FIELD OF THE INVENTION

This invention relates to measuring properties of fluid flow in pipes, such as flow rate.

BACKGROUND

An ultrasonic flowmeter is a type of flowmeter that measures the velocity of a fluid with ultrasound to calculate volume flow. Ultrasonic flowmeters are widely applied to measure flow in a variety of applications. Ultrasonic flowmeters use acoustic waves or vibrations to detect the flow traveling through a pipe. Ultrasonic energy is typically coupled to the fluid in the pipe using transducers that may be wetted (aka invasive) or non-wetted (aka non-invasive), depending upon the design of the flowmeter. This can either be done in-line, by inserting a spool piece with ultrasonic transducers into the pipe carrying the fluid, or by clamping the transducers on an existing pipe. Using ultrasonic transducers, the flowmeter can measure the average velocity along the path of an emitted beam of ultrasound by various methods.

Clamp-on meters are attractive as they can be installed without cutting the pipe or shutting down the flow, but their stability is limited, and they are unable to measure flow profiles (in contrast with expensive multi-path in-line meters), which limits their linearity at low flow speeds. Moreover, their installation typically requires complex manual alignment of the transducers and input of a variety of setup parameters (e.g., pipe dimensions and material properties, speed of sound in the fluid) by the user.

There are several different ways by which ultrasonic signals can be utilized to calculate flow rate for clamp-on meters.FIG. 1schematically shows five different non-invasive methods that can be used to measure flow in pipes110and120: transit time102, Doppler104, beam deflection106, cross correlation112and passive acoustics114.

The three main types of non-invasive ultrasonic technology currently used are Transit-Time, Doppler, and Cross-Correlation, which are explained in detail below. Passive Acoustics (e.g.,114onFIG. 1) is a technique where the transducer simply ‘listens’ to the naturally occurring vortices or eddies in the flow. By monitoring how fast they flow by the transducer, a velocity can be derived. Beam Deflection (e.g.,106onFIG. 1) works by transmitting an ultrasonic beam across the pipe which is slightly deflected by the flow. This deflection is monitored and is directly related to the velocity of the flowing medium.

Transit-Time (e.g.,102onFIG. 1andFIGS. 2A-B) is the most common type of clamp on technology and can be used for both gases and liquids. The principle is based on the time difference it takes for an ultrasonic pulse to travel a defined distance both with and against flow. The difference is directly related to the velocity of the flow and is to some extent analogous to a boat crossing a river. If the boat is travelling with the current then it will be helped across to some degree by the flowing fluid but the opposite is true when travelling against the current.FIG. 2Ais normally referred to as direct mode, i.e., the ultrasonic beam only makes one traverse across pipe210.FIG. 2Bis normally referred to as reflect mode as two traverses across pipe210are required. As shown inFIG. 2A, transducer202transmits an ultrasonic pulse across a pipeline which is received by transducer204, or vice versa. Since the sound velocity for the flowing medium and the length the ultrasound travels are both known, a simple equation can be used to calculate the time taken for the ultrasonic signal to reach the receiving transducer. This is also known as the time-of-flight of the ultrasonic signal. Similarly,FIG. 2Bshows transmission of sound from transducer202to transducer204(or vice versa) via reflection from the opposite pipe wall.

Doppler non-invasive ultrasonic flowmeters (e.g.,104onFIG. 1andFIG. 3) are based on the well-known Doppler principle and frequency shifts in the ultrasonic signal. In flow measurement, again two transducers are used and are often placed on the same side of the pipe. Most Doppler techniques require a small amount of gas or entrained solids in the flow in order to calculate a velocity. As shown onFIG. 3, transducer302emits an ultrasonic beam of a known frequency f1. This beam reflects off either the gas bubbles or solids within the flow in pipe310and is thereby frequency shifted to frequency f2. Transducer304then receives the frequency shifted ultrasonic signal and by measuring the frequency change the flow velocity can be derived.

