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Title:
FAULT DETECTION AND PREDICTION
Document Type and Number:
WIPO Patent Application WO/2019/084620
Kind Code:
A1
Abstract:
A pump including one or more pumping chambers, one or more drive mechanisms for driving the one or more pumping chambers and a logic arrangement. The first pumping chamber of the one or more pumping chambers has a first inlet check valve, a first outlet check valve and a first temperature sensor. The logic arrangement is configured to identify a leak by applying logic to at least resistance-data indicative of a resistance of the first pumping chamber to the driving and temperature-data at least based on output from the first temperature sensor.

Inventors:
REUKERS, Darren (47 Barry Road, Campbellfield, Victoria 3061, 3061, AU)
HARDING, Peter (47 Barry Road, Campbellfield, Victoria 3061, 3061, AU)
Application Number:
AU2018/051186
Publication Date:
May 09, 2019
Filing Date:
November 02, 2018
Export Citation:
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Assignee:
QUANTUM SERVO PUMPING TECHNOLOGIES PTY LTD (47 Barry Road, Campbellfield, Victoria 3061, 3061, AU)
International Classes:
F04C14/02; B26F1/26; B26F3/00; F04B9/109; F04B49/10; F04C14/28; G01M3/00
Domestic Patent References:
WO2018089299A12018-05-17
Foreign References:
US20110046902A12011-02-24
US20130167697A12013-07-04
US20150251331A12015-09-10
Attorney, Agent or Firm:
WADESON (Level 27, 101 Collins StreetMelbourne, Victoria 3000, 3000, AU)
Download PDF:
Claims:
CLAIMS

1 . A pump including two or more pumping chambers; one or more drive mechanisms for driving the two or more pumping chambers; and a logic arrangement; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the one or more pumping chambers having a second inlet check valve, a second outlet check valve and a second temperature sensor to measure a second temperature; the logic arrangement being configured to identify a leak by applying logic; and the logic including: mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber.

2. The pump of claim 1 , the first pumping chamber having a first plunger, a first cylinder and a first plunger seal; and the second pumping chamber having a second plunger, a second cylinder and a second plunger seal.

The pump of claim 2 wherein a cross-sectional area of the first plunger bstantially equal to a cross-sectional area of the second plunger.

4. The pump of claim 2 or 3 wherein the logic is to locate the leak, regardless of which the first plunger seal, the first inlet check valve, the first outlet check valve, the second plunger seal, the second inlet check valve and the second outlet check valve is leaking. 5. The pump of any one of claims 1 to 4 including plumbing for combining output from the first outlet check valve with output from the second outlet check valve.

6. The pump of any one of claims 1 to 5 including a control arrangement for controlling the one or more drive mechanisms to cause the first pumping chamber to deliver a pressure substantially equal to a pressure delivered by the second pumping chamber.

7. The pump of any one of claims 1 to 6 wherein the first temperature is a temperature of, or correlated with a temperature of, the first outlet check valve; and the second temperature is a temperature of, or correlated with a temperature of, the second outlet check valve. 8. The pump of any one of claims 1 to 7 wherein the one or more drive mechanisms includes an electric motor and a feedback sensor for providing a feedback signal indicative of at least one of a position of the motor and a speed of the motor; and the resistance-data is at least based on the feedback signal. 9. The pump of any one of claims 1 to 8 wherein the logic includes a comparison at least substantially equivalent to a comparison of one of a temperature difference across one of the pumping chambers and a pressure difference across the one of the pumping chambers to a parameter proportional to the other of the temperature difference and the pressure difference.

10. The pump of any one of claims of 1 to 9 being configured to deliver at least 345 MPa (50,000psi). 1 1 . A waterjet cutting apparatus including a cutting head; and the pump of claim 10 for suppling the cutting head.

12. A logic arrangement for a pump; the pump including two or more pumping chambers; and one or more drive mechanisms for driving the two or more pumping chambers; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the two or more pumping chambers having a second inlet check valve, a second outlet check valve and a second temperature sensor to measure a second temperature; the logic arrangement including one or more data inlets for receiving resistance-data indicative of a resistance to the driving; and temperature-data at least based on output from the first temperature sensor and the second temperature sensor; and the logic arrangement being configured to identify a leak by applying logic; the logic including mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber. 13. The logic arrangement of claim 12 wherein the logic includes a comparison at least substantially equivalent to a comparison of one of a temperature difference across one of the pumping chambers and a pressure difference across the one of the pumping chambers to a parameter proportional to the other of the temperature difference and the pressure difference.

