Ultrasonic Flowmeters - where are we now?
Engineers and flow metering specialists the world over utilize ultrasonic flow meters to measure process flowrates. The research and development and competition within the ultrasonic flow meter market over the last 20 years has served to clean up the industry’s reputation, principally damaged by the use of inaccurate instrumentation and the use of basic Doppler flow meters.
Today, ultrasonic flow meters are not used for specific applications but heavily relied upon to provide accurate flow measurement and process control throughout the world for liquids and gas.
Why choose an ultrasonic flow meter?
- Can be used on almost any liquid including water, sewage, wastewater, oil, some slurries;
- Can be retrofitted to any pipe between 10 and 5000mm: cast iron, steel, plastic, Asbestos Cement and glass.
- Accuracies close to that of electromagnetic flow meters – typically 1-2%.
- Portable and permanent flow meter options with datalogging functions
- No interruption to flow or modifications to pipework
- All the functions of traditional electronic flow meters: relays, multiple signal outputs, communication protocols, HART, MODBUS, RS232.
- Reduced installation costs
- Intrinsically safe and explosion proof flow meters for zone 1 and zone 2 areas.
- Instrumentation costs are now very competitive.
This is all very well, but everyone likes to read an actual application story – well here’s a few:
- Electromagnetic Flow Meter vs Ultrasonic
One 1200mm cast iron pipe and one 1100 steel pipe supplying most of central England with drinking water. Flow meters were required to be installed for regulatory purposes. However, due to a lack of available pipework, suitable chambers and the health and safety implications of draining a major pipeline, clamp on flow meters were used to monitor the flows through these pipes. With approximate costs of $15,000 for a flow meter, installation costs of $70,000 and third party costs of draining a major pipeline, a clamp on ultrasonic flow meter would cost less than 10-15% of that of an electromagnetic and installed in less than a tenth of the time!
Despite only having 10m upstream and 2m downstream, a two-channel ultrasonic flow meter was used to reduce inaccuracies to within 1% of the true flow.
A two-channel Panametrics DF868 was used to measure both pipes and to provide alarm relays should there be any bursts. A preliminary trial was undertaken using a Panametrics PT878 flow meter.
- Portable Ultrasonic Flow Meter vs Permanent
The costs of installing flow meters in a large manufacturing process network can be cost-prohibitive and time-consuming. It is little wonder why the portable ultrasonic flowmeter has become the standard flow measurement tool for survey work. Multiple points can be monitored within minutes to establish any network issues. With an installation time as short as 5-minutes, extensive flow surveys can be undertaken quickly and effectively. For example, RS Hydro engineers installed 28 Panametrics PT878 flow meters to monitor the supply network at a petrochemical refinery within one day. The portable flow meters were used to monitor potable and process water usage across a 100ha site over a period of one week to provide an accurate balance of water usage and leakage across the site.
Portable ultrasonic flow meters can be used as semi-permanent devices providing analog and digital signals to replace that of faulty instrumentation.
- No other device will do…
A nuclear power station was required to install flow measurement systems on 100mm pipes in high risk areas. It was obvious that a clamp on flow meter would be the only option as the transducers could be installed in a matter of minutes. Any other form of flow measurement would take hours, possibly days.
- Energy Monitoring
In todays environment, energy monitoring is now a major issue. Due to new legislation and international regulations, there is increased pressure for energy managers of all organizations and companies to monitor and reduce energy consumption. Measuring energy consumption is now an inexpensive option: ultrasonic clamp on flow meters along with two temperature probes and a heat meter allows the user to monitor heat energy consumption at any point in the system. Although spool piece flow meters are used for smaller pipes (typically <50mm), clamp on flow meters can be used on all pipe sizes without interruption to flow. The flow meter is installed without any modifications to the pipework, whilst the temperature probes can be ‘hot tapped’ whilst under pressure. The temperature differential multiplied by the flow equates to energy consumption in KWh or MWh. A network of heat meters can be monitored centrally to provide accurate information about energy consumption throughout a process or organization.
Ultrasonic flow measurement - some interesting developments…
The use of ultrasonic flow meters in open channels, rivers and streams is well-documented. Non contact sensors used in conjunction with primary structures such as v-notch weirs, flumes and other gauging structures are ubiquitous throughout the world. Contact Doppler flow meters are also commonplace, but primarily for small streams, temporary monitoring and difficult applications. The methods mentioned previously are commonly used to measure flows in small rivers of up to 5-10m in width. In rivers anc channels larger than this multi-path transit time flow meters can also be used to measure rivers up to a 1km in width. However, significant developments in signal processing has meant that open channel flow meters are more accurate than ever.
Cross-correlation open channel flowmeters - unlike standard open channel doppler flow meters, cross correlation flow meters send a series of ultrasonic signals into the liquid. After emitting the signal, the ultrasonic receiver stores the echo pattern. Shortly after, a second impulse is sent and that pattern is also stored. By mapping this shifting echo it is possible to measure to map the profile of the water, subdividing it into multiple layers. One such device can subdivide the profile into 16 different layers.
Advances in technology in pipe flow measurement have also been rapid - Cross-correlation gas and liquid flow meters: in a manner not dis-similar to the open channel method, the cross-correlation flow measurement principle has also been applied to pipe liquid flow measurement. Clamp on flow meters are used throughout the world, in almost every industry. However, there are many applications where standard clamp on flowmeters will not work due to the gas either being too low in pressure or absorbing the emitted ultrasonic signals, or the level of accuracy required is much greater than for a standard ultrasonic flow meter. There are two technologies available – for liquids, a transmitter/receiver can be connected into the pipeline to map movements of particles. In gas, two pairs of transducers act as one way transmitters and receivers. The first pair of diagonally located transducers emit and receive an ultrasonic impulse. The distortion from the first pair is recorded. The second pair, generally located 10” (250mm) downstream of the first pair also record the echo. When these echos are mapped, it is possible to measure the velocity and hence volume. In a recent project, the installation of a cross-correlation flow meter on a gas receiving station was used as a check meter for fiscal gas flow meter. The accuracy of the check meter was within 0.5% of the fiscal flow meter.
It is not difficult to see why ultrasonic flow meters are the fastest growing form of flow measurement technology. Whether your application is open channel, liquid or gas flow measurement varying technologies can be used to measure flow or mass flow without interruption to your process.
There’s always s BUT right???
Yes, but there’s only one, and put quite simply, ultrasonic flow meters are only as good as the people using them. Although many systems can be used by semi-skilled personnel, the best results come from people who are prepared to read the manual!
This article was written by R Stevens of RS Hydro Ltd
www.rshydro.co.uk
info@rshydro.co.uk
