The invention relates to a Coriolis flowmeter and further relates to a density meter utilizing the Coriolis flowmeter.
The Coriolis flowmeter is known as a direct mass flowmeter which is designed to work under the known principle that when a conduit through which a fluid to be measured flows is vibrated, a Coriolis force which is proportional to the mass flow rate is generated to give a certain effect to the movement of vibrating fluid. Generally, the Coriolis force is detected in terms of an elastic deformation or distortion of the conduit.
Since the Coriolis force is small as compared with the applied vibration force, it is required to provide a force measurement system to accurately detect the generated Coriolis force with high sensitivity. Therefore, a representative Coriolis flowmeter is designed to have a U-shaped conduit so that an enlarged deformation can be detected. However, the U-shaped conduit has a disadvantageous feature in that the fluid flowing through the U-shaped conduit is apt to undergo a pressure loss. Accordingly, most of the U-shaped conduits have not enough measurement accuracy. Moreover, it is necessary to provide a relatively large space for encasing the U-shaped conduit in a flowmeter.
For the reasons described above, a Coriolis flowmeter employing a straight conduit has been studied.
The straight conduit-Coriolis flowmeters are classified into two types: one is of single conduit type, and another is of plural conduit type. In any types, the conduit (namely, flow tube) is fixed to a supporting frame at each end. Further, a vibration generator for vibrating the conduit is provided to the center portion of the conduit, and a sensor means to detect a small deformation or distortion of the conduit caused by the generated Coriolis force is provided at a position between the vibration generator and the support frame or block.
A structure of a known Coriolis flowmeter of plural conduit type (which is shown in Japanese Patent Provisional Publication No. 3-41319) is illustrated in FIG. 12.
The Coriolis flowmeter of FIG. 12 is composed of a flow tube (i.e., conduit) 3, a counter tube 4b, and a structurally balancing auxiliary tube 4a. The counter tube 4b and auxiliary tube 4a are arranged on each side of the flow tube 3 at parallel with a space. Through the flow tube 3, a fluid to be measured flows. The flowmeter is connected to an outer flow system by means of a flange 1 provided on each side. The flow tube 3, the counter tube 4b, and the auxiliary tube 4a are fixed to a vibration control frame 9 at both ends. The flow tube 3 and the counter tube 4b are designed to have almost same resonance frequency. At the center positions of the flow tube 3 and the counter tube 4b, a vibration generator 5 is provided to give a primary flexural or bending vibration to both tubes. A pair of sensors 6a, 6b are arranged symmetrically on both sides of the vibration generator 5 along the flow tube 3. The sensors 6a, 6b have a function to detect the deformation of the flow tube 3 which is caused by the Coriolis force.
The Coriolis flowmeter having a structure such as that illustrated in FIG. 12 is vibrated by the vibration generator in the primary flexural vibration in which the nodes are placed on the support blocks at both ends of the flow tube. The Coriolis force Fc is expressed as follows:
Fc=xe2x88x922m[xcfx89]xc3x97[v]
[in which, [xcfx89] is a vector of xcfx89 (frequency) and [v] is a vector of v (flow rate)].
The present inventor has discovered that the conventional Coriolis flowmeter of plural flow tube type cannot show enough sensitivity because vibration loss of the counter tubes occurs and the vibration loss decreases the deformation or distortion of the fluid conduit to be detected to measure the flow rate.
A structure of another known Coriolis flowmeter of plural conduit type (which is shown in Japanese Patent Provisional Publication No. 11-30543) is illustrated in FIG. 13.
The Coriolis flowmeter of FIG. 13 is composed of a flow tube (i.e., conduit) 3, and a pair of counter rods 4b, 4b. The counter rods are arranged on each side of the flow tube 3 at parallel with a space. Through the flow tube 3, a fluid to be measured flows. The flowmeter is connected to an outer flow system by means of a flange 1 provided on each side. The flow tube 3 and the counter rods 4b, 4b are fixed to the flange 1 at both ends. The flow tube 3 and the counter rods 4b, 4b are designed to vibrate in opposite phase by means of vibration generators 5. A pair of sensors 6a, 6b are arranged symmetrically on both sides of the vibration generator 5 along the flow tube 3. The sensors 6a, 6b have a function to detect the deformation of the flow tube 3 which is caused by the Coriolis force.
The present inventor has discovered that the known Coriolis flowmeter having a pair of counter rods cannot show enough sensitivity because vibration loss occurs and the vibration loss decreases the deformation or distortion of the fluid conduit to be detected to measure the flow rate.
The vibration loss analyzed on the Coriolis flowmeter similar to the conventional Coriolis flowmeter of FIG. 13 is illustrated in FIG. 14. The analysis is performed by the known finite-element method. The Coriolis flowmeter of FIG. 13 is modified to have a flange having a length of one-tenth ({fraction (1/10)}) of the length of the center fluid conduit (as well as the length of each counter rod).
According to the finite-element analysis, each of the support blocks shows deformation of approximately 5% of the maximum deformation of the fluid conduit under the primary flexural vibration mode, at the position of two-fifths (⅖) from the outer end of each block in the longitudinal direction of the fluid conduit. The deformation of the support block apparently gives adverse effect to the detection of the secondary vibration occurring on the fluid conduit by the Coriolis force, and therefore the sensitivity of the flowmeter lowers.
It is an object of the present invention to provide a mass flowmeter of straight conduit type utilizing the Coriolis force which is improved in its sensitivity.
It is another object of the invention to provide a density meter utilizing the improved Coriolis flowmeter.
The present invention resides in a Coriolis flowmeter comprising a straight conduit through which a fluid to be measured flows and which has a sensor attached thereto, and two counter straight rods that are aligned on both sides of the conduit in parallel with a space, one end of the conduit and each one end of the counter rods being fixed to a common support block and another end of the conduit and each another end of the counter rods being fixed to another common support block, in which each of the conduit and counter rods has a vibration generator attached thereto for generating vibrations of the conduit and the counter rods in such manner that the conduit and the counter rods vibrate in opposite phase, and both support blocks are fixed onto a rigid substrate.
The Coriolis flowmeter of the invention can also be utilized as a density meter.
The preferred embodiments of the Coriolis flowmeter of the invention are described below.
(1) Each of the counter rods is equivalent to each other.
(2) The counter rod is equivalent to the conduit in a diameter thereof.
(3) Each support block has a length of not less than {fraction (1/10)}, specifically {fraction (3/10)} of the length of the conduit.
(4) Each support block has a length in the range of {fraction (3/10)} to {fraction (10/10)} based on the length of the conduit.
(5) Each support block has a thickness of more than the diameter of the conduit.
(6) Both support blocks have a length equal to each other.
(7) Each support block is fixed onto the substrate via an elastic element.