14. The logic arrangement of claim 12 or 13, the first pumping chamber having a first plunger, a first cylinder and a first plunger seal; the second pumping chamber having a second plunger, a second cylinder and a second plunger seal; and the logic being to locate the leak, regardless of which the first plunger seal, the first inlet check valve, the first outlet check valve, the second plunger seal, the second inlet check valve and the second outlet check valve is leaking.

15. A method of identifying a leak in a pump; the pump including two or more pumping chambers; and one or more drive mechanisms for driving the two or more pumping chambers; a logic arrangement; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the one or more pumping chambers having a second inlet check valve, a second outlet check valve and a second

temperature sensor to measure a second temperature; the method including the logic arrangement applying logic; the logic including mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber.

16. The method of claim 15 including making a comparison at least substantially equivalent to a comparison of one of a temperature difference across one of the pumping chambers and a pressure difference across the one of the pumping chambers to a parameter proportional to the other of the temperature difference and the pressure difference. 17. The method of claim 16, the first pumping chamber having a first plunger, a first cylinder and a first plunger seal; the second pumping chamber having a second plunger, a second cylinder and a second plunger seal; and the method being a method of locating the leak, regardless of which the first plunger seal, the first inlet check valve, the first outlet check valve, the second plunger seal, the second inlet check valve and the second outlet check valve is leaking.

18. The method of claim 16 or 17 including receiving, via the internet, the first temperature, the second temperature, the compression-stroke resistance of the first pumping chamber, and the comparison-stroke resistance of the second pumping chamber.

19. The method of claim 16, 17 or 18 further including signaling, the leak, in response to the identification and via the internet. 20. A pump, for pumping fluid, having a temperature sensing arrangement for providing temperature data indicative of a temperature difference associated with operation of the pump; a sensing arrangement for providing pressure data indicative of a pressure difference associated with the operation of the pump; and a logic arrangement configured to identify a fault based on the temperature data and the pressure data.

21 . A method, of identifying a fault in a pump, including obtaining temperature data indicative of a temperature difference associated with operation of the pump; obtaining pressure data indicative of a pressure difference associated with the operation of the pump; and applying logic to the temperature data and the pressure data.

Description:
FAULT DETECTION AND PREDICTION FIELD OF THE INVENTION

The invention relates to faults in pumps and/or waterjet cutting systems. Preferred forms of the invention are concerned with identifying and locating leaks in pumps for waterjet cutting systems.

The invention will be described in the context of pumps for waterjet cutting systems although variants of the invention may be applied in other contexts such as high pressure pasteurisation (HPP) and compressing gases.

BACKGROUND TO THE INVENTION The present inventor has previously developed a range of improvements in and for waterjet cutting as disclosed in international patent application nos.

PCT/AU2008/000575, PCT/AU2009/000334, PCT/AU201 1 /001 171 ,

PCT/AU2013/001033 and PCT/AU2015/000105 to which the interested reader is directed for further information. Waterjet cutting systems are complex systems based around a pump for supplying water at 345 MPa (about 50,000 psi) or more. Supplying such pressures is an extreme engineering challenge. At these pressures, water is compressable by about 12% or more.

Faults can and do occur in waterjet cutting systems from time to time. Such faults can be very costly. Typically, the system must be shut down, a technician called resulting in significant downtime before the technician even arrives on site. Once on site, identifying the fault can be difficult. By way of example, a pump might be consuming more power than expected but the cause of that additional power draw may not be immediately apparent. The present inventor has recognised that water leaks are a particularly problematic class of faults because once such a leak has developed, the leaking water, driven by the extreme pressures, will tend to erode and damage any surrounding components. As such, often when a technician arrives on site, there is significant erosion resulting in much work to do which is both expensive and time consuming.

Accordingly, preferred variants of the technologies disclosed herein aim to provide for the easy and early identification of faults, or at least to provide alternatives for those concerned with pumps and/or waterjet cutting systems.

US patent no. 6,092,370 discloses a pump, each pumping chamber of which has an inlet check valve, an outlet check valve and a plunger seal. Two temperatures are used to individually determine the operational status of the inlet check valve, the outlet check valve and the plunger seal.

