WO2015000487A1

WO2015000487A1 - Flow meter with unbroken liner

Info

Publication number: WO2015000487A1
Application number: PCT/DK2014/050193
Authority: WO
Inventors: Peter Schmidt Laursen; Søren Tønnes Nielsen
Original assignee: Kamstrup A/S
Priority date: 2013-07-02
Filing date: 2014-06-30
Publication date: 2015-01-08

Abstract

The present invention relates to an ultrasonic flow meter (1) with an unbroken liner (6) inside the flow tube. The flow tube (2) has at least one through-going opening (5) in the wall. The unbroken liner (6) is provided in a water-tight material and arranged in the flow tube to cover the entire inner surface of the flow tube. The piezoelectric transducers (11, 12) are arranged at an outside of the liner (6) in the at least one through-going opening (5) in the wall of the flow tube(2), so that ultrasonic signals are transmitted through the liner wall to enter the flow tube(2).The liner(6)is provided in a polymer selected from the group consisting of polysulphones, polysulphides and polyaryletherketones.A flow meter (1) is thereby provided which eliminates any need for sealing surfaces in relation to the ultrasonic transducers and which can sustain prolonged use in hot liquid.

Description

Flow meter with unbroken liner

FIELD OF THE INVENTION The present invention relates to an ultrasonic flow meter, and in particular to an ultrasonic flow meter comprising a liner inside the flow tube.

BACKGROUND OF THE INVENTION

In an ultrasonic flow meter, ultrasonic transducers are mounted on a flow channel in a way so that ultrasonic signals can be introduced into the flow channel and travel along a measuring section to be detected after the passage. The operation of the ultrasonic transducers is controlled by an electronic circuit, electrically connected to the transducers. A flow meter may be used as, or together with a heat meter for measuring the flow rate of hot liquid used in connection with district heating.

Different types of ultrasonic flow meters exist. In one type, the flow channel is provided by a flow tube in the form of a metal housing, typically made from brass. A metal flow tube has the advantage that it is strong and water-tight and may be used together with hot liquids, but it is relatively expensive to produce. In order to introduce ultrasonic signals into a metal flow tube, transducer inserts are mounted in openings of the wall of the flow tube. The openings in the flow tube are normally sealed by rubber seals such as O-rings provided between the transducer insert and the flow tube.

In another type of ultrasonic flow meters, the flow channel is provided by a flow tube from a polymer, which may be produced relatively in-expensively. Ultrasonic signals may be transmitted through the wall of the polymer flow tube, thereby eliminating any need for a sealing of the flow tube. Special polymer materials should however be used for the flow tube in order to ensure that it is sufficiently strong and sufficiently water-tight. On the other hand, strong polymer materials are normally not completely watertight and somewhat brittle. Moreover, polymers risk decomposing in hot liquid due to hydrolysis and as a consequence have limited lifetimes.

SUMMARY OF THE INVENTION

It would be advantageous to provide a flow meter which in a simple and in-expensive manner achieves being sufficiently strong, sufficiently water-tight and which can sustain prolonged use in hot liquid. In general it is an object of the present invention to provide an alternative to known flow meters, which can be fabricated in a cost-efficient manner.

In accordance with a first aspect of the invention, there is provided an ultrasonic flow meter arranged to measure a flow rate of a fluid, the flow meter comprising:

a flow tube with a through-going opening for passage of a fluid between an inlet and an outlet, and at least one through-going opening in a wall of the flow tube;

an unbroken liner arranged in the flow tube;

two or more piezoelectric transducers arranged at an outside of the liner in the at least one through-going opening in the wall of the flow tube, so that ultrasonic signals are transmitted through the liner; and

control circuit arranged for operating the two or more piezoelectric transducers and to generate a signal or value indicative of the flow rate of the fluid;

characterized in that the flow tube is provided in a first material which comprises a first polymer and a first filler, and the unbroken liner is provided in a second material which comprises a second polymer and optionally a second filler, and wherein the second polymer is selected from the group consisting of polysulphones, polysulphides and polyaryletherketones.

