Coriolis flowmeter having an explosion proof housing

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

Reexamination Certificate

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C439S709000

Reexamination Certificate

active

06286373

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a Coriolis flowmeter. More particularly, this invention relates to an intrinsically safe Coriolis flowmeter. Still more particularly, the present invention relates to using a secondary containment housing to create a Coriolis flowmeter that meets intrinsic safety requirements.
PROBLEM
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters have one or more flow tubes of a curved configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver applies a vibrational force to the flow tube. The force causes the flow tube to oscillate. When there is no material flowing through the flowmeter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at two different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
It is a problem to create an explosion proof Coriolis flowmeter for use in an explosive environment. In particular, it is a problem to create an explosion proof Coriolis flowmeter for large Coriolis flowmeters. For purposes of the present discussion, large Coriolis flowmeters have flow tubes of greater that a one inch diameter and operate at a resonant frequency of greater than one hundred hertz. Also for purposes of the present discussion, an explosive environment is an environment that includes a volatile material which can be ignited if a spark or excessive heat is introduced into the environment. Furthermore, an explosion proof device, such as a Coriolis flowmeter, is a device that is designed to ensure that a spark or excessive heat from the device does not ignite the volatile material in the environment.
In order to provide an explosion proof device, such as a Coriolis flowmeter, methods including encapsulation, pressurization, and flameproof containment may be used. Each of the above methods encloses a device to prevent the volatile material from contacting the device where heated surfaces of the device or sparks from circuitry in the device may cause an ignition of the material. If a material ignites inside an enclosure, any gaps or openings in the enclosure must provide a flame path of a sufficient length to cool the material as the material escapes from the enclosure. The cooling of the hot material prevents the hot material from igniting the volatile material outside the enclosure.
A second solution is to make a device intrinsically safe. An intrinsically safe device is a device in which all the circuitry in the device operates under a certain low energy level. By operating under a certain energy level, the device is ensured not to generate a spark or sufficient heat to cause an explosion even if the device fails in some manner. The power level needed to make a device intrinsically safe are determined by regulatory agencies such as UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan. However, the power requirements for vibrating flow tubes in a large Coriolis flowmeter make it very difficult to design a Coriolis flowmeter that is intrinsically safe.
One manner in which flowmeters have been made explosion proof is to enclose the electronic drive system components mounted on the flow tubes that operate above the intrinsically safe power levels. A conventional drive system has a coil and a magnet which are mounted on flow tubes opposing one another. An alternating current is then applied to the coil which causes the magnet and coil to move in opposition to one another. The current applied to the coil is above the power levels required for the drive system to be intrinsically safe. Therefore, it is possible that the current through the coil has enough power to create a spark or sufficient heat to ignite volatile material.
In order to make the drive coil explosion proof, a sleeve is placed around the coil. The sleeve is an enclosure surrounding the coil of wire and can contain an explosion ignited by a spark or heat from the coil. Any gap in the sleeve is designed to have a flame path of sufficient length to cool any material that is ignited inside the enclosure. This prevents any material ignited inside the enclosure from igniting material outside the enclosure.
In order for the sleeve and coil to be able to withstand the pressure created by an explosion, both the sleeve and the coil must be made of metal. This is a problem because metals cause eddy currents when the magnetic field is subjected to the metal. The eddy currents are caused by the alternating of the magnetic fields through the conductive medium of the metallic sleeve and coil bobbin. These eddy current cause a reduction in the available power to drive the flow tubes. The power losses due to the eddy currents maybe so great that it is impossible to create a driver that has sufficient power to drive flow tubes of a certain mass, stiffness, or frequency. Furthermore, the cost of the components for the driver increases as more expensive metal components are used.
Additionally, the conductors that connect the driver and sensors to the flowmeter electronics must also be insulated to prevent a spark from a conductor due to a break in the conductor from causing an explosion in order for a flowmeter to be explosion proof. One manner of insulating the conductors is to place a conduit of potted material on to the flow tubes. The conductors are enclosed inside the potted conduit. However, this potted conduit on the flow tubes can cause a zero stability problem in the flowmeter. Furthermore, the potted conduit is expensive and time consuming to manufacture.
For the above reasons, there is a need in the Coriolis flowmeter art for a better manner in which to make a Coriolis flowmeter that can operate in an explosive environment while operating at power levels above intrinsically safe limits.
SOLUTION
The above and other problems are solved and an advance in the art is made by the provision of a secondary containment housing for a Coriolis flowmeter sensor that is also an explosion proof container. A secondary containment housing encloses the flow tubes of the flowmeter as well as the driver, sensors, and conductors affixed to the flow tubes. A secondary explosion proof housing is a secondary containment housing that is made of a material that is able to withstand the pressure generated by an explosion caused by an ignition of volatile material inside the housing. Any gaps or openings in an explosion proof housing provide a flame path having sufficient length to cool any flames or heated material that may escape from the housing. The use of a secondary containment housing as an explosion proof enclosure, allows the removal of enclosures around the coil in the drive system. Thus,

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