It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way before the priority date. SUMMARY

One aspect of the invention provides a pump including two or more pumping chambers; one or more drive mechanisms for driving the two or more pumping chambers; and a logic arrangement; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the one or more pumping chambers having a second inlet check valve, a second outlet check valve and a second temperature sensor to measure a second temperature; the logic arrangement being configured to identify a leak by applying logic; and the logic including: mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber.

The pump may include a second pumping chamber of the one or more pumping chambers; the second pumping chamber having a second inlet check valve, a second outlet check vale and a second temperature sensor. The first pumping chamber preferably has a first plunger, a first cylinder and a first plunger seal. The second pumping chamber preferably has a second plunger, a second cylinder and a second plunger seal. Most preferably, a cross-sectional area of the first plunger is substantially equal to a cross-sectional area of the second plunger. The pump may include plumbing for combining output from the first outlet check valve with output from the second outlet check valve. The pump may include a control arrangement for controlling the one or more drive mechanisms to cause the first pumping chamber to deliver a pressure substantially equal to a pressure delivered by the second pumping chamber.

Preferably the first temperature sensor is for sensing a temperature of or correlated with the first outlet check valve; and the second temperature sensor is for sensing a temperature of or correlated with the second outlet check valve.

Preferably the logic includes mutually comparing the output from the first temperature sensor and the output from the second temperature sensor. Preferably the logic includes mutually comparing a compression-stroke speed of the first pumping chamber and a compression-stroke speed of the second pump chamber. Most preferably the logic is to locate the leak, regardless of which the first plunger seal, the first inlet check valve, the first outlet check valve, the second plunger seal, the second inlet check valve and the second outlet check valve is leaking.

The one or more drive mechanisms may include an electric motor and a feedback sensor for providing a feedback signal indicative of at least one of a position and speed of the motor. The resistance-data may be at least based on the feedback signal.

The one or more data outlets may be configured to convey data via the internet. The pump is preferably configured to deliver at least 345 MPa (50,000psi).

Another aspect of the invention provides a waterjet cutting apparatus including a cutting head and the pump for suppling the cutting head.

Another aspect of the invention provides a logic arrangement for a pump; the pump including two or more pumping chambers; and one or more drive mechanisms for driving the two or more pumping chambers; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the two or more pumping chambers having a second inlet check valve, a second outlet check valve and a second temperature sensor to measure a second temperature; the logic arrangement including one or more data inlets for receiving resistance-data indicative of a resistance to the driving; and temperature-data at least based on output from the first temperature sensor and the second temperature sensor; and the logic arrangement being configured to identify a leak by applying logic; the logic including mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber.

Another aspect of the invention provides a method of identifying a leak in a pump; the pump including two or more pumping chambers; and one or more drive mechanisms for driving the two or more pumping chambers; a logic arrangement; a first pumping chamber of the two or more pumping chambers having a first inlet check valve, a first outlet check valve and a first temperature sensor to measure a first temperature; a second pumping chamber of the one or more pumping chambers having a second inlet check valve, a second outlet check valve and a second

temperature sensor to measure a second temperature; the method including the logic arrangement applying logic; the logic including mutually comparing the first temperature and the second temperature; and mutually comparing a compression-stroke resistance of the first pumping chamber and a compression-stroke resistance of the second pump chamber. The method may include receiving, via the internet, resistance data and the temperature-data.

The method may include signaling, the leak, in response to the identification and via the internet. Another aspect of the invention provides a waterjet cutting apparatus including one or more sensors.

Another aspect of the invention provides a logic arrangement, for monitoring a condition of a waterjet cutting apparatus including one or more sensors, including a data inlet for receiving data at least based on the output from the one or more sensors.

Another aspect of the invention provides a pump, for pumping fluid, having a temperature sensing arrangement for providing temperature data indicative of a temperature difference associated with operation of the pump; a sensing arrangement for providing pressure data indicative of a pressure difference associated with the operation of the pump; and a logic arrangement configured to identify a fault based on the temperature data and the pressure data.

The pump may be a positive displacement pump including at least one pumping chamber. The pressure difference may be across the at least one pumping chamber. The at least one pumping chamber may have an outlet check valve. The temperature difference may be from a temperature of or correlated with a temperature of the fluid entering the pumping chamber to a temperature of or correlated with a temperature of the outlet check valve.

