JPH05273014A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To prevent breakdown of a measuring tube by preventing excessive application of a stress to the measuring tube when an electromagnetic flowmeter is fitted to a process piping and when a fluid temperature changes. CONSTITUTION:A ceramic measuring tube 1 is fitted in a radial direction in a plurality of O rings 14a to 14d and held in a sealed state by a case member 15. The case member 15 is pressed against each of grounding rings 5a and 5b with one O ring held between them. The axial length of the measuring tube is shorter than that of the case member and the tube is not in contact with the grounding rings. Therefore it is not subjected to an axial tension. The measuring tube is shaped in a small-thickness and simple cylinder and to be resistive to a thermal stress and it is made to have a structure of being suspended in the case member with the soft O rings interposed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter, and more particularly to an electromagnetic flow meter having a measuring tube made of a ceramic pipe suitable for measuring the flow rate of a high temperature fluid, a corrosive fluid, an abrasive fluid and the like.

[0002]

2. Description of the Related Art An electromagnetic flowmeter generates an electromotive force proportional to the product of the strength of an applied magnetic field and the flow velocity of a fluid when a conductive fluid to be measured flows perpendicular to the magnetic field. It is based on the principle that the flow rate can be measured by detecting with a pair of electrodes. The measuring pipe of this flowmeter, which comes into contact with the fluid to be measured, is generally lined with rubber or a fluoroethylene resin on the inner surface in order to have corrosion resistance and electrical insulation. Recently, an electromagnetic flowmeter using a ceramic pipe as a measuring pipe has been commercialized in order to improve heat resistance and wear resistance particularly in this portion. As a well-known example, there is a measurement value detector for a magnetic induction type flow rate measuring device disclosed in Japanese Patent Laid-Open Publication No. (Showa 58-501552).

FIG. 2 shows the structure of a conventional ceramic electromagnetic flowmeter. In the figure, reference numeral 1 is a measuring pipe made of a ceramic pipe, through which the fluid to be measured flows. A pair of electrodes 4 are attached to the measuring tube 1 so as to face each other in the radial direction of the pipe. The measuring tube 1 has thick flange portions at both ends thereof, and is fixed to the case member 7 by an organic or inorganic adhesive at the outer peripheral portion of the flange portions. Electromagnetic coils 2a, 2b and a core 3 are arranged near the outer periphery of the central portion of the measuring pipe 1, and these coils 2a, 2b
A magnetic field is generated by penetrating the inside of the measuring tube 1 by the 2b and the core 3. When the fluid flowing in the measuring tube 1 crosses this magnetic field, an electric potential is generated in the fluid, and this current is detected by the pair of electrodes 4. A wiring lead-out portion 9 is formed at the center of the case member 7, and the excitation wiring to the electromagnetic coil and the detection signal wiring from the electrode are connected to an external terminal portion via the wiring lead-out portion 9.

This electromagnetic flowmeter is sandwiched by a pair of pipe flanges 11a and 11b of a process pipe, and grounding rings 5a and 5b and gaskets 10a and 10 at both ends thereof.
It is attached by tightening with a bolt 12 and a nut 13 via b. The ground ring contacts the fluid and is connected to the ground line of the electromagnetic flow meter so that the potential of the fluid and the ground potential of the electromagnetic flow meter are common. Gasket 10a, 10
b, liquid-tightness is maintained between the process pipe flanges 11a and 11b and the grounding rings 5a and 5b, and between the grounding rings 5a and 5b and the measuring pipe 1 of the electromagnetic flowmeter.

[0005]

However, the conventional electromagnetic flowmeter has the following problems.

When mounting the electromagnetic flow meter on the process pipe,
By tightening the bolts with a torque of several thousand N · cm, the gasket is given a compressive force in the axial direction (thickness direction),
Maintains liquid tightness. Since this tightening force is received by the flange surface of the measuring pipe of the electromagnetic flow meter, the flange surface has several tens of MPa.
A large surface pressure of several hundreds MPa is applied.

