Low thermal stress balance bar for a coriolis flowmeter

Details Low thermal stress balance bar for a coriolis flowmeter Low thermal stress balance bar for a coriolis flowmeter

2000-05-02

2002-12-03

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

C073S861355

Reexamination Certificate

active

06487917

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a Coriolis flowmeter having a balance bar that can be subjected to a wide range of thermal conditions without applying stresses to the flow tube to which the balance bar is coupled.
PROBLEM
Single straight tube Coriolis flowmeters traditionally have a concentric balance bar that is coaxial with the flow tube. The balance bar vibrates 180 degrees out of phase with respect to the flow tube to counterbalance the drive mode vibration of the flow tube. The balance bar and the material filled flow tube comprise a dynamically balanced structure that vibrates at its resonant frequency. The ends of the balance bar are rigidly affixed to the flow tube via annular brace bars. Regions of no vibration, called nodes, are located in the brace bars and define the ends of the active portion of the flow tube.
The radial distance between the outer surface of the flow tube and the inner surface of the balance bar is traditionally kept small both for reasons of compactness and for tuning the resonant frequency of the balance bar. The small difference in diameter between the flow tube and the balance bar results in a connection that is very rigid.
A problem with prior art designs of balance bars is that they impose a significant thermal stress on the flow tube. There are three distinct types of thermal stress of a Coriolis flowmeter. The first is thermal shock. If a Coriolis flowmeter in a cold climate suddenly receives a hot material, the hot flow tube attempts to expand, but is restrained by the surrounding cold balance bar and flowmeter case. Prior art designs use a titanium flow tube having a low modulus of elasticity. The low thermal expansion rate and the high yield strength of titanium enable the flow tube to bear the high stress of thermal shock without damage.
The second type of thermal stress is that due to an elevated or lowered uniform temperature of the Coriolis flowmeter. This thermal stress is common in chemical or food plants where Coriolis flowmeter cases are insulated or heated so as to maintain the entire meter at the material temperature. If the entire Coriolis flowmeter were titanium, a uniform meter temperature would not result in any thermal stresses, but titanium is too expensive to use for the entire meter. Most prior art Coriolis flowmeters have a titanium flow tube because of its low expansion and low modulus of elasticity. For cost reasons they have a stainless steel balance bar and case even though titanium would be the preferred material. Thermal stress is produced in these Coriolis flowmeters at elevated uniform temperatures because these different materials have different moduli of expansion. A Coriolis flowmeter that is stress free at 70 degrees has significant stresses at a uniform 200 degrees because the stainless steel balance bar and case expand at more than twice the rate of the titanium flow tube.
In the third type of thermal loading, stress is imposed on the flow tube by a steady state thermal condition in which the material and the environment have different temperatures. A Coriolis flowmeter measuring hot material in a cold climate eventually reaches a state of thermal equilibrium in which the titanium flow tube reaches the material temperature while the balance bar is only slightly cooler. The case, however, can be much cooler depending on the ambient conditions. If the case is exposed to a cold wind, for example, the case temperature may be only a few degrees above the ambient temperature. Stresses are generated when the cool case restrains attempted expansion by the balance bar and the flow tube. Stresses are also generated when the stainless steel balance bar attempts to expand at twice the rate of the titanium flow tube.
Commercially available single straight tube flow Coriolis flowmeters must be able to withstand all three types of thermal loading without suffering permanent damage and ideally without excessive error in the material measurement. The balance bar ends are rigidly affixed to the flow tube via brace bars. This effectively divides the flow tube into three portions. The central portion, between the brace bars and within the balance bar is the active portion of the flow tube. This portion vibrates out of phase with respect to the balance bar. The two portions of the flow tube that extend from the ends of the balance bar to the case ends do not vibrate and are the inactive portions of the flow tube.
When the above described prior art Coriolis flowmeter is exposed to the first type of thermal loading, thermal shock, both the active and inactive portions of the flow tube experience the same thermal stress. This is due to the fact that neither the balance bar, which constrains the active portion of the flow tube, nor the case, which constrains the inactive portions of the flow tube change temperature or length and the three portions of the flow tube quickly attain the same elevated temperature as the material and have the same thermal stress. When the prior art Coriolis flowmeter is exposed to the second type of thermal loading in having a uniform elevated temperature, the three portions of the flow tube once again experience the same thermal stress. The balance bar and case are both stainless steel and expand at the same rate. The titanium flow tube, attempts to expand at a different rate but is restrained by the balance bar and case.
Under the third thermal condition of thermal loading, the flow tube and the balance bar nearly attain the material temperature while the case remains cold. The hot balance bar expands its length while the cold case does not. The inactive flow tube portions are between the case ends and the lengthening balance bar. The balance bar and case both have much larger cross section areas than the flow tube and force the inactive portions of the flow tube to decrease in length. Since the inactive flow tube portions are hot and if unconstrained would be increasing in length, the forced decrease in length results in stress that can even exceed the yield strength of the titanium flow tube. Meanwhile, the active portion of the flow tube is constrained at its ends by the connections to the hot stainless steel balance bar. Stainless steel has a much greater expansion coefficient than the titanium of the flow tube. Depending on the temperature differential between the balance bar and the flow tube, the active portion of the flow tube could be put in tension since the balance bar temperature is nearly equal to the flow tube temperature. It could also be put in compression as when the balance bar temperature is lower than the flow tube temperature.
The situation in which the inactive portion of the flow tube is highly stressed by temperature gradients is a problem with prior art flow Coriolis flowmeters. The problem is generally solved in prior art Coriolis flowmeters by limiting the temperature range over which the Coriolis flowmeters may be operated. This is undesirable since many customers would like to measure material flow rate at temperatures that exceed the limits dictated by thermal stress.
SOLUTION
The present invention overcomes the above and other problems by use of a balance bar that allows the stresses in the active and inactive portions of the flow tube to be as low as possible for any thermal condition. The balance bar has a middle segment that is compliant in the axial direction so that changes in length of the balance bar ends do not impose a significant axial force on the flow tube. This ensures that the thermal stresses on the active and inactive portions of the flow tube are always equal. This state of stress equality is the lowest possible stress state for the flow tube. As a result of the axially compliant balance bar, the remaining stress in the flow tube is only a function of the differential expansion between the flow tube and the case. Balance bar expansion and contraction is eliminated and has no impact on the flow tube stress.
A further advantage of the balance bar of the present invention is cost. Most prior art Coriolis flowmeters require a stainless st

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