The logic arrangement may be configured to so identify based on a comparison at least substantially equivalent to a comparison of one of the temperature difference and the pressure difference to a parameter proportional to the other of the temperature difference and the pressure difference.

Another aspect of the invention provides a method, of identifying a fault in a pump, including obtaining temperature data indicative of a temperature difference associated with operation of the pump; obtaining pressure data indicative of a pressure difference associated with the operation of the pump; and applying logic to the temperature data and the pressure data.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates a waterjet cutting system; Figure 2 is a transverse cross-section view of a pumping chamber; Figure 3 is a perspective view of an end of a pump; Figure 4 is a logic diagram;

Figure 5 is another representation of the logic; Figure 6 is another logic diagram;

Figure 7 is another representation of the logic of Figure 6; and Figure 8 schematically illustrates another pump. DETAILED DESCRIPTION

Figure 1 illustrates a waterjet cutting system 1 built around a pump 3. The pump 3 has a pump chamber 5a, 5b at each end. The chamber 5a is substantially identical to the chamber 5b. Each of the pump chambers 5a, 5b is connected to draw water from a common low pressure feed 7 and to pump water to a common high pressure feed 9.

The low pressure feed 7 is connected to draw water from a water supplying arrangement 1 1 . The arrangement 1 1 incorporates a valve 1 1 a and a rotodynamic booster pump 1 1 b arranged in parallel to each other and to connect an inlet valve 1 1 c to the lower pressure feed 7. The valve 1 1 c selectively connects the system 1 to a mains water supply. Typically, the valve 1 1 a is open and the pump 1 1 b is inactive, but the states of those components can be reversed to supply additional water to the pump 3 if need be.

An attenuator 13 connects the high pressure feed 9 to further plumbing 15 by which the pump 3 is connected to cutting head 17. The further plumbing 15 incorporates a dump valve 15a including an air inlet for dumping water when required.

The pump 3 includes a single linear actuator 3a between and mutually connecting the pumping chambers 5a, 5b. The actuator 3a preferably takes the form of a screw- member, a nut-arrangement co-operable with the screw-member, and a stator surrounding, to rotationally drive, the nut-arrangement.

By way of example, the nut-arrangement may carry magnets co-operable with the stator. The stator is preferably part of a servo motor. The nut-arrangement and screw-member are preferably a ball screw, although a roller screw would also be suitable. Whilst preferred forms of the invention incorporate a nut-and-screw arrangement and a coaxial stator, other variants are possible. By way of example, various of the advantageous technologies described in here can be applied to double acting hydraulic intensifier pumps. The actuator 3a is serviced by a lubricant loop 19 about which oil is pumped by pump 21 to lubricate the interior of the actuator 3a. First and second filters 23, 25 are mounted along the loop 19 and are separated by a heat exchanger 27 through which the oil is cooled by cooling water 29. Figure 2 is a cross-section view of the pumping chamber 5b. The chamber 5b incorporates a cylinder 29 and plunger 31 . The plunger 31 is connected to an end of the screw-member and mounted to slide through a high-pressure seal 33. The seal 33 separates the pumping chamber 5b from the interior of the actuator 3a and as such separates the water from the oil. A check valve 35 defines an inlet into the pumping chamber 35b whilst a check valve 37 defines an outlet from the chamber 5b. As the plunger 31 is reciprocally driven by the actuator 3a, the internal volume of the chamber 5b varies and the valves 35, 37 open and close to positively displace fluid through the chamber 5b. Some of the technologies described herein may be applied to positive displacement pumps other than plunger and cylinder pumps e.g. to diaphragm-type pumps.

Components 33, 35, 37 are highly stressed components at risk of leakage that leads to erosion. The inventor has recognised that monitoring a temperature of each of the pumping chambers 5a, 5b and applying logic to those temperatures and to resistance-data such faults can be efficiently identified, potentially before problematic erosion is caused.

Figure 3 is a perspective view of an end of a pump 3 illustrating a preferred temperature sensor 47. The chamber 5b has an outlet fitting 37a via which the chamber is connected to the high pressure feed 9. The sensor 47 embraces an exterior of the fitting 37a. In this example, it embraces a hexagonal portion of the exterior. Through this mounting arrangement, the sensor 47 produces an output that is highly correlated with a temperature of the check valve 37.

