Patent Description:
<CIT> discloses measuring scaling characteristics of water by means of quartz micro-balance having a plate electrode exposed to a water flow. <CIT> discloses measuring the furring capacity of a liquid by means of a quartz micro-balance having a plate electrode exposed to the liquid. <CIT> discloses measuring the corrosivity of a liquid by means of a piezoelectric crystal coated with a metal which is exposed to the liquid. <CIT> discloses a deposition monitoring system comprising two piezoelectric transducers coupled each to a metal horn exposed to a wellbore fluid, wherein one of the horns is cleaned so as to keep it free from deposition. <CIT> discloses detection of the state of a fluid by means of two piezoelectric transducers coupled to a steel membrane exposed to the fluid. <CIT> discloses monitoring corrosion in an air flow by means of quartz micro-balances covered with different metallic coatings which are exposed to the air flow. <CIT> discloses monitoring corrosion/erosion of a part of a turbomachine exposed to a working fluid, by means of a measurement element exposed to said fluid. <CIT> discloses monitoring particles in a fluid stream, wherein a corrosion/erosion sensor comprises a sample acoustic sensor and a reference acoustic sensor exposed both to near identical temperature and pressure. <CIT> discloses a fouling and corrosion detector comprising two piezoelectric elements and associated elements exposed to various fluid flows in an industrial system such as a reactor or boiler. <CIT> discloses a planar transducer for measuring corrosion/erosion in equipment such as a plant, by means of two adjacent electrically resistive elements, wherein the first one is exposed to, and the second one is physically isolated from, the environment. <CIT> discloses an electrical resistance corrosion probe comprising a sample element exposed to the fluid in a vessel or pipeline, and a reference element identical to the sample element and exposed to the same temperature, but insulated from said fluid. <CIT> discloses a contamination detector comprising a chamber housing a piezoelectric measuring resonator and a reference resonator, wherein the reference resonator is protected from external contamination. In general, machines are subject to fouling and/or erosion during their operation.

This is especially true for turbomachines, in particular single-stage or multi-stage centrifugal compressors, wherein fouling and/or erosion during operation are at least partially due to a flowing of working fluid in an internal flow path of the machine. Both fouling and erosion are due to material carried by the flowing working fluid; if flowing velocity is low erosion is also low; if flowing velocity reduces dirt tends to deposit more.

When fouling (at one or more internal places of the machine) reaches an excessive level, the machine should be stopped, cleaned and restarted; in fact, fouling inside the machine may cause for example loss of efficiency of the machine. Cleaning often requires disassembling the machine which is complicated and time-consuming and thus expensive. Therefore, at least ideally, such maintenance operation should be performed always when necessary but preferably only when necessary.

When erosion (of one or more components of the machine) reaches an excessive level, the machine should be stopped, repaired and restarted; in fact, if the eroded component should break, huge damages to the machine may occur; in any case, erosion inside the machine may cause for example loss of efficiency of the machine. Repairing always requires disassembling the machine which is complicated and time-consuming and thus expensive. Therefore, at least ideally, such maintenance operation should be performed always when necessary but preferably only when necessary.

Therefore, it would be desirable to monitor fouling and/or erosion at one or more internal places of the machine so to take the appropriate steps when they reach a predetermined level that is considered excessive.

According to one aspect, the subject-matter disclosed herein relates to a system comprising a machine and a sensor arrangement for measuring fouling and/or erosion in the machine. The sensor arrangement includes: a first piezoelectric transducer and a first plate, the first plate being fixedly coupled to the first piezoelectric transducer so to form a first single vibrating mass; the first piezoelectric transducer is arranged to be stimulated by electric signals applied to the sensor arrangement; a second piezoelectric transducer and a second plate, the second plate being fixedly coupled to the second piezoelectric transducer so to form a second single vibrating mass; the second piezoelectric transducer is arranged to be stimulated by electric signals applied to the sensor arrangement; the sensor arrangement is arranged to be installed in the machine so that the first plate is exposed to a flow of a working fluid in the machine (i.e. in the flow path of the machine) while the second plate is exposed to the working fluid but not to its flow (i.e. the fluid is still and the flow velocity is zero).

