GB2278445A

GB2278445A - Flow meters

Info

Publication number: GB2278445A
Application number: GB9310233A
Authority: GB
Inventors: Karel Svoboda
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 1993-05-18
Filing date: 1993-05-18
Publication date: 1994-11-30
Anticipated expiration: 2013-05-18
Also published as: GB2278445B; GB9310233D0

Abstract

A fluid flow meter comprises a conduit 2 within which an orifice plate 8 is secured having an opening 10. An axially slidable contoured plug 12 provides a variable restriction to the size of the opening 10. The plug 12 is biased in the upstream direction by a spring 22 such that the position of the plug is responsive to the fluid flow rate in the conduit. The position of the plug is measured by a transducer 13 comprising a ferromagnetic shaft 18 which is attached to the plug and which has a semi-circular cross-section, the orientation of which varies along its length, and a fixed ferromagnetic sensor body 24. The sensor body has a primary coil 42 and secondary coils 38, 40, the induced voltages in which are dependent upon the axial position of the shaft 18 (and therefore the plug 12), and thus give an indication of the flow rate. <IMAGE>

Description

FLOW METERS This invention relates to flow meters.

GB 1566251 discloses a variable orifice flow meter. In this type of variable orifice flow meter a flow body such as a contoured plug provides a variable closure to a fixed orifice. The plug is resiliently biased by a spring to close the orifice. The flow of fluid acts against the spring such that an increase in flow results in movement of the plug against the action of the spring so as to cause an increase in the size of the variable orifice. As the plug movement is restrained by a spring, the differential pressure increases with flow rate. In conventional variable orifice meters of this type, the differential pressure is the output signal, and consequently pressure tappings and a pressure transducer are required.

According to the present invention there is provided a fluid flow meter comprising a flow body which is movable in a conduit for displacement by fluid flowing in the conduit whereby the position of the flow body is dependent upon the fluid flow rate, a displacement transducer being provided which is responsive to displacement of the flow body and which comprises a ferromagnetic shaft and a ferromagnetic sensor body which are linearly displaceable relatively to each other along an axis, the sensor body having a primary coil and a plurality of secondary coils, the coils being spaced apart from one another about the axis, the primary coil being coupled to each secondary coil, through a separate magnetic path, each path including the shaft and the sensor body, the shaft being so configured that relative displacement between the shaft and the sensor body varies the magnetic coupling between the primary coil and the respective secondary coils thereby to provide an output signal that is dependent upon the relative positions of the shaft and the sensor body.

Thus the output signal of the transducer represents the position of the flow body, and consequently the flow rate through the duct.

Preferably, the sensor body comprises an annular member having ferromagnetic cores which project radially inwardly and about which the coils are mounted. In a preferred embodiment there is a primary coil and there are two secondary coils, the three coils being equally angularly spaced from one another about the shaft.

The shaft may have a helical configuration and may have semi-circular cross-section. Thus, the crosssection of the shaft may comprise a semi-circle which is disposed at varying angular positions along its length.

The flow body may comprise a cone which is disposed within an orifice.

For a better understanding of the present invention and to show how it may be carried into effect reference will now be made by way of example to the accompanying drawings, in which: Figure 1 shows a fluid flow meter; Figure 2 is a sectional view, taken on the line II-II in Figure 1, of a displacement transducer of the flow meter; Figure 3 shows a component of the transducer; Figure 4 shows, in longitudinal cross-section, another component of the transducer; and Figure 5 corresponds to Figure 2 but shows the transducer under different flow conditions.

The flow meter shown in Figure 1 comprises a conduit 2 having an inlet 4 and an outlet 6. An orifice plate 8 is secured within the conduit 2 and has an opening 10. An axially slidable cone shaped plug 12 provides a variable restriction to the sides of the opening 10. The plug 12 is slidable along a guide 14 which is secured to the conduit 2 by supporting members 16, only one of which is shown in Figure 1. The plug 12 is connected to a displacement transducer 13. Means 19 is provided for preventing rotation of the plug 12 on the guide 14. This means may be an anti-rotation pin or slot. The plug 12 id biassed in the upstream direction by a spring 22 which reacts against an abutment 20.

The transducer 13 comprises a ferromagnetic shaft 18 and a ferromagnetic sensor body 24. The shaft 18 is secured to the plug 12 and extends from it in the downstream direction. The sensor body 24 is in the form of a ring and surrounds the shaft 18. The shaft 18 is shown in greater detail in Figure 3. The shaft comprises an elongate body 26 having a semi-circular transverse cross-section 28. Between an upstream end 30 and a downstream end 32 of the body 26 there is a relative twisting of 90". Thus, at each point along the length of the body 26, there is a semi-circular cross-section at various angular positions. The downstream end of the shaft 18 is provided with an abutment flange 34 against which the spring 22 abuts.

