Patent Description:
This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.

Transit time ultrasonic flowmeters function by measuring the time it takes for ultrasonic signals to propagate between transducers placed at different locations around or inside a meter body through which the fluid is conveyed. The cross-sectional area of the flow passage through the meter body, the path lengths separating pairs of transducers, and the angle to the flow axis formed between pairs of transducers all play a part in the relationship between the measured times and the rate of flow.

The relationship between the measured transit times and the meter geometry is normally derived from the principle of operation. Calculation of the flow rate then depends on numerical inputs that characterize the geometry of the meter body. These inputs may be derived from design information or based on physical measurement of the meter body, and in the case of high-accuracy ultrasonic meters will normally include calibration parameters that act to correct errors arising from assumptions and/or geometric uncertainties.

In applications where the process fluid may contain contaminants or where the meter body may be prone to erosion or corrosion, the effective geometry of the meter body can be altered. If a change in geometry occurs and is not corrected by calibration or some other means, then this may result in a flow measurement error. For example, if the cross-sectional area of the flow passage is reduced, then the flow velocity may increase. And if the computation of flow rate does not account for the reduced area, the flow rate may be over-registered.

It is known in the field of ultrasonic flow metering that alteration of the internal condition of an ultrasonic meter may affect the accuracy of the meter in the manner described above. See, for example, <NPL>. Work has also been carried out with the aim of proving self-diagnosing capability for ultrasonic meters in such situations see for example, <NPL>.

In oil pipelines various potential contaminants exist, for example, in the form of paraffin wax, asphaltines and inorganic scale. In gas pipelines black powder contamination is well known. The nature of black powder contamination is varied and uncertain but may be from mill scale or corrosion products mechanically mixed or chemically combined with any number of contaminants such as water, liquid hydrocarbons, salts, chlorides, sand, or dirt. Chemical analyses of black powder contamination have revealed that it typically consists mainly of a mixture of iron oxides and iron sulphides. Furthermore, pipelines may also contain water, including salt water, which can lead to corrosion of the internal parts of ultrasonic flowmeter bodies.

Therefore, it is desirable that the internal surfaces of ultrasonic meters be resistant to corrosion and the deposition of contaminants.

<CIT> teaches an anti-scaling ultrasonic flow sensor. The flow sensor comprises a flow pipe, wherein the flow pipe is provided with two ultrasonic energy transducer installation holes in which ultrasonic energy transducers are arranged.

<CIT> teaches a method for determining the flow parameters of a streaming medium in a conduit.

The invention described in this document may be used to prevent contaminants from affecting fluid flow through a flow passage of an ultrasonic flowmeter as well as the signals used by the flowmeter to determine the fluid flow through the flow passage, both of which can introduce error into the flow measurement. The invention, in accordance with certain embodiments, involves the presence of a non-stick coating on the wetted surfaces of the flowmeter which is believed to limit the buildup of contaminants on the wetted surfaces that could affect the fluid flow and the signals of the flowmeter.

In accompanying drawings, the preferred embodiment of the invention and preferred methods of practicing the invention are illustrated in which:.

The invention comprises an ultrasonic flowmeter according to claim <NUM>. Referring now to the drawings wherein like reference numerals refer to similar or identical parts throughout the several views, and more specifically to <FIG>, <FIG> and <FIG> thereof, there is shown an exemplary ultrasonic flowmeter <NUM>. The flowmeter <NUM> comprises a meter body <NUM> including a flow passage <NUM> having wetted surfaces <NUM> through which fluid flow is to be measured. The flowmeter <NUM> comprises a non-stick coating <NUM> adhered to the wetted surfaces <NUM> of the meter body <NUM> along the length of the flow passage. The flowmeter <NUM> comprises a first transducer <NUM> and at least a second transducer <NUM> arranged around the flow passage <NUM> to transmit and receive ultrasonic energy. The flowmeter <NUM> comprises an electronic unit <NUM> designed to generate and receive electronic signals from the transducers and to process the signals in order to compute information related to the fluid flow rate through the passage. For instance, the electronic unit <NUM> may be model number 280Ci produced by Cameron International Corporation.

The flowmeter <NUM> may have a corrosion-resistant coating <NUM> adhered to the wetted surfaces <NUM> of the meter body <NUM>. The coating may be continuous of the flow passage <NUM> surface. The meter body <NUM> may include transducer housings <NUM> that also have a non-stick coating <NUM> adhered to their surface. The meter body <NUM> may include transducer housings <NUM> that also have a corrosion-resistant coating <NUM> adhered to their surface. The transducer housings <NUM> may be recessed in the wall <NUM> of the flow passage <NUM>. The transducer housings <NUM> may protrude into the flow passage <NUM>. The ultrasonic signals sent or received by the transducers pass through the coatings and are unaffected by the coatings. This is important or the coatings could not be used.