There is also disclosed a machine arranged to operate through a working fluid flowing in an internal flow path of the machine; the machine includes at least one sensor arrangement. The sensor arrangement includes: a first piezoelectric transducer and a first plate, the first plate being fixedly coupled to the first piezoelectric transducer so to form a first single vibrating mass; the first piezoelectric transducer is arranged to be stimulated by electric signals applied to the sensor arrangement; a second piezoelectric transducer and a second plate, the second plate being fixedly coupled to the second piezoelectric transducer so to form a second single vibrating mass; the second piezoelectric transducer is arranged to be stimulated by electric signals applied to the sensor arrangement; the sensor arrangement is arranged to be installed in the machine so that the first plate is exposed to a flow of a working fluid in the machine (i.e. in the flow path of the machine) while the second plate is exposed to the working fluid but not to its flow (i.e. the fluid is still and the flow velocity is zero). The first plate forms a portion of a wall of the flow path.

According to still another aspect, the subject-matter disclosed herein relates to a method as defined in claim <NUM>, for measuring fouling and/or erosion on a wall of an internal flow path of a machine; the method includes the steps of: A) repeatedly stimulating a first piezoelectric transducer by a first stimulation electric signal so that the first piezoelectric transducer generates a first electric resonance vibration, the first piezoelectric transducer being part of a first vibrating mass integrated into said wall; B) repeatedly measuring a resonance frequency of the first electric resonance vibration; C) repeatedly stimulating a second piezoelectric transducer by a second stimulation electric signal so that the second piezoelectric transducer generates a second electric resonance vibration, the second piezoelectric transducer being part of a second vibrating mass positioned close to said first vibrating mass but remote from said wall; D) repeatedly measuring a resonance frequency of said second electric resonance vibration; and E) repeatedly comparing said resonance frequency of said first electric resonance vibration and said resonance frequency of said second electric resonance vibration.

The Applicant has considered that fouling and erosion in a machine imply a mass change inside the machine: in the case of fouling, a mass of material is deposited (i.e. added) in a certain place of the machine (in particular a certain place of a component of the machine); in the case of erosion, a mass of material is removed (i.e. subtracted) from a certain place of the machine (in particular a certain place of a component of the machine).

Therefore, the Applicant has thought of monitoring fouling and/or erosion by repeating a mass measurement in the one or more places of a machine where one or more of these phenomena are likely to occur. If a mass change is detected, this means that some fouling or erosion has occurred.

There are various approaches of measuring mass, but it is very difficult to do it inside an operating machine especially in internal places where a working fluid of the machine is flowing.

The Applicant has identified an approach that is particularly effective in such circumstances, i.e. the inertial balance measurement carried out through a sensor arrangement that will be briefly described in the following.

A piezoelectric transducer and a plate are fixed together so to form a vibrating mass. After electrically stimulating the piezoelectric transducer, the vibrating mass starts vibrating mechanically, which is called "natural resonance", and consequently the piezoelectric transducer generates an electric resonance vibration; the electric resonance vibration is at a frequency depending on the overall mass of the vibrating mass, which is called "natural resonance frequency". If the overall mass of the vibrating mass changes due to e.g. fouling or erosion in the machine, also the electric vibration frequency changes, i.e. the "natural resonance frequency" changes; such frequency change may be measured and the corresponding mass change may be determined.

A sensor arrangement operating according to the above described principle may be installed inside a machine (either in a stator component of the machine or in a rotor component of the machine). In case fouling is of interest, the sensor arrangement is positioned so that dirt (due to the working fluid flow) deposits on the plate and changes the overall mass of the vibrating mass. In case erosion is of interest, the sensor arrangement is positioned so that erosion (due to the working fluid flow) acts on the plate and changes the overall mass of the vibrating mass.