The sensor body 24 is shown in transverse crosssection in Figure 2. The sensor body comprises an annular outer body 36. Within the body 36, there are provided, in the embodiment shown, three electrical coils 38,40,42. Each coil has a ferromagnetic core 37 which projects radially inwardly and is sized to leave a small gap between an end portion of the core 37 and the cylindrical envelope 39 of the shaft 18. Two of the coils are secondary coils 38,40 and one coil is a primary coil 42.

The sensor body 24 is shown in longitudinal crosssection in Figure 4. As shown, the upstream end 44 of the sensor body 24 has a smoothly curved contour so as to minimize turbulence. The curved end 44 also acts as a protecting barrier for the coils 38, 40, 42.

In use, fluid flows within the conduit 2 between the inlet 4 and the outlet 6. The resulting pressure difference across the orifice 10 causes the plug 12 to move along the duct against the action of the spring 22. The position of the plug 12 thus depends upon the rate of flow within the conduit 2. The shaft 18 is connected to the plug 12 and likewise its position depends upon the flow rate in the conduit 2.

A primary voltage is applied to the primary coil 42, shown in Figure 2, and the difference between the induced voltages in the secondary coils 38,40 is measured. The induced voltage in any one of the secondary coils 38,40 is a function of the effective air gap between that coil and the primary coil 42.

With the shaft 18 in the position shown in Figure 5, the induced voltages in the two secondary coils 38,40 are equal and so the difference is zero. However, with the shaft 18 in a different position, the induced voltages will differ as a result of the air gaps being unequal. Thus, in Figure 2 the induced voltage in secondary coil 40 is much greater than that induced in the secondary coil 38.

The angular orientation of the semi-circular cross section of the shaft 18, in the proximity of the sensor arrangement 24, is a function of the longitudinal position of the plug 12 within the conduit 2. As the flow rate within the conduit 2 increases, the orientation of the shaft cross-section rotates clockwise from the position shown in Figure 5. The induced voltage difference between the two secondary coils 38,40 therefore gives an indication of the position of the plug 12, which itself is a function of the flow rate within the conduit 2.

By appropriate calibration, the induced voltage difference can be employed to provide a direct read-out of the flow rate in the conduit.

Alternative means for measuring the linear displacement of the contoured plug 12 may be provided, for example involving the use of one or more proximity sensors in conjunction, for example, with a tapered shaft 18 instead of that shown in Figure 3.

Furthermore, although the displacement transducer 13 has been described in the context of fluid flow measurement in a conduit, it will be appreciated that the displacement transducer 13 is novel and inventive in itself and has applications in the general field of measurement of linear displacement.

Claims

1. A fluid flow meter comprising a flow body which is movable in a conduit for displacement by fluid flowing in the conduit whereby the position of the flow body is dependent upon the fluid flow rate, a displacement transducer being provided which is responsive to displacement of the flow body and which comprises a ferromagnetic shaft and a ferromagnetic sensor body which are linearly displaceable relatively to each other along an axis, the sensor body having a primary coil and a plurality of secondary coils, the coils being spaced apart from one another about the axis, the primary coil being coupled to each secondary coil, through a separate magnetic path, each path including the shaft and the sensor body, the shaft being so configured that relative displacement between the shaft and the sensor body varies the magnetic coupling between the primary coil and the respective secondary coils thereby to provide an output signal that is dependent upon the relative positions of the shaft and the sensor body.

2. A fluid flow meter as claimed in claim 1, in which the sensor body comprises an annular member having ferromagnetic cores which project radially inwardly and about which the coils are mounted.

3. A fluid flow meter as claimed in claim 1 or 2, in which there are two secondary coils.

4. A fluid flow meter as claimed in claim 3, in which the primary coil and the secondary coils are equally angularly spaced from one another about the shaft.

5. A fluid flow meter as claimed in any preceding claim, in which the shaft has a helical configuration.

6. A fluid flow meter as claimed in claim 5 in which the shaft has a semi-circular cross-section.

7. A fluid flow meter as claimed in any preceding claim, in which the flow body comprises a cone shaped plug which is disposed within an orifice of an orifice plate.

8. A fluid flow meter substantially as described herein with reference to, and as shown, in the accompanying drawings.

9. A linear transducer substantially as described herein with reference to, and as shown in, the accompanying drawings.