GB2297845A - Electromagnetic flowmeter - Google Patents

Abstract

An electromagnetic flowmeter has energising coils 14, 16 of few turns eg. ten turns or less and high current capacity eg. tens of amps. The coils may be disposed inside the flowmeter around the wall of the bore 11 and are driven via a step-down transformer 24 mounted on the outside of the fluid-carrying conduit 10 and which receives a high-voltage supply and provides a high-current low voltage supply to the coils. The coils may be in the form of a flat "spiral" of any suitable shape.

Description

IMPROVEMENTS TO ELECTRO-MAGNETIC FLOWMETERS This invention relates to electro-magnetic flowmeters.

In the field of flow measurement, electro-magnetic flowmeters are well established for the measurement of conducting fluids such as water.

Such a meter may typically comprise: - a conduit through which the fluid to be measured flows, which has a non-conducting surface in contact with the fluid, is not magnetically permeable1 and is sufficiently robust to contain the pressure of the fluid and to resist damage.

- a coil or coils energized so as to produce a magnetic field across the conduit; the coils may be energised sinusoidally by an electrical current derived from the mains power supply or they may be driven in a variety of ways from a suitable piece of electronic equipment. They may not however be energized continuously from a DC supply because the polarization which would then occur at the electrodes would make it difficult to measure the voltage developed across the conduit as the liquid passes through the magnetic field.

- preferably, a magnetic circuit of permeable material, at a radius outside the coils1 which serves both to increase the magnetic field produced in the conduit, and to shield the meter, so as to reduce its effect on adjacent equipment and the effect of adjacent electrical equipment on the meter.

- two or more electrodes to detect the e.m.f. generated by the action of the conductive medium moving in the magnetic field1 which is proportional to the mean linear velocity of the fluid.

From knowledge of the cross-sectional area of the meter bore at the plane of measurement, the volume flow-rate is calculated.

One of the principal areas of difficulty is the generation of the magnetic field. In order for the meter to have a given sensitivity, the magnetic field generated must be of given magnitude. From the required field, the ampere turns required to be developed in the coil(s) may be calculated.

In meters of this type, it is usual to construct the coils of insulated wire, of relatively small diameter, and to use many turns1 typically of the order of 150 to 350. The coil then consists of a bundle of wires, which is of substantial cross-section. The current required to drive such a coil is typically about an amp, which is readily provided.

In conventional meters where the coils are mounted outside the conduit, the conduit must be magnetically impermeable, so that the field penetrates it. In practice, a grade of stainless steel is generally used. This is a costly material, and contributes a significant proportion of the total cost of the meter. In addition, a separate magnetic circuit is generally provided, outside the coils.

It is known to mount the coils inside a tube of carbon steel, which may then also act as the magnetic circuit. When the coils are so mounted, they can no longer derive their current directly from the public supply. A magnetic field changing at mains frequency produces persistent eddy currents which circulate within the metal. This both diminishes the available magnetic field and causes an unacceptable power loss. For this reason it is preferable to drive the coils with pulses of DC where the direction of the current flow is first in one direction and then in the other. The duration of these pulses must be long enough to allow any eddy currents generated within the metal to die away, so that the magnetic field will then be stable, but not long enough for polarization at the electrodes to become a problem.

With conventional disposed multi-turn coils within a tube, the volume of the coils presents mounting difficulties. In prior designs, this construction has been complex and expensive to produce.

It has now been appreciated in this invention that the problem of mounting the coils is much reduced if the design of the coils is such that they present a very small profile to the direction of flow of the fluid, for example enabling them to be effectively "sandwiched" between the conduit and the insulating lining. This has been achieved by selecting the coils and their energising means such that the coils are of much flatter cross-section than customary. This may be achieved by using coils of a much smaller number of turns than formerly.

This means of providing compact energising coils for a flowmeter is in principle applicable regardless of whether the coils are inside or outside the conduit.

However, the current required to be developed in the coils is proportionately greater, typically of the order of tens of amps, for example, 20 to 100 amps, and it is inconvenient to draw such currents directly from the supply. In addition, it is often the case that the power supply for the meter, is mounted at a distance from the meter1 typically in a central control location, and it is not practical to transmit signals of high current and low voltage for significant distances. We have found surprisingly that by a suitable choice of one or more transformers or other matching means local to the meter, it is practicable to convert an incoming high voltage supply and provide a low impedance high current supply to the at least one coil.

