CAVITATION DETECTION

The present invention is concerned with the detection of cavitation in fluid mechanisms. In particular, the present invention is concerned with the detection of cavitation in pumps for pumping a fluid (for example a supercritical fluid) or a liquid.

Cavitation is problematic phenomenon that can occur when a pump is operated such that the pressure inside the pump drops below the vapour pressure of a liquid being pumped. Bubbles of the vapourised liquid are formed. When the bubbles collapse, damage can be caused to the pump. During severe cavitation the noise of collapsing bubbles may be audible to a human. It would, however, be advantageous to detect the onset of severe and/or damaging cavitation so that the operating conditions of the pump can be modified (a minor amount of cavitation can be tolerated in some applications).

JP 11-037979 discloses a system for detecting cavitation in fluid mechanisms such as pumps. JP 11-037979 operates by comparing successive acoustic waveform cycles from a pump. Each waveform cycle is decomposed into a plurality of coefficients. The coefficients of successive acoustic waveform cycles are compared on a term-by- term basis by taking the dot product of successive acoustic waveforms. If successive acoustic waveform cycles are sufficiently similar then cavitation is deemed not to be occurring; if successive acoustic waveform cycles are sufficiently different then cavitation is deemed to be occurring.

According to an aspect of the present invention, there is provided: a transducer; a highpass filter for highpass filtering a signal from the transducer; a reference receiver for receiving a threshold; a comparator for comparing the highpass filtered signal with the threshold.

According to other aspects of the invention, there are provided acoustic sensors and a method of detecting cavitation.

An advantage of the present invention is that less signal processing is required compared to some prior art cavitation detection methods. Another advantage of some embodiments of the present invention is that they detect cavitation in the MHz range, which allows cavitation (or the onset of violent cavitation) to be more reliably identified.

In one embodiment of the present invention, the signal from the sensor is compared with a low pass filtered version of the signal from the sensor. An advantage of this embodiment is that it is less dependent on the level of the signal from the sensor and thus this embodiment is more tolerant of uncertainties in the acoustic coupling of the sensor to a pump being monitored.

DESCRIPTION OF FIGURES

Figure Ia shows a perspective view of a pump and a motor; Figure Ib shows a view of the pump and motor which enables a piezoelectric gasket to be seen.

Figure 2 shows a view of the piezoelectric gasket of Figure Ib.

Figure 3 shows a schematic illustration of a system which may be used to detect the onset of cavitation and reduce the speed of the motor if cavitation is detected.

Figure 4 shows an example of acoustic waveforms from a pump: (i) when the pump is operating without cavitation and (ii) during the onset of cavitation.

Figure 5 shows a schematic illustration of a system based on Figure 3 but which compares differently filtered versions of the signal from a sensor.

Figure 6 shows a perspective view of a sensor that may be pressed against the housing of a pump in order to measure an acoustic signal from the pump.

Figure 7 shows a cross-sectional view of a portion of a pump in which an acoustic sensor has been embedded.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figures Ia and Ib show a pump 100 coupled to an inlet pipe 101 by a piezoelectric gasket 102. A motor 110 has a shaft 110 that drives an impellor 112 of the pump 100.

Figure 2 shows a more detailed view of the piezoelectric gasket 102. In this embodiment the piezoelectric gasket 102 is annular and has two planar faces. One of the faces is used to form a fluid seal against the pump 100; the other face is used to form a fluid seal against the inlet pipe 101.

In this embodiment the piezoelectric gasket 102 comprises a polymer piezoelectric material, for example PVDF (polyvinylidene fluoride), having a thickness of 50μm. Leads 201a, 201b are connected to respective faces of the gasket 102. The piezoelectric gasket 102 may be made from a single layer of film, a series of laminated layers or may posses a structured or patterned electrode. When the piezoelectric gasket 102 is clamped between the pump 100 and the inlet pipe 101, the piezoelectric gasket will convert acoustic signals from the pump into electrical signals. The piezoelectric gasket 102 can therefore be used to detect acoustic signals from the pump 100.

In other embodiments an electrostatic shield may be provided around the gasket 102 and/or the leads 201a, 201b to reduce the influence of external electrical signals.

In yet other embodiments, a ceramic piezoelectric material or a mixed polymer- ceramic composite material may be used instead of a polymer piezoelectric material. If a ceramic piezoelectric material is used then it may be necessary to coat the ceramic with a polymer such as rubber to prevent cracking of the ceramic when the ceramic piezoelectric material is clamped between the pump 100 and the inlet pipe 101.

