Patent Description:
A process variable transmitter generally includes or is coupled to a transducer or sensor that responds to a process variable. A process variable generally refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH and other properties. Pressure is considered to be a basic process variable that can be used to measure flow, level and even temperature.

In order to measure a fluid flow, it is often necessary to determine a number of process variables, such as process fluid temperature, process fluid static or line pressure, and process fluid differential pressure across a partial obstruction, such as an orifice plate or the like. In such instances, multivariable transmitters are commonly used to measure and monitor a plurality of process variables in order to provide calculated parameters, such a process fluid flow. Such calculated parameters are useful relative to various industrial process fluids, such as slurries, liquids, vapors and gases of chemical, pulp, petroleum, gas, pharmaceuticals, food and other fluid-type processing plants.

Multivariable process fluid transmitters generally include a differential pressure sensor as well as a line pressure sensor and/or a process fluid temperature sensor. The differential pressure sensor responds to a difference in pressure between two process fluid inputs. The line pressure sensor responds to the absolute or gauge pressure in one of the fluid inputs. The process fluid temperature sensor responds to the temperature of the process fluid with an electrical indication, such as a voltage or resistance, that is related to the temperature of the process fluid.

<CIT> describes a co-planar differential pressure sensor module comprising a pair of process connectors coupled and welded to respective pedestals and each to a base. An isolator diaphragm is coupled to each pedestal and conveys a respective process fluid pressure through a fill fluid in respective passageways. In this way, the process fluid pressures are conveyed to the differential pressure sensor without allowing the process fluid to contact the sensor. The module further comprises circuitry coupled to the pressure sensor to measure the differential pressure. The module is further capable to measure high pressures and also in high ambient pressure environments such as subsea. <CIT> describes a process variable transmitter comprising first and second pressure sensors having a vacuum cavity formed in brittle material. The vacuum cavity may include two inductor plates, which couple to electrical connection leads. The outputs of the pressure sensors are related to the deformation of the cavity. The two sensors are coupled to a line pressure and are used to sense differential pressure or to provide redundant differential pressure measurements for use in providing sensor diagnostics. The sensors are wired together by wheatstone bridges to provide error cancellation. <CIT> describes a pressure transducer comprising two pressure sensors, each having first and second pressure receiving surfaces subjected to a pressure of a plenum of a housing of the transducer.

In multivariable process fluid transmitters that include a differential pressure sensor, such transmitters typically include a pair of isolator diaphragms that are positioned in the process fluid inlets and isolate the differential pressure sensor from the harsh process fluids being sensed. Pressure is transferred from the process fluid to the differential pressure sensor through a substantially incompressible fill fluid carried in a passageway extending from each isolator diaphragm to the differential pressure sensor.

High static pressure environments can provide significant challenges for process fluid transmitters. In some cases, the bolted connection between the process fluid flange and the process variable transmitter base typically cannot seal at such high pressures due to stress limitations of the bolts and deformable seals used therebetween. When the seal is deformed or otherwise disrupted, process fluid may leak from the coupling. Currently, multivariable process fluid transmitters are not able to operate in environments rated to high line pressures, such as <NUM>,<NUM> psi (<NUM>,<NUM> MPa). Thus, current multivariable devices are not generally suitable for some process environments such as subsea use. Accordingly, in such environments, when a flow measurement or other similar measurement is desired which requires multiple process variables, multiple process fluid transmitters, such as two and sometimes three process fluid transmitters are required. Providing such transmitters involves considerable expense. Thus, for growing high-pressure markets, such as subsea oil and gas wells, it is desirable to provide a multivariable process fluid transmitter that is suitable for such environments and can provide all requisite process variables using a single device.

A multivariable process fluid transmitter module includes a base having a pair of recesses. A pair of pedestals is provided with each pedestal being disposed in a respective recess and being coupled to a respective isolation diaphragm. At least one line pressure assembly is mounted proximate one of the pedestals. The at least one line pressure assembly couples a respective isolation diaphragm to a line pressure sensor. A differential pressure sensor has a sensing diaphragm fluidically coupled to the isolation diaphragms by a fill fluid. At least one additional sensor is disposed to sense a temperature of a process fluid. Circuitry is coupled to the line pressure sensor, the differential pressure sensor, and the at least one additional sensor to measure an electrical characteristic of each of the line pressure sensor, the differential pressure sensor, and the at least one additional sensor. The circuitry is configured to provide an indication of fluid flow based on the measured electrical characteristic of each of the line pressure sensor, the differential pressure sensor and the at least one additional sensor.

