GB2545741B

GB2545741B - Flowmeter

Info

Publication number: GB2545741B
Application number: GB1522877.8A
Authority: GB
Inventors: Lorenzo Iaciofano Christopher
Original assignee: LITRE METER Ltd
Current assignee: LITRE METER Ltd
Priority date: 2015-12-24
Filing date: 2015-12-24
Publication date: 2019-11-20
Anticipated expiration: 2035-12-24
Also published as: GB201522877D0; GB2545741A

Description

FLOWMETER

The present invention relates to a flowmeter, a fluid processing system including at least one of the flowmeters, and a method for measuring a fluid flow rate. Flowmeters according to the invention can be used in, for example in the oil processing industry.

When producing oil, various chemicals are injected into crude oil to provide a degree of protection. For example, scale inhibitors are often used to prevent the build-up of scale on pipes and fittings that are used to transport the oil to the refinery. Biocides are used to prevent the build-up of organisms in the pipe. Corrosion inhibitors are used to protect pipes from corrosion, and pour point depressants are added to reduce the pour point, thereby improving the ease with which oil flows through the pipelines. Fow Dosage Hydrate Inhibitor (FDHI) is added to inhibit water based mixtures from freezing. Methanol is often used to achieve the same effect.

More exotic chemicals are sometimes used for similar purposes to those described above, with the intention of reducing cost on the way to the refinery.

Most of the chemicals additives tend to be introduced at low flow rates, typically of the order of 0.01 to 30 litres per hour. FDHI tends to have the highest injection rate, sometimes up to 17,000 litres per hour.

Some flowmeters are specially designed to measure fluid flow rates in the context of oil production systems. For example, some flowmeters are designed to monitor and control the amount (mass/volume) of chemicals added to crude oil.

Some chemical additives are supplied by means of a pulsating pump. For flowmeters used in this context, it is desirable that the flowmeter is able to cope with the pulsing nature of the pump used.

Some flowmeters operate in high pressure environments and therefore the flowmeter has to be able to withstand the high pressures that are encountered. There is a trend towards measuring the chemicals subsea rather than when the crude oil reaches the surface. Pressures can be up to 1035bar (15000psi) in modern systems injecting subsea. Topsides injection is at lower pressures typically from 14 bar (200psi) to around 415bar (6000 psi).

Not only does the flowmeter have to work at high internal pressures but also with external pressures up to 345bar (5000psi).

Control systems are often manually set from the flowmeter displayed value, either locally or from a SCADA system. However there is a trend towards automation of this process using control valves such as SkoFlo® values. A typical turndown ratio of 500:1 is measurable with the VFF positive displacement flowmeters. Often the systems are relatively slow dynamically and need only slight adjustments from day-to-day which is why manual control is still so popular. A prior art flowmeter of the type which is suitable for use in the operating conditions described above, in either subsea or topside configurations, includes: an outer housing and an inner housing. The inner housing includes an internal chamber. A rotor is housed within the chamber. The outer housing includes a cavity for receiving the inner housing, and provides a pressure rated enclosure for the inner housing. The inner housing is mounted in the cavity such that there are small gaps between outer surfaces of the inner housing and inner surfaces of the outer housing.

In use, fluid is directed to the chamber for metering purposes and into the cavity for pressure balancing purposes. Fluid within the chamber pressurises the internal surfaces of the inner housing, and fluid in the cavity pressurizes the external surfaces of the inner housing, thereby substantially balancing the inner pressure with the outer pressure. In this condition, the inner housing is referred to as a pressure balanced chamber (PBC). Thus the components located within the inner housing operate in a substantially pressure neutral environment, albeit operating at a pressure which is above atmospheric pressure.

The inventors have discovered that while flowmeters similar to the type described above operate in a generally satisfactorily manner, there is a possibility that the internal workings of the inner housing can be damaged by rapidly changing pressure differences, for example at initial start-up or a fluid change over. The risk of damage occurring increases with the time interval that the pressure within the inner housing is not equal to the pressure immediately outside the inner housing. Therefore it is desirable to minimise the amount of time that the pressure inside the inner housing does not equal the pressure outside the inner housing.

