Patent Description:
Flare gas flow measurement is important for a number of reasons including mass balance, energy conservation, emissions monitoring, and regulatory considerations. For example, natural gas is commonly associated with petroleum deposits. Gas may be released from petroleum deposits during petroleum extraction. Systems which are used to release natural gas generally operate at relatively low flow rates (purge flow conditions), but also may experience unpredictable conditions with relatively high flow rates (upset conditions). An averaging pitot tube primary element (APT) (such as the Annubar® APT available from Emerson Process Management which is suitable for measuring flow rates in upset conditions may be unable to generate a measurable differential pressure (DP) signal during purge flow conditions. Thermal mass flow sensors may be used to measure flow rates in purge flow conditions are unable to generate accurate flow rate readings during upset conditions. Measurement of flow rates may be made using ultrasonic instruments, but such instruments are typically expensive.

Natural gas is often burned at its extraction site to mitigate environmental impact and to promote worker safety. An estimated <NUM> billion cubic meters of gas are burned annually in flare systems. Oil and gas operators are required to monitor and report the amount of gas flared annually. To accurately report the amount of gas flared, flare metering applications typically target an uncertainty of +/- <NUM>% of mass flow rate of the gas that is flared.

The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. Each of <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT> presents related prior art describing flow measurement probes.

A flow measurement probe according to the invention is provided in independent claim <NUM>; a flow measurement system according to the invention comprising the flow measurement probe of independent claim <NUM> is described in claim <NUM>; a method of measuring fluid flow rate in a process according to the invention is given in independent claim <NUM>.

A typical natural gas flare system is shown in <FIG>. Flare system <NUM> comprises a gas collection header <NUM> to channel waste gas from an extraction location to a disposal point, a knock out drum <NUM> to remove a majority of condensate from the waste gas flow stream, a liquid seal <NUM> to prevent natural gas backflow from a flare stack, a flow measurement system <NUM> to measure waste gas stream mass flow for mass balance and regulatory considerations, a seal barrier <NUM> to prevent air backflow into flare stack <NUM>, and a flare tip <NUM> to ensure complete combustion of waste gas. The seal barrier <NUM> and flare tip <NUM> are within the stack <NUM> which transitions from a pipe system <NUM> leading from the liquid seal <NUM> through the flare flow measurement system <NUM> before transitioning at transition point <NUM> to the flare stack <NUM>. In one embodiment, the flare stack <NUM> is substantially vertical. Flow measurement systems such as those described below are amenable to use with a natural gas flare system such as system <NUM>. Alternatively, the device could be applied to any liquid or gas flow measurement requiring extreme turndown.

Gas systems such as natural gas disposal systems typically operate in two modes, normal operation and upset operation. In normal operation, a system, such as a flare system, constantly emits waste gas at a low flow rate known as purge flow. Purge flow operation is typically at pressures slightly above atmospheric pressure (e.g., <NUM> pounds per square inch gauge (psig)), and flow velocities typically lower than <NUM> feet per second. Ambient temperatures have some effect on purge flow. In petroleum production, unpredictable events referred to as upset events can result in a release of large quantities of waste gas to be disposed, at high flow rates, pressures, and temperatures. Under upset operation, pressures of waste gas can exceed <NUM> psig, temperatures can exceed <NUM> °F, and flow velocities can reach <NUM> feet per second.

Flow rate measurement devices such as averaging pitot tubes can be used for measurements in an upset condition, due in part to their rugged construction, proven accuracy, fast time response, and low permanent pressure loss. However, averaging pitot tube elements provide only a relatively minor restriction within the flow stream of a flare stack, and therefore have difficulty inducing sufficient differential pressure signals to make accurate flow measurement possible during normal operation under purge flow. Since low flow rate operation is nearly constant, cumulative mass flow even for normal operation can be significant, and cannot be ignored.