High pressure flow measurements using differential pressure across a primary element currently require at least two and sometimes three process variable transmitters to make the measurements. Lower pressure environments can use a single multivariable process fluid transmitter, such as those sold under the trade designations Model <NUM> or <NUM> SMV, available from Emerson Process Management, of Chanhassen, Minnesota, to measure differential pressure, line pressure and temperature in order to provide a fully compensated flow value. However, such devices are only rated to a MWP of <NUM> psi (<NUM> MPa). When a flow related value is required for a high pressure environment, defined herein as a MWP greater than <NUM> psi (25MPa) and up to and including <NUM>,<NUM> psi (<NUM>,<NUM> MPa) MWP, another approach is required. Given the high pressure of subsea environments, at least some embodiments described herein include devices or portions thereof that are suitable for direct immersion in salt water. As defined herein, "suitable for immersion in salt water" means that the material will not corrode or otherwise be impermissibly degraded in the presence of salt water for a viable product lifetime. Examples of materials that are suitable for immersion in salt water include Alloy C276 available from Haynes International Inc. , of Kokomo, Indiana under the trade designation Hastelloy C276; Inconel alloy <NUM>, available from The Special Metal Family of Companies of New Hartford, New York; and Alloy C-<NUM> available from Haynes International. Of particular interest is Alloy C276, which has the following chemical composition (by % weight): Molybdenum <NUM>-<NUM>; Chromium <NUM>-<NUM>; Iron <NUM>-<NUM>; Tungsten <NUM>-<NUM>; Cobalt <NUM> maximum; Manganese <NUM> maximum; Vanadium <NUM> maximum; Carbon <NUM> maximum; Phosphorus <NUM> maximum; Sulfur <NUM> maximum; Silicon <NUM> maximum; and balance Nickel.

As illustrated in <FIG>, multivariable sensor module <NUM> includes sidewall <NUM> coupled to base portion <NUM> and to cap <NUM>. An electrical feedthrough connector <NUM> is coupleable to electronics enclosure <NUM> and includes conductors to provide power to sensor module <NUM> as well as bidirectional communication. In some embodiments, module <NUM> may communicate over the same conductors through which it is powered.

<FIG> is a diagrammatic view of multivariable sensor module <NUM> (illustrated in <FIG>) adapted for direct immersion in sea water. Specifically, the upper portion of module <NUM>, proximate electrical connection point <NUM>, is covered with a high-pressure bearing end cap <NUM> that is constructed from a material that is suitable for direct immersion in sea water. Moreover, the high pressures associated with exposure to sea water at extreme depths are borne by end cap <NUM> which maintains its shape and integrity while so subjected. Additionally, end cap <NUM> is preferably constructed from the same material as the bottom portion <NUM> of differential pressure sensor module <NUM>. For example, if bottom portion <NUM> of module <NUM> is constructed from Alloy C276, it is preferred that end cap <NUM> also be constructed from Alloy C276. However, in embodiments where they are not constructed from the same materials, end cap <NUM> must be constructed from a material that is suitable for welding to portion <NUM> of module <NUM>. This means that either the metallurgy of the two materials must be compatible enough for welding and/or the melting points of the two materials must be close enough to each other. An additional requirement for welding different metals is the metallurgy of the resulting weld (which is different than either starting material) must also be corrosion resistant. As can be appreciated from <FIG>, sensor module <NUM> can be adapted for direct immersion in sea water relatively easily by simply welding end cap <NUM> directly to lower portion <NUM> at interface <NUM>. Accessing electrical connection point <NUM> through end cap <NUM> can be performed in any suitable manner. For example, a high-pressure glass header may be used to pass conductors through end cap <NUM> in order to couple to connection point <NUM>.