The flow meter of the present invention has a two-part flow housing, a first part being the flow tube and a second part being the liner, each of these parts having specific properties adapted to their applications.

In the context of the present invention the liner is to be understood as a cover of the inner surface of the flow tube, which would otherwise be exposed to the fluid.

A liner for a flow tube is known from Japanese Patent Application JP H05 296808 A to Tokico, Ltd. This document, however, is silent as to the combination of materials choices for the flow tube and the liner. The inventor of the present invention has realized that by providing the flow tube in a first material which comprises a first polymer and a first filler, and the unbroken liner in a second material which comprises a second polymer and optionally a second filler, and wherein the second polymer is selected from the group consisting of polysulphones, polysulphides and polyaryletherketones, the above need is fulfilled. In the context of the present invention the filler should be understood as a compound, typically in terms of particles, added to a material to improve or otherwise change the properties of the mixture material. Thus a flow meter may be obtained in an in-expensive manner which by proper choice of first material is sufficiently strong for handling and use, and sufficiently water-tight, and which due to the selected second material for the liner can sustain prolonged use in hot liquid, and thus eliminate an need for sealing surfaces relative to the ultrasonic transducers. Polymers and composite material based thereupon have varying resistance towards hydrolysis depending on the nature of the functional groups making up the backbone of the polymer, and on the amount of any filler used therein.

Thus under the conditions of sustained exposure to district heating water polymers such as polyesters and polyamides are not sufficiently resistant to hydrolysis for sustained use over several years, due to the limited stability of the ester and amide bonds to hydrolysis. Contrary hereto it has been found that polysulphones, polysulphides and polyaryletherketones show high resistance to hydrolysis even under prolonged use. On the other hand, adding a filler like carbon fibres to a polymer structure to improve its mechanical stability tends to reduce its stability to hydrolysis. Thus it is believed that the addition of a filler to the polymer structure, and even to a polymer structure which is otherwise considered watertight, provides access routes for water into the structure on the microscopic or even atomic level, thereby promoting any hydrolysis process.

The flow tube with the liner of the present invention addresses this dual problem: It provides a flow tube of high mechanical stability due to its content of filler. Simultaneously, the liner, which is selected to have high resistance against hydrolysis, protects the flow tube from hydrolysis. Also with the introduction of the liner of the present invention, the flow tube may be made from a broad range of inexpensive polymers.

Preferably the first material comprises the first filler in a first w/w-ratio in the range of 10-70%, preferably 20-50%, more preferably 30-40%. Such relative amounts of the first filler provide excellent mechanical properties of the flow tube. According to the invention the liner is preferably made of the second polymer only.

In an alternative embodiment of the invention, the second material comprises the second filler in a second w/w ratio in the range of 1-20%, preferably 1-10%, more preferably 1-5%. Such small relative amounts of the second filler provide excellent mechanical and chemical properties of the second material.

According to a preferred embodiment of the invention the first w/w-ratio is higher than the second w/w-ratio, i.e. that the first material of the flow tube has a higher content of filler than has the second material of the liner.

In an important embodiment, both the liner and the flow tube are provided in thermoplastic materials so that they may be fabricated by injection moulding, and wherein the flow tube is moulded onto the unbroken liner. By applying such so-called 2K moulding, the fabrication process may be provided in a very cost-efficient manner.

Water for district heating is typically heated up to 130°C by the utility plant, and advantageously the second material of the liner has an sustained-use temperature of at least 150°C. However, lower temperatures are sometimes used intentionally or due to heat losses, and it may be sufficient that the second material of the liner has a sustained-use temperature in the temperature range of 90-150°C.

Generally, the liner material should be resistant to hydrolysis for a time period of more than 10 years, such as 15 years, 20 years or even longer.