Thermal mass flowmeters can be used as a reliable flow measurement option for normal operation such as purge flow. A block diagram of a typical thermal mass flowmeter sensor <NUM> is shown in <FIG> and a plan view is shown in <FIG>. Thermal mass flowmeter sensor <NUM> operates on the principle of thermal convection or dispersion, and comprises two temperature sensors <NUM>, <NUM> exposed within a flow stream. One of the temperature sensors <NUM> typically uses a resistance temperature detector <NUM> and a resistive heating element <NUM>. The second temperature sensor <NUM> is a reference sensor that measures ambient temperature of the flow using resistive temperature detector <NUM>, and has a mass balancing element <NUM> to balance the mass of the resistive heating element <NUM>. For the first sensor <NUM>, flow passing by the element removes energy from the heated element through convection, as calculated by Fourier's Law.

Thermal mass flowmeters commonly measure the energy removed from the flowmeter in one of two ways. The first uses a constant current driven through the heater element <NUM>. The difference in temperature between the ambient sensor <NUM> and the heated sensor <NUM> is a measure of thermal energy loss in the instrument <NUM>. The second uses constant temperature. A feedback loop controls the current applied to the heated sensor <NUM> so as to maintain a constant temperature difference between the ambient sensor <NUM> and the heated sensor <NUM>. The current required to maintain the constant temperature difference is proportional to the thermal energy loss to the flow stream in the instrument <NUM>.

<FIG> shows a schematic of a thermal mass flow sensor <NUM>. Resistance temperature detectors <NUM> and <NUM> are connected to differential temperature monitor <NUM>, which detects a temperature differential between detectors <NUM> and <NUM>. Resistive heating element is connected to a current source <NUM> which drives a current determined by mass flow electronics <NUM>, described further below. Thermal mass flow electronics monitor and control current provided by current source <NUM> in the constant current and constant temperature operational modes of thermal mass flow sensor <NUM>.

A thermal mass flowmeter is well suited for the measurement of flow rate under relatively steady state low flow rate conditions. It can therefore accurately measure purge flow. However, in upset conditions, a thermal mass flowmeter does not function nearly as well. Thermal mass flowmeters are not generally accurate in situations involving entrained liquids. Fluid displaced from a backflow preventer such as backflow preventer <NUM> and the presence of liquid hydrocarbons within a waste gas stream adversely impact thermal mass based flow measurement accuracy during upset conditions. Flow correction coefficients for operation with different gases are not well understood and of questionable reliability.

Thermal mass flowmeters occasionally need to be removed from service for calibration recertification. However, it is typically unacceptable for safety related systems such as gas flow measurement devices to be out of service. Thermal mass measurement systems have significant sensitivity to variations in gas composition, and such variations are common in upset conditions. Upset conditions also tend to change ambient temperature conditions, increasing reliability issues with thermal mass flowmeters.

A flow measurement probe <NUM> according to one embodiment of the present disclosure is shown in <FIG>. Flow measurement probe <NUM> comprises averaging pitot tube element <NUM> having a plurality of individual upstream high pressure openings (e.g., impact tubes) and low pressure downstream openings (e.g., impact tubes) <NUM> opposite each other aligned with a flow stream, and a thermal flow measurement sensor <NUM>. The averaging pitot tube element <NUM> and the thermal mass flow sensor <NUM> are disposed in an elongate probe body <NUM>. A differential pressure is created between the upstream and downstream openings as the flow moves past the probe body <NUM>.

As can be seen in <FIG>, in one embodiment the averaging pitot tube element <NUM> and the thermal flow measurement sensor <NUM> are arranged in an elongate probe body <NUM> having a longitudinal axis <NUM>. Sensor <NUM> is positioned in cavity <NUM> which extends through the body <NUM> and allows fluid flow therethrough. The plurality of upstream openings <NUM> of the averaging pitot tube element <NUM> are distributed along the length of the elongate probe <NUM> in the direction of longitudinal axis <NUM>. This allows an average of the pressure across a cross section of piping to be developed in impulse piping. In one embodiment, the thermal flow measurement sensor <NUM> is located substantially centrally in the elongate probe <NUM>, that is, approximately equidistant from opposite edge walls of a conduit through which the fluid for the which flow rate is to be measured flows. In one embodiment, the openings <NUM> of the averaging pitot tube element <NUM> are on either side of the thermal mass flow sensor <NUM> along the length of the elongate probe <NUM> along the longitudinal axis <NUM>.