Reference now will be made in detail to various embodiments, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope of the claims. Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments, insofar as falling under the scope of the claims.

When introducing elements of various embodiments the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including", and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

Referring now to the drawings, <FIG> illustrates a schematic longitudinal cross-section view of an embodiment of a machine <NUM>, in particular a multi-stage centrifugal compressor. Machine <NUM> comprises a rotor <NUM> and a stator <NUM>; in particular, stator <NUM> surrounds rotor <NUM>. An internal flow path <NUM> is defined between stator <NUM> and rotor <NUM>, and develops from an inlet of machine <NUM> (on the left in <FIG>) to an outlet of machine <NUM> (on the right of <FIG>). During operation of machine <NUM>, flow path <NUM> is arranged to receive a working fluid at the inlet of machine <NUM>, feed it and discharge it from the outlet of machine <NUM>. In the embodiment of <FIG>, by flowing along flow path <NUM>, in particular inside flow channels of e.g. three impellers of rotor <NUM>, the working fluid causes rotation of rotor <NUM>.

Due to the flow of the working fluid in flow path <NUM>, fouling and/or erosion may occur on one or more parts of stator <NUM> and/or of rotor <NUM>.

In the embodiment of <FIG>, fouling and/or erosion are monitored for example through a first sensor arrangement <NUM> and a second sensor arrangement <NUM>. In general, the number of such sensor arrangements may vary from one to e.g. one hundred.

These sensor arrangements are positioned adjacent to flow path <NUM> (as will be explained better in the following), specifically a part of a sensor arrangement forms a portion of a wall of the flow path; any of these sensor arrangements may be mounted to stator <NUM> or rotor <NUM>. Sensor arrangement <NUM> is mounted to rotor <NUM> and a part thereof forms a portion of wall <NUM>. Sensor arrangement <NUM> is mounted to stator <NUM> and a part thereof forms a portion of wall <NUM>. While in the embodiment of <FIG>, sensor arrangements are located in an inlet region of machine <NUM>, it is to be understood that in alternative embodiments sensor arrangements may be located in the inlet region and/or in the outlet region and/or in an intermediate region of the machine.

If a sensor arrangement, like e.g. sensor arrangement <NUM>, is mounted to a stator of a machine, a wired connection is used for connecting it for example to a measurement or monitor electronic unit of the machine.

If a sensor arrangement, like e.g. sensor arrangement <NUM>, is mounted to a rotor of a machine, a wireless connection is used for connecting it for example to a measurement or monitor electronic unit of the machine. As a person skilled in the art understands a wireless connection is more complicated than a wired connection.

Referring now to <FIG>, sensor arrangement <NUM> is arranged to measure fouling or erosion and comprises at least a first piezoelectric transducer <NUM> and a first plate <NUM>; first piezoelectric transducer <NUM> and first plate <NUM> are fixedly coupled together so to form a first single vibrating mass. It is to be noted that sensor arrangement <NUM> of <FIG> is to be considered a simplified version of sensor arrangement <NUM> of <FIG>; the following explanation is useful for understanding the embodiment of <FIG>.

In the solution of <FIG>, that is a simplified version of the embodiment of <FIG>, the sensor arrangement comprises further a first support member <NUM>; first piezoelectric transducer <NUM> and first plate <NUM> are fixedly coupled to first support member <NUM> so that the first single vibrating mass is formed by the combination of first piezoelectric transducer <NUM>, first plate <NUM> and first support member <NUM>. Advantageously, first piezoelectric transducer <NUM> is fixed to a first side of first support member <NUM> designed to be remote from a flow path (<NUM> in <FIG>) of a machine and first plate <NUM> is fixed to a second side of first support member <NUM> designed to be close to a flow path (<NUM> in <FIG>) of a machine.