Another problem is that gas can get trapped within the system on start-up or when changing the fluid. Trapped gas can affect the accuracy of the flowmeter so it is desirable for gas bubbles to flow out of the flowmeter as quickly as possible, to achieve a pressure neutral state as quickly as possible. A further problem is that at low pressures a small pressure difference between the inside and the outside of the inner housing can occur.

Accordingly the present invention seeks to provide a flowmeter, a fluid processing system and a method for measuring fluid flow rate that mitigates at least one of the aforementioned problems, or at least provides an alternative to existing meters, systems and methods.

According to one aspect of the invention there is provided a flowmeter, including: an outer housing having an internal cavity; and an inner housing located in the internal cavity, said inner housing having an internal chamber housing at least part of a flow metering system, and said inner housing including at least one outer surface having at least one recess formed therein; and means for directing fluid to the chamber and the cavity, wherein the flow metering system is a positive displacement flow metering system.

The at least one recess promotes rapid submersion of the inner housing within the cavity. This minimises the amount of time that pressure imbalances occur between the interior and exterior of the inner housing. This has the benefit of protecting from damage components in the inner housing chamber during rapid changing pressure differences, for example at initial start-up. The arrangement also substantially eliminates small pressure differences occurring between the interior and exterior inner housing pressures, and therefore components inside the chamber work more efficiently at a pressure neutral state. A further advantage is that the grooves help to prevent gas bubbles from becoming trapped at start-up or fluid change. This helps to improve the accuracy of the components in the chamber.

Advantageously the inner housing can include a plurality of recesses formed in the at least one outer surface of the inner housing.

Advantageously at least one of the recesses includes a groove. Preferably a plurality of recesses, each comprise a groove.

Advantageously at least one outer surface of the inner housing can include at least one longitudinal groove. In preferred embodiments the or each longitudinal groove is arranged substantially parallel with a central axis of at least one of the outer housing and the inner housing. In preferred embodiments the or each longitudinal groove is arranged substantially perpendicularly to the plane of a metering element. Preferably at least one longitudinal groove extends along substantially the full length of the inner housing. Preferably at least one longitudinal groove extends along substantially the full height of the inner housing.

Advantageously at least one outer surface of the inner housing can include at least one groove arranged substantially transversely to the longitudinal groove. Advantageously at least one outer surface of the inner housing can include at least one groove arranged substantially transversely to a central axis of at least one of the outer housing and the inner housing. Preferably at least one outer surface includes a plurality of grooves arranged substantially transversely to the longitudinal groove and/or central axis.

Advantageously at least one outer surface of the inner housing can include at least one circumferential groove. In preferred embodiments at least one outer surface includes a plurality of circumferential grooves. Thus at least one groove extends around a perimeter of the outer surface. In preferred embodiments the or each circumferential groove is arranged substantially parallel with the plane of the metering element. Preferably each of the circumferential grooves is axially spaced apart.

In preferred embodiments the depth of the or each longitudinal groove is different from the depth of the or each circumferential groove and/or the or each transverse groove. In preferred embodiments the longitudinal groove has a greater depth than the or each circumferential groove and/or the or each transverse groove.

Advantageously at least some of the grooves, and preferably each of the grooves, are contiguous with one another. This helps to promote rapid fluid flow about the inner housing.

Advantageously the outer surface of the inner housing can include at least one curved surface. In preferred embodiments the inner housing is generally cylindrical. At least one recess is formed in the curved outer surface. In a particularly preferred embodiment the inner housing is generally cylindrical and includes at least one longitudinal groove and at least one circumferential groove formed in the curved outer surface. Typically the curved surface includes a plurality of circumferential grooves, and preferably at least three circumferential grooves. The circumferential grooves are axially spaced apart. The circumferential grooves are contiguous with the longitudinal groove.