<FIG> is a diagrammatic cross sectional view of sensor module <NUM> in accordance with embodiment of the present invention. While sensor module <NUM> is illustrated in <FIG> as being a co-planar sensor module, any suitable sensor module can be used in accordance with embodiments of the present invention. Module <NUM> includes a lower portion <NUM> that, in one embodiment, is constructed from a material suitable for immersion in salt water. While a number of materials may be suitable for immersion in salt water, one particularly suitable example is Alloy C276, set forth above. Base portion <NUM> is coupled to sidewall <NUM> and cap <NUM> to define a chamber <NUM> therein. Differential pressure sensor <NUM> is disposed in chamber <NUM> and has a pair of differential pressure sensor inputs <NUM>, <NUM> that convey process pressure to deflectable diaphragm <NUM>, which has an electrical characteristic, such as capacitance, that varies with diaphragm deflection. The electrical characteristic is measured, or otherwise transduced by circuitry <NUM> disposed proximate sensor <NUM>. Circuitry <NUM> also conditions the capacitance measurement for transmission through electrical connection point <NUM>. Circuitry <NUM> preferably includes a microprocessor as well as a process communication module for communicating over a process communication loop or segment. Examples of such communication include the Highway Addressable Remote Transducer (HART®) protocol or the FOUNDATION™ Fieldbus protocol. In some embodiments, module <NUM> may be powered over the same media through which it communicates.

As set forth above, in some embodiments, portions of module <NUM> may be adapted for immersion in salt water. Thus, the components must not only be capable resisting corrosion in such environments, but they must also be able to bear high pressure, such as <NUM> psi (<NUM>,<NUM> MPa). Base portion <NUM>, in some embodiments, is adapted for immersion in salt water. However, in all embodiments, base portion is configured to bear a high line pressure up to and including <NUM>,<NUM> psi (<NUM>,<NUM> MPa). Base portion <NUM> includes a pair of recesses <NUM>, <NUM> each having a respective pedestal <NUM>, <NUM>. An isolator diaphragm <NUM> is coupled to each pedestal <NUM>, <NUM> and conveys a respective process fluid pressure through a fill fluid, such as silicone oil, located in respective passageways <NUM>, <NUM> to a respective input <NUM>, <NUM> of differential pressure sensor <NUM>. In this way, the two process fluid pressures are conveyed to differential pressure sensor <NUM> without allowing the process fluid to contact differential pressure sensor <NUM>.

As illustrated in <FIG>, each process fluid pressure port <NUM>, <NUM> preferably includes a respective integrated process connector <NUM>, <NUM> that is welded to base portion <NUM> in order to provide a corrosion-resistant, high-pressure coupling. Each weld extends about the entire circumference of each connector such that the weld not only robustly mounts the connector to base portion <NUM>, but also seals the connector thereto. Each integrated process connector <NUM>, <NUM> includes a process fluid pressure receiving aperture <NUM> that is suitable for exposure to process fluid at pressures up to <NUM>,<NUM> psi (<NUM>,<NUM> MPa). Additionally, each pedestal <NUM>, <NUM> is also preferably welded to its respective process connector <NUM>, <NUM> before the process connectors <NUM>, <NUM> are welded to portion <NUM>. In this way, the critical process pressure retaining welds are protected inside the module from the corrosive effects of sea water exposure.

In accordance with an embodiment of the present invention, at least one of, and preferably both, pedestals <NUM>, <NUM> includes a line pressure assembly as illustrated at respective reference numerals <NUM>, <NUM>. Line pressure assemblies <NUM>, <NUM> are preferably welded to their respective pedestals <NUM>, <NUM> as indicated at reference numerals <NUM>, <NUM>. Each line pressure assembly <NUM>, <NUM> is fluidically coupled to respective passageways <NUM>, <NUM>. In this way, each line pressure assembly will be coupled to the respective line pressure at its respective process connector <NUM>, <NUM>. At least one line pressure assembly is coupled to a line pressure sensor, indicated diagrammatically in phantom at reference numeral <NUM>. The line pressure sensor may be any suitable sensor, such as a commercially available capacitance-based pressure sensor. However, given the high line pressure required for embodiments of the present invention, the line pressure sensor is adapted for high pressure operation. One such adaptation includes the utilization of a thicker deflectable diaphragm in order to adjust the gage factor for operation up to <NUM>,<NUM> psi (<NUM>,<NUM> MPa). The line pressure sensor is electrically coupled to circuitry <NUM> such that the multivariable sensor module can measure an electrical characteristic, such as capacitance, of the line pressure sensor to obtain an indication of line pressure. While only one line pressure sensor is required, it is preferred that the pedestals <NUM>, <NUM> be identical. Moreover, it is preferred that even when a single line pressure sensor is used, that both line pressure assemblies <NUM>, <NUM> be used. This reduces the number of unique components required to manufacture module <NUM>.