Advantageously the first polymer of the flow tube is selected from the group consisting of polysulphone (PSU), polyethersulphone (PES), polyphenylenesulphide (PPS), polystyrene (PS), and polyamide (PA), in particular polyamide 12 (PA 12) from 1,12-dodecandioic acid. These materials are all sufficiently strong for handling and mount in district heating pipelines.

Advantageously, the second polymer for the liner is selected from the group consisting of polysulphone (PSU), polyethersulphone (PES), polyphenylethersulphone (PPSU),

polyphenylenesulphide (PPS), polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK).

In the present context polysulphones are defined as polymers wherein sulphone groups (-S(C>2)-) make up part of the backbone structure of the polymer. Examples include but are not limited to polysulphone (PSU) polyphenylethersulphone (PPSU), and polyethersulphone (PES). In the present context polysulphides are defined as polymers wherein sulphide groups (-S-) make up part of the backbone structure of the polymer. An example hereof is polyphenylene sulphide (PPS). In the present context polyaryletherketones are defined as polymers wherein combinations of ether groups (-0-) and ketone groups (-C(O)-) make up part of the backbone structure of the polymer. Examples include but are not limited to polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). All of these three groups of polymers are characterized by not involving any ester groups (-C(O)O-) or amide groups (-C(O)N-) as part of their backbone. They are further characterized by being thermoplastic materials.

According to the invention the first filler is selected from the group consisting of graphite, carbon, carbon fibers, glass fibers and metal powder.

Also according to the invention the second filler is selected from the group consisting of graphite, carbon, carbon fibers, glass fibers and metal powder. Such first and second fillers, which in turn are selected independently, constitute reinforcing agents and provide mechanical stability to the flow tube and the liner, respectively.

In a second aspect a method of fabricating a flow meter is provided, which method comprises: - providing an unbroken liner; providing a flow tube with a through-going opening for passage of a fluid between an inlet and an outlet, and at least one through-going opening in a wall of the flow tube, the flow tube being provided to envelope the unbroken liner; arranging two or more piezoelectric transducers at an outside of the liner in the at least one through-going opening in the wall of the flow tube, so that ultrasonic signals from the piezoelectric transducers are transmitted through the liner; and arranging a control circuit for operating the two or more piezoelectric transducers and to generate a signal or value indicative of the flow rate of the fluid; characterized in that the flow tube is provided in a first material which comprises a first polymer and a first filler, and the unbroken liner is provided in a second material which comprises a second polymer and optionally a second filler, and wherein the second polymer is selected from the group consisting of polysulphones, polysulphides and polyaryletherketones.

In general the aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 illustrates an embodiment of an ultrasonic flow meter with an unbroken liner in the flow passage; Fig. 2 illustrates a flow meter installed in a pipe installation;

Figs. 3 A to 3 C illustrate embodiments of geometric configurations of the liner with respect to a disc shaped piezo electric transducer; Fig. 4 illustrates a flow meter where the shaped of the liner is such that the plane of the transducers are tilted with respect to a centre axis of the flow tube; and

Fig. 5 illustrates an embodiment where the flow tube is monolithically formed with a house for housing various components of the flow meter.

