The accuracy of open channel ultrasonic flowmeters

Real world working conditions on water sites mean that ultrasonic level sensors used on open channel flumes and weirs can be significantly in error, because of ambient changes: is there a solution?

Ultrasonic level measurement techniques have been used for around 25 years for open channel flow measurements in the water industry, working with primary devices like flumes and weirs.

The ultrasonic measurement, being non-contact, was attractive in that it offered minimal need for regular maintenance, and could work directly in the main flow stream, ie did not need the stilling well previously used for float-based level measurement systems.

But an ultrasonic level measurement device measures the distance from the transducer face to the target, the water surface.

Since the flow is derived from the height of the water above the weir, or base of the flume, or point of the V-notch, all the measurements are referred to this reference point, which is say 500mm away from the transducer face, in a small system.

How accurately will an ultrasonic system measure this distance? Obviously the flow measurement reading at very low flows is going to be relatively inaccurate, because the flow measurement is derived from the difference between two relatively large distance readings, and the instrument accuracy at say 1% gives a possible measurement error of 5mm.

Some level measurement equipment specifications can offer a better measurement repeatability, such as 0.25%, but usually specify a minimum accuracy value of around 3mm, related to the step size in the waveform, dependent on the wavelength of the ultrasound being used.

The significance of this large error at very low flows is not important, when computing the daily total flow, assuming there are some periods of high flows, where the discharge flow is at a higher level in the channel.

These higher flows can be measured hopefully to a better accuracy, and since they represent the largest contribution to the total daily flow, the MCERTS effluent discharge requirement of an overall total daily measurement accuracy of +/-8% is achievable (http://publications.environment-agency.gov.uk/pdf/GEHO0105BKAO-e-e.pdf).

The MCERTS testing of the performance of open channel level measurement devices is very much a laboratory exercise, and the practical conditions, particularly outside in the open, on a discharge to a stream for example, can be very different! This has been shown up on various water industry evaluation tests over the years.

Let’s look at some of the on-site problems that can be found.

SPEED OF SOUND.

This varies with the temperature of the air that the sound is passing through.

Every 6C change of air temperature changes the speed of sound by 1%.

Most manufacturers put a temperature sensor in the ultrasonic transmitter head to make an automatic correction.

This does not really work very well in any conditions of fairly rapidly changing temperatures, and also is really upset by the summer sun shining on the sensor head, making it rise significantly above the air temperature.

In extremes there might be up to 24C error or 4% on the 500mm range measurement, ie 20mm.

Several manufacturers, and some users, add sunshades to their sensors, to reduce this problem.

The speed of sound also varies a little with gas composition and humidity, but for open atmosphere measurements these will not be significant.

Humidity only affects the speed of sound in tanks where the liquids are heated above say 80C.

What no theory tells us, and what might only be observed on site, is how the speed of sound might be affected by the water droplets in an early morning mist, in cold conditions, rising off (relatively hot) liquids!.

REFERENCE PINS.

The answer to such temperature problems was originally sought by the use of reference pins, positioned at around 300mm from the transmitter face, giving a small reflection to enable measurement of the speed of sound over this known distance.

The variety with the tube created an ideal place for spiders to live, and interfere with the calibration, and the variety without the tube was an ideal shiny perch for passing birds, flies, butterflies et al.

Plus these reference pins have a problem with the calibration changes caused by water droplets, dirt, ice and snow, to various degrees.

SO WHAT IS THE RESULT?.

1) DAILY TEMPERATURE CHANGES.

With changing temperatures throughout a 24 hour day of maybe a total of 20C in the air, there is a lag in the temperature sensor and the accuracy correction.

A reasonable estimate might be 6C for 3 hours at each end of the day.

So this gives an error of 1% in the speed of sound used.

Take the V-notch weir used in the example quoted on the MCERTS website, with a zero level 500mm from the transducer face.

Say the flow is running at 120mm above the notch, giving a flow of 1.446 LPS.

The sensor to surface range is 380mm, so a 1% error is 3.8mm, and the indicated flow at 123.8mm is 1.563 LPS.

(Note that this example is for the temperature in the sensor head being 6C lower than the air temperature, leading to a higher flow reading than should be recorded).

The V-notch weir is the most extreme example, because of the 5/2 power law, but this flow error resulting from a 3.8mm inaccuracy in measurement is just over 8%.

This error is on top of any other errors in electronics timing, the weir itself, and the set up of the unit, and it is at a reasonably high flow rate, therefore significant in the daily total calculations.

What is the conclusion? Inevitably, the conclusion is that for accurate measurement there is a need to look closely at how the speed of sound is calculated, particularly in relation to the use of sun shields, the positioning of the temperature sensor, and also, the choice of flume/weir, with the ultrasonic sensor as close as possible to the flow.

But it is still not necessarily going to give the accuracy that might be needed.

2) SOLAR GAIN EFFECTS.

This has become identified as a significant problem, even in UK installations, and even when sunshades are used: a sunshade is not normally the complete answer.

