PUMPING SYSTEM BACKGROUND OF THE INVENTION

[0001] This invention relates to a system for pumping liquids at high pressures.

[0002] Various pumps exist for different applications. Hose pumps are commonly used for pumping dirty, viscous and abrasive slurries, waste sludge and so on. Worm pumps are employed primarily in the food industry for pumping semi-viscous liquids with a relatively fine grain structure. Bladder and diaphragm pumps find application particularly in the mining industry although they are also employed in the food and chemical industries. These pumps tolerate chemicals and abrasives and can handle relatively large solids.

[0003] Multi-stage centrifugal pumps are usually encountered in the mining industry for dewatering applications, in water treatment plants and so on. These pumps have a relatively poor abrasion resistance.

[0004] The aforementioned types of pumps, in general, are limited in respect of pressure capabilities. Although these pumps can operate at high delivery rates their efficiencies are strongly reduced when they are called upon to operate at higher pressures.

[0005] By way of contrast a positive displacement piston-type pump can generate a high pressure and flow rate with good efficiency but it suffers from poor abrasion and chemical resistance. Another adverse factor is that this type of pump has a pulsating delivery. Instantaneous and frequent flow fluctuations in liquid delivery structures, under high pressure, may fatigue the structures in short periods of time, causing unexpected damage and high repair expenses, and can pose severe safety hazards. A pressure accumulator can be employed to make the flow rate more or less constant but, at high flow volumes and high pressures, especially when pumping abrasive and corrosive liquids, this type of accumulator is expensive and calls for high maintenance.

[0006] US patent No. 2006/0239840A1 describes a combination of a bladder and piston pump. Reciprocating hydraulic pistons act as a power source via a mechanical linear crank action from a rotating cam which is driven by a prime mover. A bladder or bellows is used to pump a liquid. Although the pump can operate at a relatively high pressure, and can pump viscous, abrasive and corrosive liquids, it is, in the applicant's opinion, bulky, complicated, expensive and difficult to maintain.

[0007] In general, reciprocating piston, bladder or bellows pumps provide a pulsing delivery flow, yet multiple pumping units do reduce the pressure and flow fluctuations. Each pump unit requires two non-return valves, one for suction and one for delivery. These are costly, high wear service items.

[0008] An object of the present invention is to provide a system which can operate efficiently at a very high pressure for the pumping of liquid which may be viscous, abrasive, acidic or contain particulate material, in a substantially constant, non- pulsating delivery flow, and which uses a reduced number of service items. This type of pumping system finds application; inter alia, in the pumping of slurries, in water jetting, oil well drilling, ground solidification, dewatering and in fire fighting. These applications are exemplary only and are non-limiting.

SUMMARY OF THE INVENTION

[0009] The invention provides a liquid pumping system which includes: (a) a liquid transfer pump arrangement which includes a liquid inlet port, a liquid discharge port, a first pump chamber with a first maximum volume, a first nonreturn valve connected to and between the liquid inlet port and the first pump chamber, a first control device for pumping liquid from the first pump chamber, a second pump chamber with a second maximum volume, wherein the second maximum volume is at least half the size of the first maximum volume, the second pump chamber being connected to the liquid discharge port, a second non-return valve connected to, and between, the first pump chamber and the second pump chamber and a second control device for pumping liquid from the second pump chamber,

(b) a hydraulic fluid constant flow pump, and

(c) control structure which, responsive to liquid flow rates from each pump chamber, directs hydraulic fluid to the first and second control devices at respective controlled rates whereby, upon actuation of the first and second control devices; in a cyclical manner:

(1 ) liquid is expelled from the first pump chamber through the second nonreturn valve:

(1.1 ) at a controlled delivery rate to the liquid discharge port, and

(1.2) into the second pump chamber, and, thereafter, (2) liquid is directed from the liquid inlet port through the first non-return valve into the first pump chamber, and liquid is expelled from the second pump chamber at said controlled delivery rate to the liquid discharge port.

[0010] The pump chambers may take on any appropriate form. In one example of the invention the first pump chamber is formed by a first cylinder and a first piston which is reciprocally movable inside the first cylinder. The second pump chamber is similarly formed from a second cylinder and a second piston.

[0011] in each case each control device causes movement of the respective piston. By way of non-limiting examples each control device may, itself, comprise a respective piston and cylinder assembly, a lead screw or any equivalent adjustment mechanism.

[0012] The desired relationship between the first maximum volume and the second maximum volume may be achieved in different ways. For example the first cylinder may have a cross-sectional area which is less than two times the cross-sectional area of the second cylinder. Alternatively, if the cross-sectional areas of the cylinders are substantially the same, the stroke of the first piston may be less than two times the stroke of the second piston. Another possibility is to vary the relative cross-sectional areas of the cylinders and the strokes of the pistons to ensure that a constant flow is delivered without pressure fluctuations.