Preferred variants of the pump 3 incorporate a power and data arrangement 39 incorporating a sensor module 41 , a control module 43 and a drive module 45. The drive module 45 electrically drives the actuator 3a. The modules 41 , 43, 45 are operatively connected to each other. Preferably the modules are commonly housed although distributed arrangements are also possible.

Preferably the logic is implemented by the control module 43 although other logic arrangements are possible. The logic arrangement may be a distributed logic arrangement, potentially including components of the arrangement connected via the internet. One example of the pump includes a data outlet for conveying towards the logic arrangement data at least based on the output from the first temperature sensor and the output from the second temperature sensor. The conveyed data may directly correspond to those two outputs. Alternatively, the conveyed data may directly correspond to a difference between those two outputs. The terminology 'data at least based on output from the first temperature sensor and output from the second temperature sensor' and similar terminology as used herein embraces both possibilities.

The data outlet could be a connection arrangement for connecting to the internet. In other variants, substantially all of the logic arrangement is mounted in proximity to the actuator 3a, in which case the data outlet may be as simple as a cable. The logic arrangement preferably also takes account of feedback from the drive 45.

Each of the outlet check valves 37 opens to the high pressure feed 9 whereby the high pressure feed 9 constitutes plumbing for combining the outputs of the two check valves and the pumping chambers 5a, 5b face a substantially identical system curve. Other plumbing arrangements that lead to the two chambers facing substantially identical system curves are possible.

Preferably the drive is configured so that each of the pumping chambers 5a, 5b delivers substantially the same pressure whereby any difference in the speed of the compression-strokes of the chambers is likely to be associated with leakage. Most preferably this is achieved by the control module 43 receiving and responding to output from a pressure sensor PA configured to sense the pressure of the high pressure feed 9. As such, the modules 43, 45 together constitute a control arrangement for controlling the pump so that the pump chambers deliver the same pressure. In this example, the pressure sensor PA is mounted downstream of the accumulator 13.

When the pump chambers are controlled to deliver substantially the same pressure, the speed of the plungers becomes an indication of their resistance to the drive. If a particular chamber is leaking, typically that will reduce the resistance to the driving of the plungers whereby the plunger must be accelerated to maintain the desired pressure. The indication of plunger speed is preferably provided by the servo motor, although a sensor to directly measure the speed and/or position of the plungers (or a component, e.g. ball screw, fixed relative to the plungers) is another possibility. Compression-speed is but one example of resistance-data. Other forms of

resistance-data may be utilised e.g. in the case of a pump controlled to operate at a fixed speed, the electrical current drawn by the pump's motor(s) might be monitored to provide an indication of the resistance to the driving force provided by that motor.

Figure 4 illustrates a preferred form of the logic 100. In Figure 4, the temperature at the first and second ends of the pump are designated Τι , T2 respectively and the compression-stroke for the first and second ends of the pump are designated Vi , V2 respectively.

The logic commences with a comparison 101 to assess whether the two

temperatures are approximately equal. By way of example, the comparison may entail assessing whether the modulus of T1-T2 is less than a threshold ΔΤ. If the temperatures are approximately equal, the velocities are likewise compared at step 103. If the velocities are approximately equal, this combination of observations is taken to mean that there is no leak and the logic arrangement returns to step 101 . Thus, the steps 101 , 103 form a loop about which the logic arrangement iterates until it suggests an above-threshold difference between the temperatures or compression- stroke velocities is detected.

If the temperatures remain substantially identical and a difference in velocities is detected, the logic arrangement moves from step 103 to step 105 at which the faster end is identified. The high pressure seal (e.g. seal 33) associated with that end is likely to be leaking.

At this juncture in the logic 100 there are a variety of possible responses that may be implemented. An alarm signal may be sent and/or an interlock mechanism activated to deactivate the pump 3. Optionally the pump may be allowed to continue to operate until a second threshold velocity difference is detected whereby the pump can, for example, continue to operate with some minor leakage but automatically stop once the leakage has reached a level deemed likely to cause problematic erosion.

Whilst the logic arrangement is iterating about the loop 101 , 103, if a difference in the temperature is detected the logic arrangement moves on to step 107 at which the hotter of the two ends of the pump is identified. If the first end is hotter, the logic moves on to branch 109. If the second end is the hotter end, the logic moves on to branch 1 1 1 . The branch 1 1 1 is akin to the branch 109 but not detailed in Figure 4.

