STRAIGHT TUBE CORIOLIS FLOWMETER
FIELD OF THE INVENTION
This invention relates to a method and apparatus for providing mass flow and density compensation, as well as a 5 density determination in a straight tube Coriolis flowmeter.
All Coriolis flowmeters require compensation to correct signals generated by the Coriolis force induced displacement of the vibrating flow tube. These signals represent the phase difference between the spaced apart flow tube pick offs and are indicative of the material flow through the flowmeter. Curved and straight tube meters both need compensation for the change in elastic modulus of the flow tube with temperature. As the flow tube temperature rises, the modulus decreases and the meter becomes more sensitive. Compensation for the change in the elastic modulus is easily achieved by use of a temperature sensor on the flow tube and the appropriate compensation algorithm in the meter elec- 20 tronics.
Straight tube meters have an additional problem in that the flow tube can be put in tension or compression by unequal amounts of thermal expansion or contraction of the 2J various components of the flowmeter. Tension on the flow tube makes it less sensitive to the Coriolis force while compression makes it more sensitive. Typically thermal stress compensation has been attempted using two temperature sensors; one on the flow tube and one on the case or 3Q balance bar. The problem with the use of two temperatures sensors is that there are at least three major components which can have an impact on the thermal stress of the flow tube. If the second sensor is on the case, then the impact of the balance bar's temperature is not taken into account. 3J Likewise if the second sensor is on the balance bar, then the case temperature is not taken into account.
The use of three independent temperature sensors would be an improvement over two temperature sensors, however, three independent sensors would require three pairs of wires 40 from the sensor to the meter electronics. Extra wires can be expensive if the meter electronics is far from the sensor. Furthermore, compensation algorithms would be required to apply the appropriate weighting factors to the various temperatures, since the case temperature does not have the 45 same effect on the flow sensitivity as the balance bar temperature.
U.S. Pat. No. 4,768,384 to Flecken et al. discloses a straight tube Coriolis flowmeter which provides thermal stress compensation by the use of sensors that measure the 50 flow tube temperature, and the case temperature. A correction circuit receives the pick off signals and generates a corrected output signal that eliminates the affect of stress and temperature on the measurement result. The Flecken et al. flowmeter operates satisfactorily to provide compensation 55 for the change in elastic modulus of the flow tube. The reason is that this compensation requires nothing more than a determination of the flow tube temperature and an appropriate correction based upon known relationships between temperature, elastic modulus, and meter sensitivity. 60
The Flecken flowmeter can also determine the temperature differential between the flow tube and the case and make a stress correction of the pick off signal information. However, an assumption must be made by Flecken about the temperature of the balance bar. In a thermal steady state 65 condition, the flowing material temperature and the ambient temperature are assumed to have been constant for a long
period of time. Under this condition, the balance bar and the flow tube achieve essentially the same temperature as the flowing material temperature. In the thermal transient condition, the flowing material has a sudden change in temperature, such as when the flow is first started. Under this condition, initially, the balance bar and the case are likely to have the same temperature as the environment. The flow tube has the same temperature as the flowing material. In general, flowmeters experience both thermal transient and steady state conditions. The balance bar temperature starts at the ambient temperature and slowly changes to the temperature of the flowing material.
The compensation algorithm of the Flecken flowmeter must make an assumption regarding the balance bar temperature since its two temperature sensors are on the flow tube and the case. It therefore cannot distinguish between steady state and transient conditions of the balance bar temperature. This is a problem since the two conditions produce different stress in the flow tube and different sensitivity of the flowmeter. In the transient condition where the balance bar is initially at the case temperature, both the case and the balance bar apply force to the flow tube. In the steady state condition where the balance bar temperature is nearly equal to the flow tube temperature, the balance bar helps the flow tube resist the force applied by the case. The flow tube therefore experiences a higher stress in the thermal transient condition than in the thermal steady state condition. The best that the compensation of Flecken can do is assume the balance bar temperature to be between the flow tube and case temperatures and suffer inaccuracies at either the transient or steady state extremes.
Another prior art attempt to provide thermal stress compensation for a Coriolis flowmeter is seen in U.S. Pat. No. 5,476,013 to Hussain et al. It provides some thermal stress compensation by using parts that have the same coefficient of expansion. This eliminates thermal stresses when all of the elements are at the same temperature, but it does not address the common situation in which the different components have different temperatures. U.S. Pat. No. 5,381,697 to Van der Pol discloses a Coriolis flowmeter in which thermal stress compensation is provided, in a first embodiment, using two temperature sensors for measuring the temperature of the flow tube. A second embodiment uses a temperature sensor on the flow tube along with a length change sensor on the flow tube. This could, in theory, provide accurate thermal stress compensation. It has a problem, however, in that the means of measuring the length change in the flow tube are not as simple or reliable as temperature sensors.
In addition to the flow measurement, the density measurement of straight tube meters is also degraded by thermal stress. Coriolis flowmeters are known for providing accurate density measurements of the flowing material. Density is determined from the resonant frequency at which the flow tube is vibrated. In curved tube meters, the resonant frequency must be corrected for the change in the tube's elastic modulus with temperature. Also, a correction has to be made for the slight decrease in resonant frequency with mass flow rate as shown in U.S. Pat. No. 5,295,084. Straight tube meters require, in addition, compensation for thermal stress of the flow tube. The flow tube resonant frequency rises as it is tensioned and falls when it is compressed, like a guitar string. If these frequency changes are not compensated, a flow tube in tension will give an erroneously low reading for density and a flow tube in compression will give an erroneously high density reading. The deficiencies of the prior art meters in determining the thermal stress in the flow tube thus lead to inaccuracies in the density measurement.