The coating may be either a polymer, a fluoropolymer, a plastic, a ceramic, an epoxy, a metal, or a composite. The coating may comprise multiple layers. The non-stick coating or corrosion-resistant coating <NUM> or both may be continuous and impervious. The meter body <NUM> may be made of steel. The meter body <NUM> and the housings may be made of different materials. The housings may be made of steel. The housings may be made of titanium. The flow passage <NUM> may be cylindrical. The flow passage <NUM> may include a reduced bore section or change in cross-sectional area, such as described in <CIT>.

There may be a gap between a distal end of the transducer housing and the wall <NUM> of the flow passage <NUM>.

The present invention pertains to a method according to claim <NUM> for measuring fluid flow with an ultrasonic flowmeter <NUM>. The method comprises the steps of flowing fluid through a flow passage <NUM> having wetted surfaces <NUM> of a meter body <NUM>. The flowmeter <NUM> has a non-stick coating <NUM> adhered to the wetted surfaces <NUM> of the meter body <NUM> along the length of the flow passage. There is the step of transmitting and receiving ultrasonic energy through the flowing fluid between a first transducer <NUM> and at least a second transducer <NUM> arranged around the flow passage <NUM>. There is the step of generating electronic signals for and receiving electronic signals from the first and second transducers <NUM> and <NUM> with an electronic unit <NUM>. There is the step of processing the signals with the electronic unit <NUM> in order to compute information related to the fluid flow rate through the passage.

The present invention pertains to a method according to claim <NUM> of making an ultrasonic flowmeter <NUM>. The method comprises the steps of attaching a first transducer <NUM> and at least a second transducer <NUM> to transmit and receive ultrasonic energy to a meter body <NUM> of the flowmeter <NUM> arranged around a flow passage <NUM> of the meter body <NUM>, and to which an electronic unit <NUM> designed to generate and receive electronic signals from the transducers and to process the signals in order to compute information related to the fluid flow rate through the passage are in electrical communication. There is the step of adhering a non-stick coating <NUM> to wetted surfaces <NUM> along the length of the flow passage <NUM> of the meter body <NUM> through which fluid flow is to be measured.

There may be the step of adhering a corrosion-resistant coating <NUM> to the wetted surfaces <NUM> of the flow passage <NUM> of the meter body <NUM> through which fluid flow is to be measured.

In the operation of the invention, the problems of deposition and corrosion by providing an impermeable non-stick coating <NUM> to the internal surfaces of an ultrasonic flowmeter <NUM> is addressed. The coating either minimizes or eliminates corrosion and the deposition of paraffin wax, asphaltines, scale or black powder.

An ultrasonic flowmeter <NUM> according to an embodiment of this invention comprises a meter body <NUM> that supports at least two ultrasonic transducers and has a passage through which fluid flows. The ultrasonic transducers are connected to an electronic unit <NUM> that coordinates the transmission and reception of ultrasonic signals and the process the signals to determine ultrasonic transit times and hence information about fluid flow rate. The transducers are placed in transducer housings <NUM> that are integrated into the meter body <NUM>. Such transducers may be made of a similar material to the meter body <NUM> or may be of a different material. For example the body may be of stainless steel and the transducer housings <NUM> of titanium. The wetted surfaces <NUM> of the meter body <NUM> are those interior surfaces that are normally in contact with the process fluid. An ultrasonic flowmeter <NUM> according to the invention has an impermeable, non-stick coating
<NUM> covering the wetted surfaces <NUM> of the meter body <NUM>, inclusive of the wetted surfaces <NUM> of any integrated transducer housings <NUM>.

A variety of coating materials and methods are known in other uses have suitable properties for use in this invention. So-called fluoropolymer coatings are one example of a coating material that is suitable for this purpose. A wide variety of fluoropolymer coatings exist in the market today including PVF (polyvinylfluoride) PVDF (polyvinylidene fluoride) PTFE (polytetrafluoroethylene) PFA (perfluoroalkoxy polymer) FEP (fluorinated ethylene-propylene) ETFE (polyethylenetetrafluoroethylene) and ECTFE (polyethylenechlorotrifluoroethylene). Other types of coating that may have suitable properties include other polymers such as polyurethane, diamond-like coatings, ceramics and epoxies.

The coating may be applied as a single layer or in multiple layers. A variety of application methods may also be employed including liquid based application, thermal and plasma spraying or powder spray coating. A primer layer may be used to enhance adhesion of the non-stick layer to the interior of the meter body <NUM>. This primer layer could be a different form of polymer layer or could be a sprayed metal layer. Multiple layers may be advantageous in obtaining the required impermeable and non-stick properties at the wetted surface in combination with good adhesion to the meter body <NUM>.