In a thermal mass sensor <NUM>, the two sensor elements <NUM>, <NUM> (see <FIG>) are generally oriented in the cross sectional plane of the conduit (e.g., pipe) in which they are installed. Sensor elements <NUM>, <NUM> are positioned in cavity <NUM> of sensor <NUM> as shown in <FIG>. Sensor elements <NUM>, <NUM>, which also carry electrical wiring, are mounted in bores <NUM> shown in <FIG>. The internal averaging pitot tube impulse tubes <NUM> (<FIG>) associated with the openings <NUM> of the averaging pitot tube element <NUM> are positioned in one embodiment so as to straddle the thermal mass flow sensor <NUM>, allowing for pressure to be obtained at the multiple individual openings <NUM> of the averaging pitot tube element <NUM> for the averaging of flow rate readings. In one embodiment, the averaging pitot tube element <NUM> comprises distinct sections connected via tubing for pressure communication at the thermal mass flow sensor <NUM>.

The elongate probe <NUM> in one embodiment has a height <NUM> as shown in <FIG>. This height <NUM> is determined according to a diameter of a conduit into which the flow measurement probe <NUM> is placed. As thermal mass flow sensors, such as sensor <NUM>, typically have one or only a few measuring sensors, to ensure accuracy, their location within a flow stream is important. In a conduit, if the location of a thermal mass flow sensor is too near the edge of the conduit, the flow profile of fluid within the conduit may be such that an accurate flow rate reading is not possible. As can be seen in <FIG>, the position of the thermal mass flow sensor <NUM> in one embodiment is at or near the middle of the elongate probe <NUM> of the flow measurement probe <NUM> along its height <NUM>. This placement positions the thermal mass flow sensor <NUM> such that when the flow measurement probe <NUM> is installed in a conduit, the thermal mass flow sensor <NUM> is positioned at the center of the conduit for accurate measurements, without requiring additional measurement and determination of proper positioning.

In another embodiment, the thermal mass sensor <NUM> may be a pair or an array of thermal mass flow sensors, comprising two thermal mass sensors, or an array of thermal mass sensors, in a single cavity such as cavity <NUM>, and the flow rates for the pair or array of thermal mass flow sensors may be averaged to provide a potentially more accurate measurement of the flow rate during purge flow conditions. The thermal mass sensor <NUM> or a pair or array of thermal mass sensors such as those described are in one embodiment isolated from the upstream and downstream openings <NUM> of the averaging pitot tube element <NUM>.

As shown in greater detail in <FIG>, the upstream and downstream openings <NUM> of the averaging pitot tube element <NUM> may be formed by cross drilling. In one embodiment, plugs <NUM> are welded into cross drilled tubes that are not aligned with the fluid flow, to ensure proper impact tube alignment with the flow stream. <FIG> show cross sections along lines <NUM>-<NUM> and <NUM>-<NUM>, respectively, of <FIG> showing also conduits <NUM> for thermal wiring from thermal mass flow sensor <NUM>. The upstream and downstream openings <NUM> and their associated impulse tubes in one embodiment straddle the thermal mass flow sensor <NUM> chamber.

In one embodiment of the present disclosure, a gas flow measurement system <NUM> is provided as shown in <FIG>. System <NUM> comprises a flow measurement probe such as flow measurement probe <NUM>, described herein, having an elongate probe with each of an averaging pitot tube element <NUM> combined with a thermal mass flow sensor <NUM>. The thermal mass flow sensor <NUM> provides accurate measurement in a normal purge flow condition of the system <NUM>, and the averaging pitot tube element <NUM> provides accurate flow measurements in upset conditions of the system <NUM>. In one embodiment, outputs of the averaging pitot tube element <NUM> and the thermal mass flow sensor <NUM> covering an entire flow range are combined into a single signal indicating the flow rate. In one embodiment, this is accomplished with a transmitter <NUM> connected to receive flow information from thermal mass electronics <NUM> coupled to sensor <NUM> and differential pressure measurement system electronics <NUM> connected to the pitot tube element <NUM>. Alternatively, a terminal block and electronics stack may be used to manage the outputs from the two sensors, and in one embodiment is included in a connection head to mitigate installation complexity.

