The Buyer s Guide to Ultrasonic Flowmeters

Transcription

1 Spire Metering Technology The Buyer s Guide to Ultrasonic Flowmeters Selecting the Right Flowmeter for Your Liquid Application

2 Introduction Today s advanced ultrasonic flow meters enable the accurate measurement of liquid flow across a wide variety of process control, water resource management, and energy consumption applications. Ultrasonic flowmeters have many advantages over conventional mechanical or magnetic flowmeters, specifically: They don t have moving parts and therefore require minimal, if any maintenance. Unlike traditional flowmeters, they don t introduce obstructive components into the pipe, and therefore eliminate any interference to the flow profile and pressure. In fact, clamp on ultrasonic flowmeters require zero pipe work and never come in contact with the liquid at all. This white paper will provide you with the necessary information on today s common ultrasonic flowmeter technologies and principles enabling you to select the right ultrasonic flowmeter for your specific liquid application. Basic Flowmeter Classifications Liquid ultrasonic flowmeters can be classified in four basic ways: 1. Measurement principle: Transit-time vs. Doppler Transit-time flowmeters are based on the time difference between upstream and downstream sound propagation intervals, and provide one of the most reliable and time tested measurement principles available to date. This approach provides very good accuracy (±1-2%), and works well for clean flow applications or flow with minor particles. Applications include pure water, sea water, wash water, sewage, process liquids, oils, chemicals, and any homogeneous liquids which are capable of ultrasonic wave propagation. Doppler flowmeters are based on the Doppler Effect. They work well with suspension flows where particle concentration is above 100ppm and particle size is larger than 100um, but less than 15% in concentration. Doppler is easier to make and less accurate (±5%), thus, it is cheaper than a transittime flowmeter. 2. Device Portability: Handheld vs. Wall-mount Handheld (or portable) flowmeters are effective for flow survey, HVAC, and other applications where mobility is a critical requirement. Some handheld flowmeters can be deployed as long-term measurement solutions when and if the environment is accommodating. Wall-mount (or fixed installation) flowmeters are more suitable for applications such as process control and long-term continuous flow monitoring. The instrument enclosure is often weatherresistant or explosion-proof. These types of meters are usually cheaper than handheld devices because they are normally ordered in large quantities.

3 3. Transducer installation: Clamp-on vs. Wetted Clamp-on flowmeters are non-intrusive, easy to install and easy to maintain. The transducers are mounted outside of a pipe, and don t require the cutting or drilling of the actual pipe. Clamp-on transducers are nonintrusive and therefore cause no pollution or pressure drop in the liquid being measured. There are two varieties of wetted flowmeters, insertion and flow cell (inline type). Insertion flowmeters require the pipes to be drilled to install the transducers. However, a number of manufacturers provide hot-tapping tools which allows for the transducer installation without depressurizing the pipe or shutting down the flow. Flow cell, also called spool piece are installed in line with the pipe, through the use of special pipe fittings. The transducers are pre-installed on the flow cell, thus, installation errors are kept to a minimum. In general, wetted transducers provide better accuracy and long-term stability over clamp-on varieties, but usually cost more. 4. Transducer scheme: Single-path vs. Multi-Path Single-path flowmeters utilize one pair of transducers to form one ultrasonic path to intercept the flow in a pipe. It is suitable for small and medium size pipes. For larger pipe sizes, shorter straight pipe runs and greater accuracy, multi-path transducer installation is a good option. Multi-path flowmeters are traditionally more expensive than a single-path flowmeters, and are often implemented in applications such as custody transfer where accuracy is of paramount important. Common Measurement Technology Principles Ultrasonic flowmeters use one of four basic measurement principles. They are: 1. Transit-time Technology One of the most reliable and time tested measurement principles is Transit-time flow measurement. A typical transit-time ultrasonic flowmeter system utilizes one pair of transducers that function as both the ultrasonic transmitter as well as the receiver. We will explore Transit-time flow measurement in greater detail in the next section. 2. Ultrasonic Doppler Technology In Doppler style flowmeters, two ultrasonic transducers are employed in the system. One transmits a continuous ultrasonic wave into the flow. The other transducer receives the ultrasonic wave scattered from suspending

