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DIFFERENTIAL PRESSURE TRANSDUCER
CROSS-REFERENCE TO RELATED 5
This application claims priority from U.S. Provisional Patent Application No. 60/788,398 filed on Mar. 31, 2006, which is hereby incorporated in its entirety by reference. 10
FIELD OF THE INVENTION
The present invention relates in general to differential pressure transducers, and more particularly to a differential pres- 15 sure transducer configuration that includes a displacement sensor.
BACKGROUND OF THE INVENTION
Generally, differential pressure transducers contain a displacement sensor coupled between two thin diaphragms. The two diaphragm arrangement performs a mechanical subtraction of pressures applied to the diaphragms. The sensor measures the net motion of the diaphragms relative to the trans- 25 ducer body to determine the differential pressure. In order to prevent diaphragm rupture while maintaining the desired sensitivity to differential pressure, the volume between the diaphragms, which includes the sensor, is filled with a hydraulic fill fluid. When process line pressure is presented to one side 30 of each diaphragm, the fill fluid is pressurized to the line pressure. If the boundary of the volume between the diaphragms, including the diaphragms themselves, the electrical feed-throughs, and fill/bleed ports, are each not properly sealed, small leaks of fill fluid will occur. This will cause 35 unacceptable increases in response time, sensor output drift, and transducer non-linearity with pressure. In some cases, these changes may not be readily detected when the transducer is in service because the transducer output may remain stable at constant differential pressures. The leaking of fill 40 fluid from these known differential pressure transducers is a problem that is well known and documented.
Another deficiency in fluid-filled differential pressure transducers is a static pressure effect. A differential pressure transducer as described should output a value of zero when 45 the same process pressure is applied to both diaphragms. However, the static pressure causes the fill fluid to be pressurized, which resulting in distortions in the transducer body. These distortions cause relative motion between the diaphragms and transducer body resulting in a static pressure 50 effect. This effect causes values other than zero when both diaphragms experience equal process pressures. The distortions also produce radial forces on the diaphragms, which change the effective stiffness of the diaphragm and causes static pressure effects on span. In addition, the displacement 55 sensor is exposed to the fill-fluid pressure environment adding to the static pressure effects on both zero and span. In applications involving static pressures of several thousand pounds per square inch (psi) or greater, the requirement for a stable zero and span over the allowable range of static pres- 60 sure is difficult to achieve in practice.
Yet another deficiency in fluid-filled differential pressure transducers is the effect of hydrogen. When differential pressure transducers are operated in a hydrogen-rich environment, for example, in a hydrocarbon processing facility, the 65 hydrogen gas easily diffuses through the diaphragms and into the fill fluid. For example, if the differential pressure trans
ducer is used to measure pressure differences in a hydrogenrich high-pressure pipeline, the fill fluid will experience the large static pipeline pressure and hydrogen will diffuse through the diaphragms into the fill fluid. When the pipeline pressure is reduced, such as during scheduled shutdowns, hydrogen gas boils out of the fill fluid and forms bubbles. Since the enclosed volume of fill fluid is constant, bubbles of hydrogen within the closed volume deform the diaphragms, resulting in a calibration shift, zero offset, or in the worst case, diaphragm rupture.
The use of a fill fluid also contributes to degraded performance of differential pressure transducers when operated over a range of temperatures, as is normal in service. The volumetric expansion of liquids with changes in temperature is significantly greater than that of the metals used in construction of the transducer body. Thus, when the environmental temperature of either the differential pressure transducer or process fluids changes, the volume of the fill fluid in the transducer and capillary lines changes. Unless the changes in fill-fluid volume are perfectly balanced on both the high and low-pressure sides of the transducer, the result is significant errors in the output of the transducer. The normal method for limiting this effect is to keep the volume of the fill fluid at an absolute minimum. However, in addition to only limiting the problem and not eliminating it, this method aggravates the effect of fill fluid leakage because any loss of fluid is a more significant part of the total fluid volume.
Rather than perform a mechanical subtraction of two large pressures as described above, an alternative approach is to measure each pressure with separate gage pressure transducers and perform an electronic subtraction to calculate the differential pressure. If the full-scale differential pressure range to be measured is 400 inch H20 (15 psi) and the desired accuracy is 0.1% of the full scale range (i.e., 0.015 psi), then for applications at 3000 pounds per square inch gauge (psig) line pressure, a gage pressure transducer is required that has an accuracy of 0.015/3000=0.0005% (1:200,000). Such devices are not commercially available. Thus, electronic subtraction is not practical and mechanical subtraction of two large pressures is the only practical alternative measurement approach available with present day technology.
Presently, there is no known system or method for providing a differential pressure transducer that avoids the problems associated with the known devices listed above. The present invention as described and claimed herein, addresses the deficiencies of prior art differential pressure transducers
SUMMARY OF THE INVENTION
The present invention pertains to differential pressure transducers. In an embodiment of the invention, a differential pressure transducer includes a transducer housing, a diaphragm, a bellows, and a sensing assembly. The transducer housing, diaphragm, and bellows are coupled to form a pressure chamber. The pressure chamber is separated into two portions by the diaphragm, with each portion arranged to be filled with a process fluid. A pressure differential across the diaphragm causes displacement of the diaphragm. Such displacement is measured by the sensing assembly and used to calculate the pressure differential between the two process fluids.
In another embodiment of the present invention, the sensing assembly includes a Fabry-Perot fiber optic displacement sensor to measure displacement of the diaphragm.
In yet another embodiment of the present invention, the differential pressure transducer includes a stem coupled to the center of the diaphragm. The stem moves through the same
displacement as the center of the diaphragm along an axis passing through the center of the diaphragm. The sensor assembly is positioned to measure movement of the stem.