At this time, stress as shown in FIG. 3 is generated in the flange portion of the measuring pipe 1. This stress distribution is what the writer has confirmed and verified by numerical analysis by the finite element method, and will be described below. That is, due to this surface pressure, a compressive stress is generated in the cylindrical portion of the measuring pipe in the axial direction, and further, a large compressive stress and a tensile stress due to bending are generated in the root portion of the flange. This behavior means that a bending moment indicated by an arrow is generated in the pipe flange portion and the flange is bent in the arrow direction.

Generally, in the case of ceramics, the tensile strength is about 1/10 of the compressive strength, and the tensile stress often causes the ceramic measuring tube to break. Actually, from the position shown in FIG. It was verified to break.

For this reason, in the electromagnetic flowmeter of this structure, the upper limit of the tightening torque of the bolt when it is attached to the process pipe is often specified, and if it is tightened beyond this value, the ceramic measuring tube breaks. there is a possibility. Also,
Even when tightened within the specified torque, if one of the bolts is tightened in a so-called one-side tightened state, excessive tensile stress may occur, which may lead to breakage.

Further, since the flange portion of the measuring pipe becomes thick as in this example, when the temperature of the fluid suddenly changes, excessive thermal stress is generated in the flange portion, which also causes the damage of the ceramic measuring pipe. Become. According to the numerical analysis, the magnitude of thermal stress when a heat shock is applied increases as the wall thickness increases, so it is necessary to make the pipe thinner.

However, if the pipe is made thin, the strength against the above-mentioned bolt tightening force becomes weaker, and both cannot be solved at the same time.

The present invention solves the problems of the conventional structure, does not cause an excessive tensile stress in the ceramic measuring pipe when the electromagnetic flowmeter is attached to the process pipe, and is strong against heat shock. The purpose is to provide a total.

[0013]

According to the present invention, a measuring pipe made of a ceramic pipe is formed in a thin tubular shape having no remarkable flange portions at both ends, and one or a plurality of pairs are formed on the outer peripheral surface near the both ends. The sealant such as the O-ring is used to fix it to the case member to prevent the fluid to be measured from entering the inside of the electromagnetic flowmeter.

At this time, the outer surface of the outermost sealing material is located outside the end surfaces of the case member and the measuring pipe,
It is pressed against the flange of the pipe directly or through the grounding ring and the gasket to prevent the fluid to be measured from leaking between the case member and the pipe flange.

Further, since the axial length of the measuring pipe is slightly shorter than the axial length of the case member, when the flowmeter is fastened and fixed between the pair of flanges with a plurality of bolts,
All the tightening force is transmitted to the case member, not to the measuring tube.

With the above, it is possible to provide an electromagnetic flowmeter which is resistant to both heat shock and excessive tightening of bolts.

[0017]

When the ground ring is used in the above structure,
The tightening force of the bolts is transmitted in the order of piping flange-gasket-grounding ring- (O-ring) -case member to liquid-seal each member. At this time, since the O-ring is soft, it is crushed and the grounding ring and the case member are brought into direct contact with each other and pressed. On the other hand, if the ground ring is not used,
The piping flange directly presses the O-ring and the case part.

As described above, since the ceramic measuring tube is made slightly shorter than the axial length of the case member, it does not come into contact with the piping flange or the grounding ring, so that the tightening force is not applied to its end surface. .. Therefore, even if the bolt is tightened excessively, the measuring tube is not damaged.