4 particles (or targets). The received wave has a frequency shift comparing with the transmitted one. This shift is the so-called Doppler frequency shift, which is directly proportional to the flow velocity. Therefore, by detecting the Doppler frequency, we are able to derive the flow velocity. The flow rate of the pipe liquid is obtained by computing the product of the velocity and the cross-section area of the pipe. 3. Cross-Correlation Measurement Technology The Cross-Correlation principle provides accurate and reliable flow measurement. A turbulent flow has a cascade structure of eddies. Along the flow, the characteristics of those structures do not change much within a certain distance, called the Correlation Length. It is by tracking particles along this length, that Cross-Correlation measurement is achieved. With Cross-Correlation technology, we place two pairs of ultrasonic sensors along the flow direction to pick up the turbulence signature. The upstream sensor will detect a flow signature L/V seconds earlier than the downstream sensor, where L is the space between the two sensors and V is the flow velocity. By comparing the signals from the two sensors, we are able to determine the time delay, and thus, calculate the velocity. Due to the random nature of turbulence, the signal from the sensor is usually a random signal. In order to get more stable results, a cross-correlation-based technique must be used to estimate the time delay. 4. Acoustic Doppler Velocity Profiling Technology Three acoustic transducers are installed as shown in the left figure. The center one is usually a narrowbeam transducer. It transmits a burst of sound pulses with a certain Pulse Repetition Frequency (PRF). The other two are large angle transducers. They receive the ultrasonic wave scattered from suspending particles (or targets) of the water column insonified by both the transmitting and receiving sound beams. By using a pulse-to-pulse algorithm, we are able to obtain the Doppler frequency shift at two directions, from which a 2D velocity profile can be formed. The Basic Principles of Transit-Time Measurement One of the most widely accepted ultrasonic methods Transit-time flow measurement utilizes two ultrasonic transducers that function as both the ultrasonic transmitter and receiver. The flow meter

5 operates by alternately transmitting and receiving a burst of sound energy between the two transducers and measuring the transit time that it takes for sound to travel between the two transducers. The difference in the transit time measured is directly and exactly related to the velocity of the liquid in the pipe. To be more precise, let's assume that Tdown is the transittime (or time-of-flight) of a sound pulse traveling from the upstream transducer A to the downstream transducer B, and Tup is the transit-time from the opposite direction, B to A. The following equations hold: Tdown = ( D / sinq ) / ( c + V*cosq ), (1) Tup = ( D / sinq ) / ( c - V*cosq ), (2) Where c is the sound speed in the liquid, D is the pipe diameter and V is the flow velocity averaged over the sound path. Solving the above equations results in: V = ( D / sin2q ) * T / (Tup * Tdown), (3) Where T = Tup - Tdown. Therefore, by accurately measuring the upstream and downstream transittime Tup and Tdown, we are able to obtain the flow velocity V. Subsequently, the flow rate is calculated as following: Q = K *A* V, (4) Where A is the inner cross-section area of the pipe and K is the instrument coefficient. Usually, K is determined through calibration. From equations (3) and (4), we see that the measurement results, V and Q, are independent of fluid properties, pressure, temperature, pipe materials, etc. The sound speed term does not appear in the final equations. These characteristics, plus large turn-down ratio, no pressure drop, no moving parts, no disturbance to the flow and many other features, make ultrasonic transit-time flowmeter extremely attractive. Transducers of either type can be used in Transit-time measurement; clamp-on type or wetted type. The wetted type can be further categorized into insertion type and flow cell (or spool piece) type. A brief comparison among those types can be found in the below diagram. The transducers can be mounted in three ways: Z-method, V-method and W-method. With the Z- method, the two transducers are mounted on opposite sides of the pipe (see top figure below) and the sound pulse crosses the pipe flow once. This method is usually used for larger pipe sizes, above 12".

