Arrangement for measuring fluid flow velocity

This invention relates to an arrangement for a velocity measuring instrument for measuring fluid flow velocity, especially in pipes or pipelines, and a system including the instrument, and preferably related to high velocity gas flows, e.g. in flare systems.

In gas and oil production there is often a need for measuring the velocity of fluids passing in pipes and pipelines. In relation to gas transporting systems acoustic measurements are usually utilized, e.g. by transmitting acoustic waves in a direction having a component in the direction of the gas flow, and measuring the frequency shift or changes in the propagation time of the signal to provide the velocity of the gas. Several embodiments of this solution are discussed in US4596133.

Pressure meters have also been known for the use in flow measurements, such as the solutions described in US 2573430, WO97/30333 and US2003/0094052. These, however, has the disadvantage that they require placing obstructions in the flow which in turn may affect the flow conditions. The latter of these use both an orifice and an acoustic meter to monitor the flow velocity.

In some cases, such as flare systems, the velocity may vary within a large range of speeds. The acoustic waves transmitted through the flow do to a degree follow the flow, and above certain gas velocities the acoustics waves are deflected so that they do not reach the receiving transducer. Thus the velocity measurement is lost, and so is the possibility to measure the amount of gas transported through the pipe. This problem can occur at different velocities depending on measuring system, pipe width etc, but in a typical flare system it may arise in flow rates above 80-lOOm/s. It is an object of this invention to provide a measuring system capable of measuring gas flow velocities in the full range from low to high flow rates, especially as a supplement to existing system capable of measuring lower flow rates in gas flare system. In flare or torch system related to oil or gas production this is important, as the amount of gas burned in the torch is related to taxes connected to the CO ₂ quota, which evidently will require measurements when the flow rate through the torch is high.

The objects above are thus obtained according to the invention as stated above and being characterized as described in the independent claims.

According to the invention the flow measurement is provided by a dual system wherein the acoustic system handles the lower range while the differential

pressure (or difference between two pressure sensors) takes over. Preferably the pressure measurements make the use of the cavities in the pipe wall already provided for the acoustic sensors, but other solutions may also be used.

The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples.

Figure 1 illustrates a preferred embodiment of the invention based on the measurement of differential pressure. Figure 2 illustrates a simplified embodiment of the invention, comparing two pressure measurements. Figure 3 illustrates an alternative to the embodiment in figure 1 comparing two separate pressure sensors.

Figure 4 illustrates an alternative to the embodiment in figure 2 measuring differential pressure.

Figure 1 illustrates a section of a pipe 1 having two protruding parts A ₅ B defining channels or openings having an angle α relative to the pipe axis. Fluids F moving along the pipe hitting an opening being directed against the flow will be diverted into the opening B and have a momentum which introduces turbulence or a stationary pressure wave, as well as an increase in the pressure in the opening. Similarly fluids contained in an opening A directed along the flow will be subject to turbulence, but also be drawn out of the opening thus providing a reduced pressure therein. When the gas flows passed the channels a pressure buildup may be measured in the channel B facing the flow and a reduced pressure may be measured in the channel A directing along the flow direction. For high flow rates these pressures depend of the velocity of the flow, and may thus provide a measure related to the flow velocity.

According to the preferred embodiment the channels are provided with traditional acoustic flow velocity sensors in addition to the pressure sensors at the openings A ₅ B ₅ the openings being positioned approximately in line so as to transmit acoustic waves between the sensors. As mentioned above the acoustic sensors will provide measurements up to about 80-lOOm/s, while the pressure sensors will increase their sensitivity with the flow velocity, and thus increase the sensitivity range of the flow velocity measurements. In this preferred embodiment the openings A ₅ B are aligned because of the propagation of acoustic signals between the acoustic transducers. In

other embodiments not combined with the acoustic measure this alignment is not necessary and the pressure sensors may be positioned otherwise. Of practical reasons, and to ensure measurements in the same flow regime the pressure sensors should not be positioned too far apart. In figure 1 and figure 4 the pressure measurements are performed as a differential measurement dP between the two positions in the pipe, and the flow velocity may be calculated from dP. As an alternative one pressure sensor is positioned at each point and dP is calculated from the different measurements P _A3 PB- The chosen solution here depends on the available equipment. Both individual and differential pressure meters are available and may be chosen by a person skilled in the art according the use in each situation.

