Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Patent
1990-09-14
1992-05-19
Eisenzopf, Reinhard J.
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
296021, G01F 158
Patent
active
051136902
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flowmeter having a pressure resistant measurement tube made of a heat-resistant material and surrounded by a stainless steel casing. A magnetic device comprising at least pole pieces and windings is disposed in a sealed spaced between these two parts and one or more measuring electrodes are arranged in the measurement tube. Electromagnetic flowmeters of this type are known from German patent DE 34 23 921 A1.
Flowmeters of the aforementioned type are used to measure the rate of fluid flow and are utilized in a variety of fields including, for example, the food industry.
It is essential that the measurement tube is compression resistant, non-corrosive, structurally rigid and also a good electrical insulator. In particular ceramic materials, e.g. Al.sub.2 O.sub.3, exhibit these characteristics, though other materials, such as heat resistant plastic, are also suitable. Generally, the casing is made of stainless steel, usually stainless austentite steel (1.4301) which has highly non-corrosive properties and is readily deformable by deep-drawing. The latter property is important for the manufacture of flowmeters with steel casings because it results in low material costs. For large production runs casings of this type using austentite steel can also be cast.
A drawback of the stainless steel, especially austentite steel, is that it has a significantly higher coefficient of thermal expansion than materials such as ceramic or plastic, of which the measurement tube is made. Moreover it is very difficult to attach a stainless steel casing to a ceramic or plastic tube.
Especially when subjected to a variety of uses, when fluids must be measured over a wide range of temperatures, it is important to provide a cost effective and reliable seal between the measurement tube and the stainless steel casing. Moreover, the seal must be non-corrosive and the casing must be of the type that can be attached to the measurement tube in a cost-effective manner.
The seal of the flowmeter disclosed in German patent publication DE 34 239 21 A1 employs an expensive O-ring and the attachment occurs when the space surrounding the measurement tube is sealed. An O-ring seal has the further disadvantage that its non-corrosive properties are determined by the material of the O-ring.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to eliminate the foregoing drawbacks by providing an effective, simple and cost-effective seal between the materials of the measurement tube and the stainless steel casing. The seal should be as durable as the stainless steel casing, and it should function over a wide temperature range and up to the relatively higher operating temperatures.
The objective is achieved with an electromagnetic flowmeter, in which at least one end of the stainless steel casing is bent to form a concentric inner ring. A steel compensation ring surrounds and is affixed to the inner ring; the steel of the compensation ring has a high coefficient of thermal expansion which falls between those for the materials of the measurement tube and the stainless steel casing.
For a flowmeter with a stainless austentite steel casing the compensation ring is preferably made of a stainless ferritic steel.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention is described in greater detail with reference to the drawings, in which
FIG. 1 is a longitudinal cross-section of a flowmeter in accordance with the present invention; and
FIGS. 2a and 2b show a section of the stainless steel casing with an inner ring and a compensation ring disposed therein, before and after being attached to the measurement tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a preferred embodiment the inventive flowmeter, shown in FIG. 1 in longitudinal cross-section, includes a measurement tube 1, the ends of which define two slightly radially outwardly flared flanges 2, 3. Both ends of measurement tube 1 are connected to a conduit through which the flui
REFERENCES:
patent: 2650683 (1953-09-01), McPhee et al.
patent: 2879584 (1959-03-01), Skobel
patent: 3055465 (1962-09-01), Pulfrich
patent: 3610040 (1971-10-01), Wada
Van Bekkum Aart J.
van Nistelrooij Petrus N. J.
Eisenzopf Reinhard J.
Krohne AG
Shopbell E.
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