AU728389B2 - Improved flow meter - Google Patents

Description

IMPROVED FLOW METER The present invention relates to flow sensing apparatus, and in particular to flow meters for the detection and measurement of flows of liquids. More particularly, the present invention relates to flow meters of the turbine, or rotor, type.

Conventional turbine flow meters have a number of shortcomings. Firstly, turbine flow meters typically do not provide linear measurement characteristics over a wide range of flow rates. In particular, as the rate of flow through a turbine flow meter decreases the output signals from the turbine flow meter also decrease, but the decrease is disproportionate. As such the accuracy of measurement provided by this type of flow meter varies in accordance with the flow rate with the accuracy of measurement decreasing with decreasing flow rate. This in turn necessitates the need to use correction factors in order to calculate the true flow rate of the liquid.

i A further shortcoming with turbine type flow meters is a less than desirable life cycle, particularly given the cost of such flow meters. One particular area of the flow meter which is subject to wear are the bearings which support the rotor and provide for the rotation of the rotor. Compounding this shortcoming is the fact that the cost of reconditioning a worn flow meter is relatively high.

It is therefore an object of the present invention to provide an improved flow e meter of the turbine type which overcomes at least one shortcoming of the prior art turbine flow meters.

To this end one aspect of the present invention provides a liquid flow meter including a housing for a bore though which liquid passes, a rotor rotatably mounted within the bore by means of an axle extending from each end of the rotor and supported by a journal bearing, the axis of the rotor being in substantial alignment with the direction of fluid flow; the rotor including a number of vanes, each vane having a leading edge and a trailing edge wherein the leading edge of one vane and trailing edge of an adjacent vane lie in a substantially common plane which passes through the axis of the rotor, wherein some of the fluid to be measured flow along a portion of the axle in order to reduce the friction between the bearing and the axle.

Preferably the axle is substantially horizontal and the fluid provides a upportive film between the axle and the bearing.

Preferably the axle includes a groove to assist in the dispersement of fluid around the axle.

Preferably the groove spirals around the axle.

The present invention provides a flow meter of the turbine type which is particularly useful in the measurement of flows of liquid hydrocarbons such as petrochemical products, including liquid petroleum gas (LPG) and associated products, or for the measurement of flows of anhydrous ammonia. The invention has particular application in mobile installations such as trucks and road tankers transporting liquid hydrocarbons such as petrochemical products, including liquid petroleum gas (LPG).

The present invention will be better understood and appreciated from the following discussion of the features of a preferred embodiment. Reference is made to the accompanying drawings in which: Fig 1 is an end view of a preferred embodiment of a flow meter assembly S according to the present invention.

Fig 2 is a partial cross-sectional view A-A through the principal axis of the flow meter.

Fig 3 is a partialcross-sectional view along line B-B of Fig. 1.

Fig 4 is a graph illustrating the typical measurement accuracy over a wide range of flow rates for a flow meter designed in accordance with the present invention.

SFig 5 is a side view of a rotor and axle assembly of a flow meter designed in accordance with the present invention.

Referring to Figs 1 and 2, a preferred embodiment of a flow meter assembly according to the present invention is illustrated. The flow meter 1 is of the turbine type and includes a hollow body 2 made from stainless steel with a smooth cylindrical internal bore 3 through which the fluid passes. The flow meter 1 includes end flanges 4, 5 for the in-line installation of the flow meter in a pipe line. The flanges 4, 5 are preferably manufactured from carbon steel and include mounting holes 6 for mounting the flow meter in a pipe line. Each flange includes a centrally positioned aperture 7 for mounting the flange onto the body of the flow meter.