6 With V-method, the two transducers are mounted on the same side of the pipe and the sound pulse crosses the pipe flow twice. This is the most commonly used installation method, and is applied with pipe sizes from 1" through approximately 12". With W-method (see bottom figure below), the two transducers are mounted on the same side of the pipe. However, the spacing between the two transducers is doubled when compared with the V- method. The sound pulse is bounced twice from the other side of the pipe, thus intercepting the flow four times. This method is used for small pipe applications, usually less than 1 1/2", for better accuracy. It should be mentioned that the actual implementation of Transit-time flow measurement solutions are very complex and introduce a variety of challenges including: How to obtain accurate measurement of transit-time solutions How to reduce inconsistencies between the upstream and the downstream signal paths How to provide stable results when signal quality is degraded due to pipe age, low sound conductivity fluids, the presence of minor particles or air bubbles, and other factors How to treat the short-circuit wave (or pipe-wall born wave) How to eliminate installation errors

7 What transducers are available for high temperature applications How to select an easy-to-install and use flowmeter What should a solution cost? Selecting the Right Ultrasonic Flowmeter The most commonly used ultrasonic flowmeter is the transit-time, single-path flowmeter due to its proven accuracy, flexibility, and low cost. Depending on the applications, the selection of the right type of flowmeter may vary. You may need to answer the following questions before making the final decision: Is the pipe full? All ultrasonic flowmeters require that the pipes being measured are full of liquid. There are some effective approaches and available work-arounds for working with partially-full pipes, be sure to consult with the manufacturer or a factory trained technician. Does the liquid have particles more than 200ppm and a particle size larger than 75um (the number may be different for different manufacturers)? If yes, you might want to consider using the Doppler principle. Does the liquid temperature (particularly high heat applications) fall into the transducer temperature range? Press range in the pipe (if you selected a wetted transducer)? What is the desired level of measurement accuracy? In general, multi-path provides more accurate measurement than single path transducers, flow cell is better than insertion, and insertion is better than clamp-on. However, if you have on-site calibration facility, the story will change. Do you want the capability of moving the flowmeter instrument from one location to the next? If so, consider a portable/clamp-on solution. What output signals do you need? Analog? Digital? RS232? Data logger? What are your safety requirements and considerations? Not all flowmeters can meet the specific safety requirements of all applications, be sure to consult with the manufacturer or a factory trained technician. What is your pipe size range? Pipe material? Lining? Age? What is your flow rate range? Do you have enough straight pipe length? In order to guarantee the desired accuracy, it is normally recommended to have straight pipe equal to 15 times your pipe diameter at the measuring site. If there is a pump or valve on the upstream, the straight length should be increased. What is your budget?

8 For More Information Every flowmeter manufacturer utilizes these standard technologies and principles. As a result, there are a variety of brands on the market, and almost all flowmeter vendors provide some level of application expertise or professional services group that can answer your questions. Regardless, a thoughtful combination of quality, accuracy and cost should be the guiding criteria when selecting a manufacturer or manufacturer s rep. The author of this paper, Spire Metering Technology, is a global supplier of premium Flow Measurement and Energy Metering solutions for commercial and residential applications. In response to growing demand for lower costs and more precise measurement, Spire Metering offers one of the broadest, most affordable lines of ultrasonic and magnetic flow measurement and energy metering products in the industry today. With products made available through a global network of specialty distributors in more than 30 countries across the globe, Spire Metering Technology helps measure water resources and energy consumption for leading organizations like GE, Siemens, NASA, Honeywell, Johnson Controls, Schneider Electric, and over 1,000 customers worldwide. Spire Metering Technology is headquartered in Acton, Massachusetts (USA), 14B Craig Road, Acton, Massachusetts with facilities in Canada, China, and Brazil. For more information please call , Sales@SpireMT.com or visit us on the Web at