The temperature reading from a sensor in the ultrasonic transmitter, positioned in the full glare of sunshine above the flow stream, can be maybe 20-25 degrees hotter than the actual air.

This solar gain effect, fed into the open channel flow calculation, produces an error in the monitored flow rate and totals: the recorded flow value is typically much lower than the actual flow, (with a sensor reading a high temperature) and the under-reading error can exceed the 8% considered the limit for MCERTs installations by a long way.

Apparently such discrepancies have been reported in sunny conditions at several installations in Anglian and South West Water, and have led to some evaluation trials to quantify the problem, and test the possible solutions.

A display of the temperature being used by the flow calculation is relatively easy to see on most micro based units via the standard keyboard.

Alternative approaches are being proposed to solve this problem, but a sunshade seems a sensible first step!.

DUAL SENSOR APPROACH.

A new approach has just been announced by Pulsar Process Measurement, that uses two sensors, as a variant of the reference pin system.

One sensor is mounted higher up than the other, at a known extra distance from the liquid.

Both sensors transmit a pulse towards the liquid surface at the same time.

Only the lower sensor acts as a receiver.

This sensor detects and times the arrival of its own pulse, and then does the same for the pulse from the higher sensor.

The time between these two pulses allows the unit to calibrate for the speed of sound in the air space between the two sensors, at that moment.

Given good air mixing, all the worries of temperature changes through the day are solved.

The system is on trial in various water authorities where their applications suggest that a more accurate system is needed.

SEPARATED TEMPERATURE SENSOR.

The original Mobrey MSP90 system, in use for open channel flow metering since the late 1980s, had the capability to accept a separately installed air temperature sensor, wired into two terminals in the sensor head.

(I have to admit rather detailed knowledge of this from working for Mobrey at the time and specifying the original MSP90 unit).

Water industry evaluation trials discovered that by adding this temperature sensor on site, and positioning it in a shaded or North facing area within the flume, improved the temperature, and therefore the flow measurement accuracy considerably.

Using this technique, Anglian Water was able to upgrade the accuracy of the data available from a large number of their installations, in all weather conditions.

More recently, further trials were organized in South West Water, and these included an evaluation of the new MSP900FH unit, designed to be used with the separated temperature transmitter.

Robin Lennox of SWW confirmed that there had been some concern about the accuracy of open channel metering systems across the seasons, and initial investigations suggested that the effects of solar gain on the temperature sensors embedded in the ultrasonic transmitters were significant.

It was necessary to find a way of improving the accuracy of the measurement data.

Trials with the Mobrey Measurement MSP900FH showed that the use of a separated temperature sensor provided the improvement sought .

Results on their test rig were generally within ± 0.5% on distance measurement (for typical installation dimensions) when the sensor was sited to monitor air temperature from a properly shaded position.

RADAR?.

This could be the next step: but the current lower cost free to air radars currently seem to have around a 10mm accuracy minimum – but maybe there could be a further development to satisfy the application, if the demand grows.

Radar from Vega Controls has already been applied to tide height measurements on the Humber Estuary, it just needs scaling down a bit!.

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ATEX and IECEx flexible conduit systems from Kopex

In a major press launch this week, Kopex International presented their current plans and product developments for hazardous area electrical installations on process industry installations.

The Kopex Group includes the four major product trade names: Adaptaflex for flexible conduit systems in general application; Elkay for electrical switches and controls; Harnessflex for flexible conduit and fitting systems for cables in vehicle wiring harnesses; and Kopex for the higher specification, generally metallic cored conduit systems for protecting cables in hazardous areas, with the associated cable glands and thread converters.

Formed as Uni-Tubes in Slough in 1947, the company became part of Smiths Industries, presumably because of the Smiths interest in the major business area of car tubing and cable components.

The Kopex name originates from the Kopex machine, an innovative manufacturing system developed in the 1960s, which enabled production forming of convoluted tubing from a wide range of strip materials: the current main factory and HQ is in Coleshill in the Midlands.

With a management buyout earlier this year, the company focus is on the individual development of the major brand strengths, with significant investment already in 2007, and planned for 2008: the business growth potential for the Kopex approved conduit and cabling systems is seen as significant.

The Kopex Group is now the largest manufacturer of flexible conduit systems in the EU, having produced 14.5M metres in 2006.

The presentation of the current market figures (from IMS Research) for Europe was interesting, with Germany and UK taking around 20% each, and France, Italy, Benelux and the Nordic countries each taking around 10%, with Eastern Europe very low.

The latest ATEX137 requirements became an EU mandatory requirement in 2006, so that there should be a major demand growth, with the required upgrades in various countries.

This is where Kopex see a major spur to their business, and they have developed a range of 500 flexible conduit, cable gland and thread adaptor products with the necessary ATEX approvals.

Kopex were the first manufacturer to gain ATEX approvals for conduit systems in nylon and steel, first to introduce systems for both Exd and Exe requirements, with the associated adaptors and convertors, and also first to introduce an ATEX flameproof gland for Exd enclosures, compatible with the KopexLT flexible cable conduit system.