In some embodiments the inner housing is generally cuboid having side outer surfaces, and a plurality of recesses is formed in the side outer surfaces. The generally cuboid inner housing has four side outer surfaces. The side surfaces are arranged substantially parallel to the longitudinal axis of at least one of the inner housing and the outer housing. The housing includes an upper outer surface and a lower outer surface, both of which are transverse to the longitudinal axis. At least one longitudinal groove and at least one circumferential groove is formed in the side outer surfaces. In the context of the generally cuboid inner housing “circumferential” is to be understood as looping round the perimeter of the side outer surfaces.

In preferred embodiments the inner housing includes a main body and a cap. Advantageously at least one recess is formed in an outer surface of the main body. Advantageously at least one recess is formed in an outer surface of the cap.

In preferred embodiments the inner housing is arranged to float freely with respect to the outer housing. A flow path is provided for guiding fluid into the chamber. Fluid is supplied to the chamber for flow metering purposes.

The flow path passes through the outer housing into the inner housing. In preferred embodiments the outer housing includes a fluid inlet, which is arranged to direct fluid to an inner housing fluid inlet. Preferably a seal is located at an interface between the outer housing fluid inlet and the inner housing fluid inlet. The seal is arranged to prevent fluid entering the cavity. A flow path is provided for guiding fluid into the cavity. Fluid is supplied to the cavity to balance the pressure within the inner housing with the pressure outside the inner housing. Advantageously the flow path can be arranged such that incoming fluid is directed to the chamber prior to the cavity. That is, the flow path is arranged such that the chamber and cavity are in series, with the chamber being upstream of the cavity in the flow path. Fluid exiting the chamber flows into the cavity.

The flow path passes through the inner housing into the outer housing. In preferred embodiments the outer housing includes a fluid outlet, which is arranged to receive fluid from an inner housing fluid outlet. In preferred embodiments the inner housing fluid outlet is in fluid communication with the cavity. The arrangement is such that, in use, at least some fluid exiting the chamber, in use, flows into the cavity to fill any gaps between the inner housing and outer housing, thereby pressure balancing the inner housing.

In some embodiments a seal is located at an interface between the outer housing fluid outlet and the inner housing fluid outlet. The seal is arranged to prevent fluid entering the cavity. In this arrangement, there is no seal located at the interface between the outer housing fluid inlet and the inner housing fluid inlet. Thus incoming fluid flows into the cavity without first passing through the chamber.

Advantageously the flow metering system includes a metering element.

Advantageously the metering element is arranged for oscillatory movement.

Advantageously the metering element is arranged for rotary movement. This may be limited rotary movement, for example a restraining element may interact with metering element to restrict rotation.

The metering element preferably includes at least one of a rotor and a gear. In preferred embodiments a positive displacement rotor is provided.

Advantageously the flowmeter measures the flow rate of fluid passing through the chamber. In preferred embodiments the volumetric flow rate is measured.

Advantageously the flow metering system includes sensing means for detecting movement of the metering element. In preferred embodiments a magnet is mounted on the metering element and a sensor is provided for detecting the magnet. The sensor can include a reed switch. In preferred embodiments, the sensor is mounted in the end cap of the inner housing.

Advantageously the inner housing includes spacer means, such as at least one foot, and preferably a plurality of feet, arranged to separate an end face of the inner housing from the outer housing. The spacer means provides a space to enable fluid exiting the chamber to flow into the cavity. The outer housing typically includes a main body and an end cap. The cavity is formed in the main body, and is closed by the end cap. In preferred embodiments the or each foot on the inner housing engages an end face of the cavity.

In preferred embodiments the cavity has a transverse cross-section that substantially matches the transverse cross-section of the inner housing. For example, the transverse cross-section of the outer housing can be substantially circular for a cylindrical inner housing. The transverse cross-section of the outer housing can be substantially rectangular for a cuboid inner housing.

According to another aspect of the invention there is provided a fluid processing system, including: at least one flowmeter according to any configuration described herein; and a control system arranged to receive signals from the or each flowmeter. For example, the fluid processing system can comprise an oil processing system. The oil processing system includes at least one line for carrying oil. The fluid processing system includes at least one line for supplying an additive to the oil. At least one flowmeter is provided to monitor the flow rate of the oil. At least one flowmeter is provided to monitor the amount of additive supplied to the oil. The control system is arranged to control operation of at least one device, such as a valve or a pump, to control the quantity of additive supplied to the oil.

