JPS6014177Y2 - electromagnetic flow meter - Google Patents

Description

[Detailed description of the invention] This invention relates to an electromagnetic flowmeter, in particular an electromagnetic flowmeter that can correct flow measurement errors caused by thermal expansion of the lining applied to the inner wall of a steel pipe and obtain accurate flow measurement values. This is an attempt to provide a measurement plan.

An electromagnetic flowmeter transmitter generally uses a resin lining on the inner wall of a steel pipe as a measurement pipe, and this lining layer increases the insulation between the electrodes, effectively extracting a signal from the electrodes with a level proportional to the flow velocity. That's what I do.

By the way, there is a gap, albeit small, between the steel pipe and the lining.

Therefore, the inner diameter of the lining changes due to thermal expansion of the lining due to changes in the temperature of the fluid to be measured.

When the inner diameter of the lining changes, the cross-sectional area of the flow path changes, causing a change in the flow velocity of the fluid to be measured, and therefore the electromotive force generated at the electrode also changes according to the following equation.

Here, ΔE is change in electromotive force due to change in liquid temperature, E: electromotive force, ΔT: change in liquid temperature, linear expansion coefficient of αni lining, n
is a constant determined by the method of fixing the electrode, and when the electrode is fixed to the lining and moves with the lining, n
÷1, and if the electrode is fixed to the steel pipe and its position does not move due to expansion of the lining, then n÷2.

As can be seen from this equation, as the liquid temperature increases and the inner diameter of the lining increases, the electromotive force decreases and a span error occurs.

The purpose of this invention is to provide an electromagnetic flowmeter that can correct span errors and obtain correct flow signals even if the inner diameter of the lining changes due to changes in liquid temperature.

In this invention, a temperature corresponding to the temperature of the lining is detected by a temperature sensing element, and this detection signal is used to correct an error in the flow rate signal due to thermal expansion of the lining.

An embodiment of this invention will be described in detail below with reference to the drawings.

FIG. 1 shows an example of the configuration of a means for measuring a temperature corresponding to the temperature of the lining.

That is, in this example, only one electrode portion of the electromagnetic flowmeter transmitter is shown.

In the figure, reference numeral 1 indicates a steel pipe, and reference numeral 2 indicates a lining applied to the inner wall surface of the steel pipe 1.

This lining layer 2 is made of a resin material such as FEP or PFA.

A hole is made in the steel pipe 1 and the lining 2, and an electrode 3 is inserted from the outside through this hole.

In this example, a spring 6 is inserted between an electrode support member 4 fixed to the steel pipe 1 and an electrode support part 5, and the electrode 3 is elastically supported with respect to the steel pipe 1. This shows a case where the configuration is such that the position No. 3 also moves.

In this invention, the electromagnetic flowmeter transmitter having the lining 2 is provided with a temperature sensing element 7 for measuring a temperature corresponding to the temperature of the lining 2.

In this example, a hole is formed in the steel pipe 1, and the temperature sensing element 7 is directly buried in the lining 2 through this hole.

In particular, it is desirable that the temperature sensing element 7 be mounted closer to the electrode 3.

As the temperature sensing element 7, for example, a thermistor, other metal resistance temperature sensing element, positive temperature coefficient temperature sensing element, etc. can be used.

Another method of measuring the temperature of the lining 2 is, for example, by forming a hole in the outer wall of the steel pipe 1 and embedding the temperature sensing element 7 in this hole to measure the temperature of the steel pipe 1 and indirectly measuring the temperature of the lining 2. It can also be measured.

If the liquid temperature is relatively high, indirect measurement of the lining temperature in this manner is sufficient for practical use.

Next, a description will be given of means for correcting changes in the electromotive force generated in the electrode 3 based on the detection output of the temperature sensing element 7.

FIG. 2 shows an example. In this example, a signal proportional to the excitation current for forming a magnetic field in the oscillator and a signal proportional to the flow velocity obtained from the electrode 3 are divided by a dividing circuit. The case is shown in which the present invention is applied to an electromagnetic flowmeter that eliminates the effects of fluctuations in excitation current.

That is, in FIG. 2, 8 represents an electromagnetic flowmeter transmitter, 9 represents an excitation coil for forming a magnetic field in a direction perpendicular to the direction in which the electrodes 3 face, and 10 represents an excitation power source thereof.

On the other hand, reference numeral 11 indicates an electromagnetic flow meter converter, and as is generally well known, this converter 11 amplifies the electromotive force generated at the electrode 3 and performs synchronous rectification etc. to remove the influence of noise. This DC signal is converted into a current signal and sent to, for example, a control room.

Such an electromagnetic flowmeter converter 11 is conventionally provided with a dividing circuit 12 in order to eliminate the influence of fluctuations in the excitation current.

Therefore, in this example, a signal proportional to the excitation current supplied to the divider circuit 12, that is, this signal is generally referred to as a comparison voltage.
A case is shown in which this comparison voltage is changed by the temperature sensing element 7 to correct span fluctuations.