<FIG> shows an example of an ultrasonic flowmeter <NUM> body according to the present invention. In this embodiment, the flow passage <NUM> is cylindrical and the transducers enclosed in housings that are integrated into the meter body <NUM> at an angle of <NUM> degrees to the central axis and are slightly recessed. Alternative shapes of flow passage <NUM> can also be readily conceived. The flow passage <NUM> may have flanged end connections or some other means for integration into a pipeline or flow
system. A variety of means can be used for construction of the meter body <NUM>, including casting, welding and machining.

In one simplified form, the flow velocity, v, can be related to the measured transit times according to the following equation. <MAT>
where L is the length of the path connection transducers a and b, θ is the angle of the path and tab and tba are the transit times from a to b and vice versa. The flowrate, Q, can be found by multiplying the velocity by the cross-sectional area of the flow passage <NUM>, A, giving
<MAT>
therefore, any deposition that alters the cross-sectional area A, will affect the relationship between the flowrate and the measured transit times and thus may cause an error. Furthermore, any deposition at the interface where the sound waves pass into the fluid can alter the path angle θ and the effective path length L.

Using the example of the meter body <NUM> shown in <FIG> the coating is applied to three areas of the internal surface: the wetted surface of the cylindrical flow passage <NUM>, the surface of the cavities in which the transducer housings <NUM> reside, and the wetted surfaces <NUM> of the transducer housings <NUM> themselves. Depending on the materials and design of the meter body <NUM>, and the coating process employed, the coating process may be applied to an assembled
meter body <NUM> or it may applied separately to the housings and main part of the body prior to assembly.

The process of coating the internal surfaces of the meter body <NUM> or its constituent part will involve several steps. The following description is typical of the process.

As the flowmeter <NUM> assembly may contain component parts unsuitable for the temperatures involved in the curing process, the ultrasonic meter may be assembled after the coating has been applied to the meter body <NUM> parts. This stage will involve installation of transducer housings <NUM> and ultrasonic transducer elements, routing, connection and sealing of electrical cables, connection of electronics, factory acceptance testing of the assembly, and painting.

The total coating thickness applied is generally in the range of <NUM> to <NUM> micrometers, and typically at the lower end of the scale, and as such has no consequential effect on the acoustic properties of the assembly or its function as an ultrasonic flowmeter <NUM>, other than the beneficial effect of minimizing contamination build up on the interior surfaces of the meter body <NUM>.

Ultrasonic flowmeters <NUM> according to the current inventions has been manufactured and tested both in laboratory and field conditions. The flowmeters <NUM> in this case had eight or sixteen ultrasonic transducers arranged to form four or eight chordal paths through a flow passage <NUM> of circular cross-section with flanged end connections, similar in general to the layout shown in <FIG>. Variants in design of the flowmeters <NUM> that have been made and tested include transducer housings <NUM> recessed back from the wall <NUM> of the flow passage <NUM> with a small gap between the housing the meter body <NUM>, transducer housings <NUM> recessed but with a larger gap between the housing and the meter body <NUM>, and transducer housings <NUM> that intrude into the flow passage <NUM>. Materials used in construction of the test meters include stainless steel and titanium.

In the case of the meter with the recessed housings and the small gap, the design geometry is essentially the same as a standard meter, with the exception of the addition of the thin coating layer. Therefore the performance of a coated and an uncoated meter can be compared. <FIG> shows calibration results from coated and uncoated four-path ultrasonic
meters with a flow passage <NUM> diameter of <NUM> (<NUM> inches). These results were obtained by comparison with traceable volumetric reference standards. The primary measure of the performance of the meter is its linearity. These results show a linearity of +<NUM>-<NUM>% in both cases, demonstrating that the performance of the ultrasonic meter is not degraded by application of the non-stick coating <NUM>.

Flowmeters <NUM> according to the invention have been installed in a crude oil pipeline where it was known that there were problems relating to deposition of wax and other by-products of the hydrocarbon production and transportation, and on an offshore hydrocarbon gas production line where there had been a history of problems of wet-gas (i.e. hydrocarbon gas with free liquid components). In both cases, the meters have been visually inspected following an extended period of operational use and have been found to be in a much more pristine condition than the adjoining sections of uncoated pipe.

In the crude oil pipe line application an uncoated meter was installed prior to the coated meter installation and the effects of contamination build up were observed. The contamination effects were evaluated by three means: (<NUM>) by visual inspection; (<NUM>) by a long-term comparison of the volume transfer reported by the meter compared with that recorded in the receiving tanks; and (<NUM>) by analysis of ultrasonic fluid profile and signal diagnostics.

Visual inspection of the uncoated meter after several months of operation showed material that was adhering to the internal surfaces of the meter body <NUM>, particularly in the cavities where the recessed transducer housings <NUM> were positioned. Analysis of a sample of the material showed that it was primarily paraffin wax.