On the other hand, the measuring tube has a thin cylindrical shape,
Since there is no thick portion as in the conventional example, the temperature difference between the inner and outer surfaces of the measuring pipe is small even when the temperature of the fluid to be measured changes suddenly, and therefore the thermal stress generated in the measuring pipe is small. Further, the thermal stress generated by the temperature difference between the case portion and the measuring pipe portion is also escaped by the O-ring, so it can be almost ignored.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a sectional view of an electromagnetic flowmeter detector of the present invention. Reference numeral 1 is a measuring tube formed of ceramic as a material, and a fluid to be measured flows through the main measuring tube 1. A pair of electromagnetic coils 2a and 2b are attached to the outside of the measuring tube 1, and a core 3 is arranged outside the measuring tube 1 so as to surround the electromagnetic coils 2a and 2b. The electromagnetic coils 2a and 2b and the core 3 generate a magnetic field that vertically traverses the measuring tube 1. The electromagnetic coils 2a and 2b are generally excited by a direct current, which is a square wave and is repeatedly turned on and off periodically. At the center of the measuring tube, 1 is provided in a direction orthogonal to the magnetic field direction and the flow direction.
A pair of electrodes 4 is formed. The electromotive force generated in the fluid is detected via the electrode 4. Further, the electrode 4 may be formed in a planar shape on the outer peripheral surface of the measuring tube 1, and the electromotive force may be detected by capacitive coupling with the fluid.

The measuring tube 1 is formed of a ceramic having a high corrosion resistance such as a simple thin-walled cylindrical high-purity alumina and has a wall thickness of 5 to 10% of the inner diameter. On the other hand, the outer periphery of the measuring tube 1 near both ends is held by a case member 15 via a plurality of pairs of O-rings 14a and 14b to prevent liquid from entering the inside of the electromagnetic flow meter.

Further, both end surfaces 16a of the case member 15,
16b is a grounding ring 5a for setting the fluid to be measured to the ground potential,
5b is pressed, and the O-ring 15 is attached to its inner end.
a and 15b are sandwiched and both are liquid-tightly sealed. At this time, the outer side surfaces of the O-rings 15a and 15b are connected to the end surface 16
a) 16b, traveling more than the above-mentioned grounding rings 5a, 5b
It becomes possible to seal by being pressed against b and crushed. Since the axial length of the measuring pipe 1 is made shorter than that of the case member 15, it does not come into contact with the grounding rings 5a and 5b.

Further, the outer surfaces of the grounding rings 5a and 5b are sandwiched by the flanges 11a and 11b of the process pipe via the gaskets 10a and 10b, and are tightened with a plurality of bolts 12 and nuts 13, so that the electromagnetic flowmeter is liquid-tight. As it is, it is attached to the process pipe.

On the other hand, the case member 15 has an opening 9 at the center of the cylinder, and the opening 9 allows the electromagnetic coil 2 to move.
Wiring for supplying a current to a and 2b from the outside and wiring for taking out a signal from the electrode 4 to an external amplifying section are introduced and led out.

In the electromagnetic flowmeter having such a structure, when the electromagnetic flowmeter is clamped between the flanges 11a and 11b by using the bolt 12 and the nut 13 when the process pipe is attached, the flange 11a and the gasket member 10a are used. The compressive force applied to the ground ring 5a is the case member 1
A compressive force acts on the whole 5 in the axial direction. However, the force is not transmitted to the measuring pipe 1 due to the gap between the measuring pipe 1 and the grounding ring 5a, and the force at the time of mounting the process pipe is 1
Does not join at all.

When the temperature of the fluid to be measured in the measuring pipe 1 suddenly changes, a temperature difference occurs between the inner and outer surfaces of the measuring pipe 1 and a thermal stress is generated inside. It has been confirmed by calculation and experiments by the finite element method that the stress value is larger as the wall thickness of the measuring tube is thicker and larger as the wall thickness is uneven. Further, generally, ceramics are weaker in tensile force than compression, so it was confirmed that the inner surface was most easily damaged when it was rapidly cooled. In this embodiment, the measuring tube 1 is a thin-walled simple cylinder, and a shape in which generation of internal thermal stress is small is selected.