According to another aspect of the invention there is provided a method for measuring a fluid flow rate, including: providing a flowmeter according to any configuration described herein; supplying fluid to the chamber; and supplying fluid to the cavity, wherein the fluid flows along the or each recess formed in the outer surface of the inner housing, and substantially surrounds the inner housing, thereby substantially balancing the pressure inside the inner housing with the pressure outside the inner housing.

The method optionally includes the fluid supplied to the cavity passing through the chamber prior to entering the cavity.

The method optionally includes fluid being supplied to the cavity directly without first passing through the chamber.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is an exploded view of a flowmeter in accordance with the invention;

Figure 2 is a cross-sectional view of the flowmeter of Figure 1;

Figure 3 is an enlarged isometric view of a pressure balanced chamber included in the flowmeter of Figure 1, said pressure balanced chamber including an arrangement of grooves in an outer surface, for the purposes of balancing pressure; and

Figure 4 is a diagrammatic illustration of a positive displacement metering arrangement within the pressure balanced chamber of Figure 3.

Figure 1 shows a flowmeter 101 in accordance with the invention. The flowmeter 101 includes: an outer housing 102 including a body 103 and cap 105; and an inner housing 107.

The outer housing 102 provides a pressure rated enclosure for the inner housing 107. The body 103 has a central axis Z-Z. The body 103 includes a fluid inlet 117. The fluid inlet 117 has a first part 117a arranged substantially perpendicular to the axis Z-Z. The fluid inlet 117 has a second part 117b arranged substantially parallel to the axis Z-Z. The body 103 includes a fluid outlet 119. The fluid outlet 119 has a first part 119a arranged substantially perpendicular to the axis Z-Z. The fluid outlet 119 has a second part 119b arranged substantially parallel to the axis Z-Z. The body includes a cavity 121 for receiving the inner housing 107. The cavity 121 is substantially cylindrical and is arranged substantially coaxial with the central axis Z-Z. The fluid inlet 117 and fluid outlet 119 are each in fluid communication with the cavity 121. Preferably each of the fluid inlet 117 and fluid outlet 119 open in a closed end 122 of the cavity.

The cavity 121 is closed at its open end by cap 105 and an O-ring seal 123. The cap 105 is bolted to the body 103 by several bolts 110.

The outer housing 102 is pressure rated according to the intended use. For example, the housing can be pressure rated to withstand pressures up to 1035bar (15000psi) for use in subsea environments. Typical embodiments can be pressure rated to operate at high internal pressures, say over 690bar (10,000 psi) and to withstand high external pressures, say up to 345bar (5000psi). Flowmeters used in topside injection systems are typically rated at much lower pressures, for example pressures in the range 14 bar (200psi) to around 415bar (6000 psi). These pressure ratings can be achieved by making the housing 102 out of steel, for example stainless steel and preferably 316 stainless steel. Other potential materials include: duplex stainless steel; super duplex stainless steel S32760 (F55); F51; F53; 6%Moly; and titanium. The O-ring seal 123 can be made from Chemraz® 526, 555, FFKM of FPM.

The inner housing 107 is housed in a cylindrical cavity 121 within the body 103. The inner housing 107 includes a body 125 an Ο-ring seal 127, and an end cap 129. The end cap 129 is bolted to the body by four bolts 124. The body 125 and end cap 129 define an internal chamber 131. The body 125 includes a fluid inlet 126, which is arranged to communicate fluid from the inlet 117 in the outer housing to the chamber 131. The body 125 includes a fluid outlet 128, which is arranged to communicate fluid from the chamber 131 to the outer housing fluid outlet 119 and cavity 121. The fluid inlet 126 and fluid outlet 128 are each arranged substantially parallel with the axis Z-Z.