A long-term comparison of the volume transfer recorded by the meter versus the volumes recorded in the receiving tanks of the pipeline showed that when first installed the
uncoated ultrasonic flowmeter <NUM> was in good agreement with the receiving tanks, but as time progressed an error or 'measurement bias' developed in the flowmeter <NUM>. <FIG> shows the <NUM>-day running average error of the flowmeter <NUM> relative to the receiving tanks. It can be observed that the error is initially less than <NUM>%, which is consistent with performance expectations for this type of meter, but that as time progresses the error becomes increasingly negative, until it reaches a level of approximately -<NUM>%. This magnitude of measurement error is not acceptable in many hydrocarbon measurement applications.

<FIG> shows the gain reported for the measurement paths of the meter at three different stages of evaluation: on initial installation, after approximately <NUM> months of operation, and following cleaning of the internal surfaces that took place following the operational period. The gain describes the magnitude of amplification applied to the ultrasonic signals to maintain the amplified result within a certain range of amplitude. Therefore any change in gain is indicative of a change in the transmission of the ultrasonic signal as it passes from the transducer housing of the transmitter, through the interior of the flow passage <NUM>, and into the receiving transducer at the other side. It can be observed that the gain values are approximately <NUM> dB for paths <NUM> and <NUM> and approximately <NUM> dB for paths <NUM> and <NUM>. Following the period of normal operation these values have reduced to approximately <NUM> dB and <NUM> dB respectively. After the internal surfaces of the meter body <NUM> have been cleaned the gain values return to a level that is very close to that originally recorded confirming that the deposition on the internal surfaces of the meter body <NUM> had altered the ultrasonic signal transmission.

From the information in Figures S. <NUM> and <NUM>, and supported by the distribution of the deposition observed during visual inspection it was deduced that the mechanism by which the deposition results in a flow rate measurement error in this case was mainly by alteration of the angle of the acoustic paths. As illustrated in <FIG>, if a deposit forms in the cavity in front of an ultrasonic transducer housing, this can act to alter the effective centre and angle of the acoustic path. As the relationship between the velocity along the path and the measured transit
times is very sensitive to this angle, any uncompensated change in the path angle can result in a significant measurement error.

A coated meter according to the invention was installed in the same crude oil pipeline and monitored to determine the effectiveness of the coating in reducing the measurement errors previously observed. Following a period of more than <NUM> months, the meter was visually inspected. In comparison with the uncoated meter, the coated meter was substantially free of deposition, with any deposits present being of much smaller size. Analysis of signals diagnostics over the same period showed no substantial changes in parameters such as gain, reinforcing the findings from the visual inspection. In the same manner as previously the volume registered by the coated flowmeter <NUM> was compared with the volume recorded in the receiving tanks. In the case of the coated meter, the difference between the flowmeter <NUM> and the tanks did not show a gradual development of a large bias or error. Instead it can be observed that the difference between the meter and the tanks remains with a range of +<NUM> to -<NUM>%. Given the uncertainty inherent in performing such a comparison under field conditions, this span of results is considered consistent with expectations for high accuracy metering.

From these studies, it is believed that the invention has no negative impact on the measurement performance of the ultrasonic flowmeter <NUM> and a beneficial effect such that this level of performance can then be maintained in applications where deposition or corrosion is likely.

Claim 1:
An ultrasonic flowmeter comprising:
a first transducer (<NUM>) and at least a second (<NUM>) transducer arranged around the flow passage (<NUM>) to transmit and receive ultrasonic energy; and
an electronic unit (<NUM>) designed to generate and receive electronic signals from the transducers and to process the signals in order to compute information related to a fluid flow rate through the passage; and
a meter body (<NUM>) including a flow passage (<NUM>) having wetted surfaces (<NUM>) through which fluid flow is to be measured, characterized by said flowmeter (<NUM>) having a non-stick coating (<NUM>) adhered to the wetted surfaces (<NUM>) of the meter body (<NUM>) along the length of the flow passage (<NUM>) and including a first transducer cavity also having a non-stick coating on wetted surfaces (<NUM>) of the first transducer cavity and a second transducer cavity having a non-stick coating on wetted surfaces (<NUM>) of the second transducer cavity, the non-stick coating of the first transducer cavity and the non-stick coating of the second transducer cavity in contact with the fluid;
wherein the first transducer is enclosed in a first transducer housing (<NUM>) and is disposed in the first transducer cavity, the first transducer housing in contact with the fluid, wherein the second transducer is enclosed in a second transducer housing (<NUM>) and is disposed in the second transducer cavity, the second transducer housing in contact with the fluid;
wherein the first transducer housing (<NUM>) and the second transducer housing (<NUM>) have wetted surfaces on which the non-stick coating (<NUM>) is applied.