[0013] When the first pump chamber is full both pistons momentarily execute delivery strokes. This allows a smooth transition between delivery from the pistons in that the first piston can accelerate slowly and the second piston can be smoothly slowed to be stationary before reversing for its filling cycle.

[0014] In another form of the invention each pump chamber is formed by a respective bellows or a diaphragm such as a bladder. The bellows, or bladder, can then be changed from an extended or retracted mode by the respective control device and then enlarged to an operative configuration. If a bellows is used, the liquid medium to be pumped may be directed into an interior of the bellows, and the hydraulic power medium may be directed to a space between a wall of a cylinder and the bellows i.e. to retract the bellows. Alternatively the hydraulic power medium may be directed to inside the bellows to extend the bellows. The medium to be pumped is then displaced from a space between a cylinder and the bellows as the bellows is expanded. Thus the chamber may be inside, or outside of, the bellows.

[0015] In both cases, the internal pressures of the bellows must be kept positive, otherwise the bellows will collapse. This is done with the control structure using hydraulic throttling in main lines for the hydraulic power medium and a control cylinder. The control cylinder may either assist the bellows to extend, or restrain extension of the bellows, in a controlled way, so as to maintain a positive pressure inside the bellows. The bellows may be supported by an extension of the control cylinder. In this case the cross-sectional area of the bellows on the cylinder side is always slightly smaller. Typical ratings for commercially available bladders and bellows are 8 bar internal pressure and 25 bar burst limit. The pressure differential between the inner and outer surfaces of the bellows should therefore be controlled to ensure effective operation.

[0016] Each control device may be any suitable mechanism e.g. a small double acting control cylinder or a lead screw, preferably with a trapezoidal thread so that the thread can exert a pushing and a pulling force effectively. A combination of the aforegoing may also be employed. A hydraulic cylinder is preferred as it is small in size, yet capable of providing substantial force. A measurement of the hydraulic fluid flow into or out of the control cylinder provides accurate feedback for other control structures, such as a PLC, hydraulic valves, and so on. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention is further described by way of examples with reference to the accompanying drawings in which:

Figure 1 shows a liquid pumping system which includes a liquid transfer pump arrangement which is based on the use of cylinders and pistons, according to a first form of the invention,

Figure 2 shows a second form of the invention comprising a liquid transfer pump arrangement which is based on the use of bellows or diaphragms,

Figure 3 shows a variation of the Figure 2 system in that bellows or diaphragms, included in the liquid transfer pump arrangement, are axially aligned and not separated from each other as is the case in Figure 2, and

Figure 4 illustrates charge and discharge curves over a pump cycle.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] Figure 1 schematically shows a liquid pumping system 10 according to a first form of the invention which includes, enclosed in each case in dotted outline, a liquid transfer pump arrangement 12, a hydraulic fluid constant flow pump arrangement 14, and control structure 16.

[0019] The liquid transfer pump arrangement 12 has two principal components namely a first liquid delivery arrangement 20 and a second liquid delivery arrangement 22.

[0020] The first liquid delivery arrangement 20 includes a first cylinder 24 with a first piston 26 which, together, define a first pump chamber 28. A first control device 30 which, itself, comprises a piston 32 and a cylinder 34, is used to cause controlled movement of the piston 26. Similarly the second liquid delivery arrangement 22 includes a second cylinder 40, a second, delivery piston 42, and a second control device 44 which is constituted by a piston 46 and a cylinder 48 which operate on the delivery piston 42.

[0021] The second piston and cylinder define a second pump chamber 50.

[0022] Liquid which is to be pumped is held in a source 52 which is connected via an inlet port 54 and a first non-return valve 56 to the first pump chamber 28.

[0023] A second non-return valve 60 is connected to and between the first pump chamber 28 and the second pump chamber 50, and to a discharge port 66. An accumulator 68 connected between the inlet port 54 and the first non-return valve 56 is used to absorb pressure or flow variations in the liquid flowing from the source 52 to the first non-return valve.

[0024] The constant flow hydraulic fluid arrangement 14 includes a motor 70 which drives a pump 72 which delivers hydraulic fluid at a constant rate. The motor is also used to drive a fan 74 which provides a cooling stream of air to a heat exchanger 76 used to cool hydraulic fluid. Proportional control valves 78 and 80, included in the control structure 16, are used to regulate the flow of hydraulic fluid from the pump 72 to the liquid transfer pump arrangement 12. The pressurisation of the control devices 30 and 44 is performed from the main pump 72, or could be performed with a separate, small, on-demand pump 73.