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WO 98/13672 PCT/AU97/00627 3 Rotatably mounted within the bore of the hollow body section is a rotor 8, preferably made from a magnetic stainless steel material such as 431 stainless steel. The rotor has an outer diameter of a dimension such that the rotor is neatly located within the bore of the housing. The rotor 8 is mounted on, or includes, an axle or spindle 9 extending from each end of the rotor, with the axis of the axle 9 coinciding with the axis of the bore of the housing. The rotor and axle assembly may comprise a single axle which passes through the rotor and extends from each side of the rotor, or alternatively, may comprise a separate axle extending from each side of the rotor. In a preferred form of the invention the axle 9 is located within a central bore of the rotor by way of a press fit. The axle 9 is preferably made from a carbide material, such as cemented tungsten carbide.

The vanes 10 of the rotor 8 are helical in form, preferably with a helix angle of 450. It is further preferable that the vanes of the rotor are machined. In the preferred embodiment depicted, the rotor 8 is provided with three vanes which are equi-spaced at 1200 around the body 11 of the rotor. However it should be appreciated that the number of vanes on the rotor may be varied depending upon the flow capacity requirements of the flow meter. For example, the rotor may be provided with six or nine vanes equi-spaced around the rotor.

The flow meter 1 is reversible, or bi-directional, insofar as fluid may be passed through the meter in either axial direction and accurate measurement of the fluid flow rate will occur. The rotor and bore of the flow meter are symmetrical about a central transverse axis. Thus the measurement characteristics of the flow meter do not vary with the direction of the fluid flow through the meter.

Referring to Fig 1, for the purposes of the further description of the invention the fluid flow through the meter is taken to be from left to right as indicated by the arrows.

In accordance with one aspect of the invention, upon viewing along the axis of the rotor the leading edge 12 of each vane aligns with or overlaps the trailing edge 13 of an adjacent vane, where the leading edges of the vanes are located upstream relative to the fluid flow through the meter and the trailing edges of the vanes are located downstream. The leading edge 12 of each vane WO 98/13672 PCT/AU97/00627 4 may overlap the trailing edge 13 of an adjacent vane by an angle of up to 50 Preferably, the trailing edge 13 of a vane and the leading edge 12 of its adjacent vane are in alignment: that is, the leading and trailing edges lie in a common plane which passes through the axis of the rotor. Thus, when viewing the rotor in an axial direction the surfaces of the vanes form a continuum around the rotor such that it is not possible to view through the vanes of the rotor. This is best illustrated in Figs 3 and 5. Referring to Fig 5, assuming fluid flow through the rotor is from left to right as indicated by the arrow, the leading edges 12 of the vanes 10 are on the left side of the rotor and the trailing edges are on the right side of the rotor. In the embodiment of the rotor illustrated in Fig 5 there is a slight overlap (less than 50) of the leading edge 12a of vane 10a and the trailing edge 13b of adjacent vane 10b. The result of this vane configuration is that the fluid must pass through a vane of the rotor as it passes through the flow meter.

This design of rotor has been found to provide improved linearity and accuracy of flow measurement over a wide range of fluid flows.

Preferably the flow meter includes a plurality of flow straightening means 19 located within the bore of the meter. The flow straightening elements act to encourage laminar flow of the fluid through the bore of the flow meter. The flow straightening means comprises a plurality of planar members which extend radially from the outer surface of a central member 14 to the inner surface of the bore 3 as illustrated in Fig 1. Preferably the number of flow straightening elements equals the number of vanes on the rotor. Therefore in the preferred embodiment depicted there are three flow straightening elements located on each side of the rotor and equi-spaced around the central member 14. The flow straightening elements extend axially along the bore of the meter from adjacent the openings at each end of the flow meter.

A centrally located cylindrical member 14 extends along the principal axis of the flow meter. As shown in Fig 2, the axle 9 of the rotor 8 is rotatably mounted in journal bearings 15 contained in the central member 14. The central member 14 further contains thrust bearings 16 in the form of ball bearings which are located at each end of the axle 9 in order to accommodate for axial loads experienced by the rotor. Preferably there is point contact WO 98/13672 PCT/AU97/00627 between the thrust bearings 16 and the ends of the axle 9.

