting hydraulic circuits act conceptually as independent mechanical rockers providing a driving connection between diametrically opposed piston assemblies. It will be appreciated, however, that opposed pistons need not move exactly 1800 out of phase as in the preferred embodiment, but can be arranged to move in any predetermined phase relationship depending upon the particular number and configuration of pistons used. Leakage flow from circuits 18 is drained from the housing through drainage line 12 and made up through central supply passage 19 from an independent high pressure water supply.

The motor end of the pump (the left hand end when viewing the drawings) includes a stationary motor port &e block 20 fixe with respect to the housing and 0• comprising a driving fluid supply port 21 and a corresponding spaced apart driving fluid exhaust port 22 disposed respectively to overlap periodically with corresponding openings 23 in the proximate ends of the motor cylinders. Water under pressure is alternately admitted into successive motor cylinders 5 through 11 supply port 21 and subsequently discharged through exhaust port 22 in response to rotational movement of the barrel within the housing to effect a corresponding phase related reciprocating movement of piston assemblies 15 within respective motor and pump cylinder pairs 7 in a manner to be described more fully below.

Stationary wear plate 26 and opposed rotating wear plates 27 associated respectively with port block 20 and barrel 3 accommodate wear between the valve faces and are periodically replaced as valve clearances exceed acceptable upper limits.

*g Similarly, a stationary pump port block 30 is •disposed at the opposite end of the housing (the right hand end when viewing the drawings). The pump port block 30 includes slurry inlet port 31 and corresponding spaced apart discharge port 32 disposed respectively to &sees: overlap periodically with corresponding openings 33 in the proximate ends of pump cylinders 6. Slurry to be pumped is alternately drawn into successive pump cylinders through inlet or suction port 31 and subsequently forced under pressure through discharge port 32 in response to the reciprocating movement of the S piston assemblies and corresponding phase related rotational movement of the barrel. Once again, stationary wear plate 34 and rotating wear plate associated respectively with port block 30 and barrel 3 accommodate wear between adjacent valve faces and are periodically replaced as required to maintain valve 12 leakage flow to within predetermined acceptable limits.

An eccentric floating thrust block 36 is slidably interposed between port block 20 and barrel 3 and acts in conjunction with peripheral hydrostatic thrust bearing pockets 37 and 38 associated respectively with the motor and pump ends of the barrel to provide accurate self-adjusting hydrostatic control of the axial clearances between opposing valve surfaces in a manner more fully described later.

The cylinder openings 23 and 33, and respective motor and pump ports 21, 22 and 31, 32 are disposed such that the total cross sectional flow area exposed to each of the ports is substantially constant, irrespective of the rotational position of the barrel within the housing such that a constant volumetric flow rate of the driving fluid through the motor effects a corresponding substantially constant volumetric flow rate of slurry at the outlet of the pump.

e, o Turning now to describe the operation of the pump in more detail, the barrel is first connected to a suitable rotary prime mover such as an electric or "i hydraulic motor (not shown) by means of input drive shaft 39. The optimum rotational speed of the barrel "will be dependent upon a number of parameters including 25 those described in Australian patent specification 461204 and will typically be of the order of revolutions per minute. Motor supply port 21 is ~connected to a suitable high pressure water supply (also 13 not shown) and exhaust port 22 preferably connected to the low pressure side of that supply for recirculation.

A "make-up" volume will also be required to accommodate any leakage flow. Similarly, inlet or suction port 31 is connected to a supply of slurry and discharge port 32 connected to the pipeline along which the slurry is to be pumped.

Rotation of the barrel by the prime mover brings successi-e motor cylinder openings 23 into alignment with supply port 21 whereupon corresponding piston assemblies are successively driven to the right (when viewing the drawings) by means of the high pressure water introduced into the motor cylinders. Simultaneously, this causes slurry within the corresponding pump cylinders to be discharged under pressure through discharge port 32. The motor cylinders are larger in diameter than the corresponding pump cylinders to provide a proportionately greater driving force on the motor end of the piston S.assemblies to ensure that the pump is able to overcome 20 fluid back pressure in the slurry discharge line.