DESCRIPTION OF EMBODIMENTS

Figure 1 illustrates an embodiment of an ultrasonic flow meter in accordance with the present invention. The flow meter 1 comprises a flow tube 2 with a through-going opening for passage of a fluid between an inlet 3 and an outlet 4. The through-going opening is also referred to as a flow passage 7. The flow tube 2 further comprises two through-going openings 5 in a wall of the flow tube, with two piezoelectric transducers 11, 12 arranged in these wall openings. A control circuit 8 in the form of a printed circuit board (PCB) is arranged for operating the two or more piezoelectric transducers and to generate a signal or value indicative of the flow rate of the fluid. In the illustrated embodiment, the transducers are in direct contact with the PCB, however the transducers may also be supported by holders and connected to the control circuit by means of electrical conductors. A liner 6, in the form of an unbroken liner, is arranged in the flow tube to cover the inner surface of the flow tube 2, which would otherwise be exposed to the fluid therein. The liner 6 is provided in a water-tight material so that any fluid flowing in the in the flow tube 2 will not leak through the transducer openings 5 in the flow tube 2. The flow meter housing thus comprises two parts, a flow tube 2 and a liner 6. The flow tube is provided with holes in the wall arranged for receiving transducers 11, 12, whereas the liner 6 is unbroken with the piezoelectric transducers 11, 12 being arranged at an outside of the liner 6 in the area defined by the wall openings 5 of the flow tube 2, so that ultrasonic signals are transmitted through the liner wall to enter and exit the flow tube 2. To ensure a proper transmission of the ultrasonic signal through the liner wall, the wall thickness at least in the coupling area, i.e. the wall area through which ultrasonic signals are coupled, may be adapted in accordance with the so-called matching layer principle where the thickness of the wall is adjusted to a proper thickness to ensure proper transmission of the emitted ultrasonic waves of the ultrasonic transducer through the wall and into the flow channel, possibly under the constraint that the area should have thickness which ensures a sufficient strength. The thickness of the liner 6 may be matched to the acoustic wavelength of the emitted ultrasonic wave of the transducers 11, 12.

The ultrasonic flow meter 1 is a transit time flow meter arranged to measure a flow rate of a fluid flowing in the flow passage 7 by use of the known operation principle for transit time flow meters, i.e. where ultrasonic signals 10 are emitted at one transducer 11 and received at the other transducer 12, and where the difference in time-of-arrival between oppositely propagating signals is measured and converted into a flow rate. The piezoelectric transducers 11, 12 are operated by the control circuit 8, which based on the involved signals generate a signal or value indicative of the flow rate of the fluid. The level of signal treatment of the control circuit may vary from basic signal treatment, where processed signals are output to a further electronic unit for further signal processing, to a complete signal treatment resulting in the determination of the flow rate. Such further electronic unit may be part of the flow meter 1, or may be part of a separate calculator circuit (not shown) communicatively connected to the flow meter 1. Fig. 1 further illustrates a display 15 for displaying information relating to the consumed amount of fluid, a radio unit 16 for wireless remote reading of the meter, and a cover 9 for encapsulating the flow electronics to form an assembled flow meter. Further elements may also be present, such as one or more batteries for supplying power to electronics, temperature sensors for measuring the temperature of the flowing fluid, etc. The cover 9 may be any type of house or compartment suitable for housing the elements of the flow meter. Here the cover 9 is illustrated as a single element, but in general it may be made of more elements, such as a cup or wall part and a lid. The cover 9 may be attached to the flow tube 2 by means of clamps, or any suitable means.

The flow meter housing comprises the flow tube 2 and the liner 6. The flow tube 2 is provided in polyethersulfone with 30% w/w of glass fibers, which makes up a strong and stiff material, so that the flow meter can withstand mount in a pipe line installation for district heating. The liner 6 is made up of pure polyphenylethersulfone, which is water-tight and resistant to hydrolysis. Alternatively, the liner 6 is made up of polyphenylethersulfone with 3% w/w of glass fibers, which is still sufficiently water-tight and resistant to hydrolysis as a liner for district heating purposes.

The term water-tight is to be understood broadly, and refers to that virtually no water penetrates through the liner. In practice, zero moisture penetration may not be possible, and the term water-tight may refer to materials which comply with the standardized Ingress Protection (IP) classes of at least class IP65, but preferably higher classifications such as IP66, IP67 or even IP68 which is the highest IP class.

In an embodiment, the flow tube comprises a sealing surface 13 constituting the end surface of the flow tube for sealed connection to a pipe installation. Advantageously, the liner 6 is extended onto the sealing surface to provide a sealing means 14. In this embodiment, the liner ensures that fluid is not leaked neither through the transducer openings 5 in the wall of the flow tube 2, nor through the connection area to a pipe installation. Thus the liner 6 is a single sealing means for sealing off the entire flow tube 2.