First piezoelectric transducer <NUM> is arranged to be stimulated by electric signals applied to sensor arrangement <NUM>; for example, <FIG> shows an electric cable <NUM> electrically connected to contacts <NUM> of first piezoelectric transducer <NUM>, and arranged to feed electric signals to/from first piezoelectric transducer <NUM>. Electric cable <NUM> is arranged to feed stimulation electric signals from e.g. a measurement or monitor electronic unit to first piezoelectric transducer <NUM>. Electric cable <NUM> is also arranged to feed resonance vibration electric signals from first piezoelectric transducer <NUM> to e.g. a measurement or monitor electronic unit; a resonance vibration electric signal is a consequence of an electric stimulation, typically of a previous electric stimulation.

As can been in <FIG>, sensor arrangement <NUM> is arranged to be installed in a machine so that first plate <NUM> is exposed to a flow F of working fluid in the machine (i.e. in the flow path of the machine). Preferably, first plate forms a portion of wall <NUM> of flow path <NUM>; preferably, just after installation of sensor arrangement <NUM> (i.e. before any fouling and/or erosion), surface of first plate <NUM> is aligned with surrounding surface of wall <NUM>.

Sensor arrangement <NUM> is positioned inside a recess <NUM> of a wall <NUM> and is fixed to wall <NUM>. According to the solution of <FIG>, an annular member <NUM> is used for fixing sensor arrangement <NUM> to wall <NUM>; for example, a periphery of first support member <NUM> is held by annular member <NUM> and annular member <NUM> is screwed or fit in a hole of wall <NUM>.

Sensor arrangement <NUM> as shown in detail in <FIG> is used for measuring fouling or erosion on a wall of an internal flow path of machine <NUM>. The solution shown in <FIG> may be used also for measuring corrosion; however, in this case, is not truly a simplified version of the embodiment of <FIG>. Although in <FIG>, sensor arrangement <NUM> is mounted to a rotor wall, a similar sensor arrangement may alternatively be mounted to a stator wall.

A simplified version of an embodiment of a method for measuring fouling or erosion based on sensor arrangement <NUM> or a similar sensor arrangement, i.e. a simplified version of the embodiment of <FIG>, will be explained in the following with reference to a flow chart <NUM> of <FIG>.

Flow chart <NUM> comprises a START step <NUM> and an END step <NUM>.

The method according to flow chart <NUM> includes a preliminary step <NUM> of positioning a first single vibrating mass formed by an assembly of at least a first piezoelectric transducer (for example first piezoelectric transducer <NUM> in <FIG>) and a first plate (for example first plate <NUM> in <FIG>), the first plate forming a portion of a flow path wall.

Furthermore, the method according to flow chart <NUM> further includes the steps of:.

The repetition referred to in steps <NUM> and <NUM> corresponds to loop L1 in flow chart <NUM> of <FIG>. The loop may be repeated with a period preferably longer than <NUM> hour and preferably shorter than <NUM> day as fouling and erosion progress quite slowly; it is to be noted that the period of repetition does not need to be strictly constant, for example a variation of up to <NUM>% or <NUM>% (or even more) is acceptable.

It is to be noted that step <NUM> is carried out when assembling machine <NUM>, while steps <NUM> and <NUM> are carried out during operation of machine <NUM>, i.e. they are part of a measuring process. Loop L1 may be continuously repeated from start-up of machine <NUM> to shut-down of machine <NUM>; therefore, advantageously, loop L1 is interrupted when machine <NUM> is not operating.

Preferably, the resonance frequency of the first electric resonance vibration is greater than <NUM>.

Steps <NUM> and <NUM> may be carried out for example by a measurement or monitor electronic unit of machine <NUM> that is not shown in any figure and that may be electrically connected to electric cable <NUM> of sensor arrangement <NUM>.