As each piston assembly moves to the right under the action of the higher pressure driving fluid, water '"contained in the region bounded by the first and second 4 pistons 16 and 17 is forced through respective hydraulic circuit 18 into the corresponding region of the diametrically opposed cylinder pair. Because of the difference in diameters and hence the difference in swept volumes between the motor and pump cylirders, this effects a corresponding reciprocating movement of the opposed 14 piston assembly in the opposite direction. This in turn causes slurry to be drawn into that pump cylinder through inlet port 31 to be subsequently discharged through port 32 as described above. In this way, as each piston assembly is driven to the right by the high pressure driving fluid, the diametrically opposed piston is driven to the left so that as slurry is successively discharged from each pump cylinder, it is simultaneously drawn into an opposing pump cylinder on the suction side such that a constant volumetric flow rate of the driving fluid effects a constant volumetric flow rate of the slurry; to provide a substantially pulseless flow down o• the line.

S..

Complementary peripheral seals 28 are disposed at ~either end of the pump intermediate respective opposed valve surfaces to minimise leakage flow. This is particularly important at the discharge end of the pump to minimise leakage of the fine coal fraction which, apart from representing a commercially significant proportion of the overall coal volume, is essential to maintain the critical theological properties of the S slurry within the narrow band necessary for efficient 0 a pumping. Circumferential bearing water seals 29 isolate the water associated with the hydrostatic radial bearings from any slurry leakage.

Ideally, the hydraulic pressures and barrel speed are designed such that the pistons approach the limits of maximum excursion in both directions, without 15 actually abutting their respective end stops. However, transient pressure instabilities or fluctuations usually caused by partial port blockages can permit the pistons to behave erratically and collide with their end stops.

This induces pressure waves in the pipe line which can destabilise the slurry bed and produce clogging in the pipe. In practice therefore, it is often necessary to run the pump at significantly less than optimum efficiency in an attempt to avoid the possibility of transient shock vibrations caused by erratic pump S performance.

In order to ameliorate this problem and thereby S permit more efficient operation of the pump, each piston includes a stepped cylindrical shoulder portion

S.

nestably engageable with a corresponding frusto-conical tapered end socket 41 to provide a degree of hydraulic piston damping. As the pistons approach the extreme limits of their maximum axial excursion, stepped S shoulders 40 begin to nestably engage respective tapered sockets 41 thereby defining a progressively reducing annular cross-sectional flow area. As the shoulder proceeds into the socket, the opposing damping force increases progressively as a function of reducing flow area in accordance with the principles of thin passage rheology to damp the collision of the pistons with their respective end stops and thereby prevent undesirable shock vibrations.

The pump further includes an asymmetrically 16 disposed eccentric hydrostatic bearing pocket recessed into the surface of the stationary motor port block 20 to nestably receive a complementary eccentric annular protruding portion 46 of the floating thrust block 36 slidably disposed intermediate the port block and the barrel. Hydrostatic bearing pocket 45 includes an orifice adapted for connection to a supply source of high pressure water. In use, this provides an asymmetric hydrostatic pressure distribution on end face 47 of the barrel biased toward the discharge side of the pump. This tends to balance an opposing torque on the barrel resulting from back-pressure from the slurry in the discharge line acting on the opposite end face 48 adjacent the slurry discharge port.

This arrangement provides a self adjusting hydrostatic thrust bearing system which acts in conjunction with hydrostatic bearing pockets 37 and 38 and peripheral seals 28 to automatically reduce the thrust pressure on the barrel when the valve clearances increase and increase the thrust pressure when the respective clearances begin to close. More particularly, when the clearance spaces associated with •coo the hydrostatic bearings begin to close, leakage flow is eeoee "reduced and the bearing pressure approaches the full 25 available static pressure of the supply source to which the bearing is connected. This provides the maximum opening force on the bearing, thereby tending to increase the clearance space. Conversely, when the 17 relevant clearance begins to open, leakage flow through the bearing rises. This leads to an increase in the pressure drop across an orifice disposed in the supply line. This pressure drop correspondingly reduces the pressure available in the bearing pocket and thereby tends to reduce the clearance space.

This action automatically maintains the critical working clearances between adjacent valve faces at either end of the pump to within a precise tolerance range substantially narrower than can be achieved using conventional mechanical adjustment techniques, thereby significantly reducing valve leakage and associated problems. In addition, by balancing the twisting force couple tending to wedge the barrel within the casing, the effective service life of the expensive hydrostatic radial bearings is significantly increased. Thus, the *invention represents a significant improvement over the prior art.