An advantage of the use of a polymer piezoelectric material is that the thickness of the polymer may be more readily reduced than the thickness of a ceramic piezoelectric material. A relatively thin material will in general be better than a thicker material at detecting high frequency acoustic signals; this is because a thin material is small compared to the wavelength of a high frequency acoustic signal. For example, a 50μm

thick layer of PVDF has an upper frequency response of about 20MHz and so cannot be effectively used to sense acoustic signals having a frequency of higher than about 20MHz. At frequencies below 20MHz the thickness of a thick 50μm PVDF layer is significantly less than the wavelength of sound in, say, water. Also, a thinner material will have a higher resonant frequency (the first resonance occurs when the thickness of the material becomes equal to half of the acoustic wavelength), thus allowing higher frequency signals to be detected.

Figure 3 shows a schematic illustration of a system which may be used to detect the onset of cavitation and reduce the speed of the motor if cavitation is detected. Figure 3 shows that the electrical signal from the piezoelectric gasket 102 is amplified by an amplifier 301 and is then filtered by a highpass filter 302. (In alternative embodiments, it may be preferable to filter and then amplify the signal from the piezoelectric gasket 102.) In this embodiment the highpass filter 302 attenuates signals having a frequency of less than IMHz. In this embodiment the highpass filter 302 is a second order (i.e. attenuation increases at 12dB per octave) analogue filter although a digital filter could be used in alternative embodiments.

The output of the highpass filter 302 is connected to a detector 303. In this embodiment the detector 303 is an envelope detector and converts the signal from the highpass filter 302 into a voltage indicative of the strength of the signal from the highpass filter 302. In other embodiments, the detector 303 may comprise an amplitude sensor or an RMS (root-mean-square) detector.

The output of the detector 303 is connected to a comparator 304. The comparator 304 compares the voltage from the detector 303 with a reference signal received from a reference input 305. In this embodiment the comparator 304 subtracts the reference signal from the detected voltage. In other embodiments, the comparator 304 may divide the detected voltage by the reference signal to form a ratio.

In this embodiment, the output of the comparator 304 is connected to a motor controller 306 which controls the speed of the motor 110. In the event that the comparator 304 indicates excessive cavitation, the motor controller 305 reduces the speed of the motor 110 in order to reduce cavitation. In other embodiments, other

methods may be used to reduce cavitation. For example, the head of pressure at the inlet to the pump 100 may be increased.

Figure 4 shows an example of acoustic waveforms from the pump 100, and shows the amplitude of the acoustic waveform versus frequency. Curve 401 (the solid line) shows the amplitude of the acoustic waveform versus frequency when the pump is operating without significant cavitation. Curve 402 (the dotted line) shows the amplitude of the acoustic waveform versus frequency and during the onset of violent cavitation.

As can be seen from Figure 4, during cavitation the amplitude of the acoustic waveform increases slightly at kHz frequencies but significantly at MHz frequencies. Prior art cavitation detectors generally attempt to detect cavitation having a frequency of 100s of kHz. Embodiments of the present invention may detect cavitation having a frequency at least 0.5MHz, at least IMHz, at least 2MHz, at least 4MHz or at least 8MHz, by altering the cutoff frequency of highpass filter 302 as appropriate.

Figure 5 shows a schematic illustration of a system based on Figure 3 but which compares differently filtered versions of the signal from the piezoelectric gasket 102. Figure 5 shows that the output from the amplifier 301 is filtered by a bandpass filter 501 (instead of the highpass filter 302). In this embodiment the bandpass filter 501attenuates frequencies that lie outside the passband of IMHz to 5MHz. The output from the amplifier 301 is also filtered by a lowpass filter 502, detected by a detector 503 and used as the reference input to the comparator 304. In this embodiment the lowpass filter 502 has a cutoff frequency of IkHz and thus attenuates frequencies higher than IkHz. In alternative embodiments, the cutoff frequency of the lowpass filter 502 may instead be at most 10kHz, 100kHz or IMHz.

As those skilled in the art will appreciate, the bandpass filter 501 can conceptually be regarded as a IMHz highpass filter in series with a 5MHz lowpass filter (even though the bandpass filter 501 may actually be implemented as a single bandpass filter rather than as concatenated highpass and lowpass filters). Thus Figure 5 includes a highpass filter function that is equivalent to the highpass filter 302 of Figure 3. In some

embodiments the upper frequency response of the piezoelectric gasket 102 may be used to define the lowpass filter of the bandpass filter 501.

The system of Figure 5 compares the acoustic energy emanating from the pump 100 in the frequency range of IMHz to 5MHz with the acoustic energy below IkHz. If there is an excess of energy in the range IMHz to 5MHz then the pump 100 is deemed to be undergoing excessive cavitation and the motor controller 305 is used to reduce the speed of the motor 110 (and thus of the pump 100) accordingly. An advantage of the system of Figure 5 compared to the system of Figure 3 is that Figure 5 is more tolerant of imperfect acoustic coupling between the piezoelectric gasket 102 and the pump 100, and is also more tolerant of imperfect coupling due to the distance that the ultrasonic sound has to travel from the cavitating surface inside the pump 100 to the casing of the pump 100. The system of Figure 3 cannot account for poor acoustic coupling (poor acoustic coupling could erroneously result in an indication that cavitation is not occurring). In contrast, the system of Figure 5 normalises the energy in the frequency band IMHz to 5MHz against the energy in the frequency range below IkHz. Thus if the piezoelectric gasket 102 is not well coupled to the pump 100, the signal in the frequency band IMHz to 5MHz and the signal in the frequency range below IkHz will both be reduced. Thus the system of Figure 5 can at least partially compensate for imperfect acoustic coupling.