To establish a single flow rate output, operational information for the sensor <NUM> and averaging pitot tube element <NUM> is used in one embodiment. Averaging pitot tube elements are typically more accurate in determining flow rates at higher velocities than thermal mass sensors. In one configuration, when the averaging pitot tube element <NUM> generates a measurable differential pressure, flow rate calculations from the averaging pitot tube element are used to generate the output. In an exemplary embodiment, the differential pressure developed in averaging pitot tube element <NUM> is used, for example, by a process variable transmitter (described below) to calculate a flow rate in the system when a differential pressure exceeds a specified differential. Readings from the thermal mass flow sensor <NUM> are used, for example, by a process variable transmitter to calculate a flow rate when the differential pressure is below measurable specified differential. In this embodiment, when a specified differential pressure is detected, the flow measurement system uses the averaging pitot tube element <NUM> to determine the flow rate. Only when a specified differential pressure is not detected is the flow rate determined by the thermal mass flow sensor <NUM>. When the averaging pitot tube element <NUM> is being used to measure the flow rate, thermal mass flow sensor <NUM> may be used to measure temperature of the process fluid.

Transmitter software in one embodiment can be used to integrate the two sensor signals, from the averaging pitot tube element and the thermal mass flow sensor, to provide a user with a single flow rate output. This is accomplished in one embodiment by wiring thermal mass electronics contained in housing <NUM> signals output from the isolation manifold for differential pressure measurement <NUM> to a process variable transmitter <NUM>, which can deliver a single <NUM>-<NUM> milliAmpere output such as provided by a two-wire control loop, and/or a digital output covering an entire flow rate range. As illustrated in <FIG>, flow measurement probe <NUM> is disposed in a conduit (e.g., pipe) <NUM>. In this configuration, it can be seen that when thermal mass flow sensor <NUM> is located centrally along the height of the elongate tube <NUM>, the placement of the flow measurement probe <NUM> in the conduit <NUM> places the thermal mass flow sensor <NUM> at a desired position in the flow stream in conduit <NUM> without measurement or other placement determination. The process variable transmitter <NUM>, described further herein with respect to <FIG>, may use an algorithm to blend the thermal mass electronics output and the differential pressure measurement output together to form a single signal indicative of a flow rate for the entire flow range. The process variable transmitter <NUM> may also provide an indication as to which sensor, the averaging pitot tube element <NUM> or the thermal mass flow sensor <NUM>, is being used to provide the indicated flow rate.

Housing <NUM> has a terminal block <NUM> (see <FIG>) for thermal wiring <NUM> running from thermal mass flow sensor <NUM> through conduits <NUM> to the housing <NUM>. From the terminal block <NUM>, thermal flow electronics <NUM> determine thermal flow, and signals are sent along wiring <NUM> to process variable transmitter <NUM>.

Direct mounting of a pressure transmitter to an averaging pitot tube element such as element <NUM> is in one embodiment facilitated by installing a thermal mass flow sensor such as sensor <NUM> through a flanged opposite side support <NUM> of the averaging pitot tube element <NUM>. This flanged opposite side support <NUM> is in this embodiment an alternate mounting location for thermal mass flow electronics such as electronics <NUM>.

<FIG> is a system block diagram of a transmitter <NUM> and flow measurement probe <NUM> according to one embodiment. Transmitter <NUM> includes a loop communication circuitry <NUM>, pressure sensor <NUM>, thermal mass flow electronics <NUM>, measurement circuitry <NUM>, and controller <NUM>. Loop communication circuitry <NUM> is coupleable to a process control loop <NUM> and is adapted to communicate a process variable output related to a flow rate of gas in a pipe such as pipe <NUM>, based on a combined flow rate depending on outputs of multiple sensors of a flow measurement probe such as flow measurement probe <NUM> described above. Loop communication circuitry <NUM> can include circuitry for communicating over a wired communication link and/or a wireless communication link. Such communication can be in accordance with any appropriate process industry standard protocol such as the protocols discussed above, including both wired and wireless protocols.