The above-mentioned measurement or monitor electronic unit may process the frequency measurements carried out. For example, any time it makes a frequency measurement, it may compare the measured value with an upper threshold value and/or a lower threshold value; if any of these threshold values is exceeded it may signal such event for example to an electronic control unit of the machine and/or to an operator; such signaling may be an electronic indication (for example and electronic message sent to an electronic control unit of the machine) and/or a visual indication and/or a sound indication. It is to be noted that a frequency measurement may be subject to some pre-processing before threshold comparison or comparisons for example in order to take into account the current temperature of the components of the sensor arrangement.

Referring now to <FIG>, sensor arrangement <NUM> is arranged to measure fouling or erosion and comprises at least a first piezoelectric transducer <NUM> and a first plate <NUM>; first piezoelectric transducer <NUM> and first plate <NUM> are fixedly coupled together so to form a first single vibrating mass.

In the embodiment of <FIG>, the sensor arrangement comprises further a first support member <NUM>; first piezoelectric transducer <NUM> and first plate <NUM> are fixedly coupled to first support member <NUM> so that the first single vibrating mass is formed by the combination of first piezoelectric transducer <NUM>, first plate <NUM> and first support member <NUM>. Advantageously, first piezoelectric transducer <NUM> is fixed to a first side of first support member <NUM> designed to be remote from a flow path (<NUM> in <FIG>) of a machine and first plate <NUM> is fixed to a second side of first support member <NUM> designed to be close to a flow path (<NUM> in <FIG>) of a machine.

As can been in <FIG>, sensor arrangement <NUM> is arranged to be installed in a machine so that first plate <NUM> is exposed to a working fluid flow F in the machine (i.e. in the flow path of the machine). Preferably, first plate forms a portion of wall <NUM> of flow path <NUM>; preferably, just after installation of sensor arrangement <NUM> (i.e. before any fouling and erosion), surface of first plate <NUM> is aligned with surrounding surface of wall <NUM>.

Sensor arrangement <NUM> comprises further at least a second piezoelectric transducer <NUM> and a second plate <NUM>; second piezoelectric transducer <NUM> and second plate <NUM> are fixedly coupled together so to form a second single vibrating mass.

In the embodiment of <FIG>, the sensor arrangement comprises further a second support member <NUM>; second piezoelectric transducer <NUM> and second plate <NUM> are fixedly coupled to second support member <NUM> so that the second single vibrating mass is formed by the combination of second piezoelectric transducer <NUM>, second plate <NUM> and second support member <NUM>. Advantageously, second piezoelectric transducer <NUM> is fixed to a first side of second support member <NUM> designed to be close to a flow path (<NUM> in <FIG>) of a machine and second plate <NUM> is fixed to a second side of second support member <NUM> designed to be remote from a flow path (<NUM> in <FIG>) of a machine.

Second piezoelectric transducer <NUM> is arranged to be stimulated by electric signals applied to sensor arrangement <NUM>; for example, <FIG> shows an electric cable <NUM> electrically connected to contacts <NUM> of second piezoelectric transducer <NUM>, and arranged to feed electric signals to/from second piezoelectric transducer <NUM>. Electric cable <NUM> is arranged to feed stimulation electric signals from e.g. a measurement or monitor electronic unit to first piezoelectric transducer <NUM>. Electric cable <NUM> is also arranged to feed resonance vibration electric signals from second piezoelectric transducer <NUM> to e.g. a measurement or monitor electronic unit; a resonance vibration electric signal is a consequence of an electric stimulation, typically of a previous electric stimulation.

As can been in <FIG>, sensor arrangement <NUM> is arranged to be installed in a machine so that second plate <NUM> is exposed to a working fluid of the machine (i.e. still or at zero flow velocity) but not exposed to a flow F of the working fluid in the machine (i.e. in the flow path of the machine) (that means that should not be subject to fouling or erosion), and so that at least first piezoelectric transducer <NUM>, first plate <NUM>, second piezoelectric transducer <NUM>, and second plate <NUM> are exposed to a same temperature and to a same pressure. In the embodiment of <FIG>, even first support member <NUM> and second support member <NUM> are exposed to a same temperature and to a same pressure.