Figure 6 shows a perspective view of a sensor 600 that may be pressed against the housing of a pump in order to measure an acoustic signal from the pump. The sensor 600 may be used during the commissioning of a pump to determine operating conditions for the pump that do not involve excessive cavitation. Once the operating conditions have been determined, the sensor 600 may be more easily removed from the pump than, say, the piezoelectric gasket 102. On the other hand, an advantage of the piezoelectric gasket 102 is that it allows continuous real-time monitoring of the pump 100.

As shown, the sensor 600 comprises an acoustic coupler 601 which may be pressed against the housing of a pump. In this embodiment the acoustic coupler 601 is formed of rubber and couples acoustic energy to three PVDF layers 602a, 602b, 602c. In this embodiment each of the three PVDF layers 602 is substantially planar. The use of

three PVDF layers improves the output voltage of the sensor 600 by a factor of about three but also reduces the upper frequency limit of the sensor 600 (compared to using a single PVDF layer; the reduction in the upper frequency limit is due to the increased thickness of the three PVDF layers compared to a single layer). In this embodiment the Shore hardness of the elastomeric coupler 601 is preferably in the range 10 to 20 although in other embodiments the Shore hardness may be less than 10 or more than 20.

The sensor 600 also has, in this embodiment, a metal shank 603 which acts as a grip portion to allow a user to hold the acoustic coupler 601 of the sensor 600 against the exterior of a pump or against pipes fastened to the pump. A lead 604 is used to connect the sensor 600 to circuitry (not shown),

In alternative embodiments an acoustic sensor (not shown) may be fastened or bonded (for example using an adhesive) against the exterior of a pump. In such embodiments the acoustic coupler 601 and/or the metal shank 603 may not be required.

Figure 7 shows a cross-sectional view of a portion of a pump 700 (the majority of the pump 700 is shown in phantom lines) in which an acoustic sensor 701 has been embedded. The acoustic sensor 701 is embedded in a recess 702 in the casing 703 of the pump 700. An advantage of this embodiment is that the acoustic sensor 701 is close to the fluid in which cavitation could occur. In contrast, the embodiment of Figure 1 requires the acoustic signal to travel from the interior of the pump 100 to the piezoelectric gasket 102.

As those skilled in the art will appreciate, the present invention may be used to reduce cavitation in pumps, for example centrifugal pumps or axial pumps, or in other fluid mechanisms.

In some situations it may be advantageous to reduce the influence of electrical noise. The electrical signal from an acoustic sensor typically requires significant amplification and thus there is the risk of inadvertently picking up stray electrical signals. Although the electrostatic shield discussed above in connection with Figure 2 may be sufficient, in other embodiments a calibration step may be performed. In such

embodiments, the acoustic signal is measured under conditions under which it is known that no cavitation is occurring (for example when the motor 110 is stationary). If a signal, for example having a frequency of 2MHz, is detected under the no- cavitation condition then an electrically configurable notch filter may be used to suppress the extraneous 2MHz signal in order to avoid false alarms of cavitation when the motor 110 is running.

In some embodiments, for example when the acoustic sensor is to be fitted to plant machinery which is in use and cannot be stopped, it may be inconvenient to stop the motor(s) that drive pump(s). In such cases, it may be more convenient to detach the acoustic sensor from the body of a pump, thereby providing a "normalisation" signal in which cavitation is not occurring. Alternatively, the acoustic sensor may be allowed to dangle in air; the sensor may still pick up background electrical signals (particularly if the pump is in ah electrically noisy environment) and the background electrical noise may thus be used as a normalisation signal.

In some embodiments, if the signal from the piezoelectric gasket 102 is sufficiently strong then it may not be necessary to use the amplifier 301.

There is disclosed an apparatus and method for detecting cavitation in fluid machines, for example pumps 100. In one embodiment a piezoelectric gasket 102 is used as a sensor to sense cavitation. In some embodiments highpass filters 302, 501 are used to detect ultrasonic acoustic signals in about the MHz range. If the energy in the MHz range is excessive then cavitation is deemed to be occurring and the speed of a motor 110 may be reduced in proportion to the degree of cavitation deemed to be occurring. In another embodiment (Fig. 5) the energy in the MHz range is normalised against the energy in the kHz range. Other sensors 600, 701 are also disclosed.

The disclosures in the abstract of the present application, and the entirety of GB 0714695.4 (from which the present application claims priority), are hereby incorporated by reference.