In the Cross-Correlation technique (e.g.,112onFIG. 1andFIG. 4), two pairs of transducers402,404and406,408are positioned on a pipe with both receivers (404and408) and both transmitters (402and406) on the same side of the pipe respectively. The transducer pairs are placed in the same plane in order to maximize the potential for correlating signals. The main difference between transit-time and cross-correlation flowmeters is that one transducer is always the transmitter and one transducer is always the receiver; they do not swap roles.FIG. 4shows the transducer configurations: Both the upstream and downstream transducers transmit an ultrasonic pulse across the pipe that is modulated by naturally occurring eddies or vortices in the flow. The result is two very similar waveforms x (t) and y (t) that have distinctive peaks which are displaced by a time factor. This time factor is directly proportional to the distance between the pairs of transducers and inversely proportional to the flow velocity. To calculate the time difference a mathematical cross-correlation function is used, schematically shown by410onFIG. 4.

SUMMARY

We provide a novel clamp-on ultrasonic flow metering technique capable of measuring the flow rate and other parameters of a pipeline by selectively exciting and receiving Lamb waves in the pipe. Lamb waves, in contact with a fluid, can leak at a different rates. Furthermore, the moving fluid alters the leak rate as a function of the flow velocity. At low frequencies, the wavelength is larger, thus the penetration depth in the flow is larger. As the frequency increases, the penetration depth becomes smaller. The penetration depth provides a methodology to sample the flow velocity profile.FIGS. 8A-Dschematically show this flow metering concept. By selectively exciting and receiving guided waves in the pipe, the information of frequency/mode-dependent leak rates are recorded. The leak rate is a function of the frequency, moving fluid in the pipe, its velocity, and its direction. The flow rates can be estimated through analytical, computer based, or machine learning algorithms.

The transducers preferably provide a broad-band selective excitation of the guided axisymmetric Lamb modes. The system can include a transmitter transducer and a receiver transducer. In one mode of operation, we looked into the difference in the arrival time of the transmitted guided waves once it reaches the receiver. In another mode of operation, we look at the amplitude of the transmitted wave, which in turn provides a measure of the leak rate. Note that by switching the transmitter and receiver, we can look at the same feature in the reverse direction, in both of these cases. In turn, by doing so, we can reduce measurement error and increase accuracy. Moreover, the above mentioned methods can be combined to improve the accuracy even more. In a third mode of operation, we use both transducers in the transmit and receive modes, in which we set up standing waves in between the transducers. The modes of the standing waves are modulated by the flow through frequency-dependent leak rates. The transducers can be connected to an electrical instrument such as a network analyzer, spectrum analyzer, impedance analyzer, time-domain pulser/receiver, etc., by which means the response of the guided waves of the pipe is monitored and recorded as a function of flow velocity.

Numerous applications are possible. Clamp-on meters are applied, for instance, in the oil-and-gas, chemical, food, and semiconductor industries. They are capable of measuring gas or liquid flow without introducing contamination, leakage, fouling, pressure drops or energy losses. They are found both in permanent installations, e.g. where hygiene, continuous operation, maintenance and cost are a major consideration, as well as in temporary use, as a tool for checking, servicing and maintaining existing flowmeters.

Significant advantages are provided. This approach provides a non-invasive flow rate measurement, without the need to cut through and insert a sensor into the pipe. Since this is a clamp-on technique, it can be used as a temporary, semi-permanent or permanent method of measurement. It can be used to measure other flow characteristics such as pressure, temperature, composition, bubble content, velocity profile, etc. It can measure both gas and liquid flows. Our method can bring clamp-on sensors closer to in-line (i.e., wetted/invasive) sensors in terms of performance and applicability. It can greatly simplify the installation procedure and enhance safety. It can improve the accuracy and repeatability in the presence of changes in temperature, density, and composition, reducing maintenance needs. Moreover, the unique capability to measure the velocity profile is unavailable in current clamp-on meters. It also provides better sensitivity to the flow velocity with a wide dynamic range (i.e., can be used to measure very slow flow to very fast flow speeds). It can enable the accurate measurement of both laminar and turbulent flows, thus enhancing the linearity across a wide range of flow speeds. These improvements are expected to expand the application space of clamp-on flowmeters and thus offer an important competitive advantage to existing technologies. The availability of high-performance clamp-on sensors will also be a competitive advantage to end-users, e.g. in the oil-and-gas, and chemical industries.