In some examples, a temperature sensor, such as sensor <NUM> can be provided and coupled to electronics <NUM> in order to provide an electrical indication related to the temperature of the process fluid. Temperature sensor <NUM> can be any suitable type of temperature sensor, such as a resistance temperature detector (RTD), thermocouple, thermistor or any other suitable device that has an electrical characteristic or value that changes with temperature. Preferably, temperature sensor <NUM> is immersed in the fill fluid in the oil fill system. Due to its immersion in the oil fill system and its proximity to the isolator, temperature sensor <NUM> can be used, along with the differential pressure sensor signal and the line pressure sensor signal to provide a fully compensated flow measurement.

In another example, the temperature sensor may be positioned at any other suitable position within module <NUM> and a second line pressure sensor could be located at the second line pressure assembly. The utilization of a second line pressure sensor provides redundancy such that if one of the line pressure sensor should fail, the second line pressure sensor could be used. Additionally, the two line pressure sensors can also be used to provide a verification of the differential pressure sensor output. Alternately, the two line pressure sensors could be used to provide a redundant differential pressure reading based on the difference between the two line pressure sensor measurements. While such a derived differential pressure sensor reading would be less accurate than a direct reading from the differential pressure sensor, it could still provide useful differential pressure information when the differential pressure sensor has failed or is otherwise unavailable. Such redundancy is particularly advantageous in subsea applications and/or other hostile or challenging environments where immediate access to the module is not a trivial endeavor.

In an embodiment according to the invention, a second line pressure sensor is used and is positioned at the second line pressure assembly. However, instead of being coupled to line pressure, the second line pressure sensor is sealed in a vacuum or near vacuum. Thus, the second line pressure sensor will react to non-pressure-related variables, such as temperature and/or stresses on the sensor module in much the same way as pressure sensor that is coupled to line pressure. As such, when the output of the second sensor is subtracted from the line pressure sensor, the result is a compensated for temperature effects. Thus, in this embodiment, a temperature sensor may not be necessary. Further, the output of the vacuum sealed sensor could be used to provide a direct indication of temperature.

Claim 1:
A multivariable process fluid transmitter module (<NUM>) comprising:
a base (<NUM>) having a pair of recesses (<NUM>, <NUM>),
a pair of pedestals (<NUM>, <NUM>), each pedestal being disposed in a respective recess (<NUM>, <NUM>) and being coupled to a respective isolation diaphragm (<NUM>) and configured to receive a pressure of a process fluid as a line pressure and configured to be bearing high pressure up to at least <NUM> MPa (<NUM>,<NUM> psi);
at least one line pressure assembly (<NUM>, <NUM>) mounted proximate one of the pedestals (<NUM>, <NUM>), the at least one line pressure sensor assembly (<NUM>, <NUM>) coupling a respective isolation diaphragm (<NUM>) to a first line pressure sensor (<NUM>);
a differential pressure sensor (<NUM>) having a sensing diaphragm (<NUM>) fluidically coupled to the isolation diaphragms (<NUM>) by a fill fluid;
at least one additional sensor disposed to sense a variable of the process fluid;
circuitry (<NUM>) coupled to the first line pressure sensor (<NUM>), the differential pressure sensor (<NUM>), and the at least one additional sensor to measure an electrical characteristic of each of the first line pressure sensor (<NUM>), the differential pressure sensor (<NUM>), and the at least one additional sensor; and
wherein the circuitry (<NUM>) is configured to provide an output related to the first line pressure sensor (<NUM>), the differential pressure sensor (<NUM>) and the at least one additional sensor,
wherein the at least one additional sensor comprises a second line pressure sensor positioned at a second line pressure assembly, and
wherein the second line pressure sensor is sealed in a vacuum or near vacuum instead of being coupled to line pressure, so that it reacts to non-pressure-related variables, such as temperature and/or stresses on the sensor module (<NUM>).