Along branch 109, the compression-stroke velocities are compared at step 1 13. The threshold velocity difference AV may or may not be the same as the threshold of AV in step 103. If one end of the pump is hot but both ends are moving in substantially the same velocity, that is regarded as an indication that both the inlet and outlet valves (e.g. valves 35, 37) of the hotter end are leaking. If there is a velocity difference, the logic 100 moves on to step 1 15 at which the faster end is identified. If the hotter end is the faster end, that is an indication that the inlet valve on the hotter end is leaking. The additional velocity is resultant from the drive 45 accelerating the plunger to maintain the delivery pressure whilst water is escaping past the inlet check valve. The reversal of fluid through the leaking (but still restrictive) check valve produces heat. If the hotter end of the pump is the slower end, that is an indication that the outlet valve on that end is leaking. The difference in velocity arises from the control arrangement 45 accelerating the plunger on the compression-stroke of the colder end to compensate for leakage through the outlet valve of the hot end. Heat is generated by the water being forced through the leaking, but restrictive, outlet valve. As per the discussion in respect of leaking seals being identified, once one or more valves have been identified as leaking, alarms may be raised, a monitoring mode may be entered and/or an interlock activated.

Figure 5 is another representation of the logic 100. Figure 5 is a 2D plot in which the temperature difference and the velocity difference are plotted on the horizonal and vertical axes respectively. The central region represents fault-free operation whereas a departure from the central region is suggestive of a fault:

• a departure along the vertical axes is suggestive of one seal or the other

leaking; · a departure along the horizontal axes is suggestive of both check valves at one end of the pump leaking; and

• a departure along a diagonal is suggestive of one of the check valves leaking.

Preferred forms of the logic 100 preferably include allowances for the transient behavior of the pump. One option may entail delaying for a predetermined period of time, e.g. 1 minute, after the pump has been activated before moving to step 101 . In addition, or as an alternative to a predetermined delay period, the control

arrangement may be configured to move to step 101 only after certain parameters of the pump have met one or more predefined stability criterion. In addition or as an alternative to these possibilities, the thresholds applied as part of the logic (at steps 101 , 103 and 1 13 in this example) may vary as a function of time and/or one or more other parameters.

Preferred forms of the invention monitor the sensor data to not only detect but predict the occurrence of faults. By way of example, a rate at which a relative difference in parameters is growing may be monitored to predict when that difference will cross a threshold deemed to be problematic. Preferred variants of the waterjet cutting system 1 incorporate one or more of the following sensors:

Instrumenting the system in this way can enable one or more of the following faults to be identified, located and/or predicted:

Blocked Oil Filters or Damaged Oil Pump

Premature ball screw and bearing failure

Shortened seal life

Insufficient cutting water supply (either low pressure or insufficient volume) Cavitation damage to seals, cylinders and check valves Low air pressure

Leaking dump valve and or Cutting Head causing damage to the needle and seat Low oil level

Damaged ball screw or bearings Leaking needle and seat in Dump Valve

Erosion damage to the dump valve

High motor torque Damaged ball screw or bearings

Damaged plunger

This instrumentation enables another approach to fault identification that may be utilised in addition to or as an alternative to the previously described approaches. The present inventors have recognised that in normal, fault-free, operation the temperature differences within various portions of the pumping system are related to pressure differences within the pumping system and as such deviations from these normal relationships can be used to identify faults. A preferred implementation of the concept entails determining a pressure rise ΔΡ across the chamber 5b by subtracting the pressure of the fluid entering the chamber 5b (as determined by sensor P3) from the outlet pressure (as determined by the sensor PA). A temperature difference ΔΤ between the outlet check valve 37 and the inlet water temperature is determined by comparing the outputs of the sensors T2, T7.

In normal operation these factors are relatable in accordance with the following relationship: ΔΤ = ΚΔΡ

Accordingly ΚΔΡ is a parameter proportional to the pressure difference ΔΡ that may be conveniently compared to the temperature difference to identify a fault as follows:

|ΔΤ - ΚΔΡ| < C wherein C fault is a criterion for the identification of a fault. The following are equivalent expressions of the same comparison:

|ΔΤ/Κ - ΔΡ| < C

|ΔΤ/ΔΡ - K| < C"

The K value may be selected so that ΔΤ corresponds to the adiabatic temperature rise. Alternatively it may be a calibrated value or another value otherwise preset to suit a particular application.