Fig. 2 schematically illustrates the flow meter 1 installed in a pipe installation 20. The flow meter 1 is installed in the pipe line in a sealed manner by use of the sealing part 14 of the liner 6. The sealing part 14 is squeezed in between the end section of the flow meter 1 and the end section of the pipe installation 20. These end sections may be shaped to provide suitable sealing surfaces. The flow meter 1 is attached to the pipe installation 20 be means of a connection means 21, such as a union nut or other suitable means. In this manner, the flow passage becomes part of the pipe line for supplying a flowing fluid 22 to a consumer. In general embodiments, the ultrasonic flow meter may be or may be part of a charging consumption meter, e.g. a heat meter or energy meter, where the consumption meter is arranged for measuring consumption data of a supplied utility used as a basis for billing. The consumption meter may be used in connection with district heating or district cooling. The consumption meter may be a legal meter, i.e. a meter which is subdued to regulatory demands. Such regulatory demands may be demands to the precision of the measurements.

Figs. 3 A to 3 C illustrate different embodiments of geometric configurations of the liner 6 with respect to a disc shaped piezo electric transducer 11. In important embodiments, the piezoelectric transducers are monolithic piezoelectric transducers of a ceramic material, such as lead zirconate titanate (PZT) based transducers. Such transducers are disc or cylinder shaped, thus comprise two opposite parallel flat surfaces.

Fig. 3 A illustrates an embodiment where the liner6 is flat in a contact area with the piezoelectric transducer 11, so that a front surface of the piezoelectric transducer 11 and the outer surface of the liner 6 are plane parallel in the contact area.

Fig. 3B illustrates an embodiment where also the inner surface of the liner is flat in an area covering a contact area with the piezoelectric transducer 11, so that a front surface of the piezoelectric transducer 11 and the inner surface of the liner 6 are plane parallel in the area covering the contact area.

Fig. 3C illustrates an embodiment wherein the flat surface of the transducer is adapted to the curved outer surface of the liner by means of an adaptor piece 30 provided in the area between the outer surface of the liner 6 and the outer surface of the piezoelectric transducer 11.

In a not shown embodiment, the inner surface of the liner is flat, as in Fig. 3B, whereas the outer surface of the liner is curved. In such a situation, an adaptor piece may also be used to match the flat transducer 11 to the curved liner 6. The manufacturing of such surfaces is disclosed below with embodiments of fabricating a flow meter. In Fig. 1, the flow meter comprises a flow insert 17 comprising two or more reflectors 18, 19 for directing an ultrasonic signal from an emitting piezoelectric transducer 11 to a receiving piezoelectric transducer 12 in a manner so that the ultrasonic signal propagates parallel with the direction of the flow tube 2, i.e. parallel with the centre axis of the flow tube 2. The shown insert 17 comprises reflectors 18, 19 for directing the signal, as well as a centre piece in the form of a flow reducer. Thus a flow insert 17 is used in embodiments where the transducers 11, 12 are placed in the same plane in a manner so that the centre axis of the flow tube 2, the plane of the mounting area of the transducers 11, 12, and the plane of the PCB of the control circuit 8 all are parallel. In such a situation, the piezoelectric transducers 11, 12 emit ultrasound perpendicular to the flow tube 2 and reflectors 18, 19 are needed to direct the ultrasonic signal along the flow passage. Other types of flow inserts may also be used.

Fig. 4 illustrates an alternative embodiment where the shape of the liner 6 is such that the planes of the transducers 11, 12 are inclined with respect to a centre axis of the flow tube 2. In this situation, the two transducers 11, 12 partly oppose each other, and the ultrasonic signal may be reflected by an inside of the flow tube 2 so that a signal emitted at one transducer 11 arrives at the other transducer 12. This naturally requires that the material of the flow tube 2 is selected so that ultrasound is reflected. Fig. 4 illustrates a situation where the signal is reflected once to travel along a "V", however other configurations can be envisioned, e.g. where the signal is reflected three times to travel along a "W".