Typically, a field device such as transmitter <NUM> is located at a remote location in a process facility, and transmits a sensed process variable back to a centrally-located control room. Various techniques can be used for transmitting the process variable, including both wired and wireless communications. One common wired communication technique uses a two-wire process control loop <NUM> in which a single pair of wires is used to both carry information as well as provide power to the transmitter <NUM>. One technique for transmitting information is by controlling the current level through the process control loop <NUM> between <NUM> mA and <NUM> mA. The value of the current within the <NUM>-<NUM> mA range can be mapped to corresponding values of the process variable. Example digital communication protocols include HART® (a hybrid physical layer consisting of digital communication signals superimposed on a standard <NUM>-<NUM> mA analog signal), FOUNDATION™ Fieldbus (an all-digital communication protocol promulgated by the Instrument Society of America in <NUM>), Profibus communication protocol, or others. Wireless process control loop protocols, such as radio-frequency communication techniques including WirelessHARTO in accordance with the IEC <NUM> standard, may also be implemented. Process control loop <NUM> in <FIG> represents either or both of wired and wireless embodiments of communication connections between transmitter <NUM> and a user interface.

Pressure sensor <NUM> includes pressure input ports coupled to averaging pitot tube element <NUM> through impulse piping <NUM>. Sensor <NUM> can be any device that has an electrical characteristic that changes in response to changes in applied pressure. For example, sensor <NUM> can be a pressure sensor in which a capacitance changes in response to the differential pressure applied between input ports. Thermal mass flow electronics <NUM> receive data from the thermal mass flow sensor <NUM>.

Measurement circuitry <NUM> is coupled to sensor <NUM> and electronics <NUM> and is configured to provide sensor outputs based on the signals to controller <NUM>. Measurement circuitry <NUM> can be any electronic circuitry that can provide a suitable signal related to differential pressure. For example, measurement circuitry can be an analog-to-digital converter, a capacitance-to-digital converter or any other appropriate circuitry.

Controller <NUM> is coupled to measurement circuitry <NUM> and loop communication circuitry <NUM>. Controller <NUM> is adapted to provide a process variable output to loop communication circuitry <NUM>, which output is related to the sensor outputs provided by measurement circuitry <NUM>. Controller <NUM> can be a programmable gate array device, a microprocessor, or any other appropriate device or devices. Although loop communication circuitry <NUM>, measurement circuitry <NUM> and controller <NUM> have been described with respect to individual modules, it is contemplated that they can be combined such as on an Application Specific Integrated Circuit (ASIC). In an exemplary embodiment, memory <NUM> is included and is coupled to controller <NUM> for storage of computer readable instructions, parameter values, etc. used to configure controller <NUM> and/or measurement circuitry <NUM>. In some such embodiments, configuration information for sensors such as averaging pitot tube element <NUM> and thermal mass flow sensor <NUM> is stored in memory <NUM>.

The controller <NUM> is configured in one embodiment to determine a flow rate of the process flow using the input from the differential pressure sensor <NUM> when a differential pressure is at least a defined measurement threshold, and to determine the flow rate using the input from the thermal mass flow sensor determined by the thermal mass flow electronics <NUM> when the differential pressure is less than the defined measurement threshold. Inputs from the thermal mass flow sensor and the differential pressure sensor are provided in one embodiment by a flow measurement probe such as flow measurement probe <NUM> described above, having a thermal mass flow sensor integrated with an averaging pitot tube element in an elongate probe.

In one embodiment, the process variable transmitter is configured to switch between calculating a flow rate based on a differential pressure between the plurality of upstream and downstream openings when the differential pressure exceeds a measureable differential, and calculating a flow rate based on readings of the thermal flow measurement sensor when the differential pressure is below the measurable differential. This switching may be automatic, such as when a certain threshold is met, or may be selectable, such as by a user.

Further, some calculation of flow rate, although not as accurate, may be made using readings from either sensor if the other is unavailable. A process variable transmitter such as transmitter <NUM> may monitor the averaging pitot tube element and thermal mass flow measurement sensor of a flow measurement probe such as probe <NUM>, and if one element fails, or begins to provide readings indicative of failure, the process variable transmitter may in one embodiment provide a warning of the failure or impending failure.