The sensors P3, PA are one example of a sensing arrangement for providing pressure data indicative of a pressure difference. Other examples are possible. In another example wherein the inlet pressure is constant the pressure sensor PA is itself a sensing arrangement providing pressure data indicative of the pressure difference across the pumping chamber 5b. In yet another example, the actuator 3A may be the sensing arrangement. An output from the actuator indicative of the motor's drive torque is indicative of the outlet pressure in that it may be directly related to the outlet pressure via the pitch of the ball screw and the diameter of the plunger 31 .

Figure 200 is a logic diagram illustrating logic 200 by which the pressure data is combined with logic 100' akin to logic 100 for improved fault finding. The logic 200 is separately applied to each of the two cylinders of the pump.

At step 201 output from the pressure sensors P3, PA is processed to determine the pressure rise ΔΡ across the cylinder. At step 203 that value is multiplied by predetermined constant K to determine an expected temperature rise ΔΤ. At step 205 that ΔΤ and the inlet water temperature (provided by temperature sensor T7) are summed to produce an anticipated outlet temperature Tref.A-

At step 207 the anticipated outlet temperature T re f,A and the outlet temperature from the other end of the pump T re f,B are taken into account to determine a reference temperature TRef for the cylinder under consideration. The processing step 207 could take a variety of forms. In one example, a lowest of the two inputs may be selected. In another example the two inputs may be averaged. From this juncture forward the logic may substantially follow the logic 100 including a comparison 101 Of the cylinder temperature to the reference temperature TRef and a comparison of the velocities at 103'.

Figure 7 plots the output of the logic 200 when applied to one end of the pump. As illustrated, the logic enables a leaking one of the two check valves and the seal at the one end of the pump to be specifically identified.

The pump 3 incorporates a single drive mechanism in the form of linear actuator 3a driving two pumping chambers 5a, 5b. The pumping chambers 5a, 5b are plumbed in parallel to each other and are utilised to pump water for waterjet cutting. Figure 6 illustrates a contrasting example. The pump 100 incorporates a trio of pumping chambers 105a, 105b, 105c plumbed in series for pumping a gas. Each of the chambers has a respective drive 103 incorporating a rotationally driven crank 103a and a conrod 103b connecting the crank 103a to a piston 131 of the pump chamber 105b. Piston-cylinder arrangements are alternatives to plunger-cylinder

arrangements.

The chamber 105b has its inlet check valve 135 mounted along a line communicating the interior of the chamber 105a to the interior of the chamber 105b. As such, the chambers 105a, 105b share the check valve 135; the chamber 105a has the check valve 135 as its outlet check valve.

The drive 1 03 further includes a combustion engine arranged to rotationally drive the crank 103a. The engine incorporates a governor by which the engine is throttled to deliver a constant shaft speed. In this example, a position of the throttle is resistance- data indicative of the resistance to the drive of the pumping chamber. Applying strain gauges, or another suitable sensing arrangement, to measure the drive torque is another option. Drive-power, e.g. shaft-power or electric motor input power, is also a possibility. By instrumenting the pump 100 and applying logic to the resistance data and temperature data from the instruments, faults in the pump 100 may be identified in a manner akin to the manner described in respect of the pump 3.

The pump 3 incorporates chambers 5a, 5b which are substantially identical to each other and plumbed in parallel between the water supplying arrangement 1 1 , the attenuator 13.

In another advantageous embodiment of the principles disclosed herein that is suited to compressing gases, the chamber 5a has an effective cross-sectional area about double the effective cross-sectional area of the chamber 5b and the chambers are plumbed in series via a suitable heat exchanger. Gas is compressed through the chamber 5a, cooled through the heat exchanger and then further compressed through the chamber 5b. In embodiments such as these, the speed of the plungers may well be asymmetric and the logic 100 adjusted accordingly. By way of example the test at step 103 might be replaced by |Vi-1 .5V2| < AV. Likewise a different relationship between the two temperatures might be applicable. Whilst various examples have been described, the invention is not limited to these examples. Rather, the invention is defined by the claims.