According to the embodiment of <FIG>, sensor arrangement <NUM> may comprise further a first cavity <NUM>. The first single vibrating mass, i.e. the combination of elements <NUM>+<NUM>+<NUM>, is positioned on a first side of first cavity <NUM>, while the second single vibrating mass, i.e. the combination of elements <NUM>+<NUM>+<NUM>, is positioned on a second side of first cavity <NUM>; the second side is different from the first side.

Preferably and as shown in <FIG>, the second side is opposite to the first side. Advantageously, a hollow separation member <NUM> is fit between the first single vibrating mass and the second single vibrating mass; a transversal cross-section (not shown in <FIG>) of member <NUM> may have the shape of a circle or a polygon.

Advantageously, a separation wall component (not shown in <FIG>) may be located inside first cavity <NUM>; such separation wall component is aimed at avoiding or at least limiting frequency interactions between the first vibrating mass and the second vibrating mass which are unwanted. Such separation wall component may take the form for example of disk made of stainless steel and fixed at its boundary to the hollow separation member <NUM>; in this way, first cavity <NUM> is divided into two sub-cavities.

According to the embodiment of <FIG>, sensor arrangement <NUM> may comprise further a second cavity <NUM>. The second single vibrating mass, i.e. the combination of elements <NUM>+<NUM>+<NUM>, is positioned also on a side of second cavity <NUM>.

Advantageously, first cavity <NUM> is arranged to be in fluid communication with a working fluid flow path (<NUM> in <FIG>) of the machine; in this way, especially second piezoelectric transducer <NUM> is exposed approximately to a same temperature and to a same pressure as first piezoelectric transducer <NUM>.

Advantageously, second cavity <NUM> is arranged to receive the working fluid of the machine; in this way, especially second plate <NUM> is exposed approximately to a same temperature and to a same pressure as first plate <NUM>. In the embodiment of <FIG>, working fluid passes first through a annular duct <NUM> (read afterwards) and then through a plurality of hole ducts <NUM> (read afterwards). It is to noted that the working fluid slowly diffuses into cavity <NUM> passing through duct <NUM> and ducts <NUM>; in the way, the working fluid inside cavity <NUM> is still and therefore does not erode plate <NUM>; in this way, dirt carried by the working fluid gradually deposits on the walls of duct <NUM> (that are at an appropriate distance) and when the working fluid enters into cavity <NUM> it is free (or almost free) from dirt so that it does not foul plate <NUM>.

From the mechanical point of view, sensor arrangement <NUM> may comprise a tubular shell <NUM> arranged to be fit in a recess <NUM> of a wall <NUM> of a flow path <NUM> of a machine. Tubular shell <NUM> surrounds both first cavity <NUM> and second cavity <NUM>. Separation member <NUM> may be fit into an inner annular recess of tubular shell <NUM> together with a periphery of first support member <NUM> and a periphery of second support member <NUM>.

Advantageously, tubular shell <NUM> has a larger cross-section at its inner zone so to fix inside recess <NUM> and a small cross-section at its outer zone so to define an annular duct <NUM> between an inner surface of recess <NUM> and an outer surface of tubular shell <NUM>. Furthermore in this case, shell <NUM> has, at its outer zone, a plurality of hole ducts <NUM> extending from annular duct <NUM> to second cavity <NUM>.

As, according to this embodiment, a second cavity <NUM> is arranged to receive the working fluid of the machine, it is preferable to provide a draining channel <NUM>, for example at an inner end of tubular shell <NUM>, arranged to drain liquid from second cavity <NUM>; such liquid may be due to partial condensation of the working fluid.

As, according to this embodiment, a second cavity <NUM> is arranged to receive the working fluid of the machine, it is preferable to provide a liquid detector <NUM> positioned in second cavity <NUM> and arranged to detect for example when liquid in second cavity <NUM> exceeds a predetermined quantity or level. <FIG> shows an electric cable <NUM> electrically connected to liquid detector <NUM>, and arranged to feed electric signals from liquid detector <NUM> to e.g. a measurement or monitor electronic unit.

