Low thermal stress case connect link for a straight tube...

Measuring and testing – Volume or rate of flow – Mass flow by imparting angular or transverse momentum to the...

Reexamination Certificate

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Reexamination Certificate

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06397684

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a straight tube Coriolis flowmeter and in particular to a case connect link that provides decreased thermal stress for a straight tube Coriolis flowmeter.
PROBLEM
It is known that the elements internal to a straight tube Coriolis flowmeter are subject to stress when the flowmeter is subject to conditions in which temperature differentials are developed within a part or between various parts of the flowmeter. These temperature differentials can cause a part to expand/contract and either damage itself or other parts to which the expanding/contracting part is coupled. For example, the expansion/contraction of the flow tube by an amount different than the expansion/contraction of the flowmeter elements to which the flow tube is connected can stress the flow tube beyond the limits of its permissible deformation. Thus, if the flow tube expands an excessive amount with respect to the case and its end flanges, the flow tube may buckle. Conversely, if the flow tube contracts an excessive amount with respect to the case end flanges, it may develop cracks or tears or it may yield and be permanently deformed.
Attempts have been made to minimize problems associated with thermal contractions/expansion in flow tubes of Coriolis flowmeters. One solution utilizes materials having similar coefficients of expansion so that the expansion/contraction among all elements is uniform. Another solution uses a prestressed flow tube so it can change in length a modest amount without excessive internal stresses. Another solution uses bellows near the flow tube ends so the flow tube can change in length without undo stress. Another solution provides bends in the flow tube so that length changes are absorbed by the flow tube segment containing the bend. Another solution slidably mounts the ends of the flow tube to the case ends. These solutions reduce the problems associated with thermally induced length changes of the flow tube with respect to the elements of the flowmeter to which the flow tube is connected. However, these solutions have not solved the problems of thermally induced diameter changes in the flow tube. These diameter changes cause stresses in other Coriolis flowmeter elements including those that couple the flowmeter case to the vibratory elements of the flowmeter including the flow tube and its surrounding balance bar.
It is known to use case connect links to couple the vibratory end nodes of a flow tube to the flow tube case. This is done to prevent the end nodes from vibrating excessively during conditions of unbalance. Excessive vibration of the end nodes is undesirable because it causes changes in the flow sensitivity of the meter. Undesirable vibratory deflections at the brace bar (wherein the end nodes reside) is prevented by coupling the brace bar to a connecting link oriented transversely to both the vibratory direction of the flow tube and the axial direction of the flow tube. One end of the case connect link is connected to the case inner wall; the other end of the case connect link is connected to the brace bar or the end of the balance bar. The prior art case connect link is a relatively thin planar leaf spring member which is flexible in a twist mode and does not inhibit the rotation of the flow tube and balance bar at the desired vibration nodes. However, the case connect link prevents the end nodes from translating in the drive direction under conditions of unbalance. Each link prevents its respective end node from translating by rigidly coupling the end node to the relatively large mass of the case. The case connect link thereby improves the accuracy of the meter in measuring materials of higher or lower density than the nominal. A flowmeter having a case connect link of this type is shown in patent EP-0759542A1, published Feb. 26, 1997.
In Coriolis flowmeters of the straight tube type that employ case connect links, the flow tube extends for the entirety of the length of the flow meter with the flow tube ends being connected to case end flanges. The balance bar is typically parallel to the flow tube and may either be a separate parallel member or a cylindrical member surrounding the flow tube. The balance bar is shorter in length than the flow tube so that each end of the balance bar is connected by a brace bar to a near end portion of the flow tube. The brace bar typically is a relatively short circular member that extends transversely from the balance bar end to the near end location of the flow tube.
The vibratory node of the flow tube/brace bar combination normally resides in the brace bar during conditions of vibratory balance. Without case connect links, when materials of a relatively higher or relatively lower than nominal density flow in the flow tube, the vibratory node may move from the brace bar axially inward towards the flow tube center for heavy material or axially outward towards the end flanges for relatively light materials. These conditions of vibratory unbalance cause the brace bar to vibrate as part of the vibratory system and, in so doing, to shorten or lengthen the vibrating length of the flow tube to which it is connected. This change in length of the active portion of the flow tube is undesirable since it produces undesired changes in the flow sensitivity of the flow meter by altering the distance between the nodes and the pickoffs.
The use of case connect links positioned transversely with respect to the drive direction of the flow tube and the tube axis forces the vibration nodes of the flow tube/brace bar/balance bar combination to remain in the brace bar. The use of case connect links achieves the goal of minimizing undesired vibrations of the brace bar regions within the flow meter during conditions of vibratory unbalance associated with the processing of heavier or exceedingly light materials.
Although the use of case connect links minimizes undesired vibrations in straight tube Coriolis flowmeters, the case connect links are subject to structural damage when the temperature of the material in the flow tube differs from the case temperature. Under such conditions, the brace bar end of the case connect links may move in the radial direction a different distance than the case end due to different amounts of thermal expansion/contraction of the flow tube diameter. This results in the case connect links being subject to compressive or tensile loads that may stress and damage them or the elements to which they are connected.
It can therefore be seen that it is a problem to minimize structural damages to these case connect links resulting from differing amounts of thermal contractions/expansions of the case and the brace bar region.
It can be seen from the above that an aspect of the invention is a Coriolis flowmeter comprising:
a flow tube;
a balance bar oriented substantially parallel to the longitudinal axis of said flow tube;
first and second ends of said balance bar coupled to opposing near end portions of said flow tube;
a case containing said balance bar and said flow tube;
case connect link means coupling said first and second ends of said balance bar to an inner wall of said case;
at least one out of plane bend in said elongated case connect link means for enabling changes in the effective dimension of said case connect link means in response to thermal differences between said flow tube and said case.
Another aspect is that said flow tube is substantially straight.
Another aspect is that said balance bar is substantially cylindrical and surrounds said flow tube.
Another aspect is that said case is cylindrical and oriented substantially parallel to a longitudinal axis of said flow tube.
Another aspect is that said case connect link means is elongated and substantially flat and has a longitudinal axis oriented substantially perpendicular to the longitudinal axis of said flow tube and said balance bar;
first and second ends of said elongated case connect link means couple said first and second ends of said balance bar to an inner wall of said case; and
at least one out of plane bend in said elo

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