It will be appreciated, however, that a variety of 20 asymmetric port configurations including one or more 555 bearing pockets disposed at either or both ends of the barrel colild be used. Furthermore, since the peripheral 4*SS •t •seals have a substantial effect on pressure distribution .0I.00 between the value surfaces, they can also be used in conjunction with the hydrostatic bearing pockets in a pressure balance fu.,ction as well as a sealing function to minimise slurry leakage.

The pump further includes a pressure monitoring 18 system comprising pressure transducers (not shown) to monitor pressure flunctuations within the pump and position transducers (also not shown) to monitor the positions of the pistons within their respective cylinders. These transducers form part of a feedback control system which can be set up to automatically adjust the pump speed in response to vibration levels exceeding predetermined acceptable limits to prevent erratic pump performance and the possibility of consequential line blockages and damage to the pump whilst permitting the pump to operate at close to maximum efficiency. In addition, the position transducers of the positive displacement pistons can be used to provide useful and accurate information on important operating parameters such as instantaneous flow rate, volumetric efficiency, piston speed and the like.

Although this invention has been described with reference to specific examples, it will be appreciated 20 by those skilled in the art that the invention may be

C..

embodied in many other forms. For example, the motor and pump cylinder pairs need not be parallel to the oeC rotational axis of the barrel, but could be splayed so as to diverge radially outwardly in a frusto-conical 25 array.

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Claims (9)

1. A positive displacement pump including: a housing; a barrel rotatably mounted within the housing and including a plurality of peripherally spaced motor cylinders and a corresponding number of pump cylinders in respective coaxial alignment with said motor cylinders forming a peripheral array of generally longitudinally extending spaced apart motor and pump cylinder pairs; a plurality of double acting piston assemblies each comprising a first piston disposed within a respective motor cylinder and a second piston interconnected with and spaced apart from said first piston and disposed within a corresponding pump cylinder, said piston assemblies being sealingly and slidably mounted for reciprocation within respective motor and pump cylinder Spairs; o interconnecting means associated with said motor a S 20 and pump cylinder pairs whereby axial displacement of 6o6 any one of said piston assemblies effects a corresponding axial displacement of another of said piston assemblies in predetermined phase relationship; 666*66 *o 6o a stationary motor port block fixed with respect to S25 said housing and including a driving fluid supply port and a driving fluid exhaust port disposed respectively to overlap periodically with corresponding openings in said motor cylinders whereby driving fluid under !u c Iv r or I pressure is alternately admitted into successive motor cylinders through said supply port and discharged through said exhaust port in response to rotational movement of said barrel within said housing thereby to effect a corresponding phase related reciprocating movement of said piston assemblies; a stationary pump port block fixed with respect to said housing and including an inlet port and a discharge port disposed respectively to overlap periodically with corresponding openings in said pump cylinders whereby fluid to be pumped is alternately admitted into 0e SW S. successive pump cylinders through said inlet port and "-7a discharged under pressure through said discharge port in response to said reciprocating movement of said piston 0* .C assemblies; said cylinder openings and said ports being disposed such that the total cross sectional flow area 55@05S exposed to each of said ports is substantially constant irrespective of the rotational position of said barrel se. within said housing such that a constant volumetric flow rate of said driving fluid tends to effect a ~corresponding substantially constant volumetric flow fd rate of said fluid to be pumped; and at least one asynmetrically disposed hydrostatic bearing pocket recessed into an axial bearing surface associated with said barrel and adapted for connection to a supply source of fluid under pressure to provide an asymmetric hydrostatic pressure 21 I distribution on an end face of the barrel tending to balance an opposing torque resulting from pressure from said fluid to be pumped acting on the barrel adjacent said discharge port.

2. A pump according to claim 1 wherein the peripherally spaced motor and pump cylinder pairs are substantially parallel to the rotational axis of the barrel.

3. A pump according to claim 1 or claim 2 wherein said interconnecting means comprises a plurality of S. independent hydraulic fluid circuits connecting central regions of respective diametrically opposed motor and pump cylinder pairs bounded by corresponding first and second pistons of respective piston assemblies whereby axial displacement of one of the piston assemblies effects a corresponding axial displacement of a diametrically opposed piston assembly in the opposite S 1oi0 direction.