The inner housing 107 is typically made from similar materials to the outer housing 102. A rotor 133 is housed within the chamber 131. The rotor 133 is disc-shaped. The plane of the disc-shaped rotor 133 is substantially perpendicular to the central axis Z-Z. The rotor 133 includes a guide peg 147 located on its lower side. The guide peg 147 is constrained to move in a circular groove 149 in the body 125. Fluid flow into the chamber 131 causes the rotor 131 to move within the chamber 131. The rotor 133 includes an annular groove on its underside, which holds and transports fluid from the chamber inlet 126 to the outlet 128. Some fluid is also transported in a cavity formed between a rotor wall 143 and a chamber wall 145. A plate 151 extends radially inwardly from the chamber wall 143. The rotor 133 includes a radial slot 153. The plate 151 engages the slot 153 in the rotor and modifies the rotation to that of an oscillation as flow passes through the chamber 131. It is this oscillation that produces the compartmentation of the fluid into positively displaced pockets. The arrangement is a positive displacement meter. Movement of the rotor 133 is illustrated diagrammatically in Figure 4. A strong permanent magnet (not shown) is mounted on a top side of the rotor 133, directly above the guide peg 147. Accordingly, the magnet moves along a similar circular path to the peg 147, which allows it to activate and deactivate a reed switch sensor located in the cap 129. A volt-free contact closure output signal is given for each oscillation which represents a volumetric increment. From this signal it is possible to determine the volumetric flow rate of the fluid. The fluid is transported in a positive manner at all times. The typical metering repeatability is better than 0.2% and a meter accuracy of 1% actual reading is usually obtained over a substantial flow range. For lowest flows the meter will under-read the actual flow in a consistent manner. This allows an improved wide-range system accuracy to be gained by the use of a linearizing electronics instrument such as the Fitre Meter FPod.

Thus movement of the rotor 133 is used to determine the volumetric flow rate of the fluid. Movement of the rotor 133 is sensed by the sensor. The sensor generates an output signal representing an increment of volume flow, which can be used to calculate the volumetric flow rate. With other data, is possible to convert the volumetric flow rate into a mass flow rate.

The sensor signal can be provided to a display device and/or to a control system, for example a control system in a fluid processing system. Signals can be communicated via a wired or wireless communications means.

The external shape of the inner housing 107 is substantially cylindrical. The outer curved surface 137 of the inner housing includes a longitudinal groove 139 formed therein. The longitudinal groove extends along substantially the full length of the outer curved surface 137 of the inner housing, and is formed in the cap 129 as well as the body 125.The outer curved surface 137 includes a plurality of circumferential grooves 14 la-141c formed therein, three are shown in Figure 4. The circumferential grooves 141a-141b are spaced apart along the axis of the inner housing, and preferably are evenly spaced. The circumferential grooves 141a-141b are formed in the outer surface of the body 125. The longitudinal groove 139 has a slightly larger depth than the circumferential grooves 141 a-141c.

At the end 155 of the inner housing opposite to the cap 129, there are four feet 157, which protrude from that end 155 of the housing. The feet 157 space the end 155 of the inner housing from the closed end 122 of the cavity.

The inner housing 107 is mounted in the cavity such that there a small circumferential gap, typically greater than 0.1mm between outer curved surfaces 137 of the inner housing, and the inner curved surface of the body 103, which defines cavity 121. There is also a small gap, typically greater than 0.1mm between the outer surface of cap 129 and inner surface of cap 105. Typically the gaps are less than 100mm.

In use, fluid entering the body 103 via the fluid inlet 117 enters into the chamber 131. An O-ring seal 135 prevents incoming fluid from entering into the cavity 121. Fluid entering the chamber 131 causes the rotor 133 to move within the chamber 131. The rotor 133 directs the incoming fluid to the fluid outlet 128, and fluid flows out of the chamber 131, to outlet 119.

The sensor detects movement of the rotor 133 in the manner described above, and the signal is interpreted to determine a fluid flow rate. This is displayed on the display device and/or provided to a fluid control system.