[0025] Fluid flow to the control devices takes place via pressure regulator valves 82 and 84, a change over valve 86, and flow meters 88 and 90 respectively.

[0026] An end of the stroke of each control cylinder (in the respective devices 30 and 44) is detected with respective switches 91 and 92. Data generated by the operation of the switches is supplied to the control structure which is calibrated during each return stroke of each of the cylinders by means of a microprocessor or programmable logic controller (PLC) 100 which is included in the control structure.

[0027] When the pistons 26 and 42 are extending or retracting, flow counts from the flow meters 88 and 90 provide feedback to the PLC 100 which controls the hydraulic system and which ensures a constant output flow from the discharge port 66. Additionally, pressure transducers Pi to P ₄ monitor the pressures in the chambers, and feed data thereon to the PLC 100 which, in response to this data, adjusts the hydraulic system and the pressures in the control cylinders to maintain optimum pressure balances.

[0028] The pumping system includes at least one pressure filter 96 for the hydraulic fluid delivered by the pump 72.

[0029] The maximum volume of the second pump chamber 50 is preferably at least half the size of the maximum volume of the first pump chamber 28. This can be achieved in various ways. For example, the cross sectional area of the cylinder 24 may be at least two times the cross sectional area of the cylinder 40. Alternatively, the strokes of the pistons 26 and 42 can be varied. It is a|so possible to use a combination of different stroke lengths with different cross-sectional areas to ensure that the maximum volume of the second pump chamber is at least half the size of the maximum volume of the first pump chamber.

[0030] If the filling cycle for the first chamber is fast, then the second chamber could be correspondingly small and would act only to discharge during a short period of time as the first chamber is being filled. This, however, is not beneficial because high pressure spikes would be produced in the inlet port 54. [0031] In use of the pumping system, liquid from the inlet port 54 flows through the first non-return valve 56 into the first pump chamber 28. The first piston 26 is retracted by the pumping action of the first control device 30 which operates, under the control of the controller 100, using energy from hydraulic fluid delivered by the pump 72. This description applies when the pump unit is self-priming. If the inlet line is pressurized then the piston retraction is assisted by the inlet pressure and the control device only regulates the filling rate and the speed.

[0032] Once the chamber 28 is fully charged the piston 26 reverses direction and discharges liquid from the chamber 28 through the second non-return valve 60. Liquid cannot flow to the source due to the action of the first non-return valve 56. The controller 100 regulates the flow rate from the chamber 28 and the pressure of the liquid delivered from this chamber. The liquid is delivered, firstly, to the discharge port 66 and, secondly, into the second pump chamber 50. The volume of liquid which flows into the chamber 50, and the rate at which this liquid flows, are regulated by the second control device 44 i.e. by retraction of the piston 42. When the piston 42 is retracting, the hydraulic oil behind it is regenerating and adds into the fluid delivery supply from the pump 72, speeding up the discharge stroke of the piston 26.

[0033] As the maximum volume of the second pump chamber 50 is at least half the size of the maximum volume of the first pump chamber 28, liquid continues to flow from the first pump chamber 28 to the discharge port 66 once the second pumping chamber 50 is full. At this point the piston 42 is reversed and liquid is then expelled from the second chamber 50 to the discharge port 66. Liquid cannot return to the first pump chamber 28 because of the blocking action of the second non-return valve. The pistons 26 and 42 momentarily and simultaneously perform respective discharge strokes. When the piston 26 stops, the non-return valve 60 closes and the piston 42 speeds up on its delivery stroke. The piston 26 then starts its suction stroke. The rate at which liquid is delivered from the second pump chamber 50 is regulated by the controller 100, and by the supply of hydraulic fluid from the fixed displacement pump 72. The delivery of liquid from the first pump chamber 28 is correspondingly reduced, ultimately to zero, to ensure that the delivery rate through the port 66 is kept effectively constant.

[0034] As the piston 42 advances the piston 26 is retracted so that the first pump chamber 28 is recharged. The piston 26 is reversed shortly before the piston 42 reaches the end of its delivery stroke. To compensate for the opening and closing times of the non-return valves and to achieve a pulseless delivery shift between the pump chambers, the pistons 26 and 42, as noted, perform momentarily and simultaneously, respective liquid delivery strokes.

[0035] The non-return valves 56 and 60 are not used for throttling liquid flow. These valves are piloted fully open or fully closed, according to requirement. This characteristic effectively eliminates "sand blasting" on the components of the valves and increases the life expectancy of the valves. Thus the non-return valves only control liquid flow into or out of the first pump chamber 28.

[0036] The aforementioned process is repeated in a cyclical manner so that a constant liquid flow at a desired operating pressure is achieved. The hydraulic fluid is separated from the liquid which is pumped and it is possible therefore to configure the cylinders 24 and 40, and the pistons 26 and 42, respectively, so that viscous, abrasive and acidic liquids can be pumped.