A further aspect of the present invention is that fluid communication is provided between the journal bearings 15 and the bearing surfaces of the axle 9 of the rotor. A transversely extending passageway 17 is provided in the central member which communicates fluid from the fluid stream passing through the meter to the bearings 15. By accurate control of the tolerances between the bearing surfaces of the rotor axle 9 and the journal bearings 15 a liquid film is provided between the bearing surfaces of the axle and the journal bearings which in turn reduces friction between the surfaces and enhances bearing life.

For example, for a 6 millimetre diameter axle a tolerance of approximately 0.1 millimetres (0.004 inches) between the outer diameter of the axle and the inner diameter of the journal bearings has been found to be effective. The fluid passes out from the bearing area via a gap 18 between the end of the central member 14 and the end of the rotor 8. As can be seen in Fig 2, the design of the rotor bearings is the same on each side of the rotor.

Referring to Fig 5, the bearing surfaces of the rotor axle 9 may be provided with a groove 23 to promote fluid flow between the axle and the journal bearings. Preferably the groove is a helical groove extending over the bearing surfaces of the axle 9.

Preferably the rotor axle 9 is made from a carbide material whilst the journal bearings are made from a teflon based, bronze impregnated material.

The material is preferably an extruded teflon-bronze material comprising approximately 80% teflon and 20% bronze.

The flow meter includes a sender unit or pulse generator 20 to generate a signal proportional to the rate of flow through the meter. The pulse generator includes a coil 21 and a magnet 22 which creates a magnetic field in the region of the rotor. The sender unit may further include a signal amplifier. The rotor is made from a magnetic stainless steel material and thus rotation of the rotor within the magnetic field creates an electrical signal. This signal is proportional to rotor speed, and in turn the rate of flow of liquid through the meter.

WO 98/13672 PCT/AU97/00627 6 Advantageously a flow meter according to the present invention provides linear, or near linear, flow measurement characteristics over a wide range of measurement. In particular, it has been found that accuracy of within 0.5% can be achieved over the flow rate range. Referring to Fig 4, the typical accuracy of a flow meter according to the present invention is depicted as a function of flow rate (measured as a percentage of maximum flow rate). As is shown, the flow meter provides near linear measurement characteristics from as low as 20% of the rated maximum flow. Whilst the flow meter can be incorporated into flow meters of varying capacities, it has been found particularly applicable in the measurement of flow rates from 15 litres per minute up to 1000 litres per minute.

Also, the design of the bearings of the flow meter provide the meter with an operational life of up to five times that of a conventional flow meters when used in the measurement of flows of petrochemical products such as liquid petroleum gas (LPG) and associated products. Testing to date has shown there to be negligible bearing wear after total flows of 300 million litres. Importantly this reduces operational costs and also provides increased reliability and accuracy over a long life period. Furthermore, at the end of the operational life of the meter it can be rebuilt for a fraction of the cost of conventional flow meters.

Claims (4)

1. A liquid flow meter including a housing for a bore though which liquid passes, a rotor rotatably mounted within the bore by means of an axle extending from each end of the rotor and supported by a journal bearing, the axis of the rotor being in substantial alignment with the direction of fluid flow; the rotor including a number of vanes, each vane having a leading edge and a trailing edge wherein the leading edge of one vane and trailing edge of an adjacent vane lie in a substantially common plane which passes through the axis of the rotor, wherein some of the fluid to be measured flow along a portion of the axle in order to reduce the friction between the bearing and the axle.

The liquid flow meter of claim 1 wherein the axle is substantially horizontal and the fluid provides a supportive film between the axle.and the bearing.

3. The liquid film of claim 1 or 2, wherein the axle includes a groove to assist in the dispersement of fluid around the axle.

4. The liquid flow meter of claim 3 wherein the groove spirals around the axle. DATED this 30th day of October, 2000 ACME FLUID HANDLING PTY LTD WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA LCG:SJM:PcP P7153AU00