At start up, some of the fluid exiting the chamber 131 enters the cavity 121 and fills any gaps between the outer surfaces of the inner housing 107 and the inner surfaces of the outer housing 102, thereby pressurizing the outer surfaces of the inner housing. Fluid within the chamber 131 pressurises the internal surfaces of the inner housing 107, thereby substantially balancing the inner and outer pressures of the inner housing 107. In this condition, the inner housing 107 is referred to as a pressure balanced chamber (PBC). Thus the components located within the inner housing 107 operate in a substantially pressure neutral environment, albeit operating at a pressure which is above atmospheric pressure. The inner housing 107 is separate from the outer housing 102 and, in this condition, the inner housing 107 effectively floats within the cavity 121. However movement of the inner housing 107 with respect to the outer housing 102 is minimal since the position of the inner housing 107 is highly constrained by the walls of the outer housing 102 and pressure of the fluid within the cavity 121.

The inventors have found that by providing an arrangement of grooves 139,141a-141c in the outer curved surface 137 of the inner housing rapid submersion of the inner housing 7 within the cavity 121 is achieved. This minimises the amount of time of pressure imbalances between the interior and exterior of the inner housing 107. This has the benefit of protecting components in the chamber 131 from damage during rapid changing pressure differences, for example at initial start-up. The arrangement also eliminates small pressure differences occurring between the interior and exterior inner housing pressures, which helps components inside the chamber 131 to work more efficiently. A further advantage is that the grooves help to prevent gas bubbles from becoming trapped at start-up or fluid change. This helps to improve the accuracy of the components in the chamber 131. In prior art devices, which do not include grooves formed in the outer surface of the inner housing, trapped gas or fluid is not pressurised at the time it is released from its trapped state within the chamber 131, and it contaminates the fluid passing through the system.

Although the present invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Furthermore, it will be apparent to the skilled person that modifications can be made to the above embodiment that fall within the scope of the invention.

For example, a different arrangement of grooves from that shown in the Figures can be used to promote rapid submersion of the inner housing. For example, at least one additional longitudinal groove can be provided. A different number of circumferential grooves can be provided. At least one non-circumferential transverse groove can be provided.

While the embodiment above has been described with reference to a positive displacement flowmeter including a rotor, the invention is applicable to positive displacement flowmeters having a different oscillating element(s), for example a gear flowmeter. Instead of the inner housing being generally cylindrical, the inner housing can have other shapes. For example, the inner housing can have a generally cuboid outer shape. Preferably the generally cuboid inner housing has radiused edges. The recesses are formed in side surfaces of the generally cuboid housing. The side surfaces being equivalent to the curved surface of the cylindrical inner housing. The side surfaces are arranged substantially parallel with a central axis of at least one of the inner and outer housings.

The O-ring seal 135, which is located at the interface between the outer housing fluid inlet 117 and the inner housing fluid inlet 126, can be located at the interface of the inner housing outlet 128 and outer housing outlet 128. In this arrangement incoming fluid floods the cavity 121 instead of the outgoing fluid. The fluid flows directly into the cavity. It does not first have to flow through the chamber.

Claims

1. A flowmeter, including: an outer housing having an internal cavity; and an inner housing located in the internal cavity, said inner housing having an internal chamber housing at least part of a flow metering system, and said inner housing including at least one outer surface having at least one recess formed therein; and means for directing fluid to the chamber and the cavity; wherein the flow metering system is a positive displacement flow metering system.

2. A flowmeter according to claim 1, including a plurality of recesses formed in the at least one outer surface of the inner housing.

3. A flowmeter according to claim 2, wherein each recess comprises a groove.

4. A flowmeter according to claim 3, including at least one longitudinal groove.

5. A flowmeter according to claim 4, including at least one groove arranged transversely to the longitudinal groove.

6. A flowmeter according to any one claims 3 to 5, including at least one circumferential groove.

7. A flowmeter according to claim 6, including a plurality of circumferential grooves, wherein the circumferential grooves are axially spaced apart.