On the other hand, due to the temperature difference and the linear expansion coefficient difference between the measuring pipe 1 and the case member 15, the dimensions of the measuring pipe 1 and the case member 15 should change in the axial direction and the radial direction, and thermal stress should be generated between them. Since the structure is such that it floats in the case member 15 via the O-ring, only the gap between the two changes, and no force is applied to the measuring tube 1.

If the grounding rings 5a and 5b in this figure are unnecessary, the gaskets 10a and 10b may be removed and the case side surfaces 16a and 16b may be brought into direct contact with the piping flanges 11a and 11b.

On the other hand, when the fluid to be measured is corrosive, according to this embodiment, the fluid to be measured can be sealed with the O-ring, so that no special corrosion resistance is required for the material of the case member and stainless steel is used. It can be formed of a general-purpose material such as steel. Further, according to this structure, the grounding ring may be a simple flat plate, and if the grounding ring is formed of a foil of a corrosion-resistant material of noble metal such as platinum, it is possible to easily provide an electromagnetic flowmeter having corrosion resistance.

Further, at this time, the outer O-rings 14a, 1
If 4d is attached easily for replacement, an O-ring can be selected and specified depending on the corrosiveness of the fluid to be measured.

[0031]

According to the present invention, an excessive force is applied to the measuring pipe even when an excessive tightening force is applied when the process pipe is attached or when a high or low temperature measuring liquid flows. There is an effect that it is possible to provide a highly reliable electromagnetic flowmeter that can prevent the fluid and can easily cope with a corrosive fluid to be measured.

[Brief description of drawings]

FIG. 1 is a side sectional view of an electromagnetic flow meter of the present invention.

FIG. 2 is a side sectional view of a conventional electromagnetic flow meter.

FIG. 3 is an explanatory diagram showing a stress distribution of a ceramic measuring tube of a conventional electromagnetic flowmeter.

FIG. 4 is an explanatory view showing a broken state of a ceramic measuring tube of a conventional electromagnetic flowmeter.

[Explanation of symbols]

1 ... Measuring tube, 5a, 5b ... Grounding ring, 14a, 14
b, 14c, 14d ... O-ring, 15 ... Casing member.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sozo Fujimoto 832 Nagakubo, Horiguchi, Katsuta City, Ibaraki Prefecture

Claims (7)

[Claims] Claim: What is claimed is: 1. An electromagnetic field comprising a ceramic pipe, which is mounted between a pair of flanges provided in the middle of a pipe and held in a case member of a flowmeter for measuring a flow rate, as a measuring pipe for guiding a fluid to be measured. In a flowmeter, an electromagnetic flowmeter characterized in that an outer peripheral portion near both ends of the measuring pipe and the case member are sealed with a seal member so that axial pressure cannot be applied to the end face of the measuring pipe. 2. The electromagnetic flowmeter according to claim 1, wherein the seal member comprises a pair or a plurality of pairs of O-rings. 3. The electromagnetic flowmeter according to claim 1, wherein the outermost surface of the seal member travels outside the both end surfaces of the case member and the measuring pipe. 4. The electromagnetic flowmeter according to claim 2, wherein at least a pair of outer sides of the plurality of O-rings have a structure that can be easily replaced. 5. The electromagnetic flowmeter according to claim 1, wherein the measuring pipe is a pipe having a substantially uniform and thin wall without thick flanges at both ends. 6. The electromagnetic flowmeter according to claim 1, wherein the case member is slightly longer in the axial direction than the measuring tube. 7. A ceramic pipe clamped between a pair of flanges provided in the middle of a pipe with a plurality of bolts and held in a case member of a flow meter for measuring the flow rate of a fluid flowing in the pipe. In an electromagnetic flowmeter that uses as a measurement pipe for guiding the fluid to be measured, a seal between the pipe and the flowmeter is directly between the flange and the case member, or by a pair or a plurality of pairs of gaskets sandwiched between them. An electromagnetic flowmeter characterized in that the tightening force of the bolt is not applied to the end face of the measuring pipe.