[0037] Pumping takes place at the high efficiencies and pressures associated with positive displacement piston pumps, but at an effectively constant flow rate. It is envisaged that a pumping system of the kind described ca operate-; at pressures, of up to 40 MPa with a continuous duty cycle and at a flow rate of up to 10000 litres per hour, at an efficiency exceeding 90%. These performance characteristics can be achieved using components which, for practical reasons, can be considered to be standard components. A fixed displacement hydraulic pump may be replaced with a variable displacement pump in cases where the pumping rate must be adjustable. A reduction of the displacement does however constitute a reduction of pump efficiency.

[0038] Despite the benefits of the arrangement shown in Figure 1 the pump chambers 28 and 50 are constituted by relatively large, and hence expensive, cylinders and corresponding pistons. Figure 2 illustrates a modified pumping system 10A which has many similarities to the system 0 and which, for this reason, is not fully described. Where applicable like reference numerals are used to designate like components.

[0039] The cylinder 24 and piston 26 are replaced, respectively, by a vessel 24A and a bellows or diaphragm 26A. Similarly, the cylinder 40 is replaced by a vessel 40A, and the piston 42 is replaced by a bellows or diaphragm 42A. Control devices 30A and 44A act directly on the respective bellows 26A and 42A. The bellows 42A has an effective maximum volume which is at least half the size of the maximum volume of the bellows 26A. The system 0A functions in a manner which is similar to that described in connection with Figure 1 . However, the use of the bellows or diaphragms means that an expensive and bulky piston and cylinder arrangement is not required. Additionally, mechanical seals which are subject to wear, under abrasive conditions, are eliminated. [0040] Figure 3 shows another system 10C, which has substantial similarities to the system 0A. With the system 10A, the bellows 26A and 42A are located in separately constructed and spaced apart vessels 24A and 40A respectively. In the system 0C the bellows are in line with each other. Volumes 28B and 50B on outer sides of the bellows are linked via a port arrangement 106 which facilitates fluid transfer between these volumes. In other respects, however, the respective control devices 30A and 44A are used, essentially as has been described, to control expansion or retraction (collapse) of the bellows. The system 10C preferably includes pressure transducers Pi to P , similar to those shown in Figure 1 , which function to monitor pressure balances in the system and to ensure that appropriate data is fed to the PLC 100 which, in turn, acts to maintain optimum pressure balances within the system.

[0041] A preferred mounting orientation of the designs in Figure 2 and Figure 3, is vertical, so that the mass of the pistons and the mass of the bellows do not contribute to uneven wear on, and premature failure of, the components. If concrete or heavy solids are to be pumped, the pump chambers and non-return valve locations must be altered accordingly, so that solids do not start building up inside the pump chambers.

[0042] Figure 4 graphically depicts aspects of the pumping sequence referred to, over a pump cycle T with a cycle time of, say, 4 seconds. Assume the first chamber 28 has a maximum volume of 20 litres and that the second chamber 50 has a maximum volume of 11.5 litres. The first chamber is recharged over an interval from T1 to T2 and discharges from T3 to T4. The second chamber is recharged during an interval from T5 to T6 and discharges from T6 to T5. [0043] The pump chambers simultaneously discharge over the intervals T3 to T5 and from T6 to T4.

[0044] A dotted line D represents the net outflow from the first chamber i.e. 8.5 litres (equal to 20 - 11.5 litres).

[0045] The discharge rate per cycle is thus 1 .5 plus 8.5 equals 20 litres.

[0046] With a cycle time of 4 seconds, there are fifteen cycles per minute. The discharge rate is thus 15 X 20 = 300 litres per minute. The discharge rate is continuous and constant because of the use of the fixed displacement pump 72.

[0047] When the bellows are used the speeds of the pistons adjust automatically to match the hydraulic fluid flow rate from the pump 72. During the brief intervals that the pistons work simultaneously the speed of each piston alters accordingly and the discharge flow rate is maintained effectively at a constant value.

[0048] The flow and pressure fluctuations in the incoming low pressure liquid port 54 can be minimized by suitably dimensioning the chambers 28 and 50. If the volume of the discharge chamber 50 is increased, the period taken to re-charge the inlet chamber 28 is increased. The pressure accumulator 68, see Figure 1 , is used to level the intake flow fluctuations and pressure spikes.

[0049] If the input flow rate is required to stay reasonably constant, a third- liquid delivery arrangement could replace the accumulator 68. This delivery arrangement is not shown for it would be of similar design to the high pressure arrangements 20 and 22. The liquid delivery arrangement and the control device would then be linked to the PLC 100 and would be powered by the return flow from the high pressure pump unit.