DESCRIPTION OF THE DRAWINGS 5
Operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:
FIG. 1 is a schematic view illustrating a first known differ- 10 ential pressure transducer;
FIG. 2 is a schematic view illustrating a second known differential pressure transducer;
FIG. 3 A is a schematic view of an exemplary embodiment of a differential pressure transducer in accordance with the 15 present invention;
FIG. 3B is a schematic of the diaphragm shown in FIG. 3 A.
FIG. 3C is a schematic view of another exemplary embodiment of a differential pressure transducer in accordance with the present invention; 20
FIG. 3D is a schematic view of yet another exemplary embodiment of a differential pressure transducer in accordance with the present invention;
FIG. 4 is a model based on the present invention used for finite element stress analysis; 25
FIG. 5 is an expanded schematic view of a fiber optic sensor subassembly used in conjunction with the present invention;
FIGS. 6A through 6C are schematic views of a sensing system with a light signal processor used in conjunction with 30 the present invention;
FIG. 6D is a graph of a signal reading from a photodetector array after electronic filtering; and
FIG. 7 is a graph plotting signal processor output signal versus displacement for a Fabry-Perot fiber optic displace- 35 ment sensor according to the present invention.
While the present invention is described with reference to 40 embodiments described herein, it should be clear that the present invention is not limited to such embodiments. Therefore, the description of the embodiments herein is merely illustrative of the present invention and will not limit the scope of the invention as claimed. 45
FIG. 1 illustrates a first known differential pressure transducer having a differential pressure cell 110 comprising two spaced diaphragms 122 connected to a housing by seals 114. Fill fluid 116 and a sensor 120 are contained between the diaphragms 122 as well as a sensor lead wire 112. Diaphragm 50 stops 124 are employed outside of the diaphragms 122.
FIG. 2 illustrates a second known transducer, which is also a fluid-filled differential pressure transducer. The welded assembly 210 comprises isolation diaphragms 231, a sensing diaphragm 233, fill fluid 216, lead wires, and high-pressure 55 and low-pressure fill tubes 225. The measured change in capacitance between the sensing diaphragm 233 and high and low pressure metallized surfaces 237 is directly proportional to the pressure difference across the transducer 210. The cell further comprises process chambers 229 spaced away from 60 convoluted pressure plates 235. An electrical insulator 221 is also provided as well as ceramic inserts 228.
The fill tube penetrations 225 are the locations most likely to leak fill fluid over time. For example, in a process plant pipeline application the pressure on the fill fluid 216 is nomi- 65 nally 3000 psi at operating pressure. If the fill tube diameters 225 are kept small, the force acting on the fluid 216 to push it
out of the fill tube penetrations 225 will also be small. Nevertheless, a good seal is difficult to maintain. Fill fluid leaks are also possible between the outside diameter of the fill tube 225 and the glass insulating material 221 and along the boundaries between the insulating material 221 and metal housing 211. Thus, it is very difficult to make a totally leaktight seal, since penetrations through the high-pressure boundary must be made.
One approach to solving some of the deficiencies of the prior art is to place a rigid piston between a pair of diaphragms. Such an arrangement eliminates the need for fill fluid. A differential pressure transducer arranged with a rigid piston is described in U.S. patent application Ser. No. 11/105, 670, filed on Apr. 14, 2005, and titled "DIFFERENTIAL PRESSURE TRANSDUCER WITH FABRY-PEROT FIBER OPTIC DISPLACEMENT SENSOR," which is hereby incorporated in its entirety by reference.
The present invention provides an alternative to the use of the fill-fluid differential pressure transducers presented in FIGS. 1 and 2. The present invention utilizes a single diaphragm, which is used to separate a pressure chamber in a transducer housing into two portions. The first portion of the pressure chamber is filled with a first process fluid that exposes a first side of the diaphragm to a first pressure. The second portion of the pressure chamber is filled with a second process fluid that exposes a second side of the diaphragm to a second pressure. When the first and second pressures are unequal, the diaphragm deflects or moves toward the lower pressure process fluid. A sensing assembly positioned to observe the diaphragm detects such movement. The differential pressure across the diaphragm is calculated based on the magnitude of movement of the diaphragm.
In an embodiment, the first and second portions of the pressure chamber are each sealed by a bellows, which is coupled to the transducer housing and the diaphragm. The bellows seals the pressure chamber such that process fluid does not interact with either the sensing assembly or the portion of the diaphragm observed by the sensing assembly. Such an arrangement allows the sensing assembly to be positioned such that it is only exposed to ambient pressures and eliminates the need for a sensor to detect movement through a fill fluid or process fluid.
The sensing assembly may optionally include an optical sensor, such as a Fabry-Perot fiber optic displacement sensor, to detect and measure movement of the diaphragm. Although the present invention as described incorporates an optical sensor, specifically a Fabry-Perot fiber optic sensor, it will be readily understood by those skilled in the art that any sensor, whether optical, mechanical, electrical, or the like, may be used with the present invention, provided it is capable of measuring displacement of a diaphragm.
A stem may optionally be coupled to the diaphragm, along a central axis passing through the diaphragm. The stem is arranged such that deflection of the diaphragm moves the stem proportionally to the diaphragm displacement and linearly along the axis passing through the diaphragm. The stem may extend towards the Fabry-Perot sensor and include a reflective surface to optically interact with the sensor to detect movement of the stem.
In an embodiment, mechanical stops may be used to limit movement of the diaphragm. The diaphragm may be exposed to high pressures such as, for example, 3000 psi. The inclusion of stops may prevent the diaphragm from deforming excessively or rupturing when exposed to such high pressures. In addition, in order to extend the service life and