Figures 2 and 4 show a solution in which only one of the pressures measuring channels is positioned in a channel having an angle relative to the flow direction or pipe axis. This may reduce the sensitivity, but may be used in some situations, for example if the the acoustic sensors are small and may be positioned with an angle relative to the cavity in which they are located. The pressure measured by the sensor directed perpendicular relative to the flow direction may be positioned close to the inner pipe wall, thus not defining an opening or recess therein, and may be constituted by a pressure sensor typically present in the pipe for monitoring the pressure conditions. As indicated in the figures the channels A ₅ B are constituted by standard mounts for acoustic sensors or pressure sensors. The dimensions of the channels will thus in the preferred embodiment of the invention be related to existing features in the pipe. The size of the opening may however, be adjusted by adjusting the position of the acoustic sensor or a plug in the channel, so as to obtain a chosen channel depth. The specific depth may vary with the present conditions and be chosen to minimize such disturbances from turbulence and waves in the opening. Fluctuating pressure signals may also be handled by software filtering the measured pressure e.g. to provide a mean pressure signal over a chosen time period.

The pressure sensors may be of any available type adapted to operate within the pressures related to the specific use, and with a sensitivity chosen accordingly. The response time of the sensor may also be taken into consideration if pressure fluctuations, e.g. due to standing waves or turbulence, is to be monitored. The pressure sensors per se are therefore not described in any detail here.

The angle of the openings α will in the preferred embodiment incorporating acoustic sensors be approximately 45 degrees, according to a specific solution the upstream opening in the first position A has an angle of 48 degrees while the downstream opening in position B has an angle of 42 degrees, thus compensating to a certain degree for the deviation in the direction of the acoustic wave because of the fluid flow.

The dimensions and depth of the openings and channels may be adapted to the specific situations, so as to optimize the conditions to reduce disturbances from turbulence etc in the channel. For example the depth may be adjustable in per se known ways by adjusting the position of the sensors inside the channel or means may be used to control the gas movements therein.

In addition to the pressure and possible acoustic sensors in the openings temperature sensors may be provided for monitoring the conditions in the flow. The temperature sensor may be integrated into the acoustic transducers or pressure sensors, or may be provided as a separate unit, e.g. for positioning in a sensor holder adapted to receive all of these sensors and transducers. In addition to monitor the general conditions in the flow the temperature sensors will aid the measurements providing a possibility for compensating for temperature induced changes in the propagation velocity of the acoustic signals. According a typical example of the preferred embodiment of the invention the transducer holders constituting the channels have inner diameters of 49mm and maximum depths of 200mm. In flare applications the pipe diameter is typically 18" and the line pressure is typically maximum 5barA at maximum flow velocity.

To summarize the invention thus relates to an arrangement for measuring fluid flow velocity, especially in pipes or pipelines, the pipe wall comprising at least two openings with corresponding cavities for receiving acoustic transducers and pressure sensors or probes therein, where the cavities have an angle with respect to the flow direction being different from 90° and that the sensor or probe is positioned so as to define a cavity for the flowing fluid inside the opening. The transducers, sensors or probes are coupled to a calculation unit being adapted to calculate the flow velocity based on pressure difference between openings as well as the differences in acoustic velocity information. The calculation unit also being adapted to provide an output based on said pressure and acoustic measurements indicating the flow velocity.

According to the preferred embodiment of the invention the acoustic velocity measurement includes two acoustic transducers positioned on opposite sides of the flow, the line between said transducers having an angle relative to the flow between in the range of 15-75°, preferably approximately 45°, relative to the flow direction, and being adapted transmit acoustic energy between said transducers. The calculation unit is in a per se known way adapted to calculate the flow rate based on the acoustic signal transmitted and received at said transducers. The acoustic transducers are positioned in the cavities in the pipe wall having axis being essentially parallel to the line between said transducers and wherein said pressure sensors are positioned in said recesses.