Measuring and testing – Volume or rate of flow – Using rotating member with particular electrical output or...
Reexamination Certificate
1998-08-31
2001-06-26
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Using rotating member with particular electrical output or...
Reexamination Certificate
active
06250167
ABSTRACT:
BACKGROUND
The present invention relates to fluid flow meters for measuring the rate of fluid flowing through a pipe or conduit, and more particularly, to gas utility meters utilizing a vaned rotor and a non-contacting sensor.
Flow meters of a type commonly used as commercial utility meters include a housing enclosing a vaned rotor mounted on an axle connected to a tachometer. Fluid flow through the meter causes the rotor to rotate at a speed proportional to the flow rate. The tachometer generates a signal proportional to the rotor speed to indicate volumetric flow rate of the fluid through the meter.
A disadvantage with such meters is that a certain amount of leakage occurs from the bore through which the axle extends from the meter body. While this leakage is minimized by packing the axle to seal the bore, this does not solve the problem completely. Furthermore, such packing adds to the cost of the meter, has a limited lifespan, and when it fails can contaminate the liquid or gas flowing through the meter. In addition, the packing may be less effective at relatively high fluid pressures.
In response to such problems, fluid flow meters have been developed which do not require a mechanical connection between the rotor and the tachometer and thus can be completely sealed within the housing. This can be accomplished either by fabricating the rotor blades from a magnetic or conductive material or by mounting a magnetic or conductive element on one or more of the blades of the rotor. A sensor is positioned outside the meter body which generates a signal or pulse in response to the vanes of the rotor passing through a magnetic or electric field produced by the sensor, and a corresponding fluid flow rate is determined from the rate of pulses generated. Typical systems of this type use vane-mounted magnets or aluminum vanes and a sensor which detects a change in magnetic flux caused by a passing vane. Other systems have been developed which use radio frequency sensors. Irrespective of the mechanism employed, pulses received from the sensor are processed to determine the volumetric flow rate. Specifically, the pulses are counted to determine the total volume that has passed through the meter.
While such non-contact systems for measuring fluid flow rate solved many problems encountered with the earlier, mechanically connected flow meters, these systems have disadvantages. Since the accuracy of such magnetic or radio frequency based flow meters depends upon the ability of the sensor to detect a passing rotor through a magnetic or RF field, the body of the flow meter is made of a material which does not alter or distort the field. Furthermore, these flow meters are often subjected to highly corrosive environments and must withstand fluid pressures of up to 1440 psig. In most cases, these requirements have been met by fabricating the meter bodies entirely from non-magnetic stainless steel or other non-magnetic material. However, the use of stainless steel or other materials for the entire meter body increase the material cost, as well as the cost of fabrication. Also, the use of some alternative non-magnetic materials react with the fluid that is being measured, and therefore have an undesirably short useful life.
Attempts have been made to avoid the use of non-magnetic bodies by using a sensor encased in a non-magnetic well. This well is installed in an opening through the body which can be made of less expensive magnetic material. A seal must be provided between the body and sensor. A problem encountered with such prior flow meters is that, should a sensor need to be removed from the housing for repair or replacement, it is necessary to depressurize the entire line where the meter is located, since the sensor module itself forms part of the meter body and its removal creates an opening. In most cases, this requires down time.
Accordingly, there is a need for a flow meter of the non-contacting type which can be made of a conventional, relatively inexpensive and robust material, but will not interfere with the magnetic field detected by the sensor, is corrosion resistant, can withstand high pressures, allows removal of the sensor without requiring depressurization of the fluid line, and can safely be removed without endangering service personnel.
SUMMARY OF THE INVENTION
The present invention is a sensor assembly for a sealed, rotor-type flow meter which is fabricated from a material which does not interfere with the signals produced by the rotor. While the preferred embodiments of the fluid flow meters of the present invention herein described are designed for a gas turbine flow meter, the usefulness of the invention in flow meters designed for water or other fluids or with rotors other than a turbine impeller will be understood to those skilled in the art.
The sensor assembly includes a sensor well received within the bore of a flow meter body boss, and a sensor fitted within a bore in the well. The well is generally cylindrical in shape and is shaped to extend through the cylindrical base, meter body and internal rotor housing. The meter housing is made of a ferromagnetic material. The well includes a face in close proximity to the rotor which is sufficiently thin to enable the sensor to detect the passing rotor vanes, but is strong enough to withstand internal gas pressures of up to 1440 psig. In a preferred embodiment, the face is shaped to fit the contour of the inner face of the internal rotor housing, thereby creating a smooth path for rotation of the rotor and enabling the sensor to be located as close as possible to the path of the rotor blades, thereby increasing the strength of the signal detected by the sensor. A sealing device, such as an O-ring, preferably is provided between the sensor well and the body boss to ensure a gas-tight seal.
The sensor well of the present invention can be attached to the meter body boss in a number of ways. In one embodiment, the sensor well is permanently attached to the body boss by a weld, braze or epoxy. Such a permanent attachment eliminates the need for a sealing O-ring. However, in some instances removal of the entire sensor well will be required. In these cases, even if the meter can be effectively bypassed without a shut down, it could be difficult to ensure that the line containing the meter has been completely depressurized and purged. Thus, the person performing such maintenance can be exposed to a dangerous release of pressurized gas when the meter body is disassembled.
This problem is overcome in another embodiment of the invention in which the sensor well is removably attached to the body boss by bolts. The bolts pass through bores in the sensor well and thread into the body boss. In a preferred embodiment, the bolts are countersunk into the well body and are retained therein during unthreading by a dowel pin. This requires all of the bolts to be unthreaded in unison by alternately and incrementally turning each bolt. In so doing, the O-ring seal will be broken prior to any of the bolts being removed. If present, gas sensor will begin escaping past the seal. The noise will warn the operator to stop removal of the well and to depressurize the line by accepted means, thereby preventing a blowout of the well by the internal sensor in the meter housing. Alternately, the retaining elements such as a retaining ring, or a swedged or staked-over lip may be used instead of dowel pins. A retaining ring or stop positioned on the lower part of the fastener near the body is also effective.
In an alternate embodiment, the sensor well is threaded into the body boss such that the partial removal of the well from a meter on a pressurized line causes an audible warning sound or visual spray, warning the operator of sensor in the line. This warning signal is caused by the seal moving away from its seat before the threads disengage completely from the boss.
The sensor can be retained in the sensor well in a number of suitable ways, so long as the sensor can easily be disconnected from the sensor well for repair or replacement. In a preferred embodim
Corpron Gary P.
Ometz Lewis C.
Peace Daniel W.
Fuller Benjamin R.
M & FC Holding Company
Patel Jag-Dish
Thompson Hine LLP
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