The comparison voltage is generally obtained by extracting a current proportional to the excitation current using a current transformer 13 and passing the current through an input resistor 14 of the divider circuit 12.

A series circuit consisting of a fixed resistor 15 and a temperature sensing element 7 is connected in parallel with the input resistor 14.

With this connection, when the temperature of the lining 2 rises, if the temperature sensing element 7 is a thermistor, its resistance value will decrease, and the current flowing through the series circuit of the fixed resistor 15 and the temperature sensing element 7 will increase. Since the current flowing through the input resistor 14 decreases, the comparison voltage developed across the input resistor 14 decreases.

Therefore, the ratio of the flow rate signal to the excitation current is corrected by the reduction in the comparison voltage, and the reduction in span due to thermal expansion of the lining 2 can be corrected.

To explain this in more detail, the resistance value of the input resistor 14 is R8, the resistance value of the fixed resistor 15 is R1, the temperature sensing element 7 is
Let the resistance value be RsN, and let the comparison voltage generated across the input resistor 14 of the divider circuit 12 be .

R8 so that bill t Rg<R□! R1? If Rs is selected, the comparison voltage (2) changes depending on the temperature change of the lining 2.

When a thermistor is used as the temperature sensing element 7, it has a negative temperature coefficient, so the resistance value R of the fixed resistor 15 and the temperature sensing element 7 is
By appropriately selecting □ and R8, the temperature change in the comparison voltage ■ can be corrected for the change in the electromotive force E due to the expansion of the relining 2, which is the same as the temperature change in the electromotive force E explained in the first equation. can.

In the embodiment shown in Fig. 2, the comparison voltage was obtained using the current transformer 13, but there are other methods for obtaining the comparison voltage, such as directly detecting the change in the magnetic field with a coil, or obtaining the comparison voltage by excitation. Possible methods include inserting a resistor in series in the current path and extracting the voltage generated across the resistor. Regardless of which method is used to extract the comparison voltage, the span error can be corrected by the temperature sensing element 7. Can be done.

FIG. 3 shows another embodiment of this invention.

In this example, a case is shown in which the gain of the flow signal amplifier provided in the converter 11 is changed in response to changes in the temperature of the lining 2.

That is, the signal from the oscillator 8 is amplified by the preamplifier 16 and its amplified output is supplied to the temperature compensation amplifier 17.

The temperature correction amplifier 17 has a negative feedback circuit 18 , and the temperature sensing element 7 is connected in parallel to a voltage dividing resistor 19 of the negative feedback circuit 18 .

If the temperature sensing element 7 is a thermistor, its resistance value decreases as the temperature of the lining 2 increases.

Therefore, the amount of negative feedback of the amplifier 17 decreases, and the gain of the amplifier 17 increases in accordance with the decrease in the resistance value of the temperature sensing element 7.

Therefore, if the resistance value of the temperature sensing element 7, the resistance value of the voltage dividing resistor 19, and the resistance values of the resistor 20 etc. connected in series with this resistor 19 are appropriately selected, the span error due to thermal expansion of the lining 2 can be reduced. can be corrected.

FIG. 4 shows yet another embodiment of this invention.

In this example, a stabilized constant current is supplied to the excitation coil 9 of the transmitter 8 by the constant current circuit 21, and the dividing circuit as explained in FIG. 2 is not provided.

In other words, the case is shown in which a constant current is supplied from the DC power supply 22 to the excitation coil 9 through the constant current circuit 21, and the direction of the excitation current is changed by the switching circuit 23 at a constant cycle, for example, several Hz.
In the case of such a constant current excitation method, the output current value of the constant current circuit 21 is changed by a change in the resistance value of the temperature sensing element 7.

Even in this manner, the span error caused by the temperature rise of the lining 2 can be corrected.

As explained above, according to this invention, it is possible to correct the span change due to temperature change of the electromagnetic flowmeter transmitter having the lining 2, and the effect is very useful in practical use.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing an example of installing a temperature sensing element in an electromagnetic flowmeter according to this invention, and Fig. 2 illustrates an example of means for correcting a span error using a detection signal of a temperature sensing element in an electromagnetic flowmeter according to this invention. FIGS. 3 and 4 are connection diagrams showing other examples of means for correcting the span error based on the detection signal of the temperature sensing element in the electromagnetic flowmeter of this invention. 1... Steel pipe, 2... Lining, 3.
-----tti, 7...Temperature sensing element.

Claims (1)

[Scope of utility model registration request] In an electromagnetic flowmeter that uses a measuring tube whose inner wall surface is lined with a resin material, a temperature sensing element detects a temperature corresponding to the temperature of the lining, and a detection signal from the temperature sensing element is used to automatically detect the temperature. An electromagnetic flowmeter comprising a correction means for correcting a flow rate signal ratio to an excitation current, and correcting a span error based on thermal expansion of the lining.