4. A pump according to claim 3, including four peripherally spaced motor and pump cylinder pairs, each connected to a diametrically opposed pair by means of an independent hydraulic fluid circuit. m4

5. A pump according to any one of the preceding claims wherein the hydrostatic bearing pocket is generally in the form of an eccentric annulus recessed into the motor port block to nestingly receive a complementary asymmetric floating thrust block slidably disposed intermediate the port block and the barrel. 22

6. A pump according to any one of the preceding claims wherein said at least one hydrostatic bearing pocket is disposed such that said asymmetric hydrostatic pressure distribution is biased toward a discharge side of the pump.

7. A pump according to claim 5 or claim 6 wherein the axial centre of pressure of the bearing pocket and the corresponding centre of pressure from the fluid to be pumped acting on respective end faces of the barrel are substantially colinear.

8. A pump according to any one of the preceding claims, wherein each piston includes a stepped shoulder portion nestingly engageable with a corresponding generally frusto-conical tapered end socket, thereby defining a progressively reducing generally annular cross sectional flow area as the pistons approach their respective limits of axial excursion, to provide a degree of hydraulic piston damping.

9. A pump according to any one of the preceding claims wherein said at least one asymmetrically disposed bearing pocket acts in conjunction with peripheral hydrostatic thrust bearings asssociated respectively with the motor and pump ends of the barrel to provide self adjusting hydrostatic control of axial clearances between opposing valve surfaces associated with end faces of the barrel whereby thrust pressure on either end face of the barrel tends to be reduced when the respective clearances increase, and thrust pressure S. S S S. S*S S. 55 *S. 5 Ar /Vr 23 tends to be increased when the respective clearances are reduced. A positive displacement pump, substantially as hereinbefore described with reference to the accompanying drawings. DATED this 13th day of JANUARY, 1994 ABB MINERAL SLURRY TRANSPORTATION At'orney: STUART M. SMITH Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS 4 444 4 4 oV s 24 ABSTRACT A positive displacement pump including: a housing; a barrel rotatably mounted within the housing and including a plurality of peripherally spaced motor cylinders and a corresponding number of pump cylinders in respective coaxial alignment with said motor cylinders forming a peripheral array of generally S longitudinally extending spaced apart motor and pump cylinder pairs; a plurality of double acting piston assemblies each Scomprising a first piston disposed within a respective motor cylinder and a second piston interconnected with and spaced apart from said first piston and disposed within a corresponding pump cylinder, said piston assemblies being sealingly and slideably mounted for reciprocation within respective motor and pump cylinder pairs; interconnecting means associated with said motor and pump cylinder pairs whereby axial displacement of any one of said piston assemblies effects a corresponding axial displacement of another of said piston assemblies in predetermined phase relationship; a stationary motor port block fixed with respect to said housing and including a driving fluid supply port and a driving fluid exhaust port disposed respectively to overlap periodically with corresponding openings in said motor cylinders whereby driving fluid under 25 0 U pressure is alternately admitted into successive motor cylinders through said supply port and discharged through said exhaust port in response to rotational movement of said barrel within said housing thereby to effect a corresponding phase related reciprocating movement of said piston assemblies; a stationary pump port block fixed with respect to said housing and including an inlet port and a discharge S port disposed respectively to overlap periodically with 41Q, corresponding openings in said pump cylinders whereby fluid to be pumped is alternately admitted into 00 successive pump cylinders through said inlet port and discharged under pressure through said discharge port in response to said reciprocating movement of said piston assemblies; said cylinder openings and said ports being 0 0• 00 0 disposed such that the total cross sectional flow area exposed to each of said ports is substantially constant irrespective of the rotational position of said barrel within said housing such that a constant volumetric flow rate of said driving fluid tends to effect a corresponding substantially constant volumetric flow rate of said fluid to be pumped; and at least one asymmetrically disposed hydrostatic bearing pocket recessed into an axial bearing surface associated with said barrel and adapted for connection to a supply source of fluid under pressure to provide an asymmetric hydrostatic pressure 26 distribution on an end face of the barrel tending to balance an opposing torque resulting from pressure from said fluid to be pumped acting on the barrel adjacent said discharge port. b P 27