Fig. 5 illustrates an embodiment where the flow tube 2 is monolithically formed with a house (or cup) 50 for housing the control circuit for operating the two or more piezoelectric transducers, as well as the transducers, and possible other elements, such as display, battery, etc. The liner 6 and the house 50 may be formed by injection moulding, as will be discussed below in further detail.

Fig. 5 further illustrates a cover 51 and a seal 53, e.g. a rubber seal, such as an O-ring, to form a sealed cavity 52 for housing electronic elements to operate the flow meter 1. The flow meter 1 may further comprise an attachment means (not shown) for attaching the cover firmly to the house 50. Such attachment means may be a lock ring. This embodiment provides a flow meter where the electronic elements are comprised within a cavity or compartment 52 of a house 50 which only has a single sealing surface towards the exterior. The sealing towards the flow tube 2 is made up by the unbroken liner 6. Thus only a single sealing element (the O-ring) is used to seal between the rim of the cavity and the cover. A flow meter may be fabricated according to the method of the second aspect of the invention.

By providing the flow tube and the liner in thermoplastic materials, the liner may be provided in a first moulding step, and the flow tube may be provided in a second moulding step, where the flow tube 2 is moulded onto the liner 6. Any shaping needed, e.g. as illustrated in Fig. 3A and 3B, can be provided by using a proper mould shape.

In an alternative mould process, the flow tube 2 is provided in a first moulding step, and the liner 6 is provided by injection moulding it directly onto the flow tube 2. This may be done by inserting a core element into the through-going opening for passage of a fluid of the flow tube 2; and injection moulding the liner 6 in the space between the core element and the flow tube 2.

Further alternatively the unbroken liner 6 is provided in the flow tube 2 by insertion through either the inlet 3 or the outlet 4 of the through-going opening for passage of a fluid.

The embodiment of Fig. 5 may be provided in the injection moulding process where the mould is shaped to further define the house structure 50, so that the flow tube 2 and the house 50 are moulded in the same step to form a monolithic unit. In this manner a complete flow meter may be injection moulded in a two-step mould process.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The invention can be implemented by any suitable means; and the scope of the present invention is to be interpreted in the light of the accompanying claim set. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An ultrasonic flow meter (1) arranged to measure a flow rate of a fluid, the flow meter comprising: - a flow tube (2) with a through-going opening for passage of a fluid between an inlet (3) and an outlet (4), and at least one through-going opening (5) in a wall of the flow tube; an unbroken liner (6) arranged in the flow tube; - two or more piezoelectric transducers (11, 12) arranged at an outside of the liner in the at least one through-going opening in the wall of the flow tube, so that ultrasonic signals are transmitted through the liner; and control circuit (8) arranged for operating the two or more piezoelectric transducers and to generate a signal or value indicative of the flow rate of the fluid; characterized in that the flow tube is provided in a first material which comprises a first polymer and a first filler, and the unbroken liner is provided in a second material which comprises a second polymer and optionally a second filler, and wherein the second polymer is selected from the group consisting of polysulphones, polysulphides and polyaryletherketones.

2. The ultrasonic flow meter according to claim 1, wherein the first material comprises the first filler in a first w/w-ratio in the range of 10-70%, preferably 20-50%, more preferably 30-40%.

3. The ultrasonic flow meter according to any of claims 1 and 2, wherein the second material comprises the second filler in a second w/w ratio in the range of 1-20%, preferably 1-10%, more preferably 1-5%.

4. The ultrasonic flow meter according to any of the preceding claims, wherein the first w/w-ratio is higher than the second w/w-ratio.

5. The ultrasonic flow meter according to any of the preceding claims, wherein the second material has a sustained-use temperature of at least 150°C.

6. The ultrasonic flow meter according to any of the claims 1-4, wherein the second material has a sustained-use temperature in the temperature range of 90-150°C.