Sensor arrangement <NUM> as shown in detail in <FIG> is used for measuring fouling or erosion on a wall of an internal flow path of machine <NUM>. Although in <FIG>, sensor arrangement <NUM> is mounted to a stator wall, a similar sensor arrangement may alternatively be mounted to a rotor wall.

An embodiment of a method for measuring fouling or erosion based on sensor arrangement <NUM> or a similar sensor arrangement will be explained in the following with reference to a flow chart <NUM> of <FIG>.

The method according to this embodiment is significantly similar to the method previously described. In fact, the one previously described is based on a sensor arrangement comprising one vibrating mass and this embodiment is based on a sensor arrangement comprising two vibrating masses, i.e. a first vibrating mass and a second vibrating mass.

As far as the first vibrating mass is concerned, the method according to flow chart <NUM> includes a preliminary step <NUM> of positioning a first single vibrating mass formed by an assembly of at least a first piezoelectric transducer (for example first piezoelectric transducer <NUM> in <FIG>) and a first plate (for example first plate <NUM> in <FIG>), the first plate forming a portion of a flow path wall.

Still as far as the first vibrating mass is concerned, the method according to flow chart <NUM> further includes the steps of:.

As far as the second vibrating mass is concerned, the method according to flow chart <NUM> includes a preliminary step <NUM> of positioning a second single vibrating mass formed by an assembly of at least a second piezoelectric transducer (for example second piezoelectric transducer <NUM> in <FIG>) and a second plate (for example first plate <NUM> in <FIG>), the second plate being close to the first plate but remote from a flow path wall. In particular, while the first plate is exposed to a flow of a working fluid in the machine (i.e. in the flow path of the machine), the second plate is exposed to the working fluid but not to its flow (i.e. the fluid is still and the flow velocity is zero).

Still as far as the second vibrating mass is concerned, the method according to flow chart <NUM> further includes the steps of:.

As shown in <FIG>, the preferable sequence of the above-mentioned steps is: step <NUM>, step <NUM>, step <NUM>, step <NUM>, step <NUM>, step <NUM> and step <NUM> (that will be explained below).

It is to be noted that steps <NUM> and <NUM> are carried out when assembling machine <NUM>, while steps <NUM> and <NUM> and <NUM> and <NUM> are carried out during operation of machine <NUM>, i.e. they are part of a measuring process.

Preferably, the positioning at step <NUM> and the positioning at step <NUM> lead to the first single vibrating mass and the second single vibrating mass being exposed to a same temperature and to a same pressure.

Advantageously, the method according to flow chart <NUM> further includes the step of:
E) step <NUM>: repeatedly comparing the resonance frequencies, in particular the resonance frequency of the first electric resonance vibration and the resonance frequency of the second electric resonance vibration.

The repetition referred to in steps <NUM> and <NUM> and <NUM> and <NUM> and <NUM> corresponds to loop L2 in flow chart <NUM> of <FIG>. The loop may be repeated with a period preferably longer than <NUM> hour and preferably shorter than <NUM> day as fouling and erosion progress quite slowly; it is to be noted that the period of repetition does not need to be strictly constant, for example a variation of up to <NUM>% or <NUM>% (or even more) is acceptable.

Loop L2 may be continuously repeated from start-up of machine <NUM> to shut-down of machine <NUM>; therefore, advantageously, loop L2 is interrupted when machine <NUM> is not operating.

Preferably, the resonance frequency of the first electric resonance vibration and the resonance frequency of the second electric resonance vibration are greater than <NUM> even if not always identical.

According to a first possibility, the resonance frequency of the first electric resonance vibration and the resonance frequency of the second electric resonance vibration are identical (or almost identical) when the first plate (for example first plate <NUM> in <FIG>) has no fouling or is not eroded. A frequency difference exists and may be measured after fouling or erosion.