Thus the invention provides an electro-magnetic flowmeter comprising a conduit through which a flow of fluid to be measured is to be passed, a coil or coils or relatively few turns and high current capacity for generating a periodic magnetic field through the fluid transversely of the direction of said flow, means for receiving a voltage induced in the fluid due to the movement of the fluid in the magnetic field, and matching means disposed on or adjacent the conduit for receiving a high voltage low current supply and providing a high current low voltage supply to the coil or coils.

The matching means may be a transformer or other device having suitable characteristics, for example a switched mode power supply.

The matching means may provide to the coil or coils high current low voltage DC pulses of sufficient duration for each pulse to establish a steady magnetic field without significant polarization of the fluid at the means for receiving the induced voltage. Thus the transformer may be a pulse transformer of suitably high inductance.

By a "coil having relatively few turns" we mean few in comparison with the conventional 150 to 350 turns, for example, no more than ten turns at most, and preferably five turns or less. Indeed, there may be just a single turn.

The or each coil preferably has its tums disposed side by side in a single layer, for example in the form of a spiral. The term spiral as used herein is not limited to the strict mathematical meaning. It is intended to include any arrangement in which one turn is disposed circumferentially and outwardly of the preceding turn; the turn may be of any suitable shape eg.

substantially circular, elliptical, rectangular or diamond-shaped, as appropriate to the particular application.

The coil or each coil may be contained in insulating material, and thus if the conductor of the coil is an insulated wire the insulating material may be such as to adhere to the insulated surface of the wire. The wires may be of round cross-section, but advantageously are of flattened form.

It is preferred however that the conductor of the coil or each coil is of strip-like cross-section. The strip-like conductor may be folded to effect changes in the direction thereof. Alternatively, the coil or each coil may be cut (eg. pressed or stamped) from a single sheet of material.

When the coil or each coil is disposed within the conduit preferably it is disposed around and conforms to the inner surface of the conduit.

The coil or each coil may be disposed between the inner wall of the conduit and an insulating lining thereof, or may be embedded in said lining.

The conduit may be of magnetically permeable material such as carbon steel whereby to form a magnetic circuit with the at least one coil.

Altematively, the conduit may be of impermeable material such as stainless steel, a magnetic screen of relatively high permeability material preferably being provided around and outside it.

Regardless of the permeability of the material, it may be of poor electrical conductivity. A "duplex" stainless steel (one containing a mixture of two or all of ferrite, austenitic and martensitic phases) may exhibit a useful combination of magnetic permeability, and electrical resistivity, both properties falling between those of carbon steel and type 304 stainless steel.

The preferred forms of the invention can have the following advantages over known flowmete rs: - simplicity of construction and therefore cheapness.

- ailows the use of cheaper conduit materials than is customary.

- greater robustness, due to the simplicity of the design, in particular resistance to damage by loading from above when buried.

The invention will now be described merely by way of example with reference to the accompanying drawings wherein Figure 1 is a perspective view of a flowmeter according to the invention; Figure 2 shows a coil of the flowmeter of Figure 1; and Figure 3 is a section through the wall of the flowmeter, in an axial plane on arrows 3-3 of Figure 1.

Referring to Figure 1, an electro-magnetic flowmeter consists of a length of conduit (10) having a bore (11) and flanges (12) at each end so that it may be incorporated eg. in a buried pipe line. The conduit may be of considerable internal diameter eg. from about 0.5 meter, up to perhaps 3 meters. The conduit in this example is of carbon steel. Installed within the conduit are two saddle-shaped coils (14, 16) which extend circumferentially around the internal wall of the bore (11) and also axially thereof. The developed form of each coil is shown in Figure 2; each is in the form of a rectangular spiral of copper strip, having been stamped from a single sheet.

For a conduit of nominal bore 800 mm, the overall axial length (18) is of order 1100 mm and the overall width (20) about 500 mm. The strip width (22) is 25 mm and it is 1.6 mm thick; that is to say its width is several times its thickness, eg. ten or more times.

The coils are energised from a transformer (24) mounted in an armoured enclosure on the outside of the conduit between the flanges. The transformer receives high voltage from a suitable pulsed DC supply, drawing about 0.5 amps from it, and delivers (say) 25 amps at 0.5 volts, thus appearing to the coils as a low-impedance supply. The high voltage supply may be as described in our UK paten specification 2271639A.