7. The ultrasonic flow meter according to any of the preceding claims, wherein the first polymer is selected from the group consisting of polysulphone (PSU), polyethersulphone (PES),

polyphenylenesulphide (PPS), polystyrene (PS), and polyamide (PA), in particular polyamide 12 (PA12).

8. The ultrasonic flow meter according to any of the preceding claims, wherein the second polymer is selected from the group consisting of polysulphone (PSU), polyethersulphone (PES),

polyphenylethersulphone (PPSU), polyphenylenesulphide (PPS), polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK).

9. The ultrasonic flow meter according to any of the preceding claims, wherein the first filler is selected from the group consisting of graphite, carbon, carbon fibers, glass fibers and metal powder.

10. The ultrasonic flow meter according to any of the preceding claims, wherein the second filler is selected from the group consisting of graphite, carbon, carbon fibers, glass fibers and metal powder.

11. The ultrasonic flow meter according to any of the preceding claims, wherein the liner is flat in a contact area with the piezoelectric transducers, so that a front surface of the piezoelectric transducer and the outer surface of the liner are plane parallel in the contact area.

12. The ultrasonic flow meter according to any of claims 1-10, wherein the liner has a curved outer surface and where the piezoelectric transducer is disc-shaped, and wherein an adaptor piece is provided in the area between the outer surface of the liner and the outer surface of the piezoelectric transducer.

13. The ultrasonic flow meter according to any of the preceding claims, wherein an inner surface of the liner is flat in an area covering a contact area with the piezoelectric transducers, so that a front surface of the piezoelectric transducer and the inner surface of the liner are plane parallel in the area covering the contact area.

14. The ultrasonic flow meter according to any of the preceding claims, wherein the flow tube is provided with a sealing surface (13) and wherein the liner is extended onto the sealing surface to provide a sealing means (14).

15. The ultrasonic flow meter according to any of the preceding claims, wherein the flow meter further comprises a flow insert (17) comprising two or more reflectors (18) for directing an ultrasonic signal from an emitting piezoelectric transducer to a receiving piezoelectric transducer.

16. The ultrasonic flow meter according to any of the preceding claims, wherein the surface of the liner in a contact area with the piezoelectric transducers is inclined with respect to a centre axis of the flow tube in a manner so that an ultrasonic signal emitted from a first transducer is reflected at an inner surface of the flow tube to be directed towards a second piezoelectric transducer.

17. The ultrasonic flow meter according to any of the preceding claims, wherein the flow tube is monolithically formed with a house (50) for housing the control circuit for operating the two or more piezoelectric transducers.

18. A method of fabricating a flow meter, the method comprises: providing an unbroken liner (6); providing a flow tube (2) with a through-going opening for passage of a fluid between an inlet (3) and an outlet (4), and at least one through-going opening (5) in a wall of the flow tube, the flow tube being provided to envelope the unbroken liner; arranging two or more piezoelectric transducers (11, 12) at an outside of the liner in the at least one through-going opening in the wall of the flow tube, so that ultrasonic signals from the piezoelectric transducers are transmitted through the liner; and arranging a control circuit (8) for operating the two or more piezoelectric transducers and to generate a signal or value indicative of the flow rate of the fluid; characterized in that the flow tube is provided in a first material which comprises a first polymer and a first filler, and the unbroken liner is provided in a second material which comprises a second polymer and optionally a second filler, and wherein the second polymer is selected from the group consisting of polysulphones, polysulphides and polyaryletherketones.

19. The method according to claim 18, wherein the unbroken liner is provided in the flow tube by insertion through either the inlet or the outlet of the through-going opening for passage of a fluid.

20. The method according to claim 18, wherein the unbroken liner and the flow tube are provided by injection moulding, and wherein the flow tube is moulded onto the unbroken liner.

21. The method according to claim 20, further comprising injection moulding a house for housing the control circuit for operating the two or more piezoelectric transducers in the same moulding step as the moulding step of the flow tube to form a monolithic unit of the flow tube and the house.