According to a second preferable possibility, the resonance frequency of the first electric resonance vibration and the resonance frequency of the second electric resonance vibration are different when the first plate (for example first plate <NUM> in <FIG>) has no fouling or is not eroded; this difference is preferably greater than <NUM>. The frequency difference increases or decreases and may be measured after fouling or erosion.

Steps <NUM> and <NUM> and <NUM> and <NUM> may be carried out for example by a measurement or monitor electronic unit of machine <NUM> that is not shown in any figure and that may be electrically connected to electric cables <NUM> and <NUM> of sensor arrangement <NUM>.

The above-mentioned measurement or monitor electronic unit may process the frequency measurements carried out as well as compare, for example subtract, the measured resonance frequencies (see step <NUM>). For example, any time it makes a frequency subtraction between a resonance frequency of the first electric resonance vibration and a resonance frequency of the second electric resonance vibration, it may compare the subtracted value with an upper threshold value and/or a lower threshold value; if any of these threshold values is exceeded it may signal such event for example to an electronic control unit of the machine and/or to an operator; such signaling may be an electronic indication (for example and electronic message sent to an electronic control unit of the machine) and/or a visual indication and/or a sound indication.

It is to be noted that, advantageously, the use of two vibrating masses (at the same temperature and pressure) allows an automatic compensation of the frequency measurements; therefore, some pre-processing before threshold comparison or comparisons may be unnecessary.

As already explained, sensor arrangements identical or similar to sensor arrangement <NUM> and sensor arrangement <NUM> may be advantageously installed and used in machines, preferably turbomachines, more preferably single-stage or multi-stage centrifugal compressors.

Any machine may include one or more such sensor arrangements.

Furthermore, such a machine may include a measurement or monitor electronic unit or be associated to a measurement or monitor electronic unit; the same unit may be connected (through wired and/or wireless connections) to one or more such sensor arrangements.

Claim 1:
A system comprising a sensor arrangement (<NUM>, <NUM>) for measuring fouling and/or erosion in a machine (<NUM>) and a machine (<NUM>) in which the sensor arrangement is installed, the system comprising:
- a first piezoelectric transducer (<NUM>, <NUM>),
- a first plate (<NUM>, <NUM>), the first plate (<NUM>, <NUM>) being fixedly coupled to the first piezoelectric transducer (<NUM>, <NUM>) so to form a first single vibrating mass (<NUM>+<NUM>+<NUM>, <NUM>+<NUM>+<NUM>),
- a second piezoelectric transducer (<NUM>), and
- a second plate (<NUM>), the second plate (<NUM>) being fixedly coupled to the second piezoelectric transducer (<NUM>) so to form a second single vibrating mass (<NUM>+<NUM>+<NUM>);
wherein the first piezoelectric transducer (<NUM>, <NUM>) is arranged to be stimulated by electric signals applied (<NUM>, <NUM>) to the sensor arrangement (<NUM>, <NUM>);
wherein the sensor arrangement (<NUM>, <NUM>) is installed in the machine (<NUM>) so that the first plate (<NUM>, <NUM>) is exposed to a flow (F) of a working fluid in the machine (<NUM>);
wherein the second piezoelectric transducer (<NUM>) is arranged to be stimulated by electric signals applied (<NUM>) to the sensor arrangement (<NUM>);
wherein the sensor arrangement (<NUM>) is installed in the machine (<NUM>) so that the second plate (<NUM>) is exposed to the working fluid of the machine (<NUM>) and not exposed to a flow (F) of said working fluid in the machine (<NUM>);
wherein the second single vibrating mass (<NUM>+<NUM>+<NUM>) is equal to the first single vibrating mass (<NUM>+<NUM>+<NUM>); and
wherein the sensor arrangement (<NUM>) is installed in the machine (<NUM>) that the first piezoelectric transducer (<NUM>), the first plate (<NUM>), the second piezoelectric transducer (<NUM>), the second plate (<NUM>) are exposed to a same temperature and to a same pressure.