Patent Application
20210396560
Industrial process control systems are used to monitor and control industrial processes that produce or transfer fluids or the like. In such systems, it is typically important to measure “process variables” such as temperatures, pressures, flow rates, and others. Process variable transmitters are used to measure such process variables and transmit information related to the measured process variable back to a central location such as a central control room.
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.
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. Multivariable process fluid transmitters are used in a variety of flow measurement applications including, without limitation, single phase flowmeters, multiphase flowmeters, and wet-gas flowmeters.
Single phase flowmeters measure any type of single fluid systems, such as oil, water, chemical injection fluids. Typically, these applications only include a differential pressure sensor, but incorporating a multi-variable transmitter would enable fully-compensated flow measurements (flow measurements compensated for density differences driven by static pressure or temperature changes). Multiphase flowmeters measure flow of one or more process fluids with different phases (i.e. saturated steam/water). Wet-gas flowmeters are flowmeters used for measuring natural gas/water combinations. These flowmeters can use different primary elements than a multiphase flowmeter, but still use a differential pressure measurement in combination with a static pressure measurement and a temperature measurement.
High static pressure environments, such as subsea applications, can provide significant challenges for process fluid transmitters. In such environments, when a multiphasic flow measurement or other similar measurement is desired which requires multiple process variables, multiple process fluid transmitters are required. Providing such transmitters involves considerable expense and complexity. For example, subsea multiphase flowmeters typically incorporate many sensor inputs. The three primary sensor inputs for measuring process fluid flow are differential pressure (DP), line pressure (LP), and temperature. These three sensor inputs are typically provided by two different devices; a differential pressure transmitter and a pressure-temperature (PT) transmitter. The differential pressure transmitter is typically coupled to the flowmeter using two flanged connections and the PT transmitter is coupled to the flowmeter using another flange connection. These flanges are large in order to operate under the extreme process pressures and in the crushing pressure of subsea environments. For example, the flanges are so large and heavy that they drive the final size of the multiphase flowmeter.
Since differential pressure and process temperature are critical inputs for flow calculation, the subsea process industry has generally required these to be redundant measurements. This is at least partially due to the cost and complexity of servicing equipment in the subsea environment. Thus, redundant DP measurements are made from two separate transmitters. However, the PT transmitter usually provides redundancy by employing a pair of temperature sensors. Such a device is termed a dual-PT or PTPT device. While embodiments will generally be described with respect to a subsea multiphase flowmeter, it is noted that embodiments are applicable to other types of flowmeters as well.
A multivariable transmitter is provided for measuring multiple process fluid variables. The multivariable transmitter includes a metal housing constructed from a material suitable for exposure to a corrosive material. A differential pressure sensor is disposed within the metal housing. A line pressure sensor is also disposed within the metal housing. Measurement circuitry is operably coupled to the differential pressure sensor and the line pressure sensor to provide differential pressure and line pressure outputs. A temperature probe has a sheath constructed from a material suitable for exposure to the corrosive material. The temperature probe is electrically coupled to circuitry within the metal housing and is physically coupled to the metal housing via a high-pressure coupling.
In accordance with embodiments provided herein, a subsea multivariable system is provided that measures differential pressure, absolute pressure, and process temperature using two process penetrations rather than three. Reducing the number of process penetrations will eliminate a potential process leak path and thereby improve the safety of the flowmeter. Further, eliminating an extra measurement device will also reduce the size and weight of the flowmeter by approximately up to 50 pounds. Further still, as will be described below, some embodiments provided herein can employ redundant process temperatures measurements using two dual temperature elements, such as RTDs, in the same temperature sensor thus improving measurement redundancy and reliability.
Some efforts in this area have already been provided in the form of U.S. Pat. No. 9,234,776 assigned to the assignee of the present application. This patent teaches the utilization of a line pressure sensor in combination with a differential pressure sensor and temperature measurement ability in a subsea transmitter. Accordingly, at least some embodiments described herein may be considered improvements to the aforementioned U.S. patent. The structure described in the '776 incorporates a circuit card assembly that has the ability to measure a 4-wire RTD temperature sensor. Embodiments provided herein generally set forth a non-invasive process fluid temperature measurement regime in combination with a very robust coupling of the temperature sensor to the multivariable transmitter.
Examples of materials that are suitable for immersion in saltwater and other corrosive materials include alloy C276 available from Haynes International Inc., of Kokomo, Ind. under the trade designation Hastelloy C276, Inconel Alloy 625, available from the Special Metals families of companies of New Hartford, N.Y.; and alloy C-22 available from Haynes International. Of particular interest is alloy C276 which has the following chemical composition (by percentage weight): Molybdenum 15.0-17.0; Chromium 14.5-16.5; Iron 4.0-7.0; Tungsten 3.0-4.5; Cobalt 2.5 maximum, Manganese 1.0 maximum; Vanadium 0.35 maximum; Carbon 0.01 maximum; Phosphorous 0.04 maximum; Sulfur 0.03 maximum; Silicon 0.08 maximum; and balance Nickel.
Base portion 158 is coupled to sidewall 160 and endcap 162 to define a chamber 164 therein. Differential pressure sensor 166 is disposed in chamber 164 and has a pair of differential pressure sensor inputs 168, 170 that convey process fluid pressure to deflectable diaphragm 172, which has an electrical characteristic, such as capacitance or resistance, that varies with diaphragm deflection. The electrical characteristic is measured, or otherwise transduced, by circuitry 174 disposed proximate sensor 166. Circuitry 174 also conditions the measurement for transmission through electrical connection point 176. Circuitry 174 may include a microprocessor as well as a process communication module for communicating over a process communication loop or segment. Examples of such process communication include the Highway Addressable Remote Transducer (HART®) Protocol or the FOUNDATION™ Fieldbus Protocol. Circuitry 174 is coupled to output circuit card 178 which is mounted within cover 180. Output circuit board 178 is configured to convey process measurements to remote circuitry, such as a flow computer 102. Additionally, output circuit board 178 also includes one or more inputs for coupling the multivariable transmitter with one or more temperature sensors. As shown in
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These temperature sensors are available with subsea compatible housings formed of suitable materials, such as Inconel or other materials that are suitable for direct immersion in saltwater and other corrosive materials. The MI cable 188 for temperature sensor 182 is welded, as shown, into the housing 185 of multivariable transmitter 150 at weld 186. The individual conductors of the MI cable 188 are either directly soldered into output circuit card 178 of multivariable transmitter 150 or may employ a flexible cable or other intermediary to electrically couple to output circuit card 178.
Embodiments provided herein generally employ a single or dual element RTD 184 that is thermally coupled to process fluid without requiring an additional process penetration. There are several options for where the temperature sensor itself may be installed. However, such installations generally mount the temperature sensor without generating a process penetration and thus the associated high-pressure seal required. Since a process penetration is not required to measure temperature, the large flange connections used for pressure sensors are not required, further reducing the size and increasing the reliability of the system.
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Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