8. A flowmeter according to claim 1, including a plurality of recesses, the recesses comprising at least one longitudinal groove; and at least one groove arranged transversely to the longitudinal groove and/or at least one circumferential groove; wherein the depth of the or each longitudinal groove is different from the depth of the or each circumferential groove and/or the or each groove arranged transversely to the longitudinal groove.

9. A flowmeter according to any one claims 3 to 8, wherein at least some of the grooves are contiguous with one another.

10. A flowmeter according to any one of the preceding claims, wherein the inner housing is generally cylindrical, and a plurality of recesses is formed in a curved outer surface.

11. A flowmeter according claim 10, including at least one longitudinal groove and at least one circumferential groove formed in the curved outer surface. f2. A flowmeter according to any one of claims 1 to 9, wherein the inner housing is generally cuboid having side outer surfaces, and a plurality of recesses is formed in the side outer surfaces.

13. A flowmeter according claim 12, including at least one longitudinal groove and at least one circumferential groove formed in the side outer surfaces.

14. A flowmeter according to any one of the preceding claims, wherein the inner housing includes a main body and an end cap.

15. A flowmeter according to claim 14, including at least one recess formed in an outer surface of the main body.

16. A flowmeter according to claim 14 or 15, including at least one recess formed in an outer surface of the end cap.

17. A flowmeter according to any one of the preceding claims, wherein the inner housing is arranged to float freely with respect to the outer housing.

18. A flowmeter according to any one of the preceding claims, wherein the outer housing includes a fluid inlet and a fluid outlet, and the inner housing includes a fluid inlet and a fluid outlet, wherein the outer housing fluid inlet is in fluid communication with the inner housing fluid inlet and the outer housing fluid outlet is in fluid communication with the inner housing fluid outlet; and including a seal located at an interface between the outer housing fluid outlet and the inner housing fluid outlet that is arranged to prevent fluid entering the cavity, and wherein the outer housing fluid inlet is in fluid communication with the chamber and the cavity.

19. A flowmeter according to any one of claims 1 to 17, wherein the outer housing includes a fluid inlet and a fluid outlet, and the inner housing includes a fluid inlet and a fluid out, wherein the outer housing fluid inlet is in fluid communication with the inner housing fluid inlet and the outer housing fluid outlet is in fluid communication with the inner housing fluid outlet including a seal located at an interface between the outer housing fluid inlet and the inner housing fluid inlet that is arranged to prevent fluid entering the cavity, wherein the inner housing fluid outlet is in fluid communication with the chamber and the cavity.

20. A flowmeter according to any one of the preceding claims, wherein the flow metering system includes a metering element.

21. A flowmeter according to claim 20, wherein the metering element is arranged for oscillatory movement.

22. A flowmeter according to claim 20 or 21, wherein the metering element is arranged for rotary movement.

23. A flowmeter according to any one of claims 20 to 22, wherein the metering element includes at least one of a rotor and a gear.

24. A flowmeter according to any one of claims 20 to 23, wherein the flow metering system includes sensing means for detecting movement of the metering element.

25. A flowmeter according to claim 24, including a magnet mounted on the metering element and a sensor for detecting the magnet.

26. A flowmeter according to any one of the preceding claims, wherein the inner housing includes spacing means, such as at least one foot, arranged to separate an end face of the inner housing from the outer housing.

27. A flowmeter according to any one of the preceding claims, wherein the cavity has a transverse cross-section that substantially matches the transverse cross-section of the inner housing.

28. A fluid processing system, including: at least one flowmeter according to any one of the preceding claims; and a control system arranged to receive signals from the or each flowmeter.

29. A method for measuring a fluid flow rate, including: providing a flowmeter according to any one of claims 1 to 27; supplying fluid to the chamber; and supplying fluid to the cavity, wherein the fluid flows along the or each recess formed in the outer surface of the inner housing, and substantially surrounds the inner housing, thereby substantially balancing the pressure inside the inner housing with the pressure outside the inner housing.

30. A method according to claim 29, wherein fluid supplied to the cavity passes through the chamber prior to entering the cavity.

31. A method according to claim 29, wherein fluid is supplied to the cavity directly without first passing through the chamber.