A pair of electrodes, only one shown at (26) are arranged orthogonally to the coils (14, 16) to detect in conventional manner the voltage induced in a conductive medium flowing through the conduit.

Telemetry and/or processing circuitry (28) delivers the signal obtained from the electrodes to a remote station by conventional means not shown.

Referring to Figure 3, turns (30 to 36) of the coil (14, 16) are insulated from the conduit wail, in this case by being disposed on an insulating pad (38) around the inside of the wall of the conduit (10). The turns (30 to 36) and the pad (38) are set in a recess in an intermediate lining (40) of rubber material and enclosed in a further inner lining (42). Thus the turns of the coil are sandwiched between the wall of the conduit and the non-conductive lining (42).

Being of carbon steel the conduit is of high permeability and thus acts as a magnetic circuit containing the flux generated by the coils and concentrating it in the fluid. The conduit also serves to shield the meter from external interference eg. due to the magnetic effects of passing road traffic and to prevent electro-magnetic interference being caused to neighbouring plant and equipment by the flowmeter.

Variations on the preferred construction are of course possible. For example, instead of two coils facing each other, one coil may be used.

Such a coil would extend around the conduit from say 7 o'clock to 5 o'clock, or from 11 o'clock to 1 o'clock in Figure 1, and would extend axially of the conduit as in that figure.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The appended abstract as filed herewith is included in the specification by reference.

Claims (21)

1. An electro-magnetic flowmeter comprising a conduit through which a flow of fluid to be measured is to be passed, a coil or coils or relatively few turns and high current capacity for generating a periodic magnetic field through the fluid transversely of the direction of said flow, means for receiving a voltage induced in the fluid due to the movement of the fluid in the magnetic field, and matching means disposed on or adjacent the conduit for receiving a high voltage low current supply and providing a high current low voltage supply to the coil or coils.

2. A flowmeter as claimed in Claim 1 wherein the matching means provides to the coil or coils high current low voltage DC pulses of sufficient duration for each pulse to establish a steady magnetic field without significant polarization of the fluid at the means for receiving the induced voltage.

3. A flowmeter as claimed in Claim 1 or claim 2 wherein the matching means is a transformer.

4. A flowmeter as claimed in any preceding claim wherein in operation the current in the coil or coils is of order tens of amps.

5. A flowmeter as claimed in any preceding claim wherein the coil or each coil comprises ten turns or less.

6. A flowmeter as claimed in Claim 5, wherein the coil or each coil comprises five turns or less.

7. A flowmeter as claimed in Claim 6, wherein the coil or each coil consists of a single turn.

8. A flowmeter as claimed in any preceding claim wherein the coil or each coil has its turns disposed side by side in a single layer.

9. A flowmeter as claimed in Claim 8 wherein the coil or each coil is in the form of a spiral.

10. A flowmeter as claimed in Claim 8 or 9, wherein the coil or each coil is contained in insulating material.

11. A flowmeter as claimed in any preceding claim wherein a conductor of the coil or each coil is of strip-like cross-section.

12. A flowmeter as claimed in Claim 11 wherein the conductor is folded to effect changes in the direction thereof.

13. A flowmeter as claimed in Claim 11 wherein the coil or each coil is cut from a single sheet of material.

14. A flowmeter as claimed in Claim 8, 9, or 10, wherein the turns are insulated wires.

15. A flowmeter as claimed in any preceding claim wherein the coil or each coil is disposed within the conduit, and is insulated from the flow.

16. A flowmeter as claimed in Claims 8 and 15 wherein the coil or each coil is disposed around and conforms to the inner surface of the conduit.

17. A flowmeter as claimed in Claims 15 or 16, wherein the coil or each coil is disposed between the inner wall of the conduit and an insulating lining thereof, or is embedded in said lining.

18. A flowmeter as claimed in Claims 15, 16, or 17 wherein the conduit is of magnetically permeable material whereby to form a magnetic circuit.

19. A flowmeter as claimed in Claims 15, 16, or 17 wherein the conduit is of magnetically impermeable material and a magnetic screen of relatively highly permeable material is provided outside the conduit.

20. A flowmeter as claimed in any preceding claim wherein the conduit is of a material which has poor electrical conductivity.

21. A flowmeter substantially as herein described with reference to the accompanying drawings.