A method <NUM> of measuring fluid flow rate in a process is shown in flow chart form in <FIG>. Method <NUM> comprises in one embodiment measuring a flow rate of the fluid with a first sensor of an averaging pitot tube when a differential pressure is at least a defined measurement threshold in block <NUM>, and measuring the flow rate of the fluid with a second sensor of the averaging pitot tube when the differential pressure is less than the defined measurement threshold in block <NUM>. Measuring with a first sensor in one embodiment comprises measuring with pressure sensor attached to the averaging pitot tube, and measuring with a second sensor comprises measuring with a thermal mass flow sensor within the averaging pitot tube. As thermal mass flow sensors are sensitive to ambient temperature, in one embodiment, the second sensor is isolated from the first sensor to mitigate signal loss through conduction.

The time response of differential pressure flow technology such as averaging pitot tube element <NUM> is superior to the time response of thermal mass flow measurement technology such as sensors like sensor <NUM>. Therefore in one embodiment the output from the averaging pitot tube element <NUM> is used when a measureable differential pressure magnitude is present. During measurement range overlap when each of the averaging pitot tube element <NUM> and thermal mass flow sensor <NUM> can perform a flow rate measurement, a comparison can be made between the outputs of each sensor to enable verification of the thermal mass flow sensor without downtime. As traditional thermal mass flowmeters occasionally need to be removed from service for calibration recertification, the ability to calibrate without down time makes embodiments of the present disclosure well-suited for safety related systems such as flare flow measurement systems that have strict in service requirements. Comparison in one embodiment comprises comparing output from the averaging pitot tubes <NUM> and the thermal mass flow sensor <NUM> when a flow rate is within a range of operational measurement for each of the averaging pitot tube element and the thermal mass flow sensor, and calibrating the thermal mass flow sensor using the averaging pitot tubes reading when the flow rate is within the range of measurement for each of the sensors. Comparison as has been described may also in one embodiment be used to detect a bad sensor in a diagnostic manner.

Upset events often introduce impurities into the flow stream, such as debris, entrained liquids such as hydrocarbons, varying gas composition, as well as changing flow rates and ambient temperatures. Fluid in a thermal mass flow sensor such as that displaced from a backflow preventer and the presence of liquid hydrocarbons within the flow stream can negatively affect the performance of thermal mass flow sensors. Debris can lodge in impact tube openings or impact tubes of the APT sensor. Each of these occurrences can affect operation of one or both APT and thermal mass flow sensors. In various embodiments, fluids are removed from a thermal mass flow sensor after an upset event. In various embodiments, debris is cleared from impact tube openings and/or impact tubes of an APT sensor after an upset event. Each of these operations may be done, for example, by purging a gas such as air or nitrogen through the appropriate sensor.

A flow measurement system such as those described herein is in one embodiment located at grade just upstream of a transition such as transition <NUM> in <FIG> to a vertical stack such as stack <NUM> in <FIG>. This location ensures easy accessibility for service personnel. Flares are generally located in a remote location and are connected to the rest of the system by long sections of pipe. These long sections allow for a sufficiently long straight run of fluid therein to ensure a stable flow profile at the measurement point by a flare flow measurement system such as those described with respect to <FIG>.

Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. Further, while the flow measurements described herein have referred to natural gas flow in a conduit, and the measurement of flare gas flow rates in a flare system, flow of process fluids other than natural gas is amenable to measurement using the structure and methods described herein, without departing from the scope of the disclosure.

Claim 1:
A flow measurement probe (<NUM>), comprising:
an elongate probe comprising an averaging pitot tube element (<NUM>) having a plurality of upstream and downstream openings (<NUM>) arranged along a length of the elongate probe; and
at least one cavity (<NUM>) extending through the elongate probe allowing fluid flow therethrough, the at least one cavity positioned between at least two of the plurality of upstream and downstream openings (<NUM>); and
a thermal flow measurement sensor (<NUM>) disposed in the at least one cavity (<NUM>) and within the elongate probe.