The ATEX approved systems are basically for use in Europe, to meet the EU requirements that arise from ATEX137.

There is still a lot of education needed of the users, who do not necessarily realise that the legislation applies to all plants who have areas where hazardous dusts or gases might be present: indeed some grain, food and flour mills, or plastics processors, may not have realised the hazard present in the powders on their sites.

Kopex have produced literature trying to explain the requirements, and explaining their different product ranges available with the necessary approvals for flexible conduit systems.

Outside Europe, the difficulties of the different international electrical regulations and approvals have held up the use of standard designs of plant and working practices for international companies, such as Shell, BP, Rolls Royce and Siemens.

The IECEx international approval system has been developed to make such standardisation possible, allowing one equipment approval to be acceptable to all.

With countries like Australia, USA, S Africa, China and Russia now accepting IECEx approved equipment and installations, this is becoming a world-wide reality, and Kopex has ensured that their latest products are designed to meet both the ATEX and the IECEx approval requirements.

Contracts using IECEx are already being one by Kopex in Australia, in the mining industry, and in Saudi Arabia and the Gulf, where the previous dominance of US codes of practice has disappeared.

Kopex advised that the US National Electrical Code Appendix 5 has enabled the use of IECEx approved equipment in electrical installations.

Combustible dust accidents reported by OHSA

This article presents examples from an OSHA report on ‘Combustible Dusts in Industry’ to illustrate the hazards of dust explosions, in relation to the current DSEAR requirements for hazard assessments.

The OSHA report entitled ‘Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions’ is a Safety and Health Information Bulletin (SHIB) that highlights the hazards associated with combustible dusts.

The examples are from actual accidents reported in the USA.

1) Organic Dust Fire and Explosion: Massachusetts (3 killed, 9 injured).

In February 1999, a deadly fire and explosion occurred in a foundry in Massachusetts.

The Occupational Safety Health Administration (OSHA) and state and local officials conducted a joint investigation of this incident.

The joint investigation report indicated that a fire initiated in a shell moulding machine from an unknown source and then extended into the ventilation system ducts by feeding on heavy deposits of phenol formaldehyde resin dust.

A small primary deflagration occurred within the ductwork, dislodging dust that had settled on the exterior of the ducts.

The ensuing dust cloud provided fuel for a secondary explosion which was powerful enough to lift the roof and cause wall failures.

Causal factors listed in the joint investigation report included inadequacies in the following areas: Housekeeping to control dust accumulations; Ventilation system design; Maintenance of ovens; and, Equipment safety devices.

2) Organic Dust Fire and Explosion: North Carolina (6 killed, 38 injured).

In January 2003, devastating fires and explosions destroyed a North Carolina pharmaceutical plant that manufactured rubber drug-delivery components.

Six employees were killed and 38 people, including two firefighters, were injured.

The U.S Chemical Safety and Hazard Investigation Board (CSB), an independent Federal agency charged with investigating chemical incidents, issued a final report concluding that an accumulation of a combustible polyethylene dust above the suspended ceilings fueled the explosion.

The CSB was unable to determine what ignited the initial fire or how the dust was dispersed to create the explosive cloud in the hidden ceiling space.

The explosion severely damaged the plant and caused minor damage to nearby businesses, a home, and a school.

The causes of the incident cited by CSB included inadequacies in: Hazard assessment; Hazard communication; and Engineering management.

The CSB recommended the application of provisions in National Fire Protection Association standard NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, as well as the formal adoption of this standard by the State of North Carolina.

3) Organic Dust Fire and Explosion: Kentucky (7 killed, 37 injured).

In February 2003, a Kentucky acoustics insulation manufacturing plant was the site of another fatal dust explosion.

The CSB also investigated this incident.

Their report cited the likely ignition scenario as a small fire extending from an unattended oven which ignited a dust cloud created by nearby line cleaning.

This was followed by a deadly cascade of dust explosions throughout the plant.

The CSB identified several causes of ineffective dust control and explosion prevention/mitigation involving inadequacies in: Hazard assessment; Hazard communication; Maintenance procedures; Building design; and, Investigation of previous fires.

4) Metal Dust Fire and Explosion: Indiana (1 killed, 1 injured).

Finely dispersed airborne metallic dust can also be explosive when confined in a vessel or building.

In October 2003, an Indiana plant where auto wheels were machined experienced an incident which was also investigated by the CSB.

A report has not yet been issued, however, a CSB news release told a story similar to the previously discussed organic dust incidents: aluminium dust was involved in a primary explosion near a chip melting furnace, followed by a secondary blast in dust collection equipment.

5) Related Experience in the Grain Handling Industry.

In the late 1970s a series of devastating grain dust explosions in grain elevators left 59 people dead and 49 injured.

In response to these catastrophic events, OSHA issued a ‘Grain Elevator Industry Hazard Alert’ to provide employers, employees, and other officials with information on the safety and health hazards associated with the storage and distribution of grain.

In 1987, OSHA promulgated the Grain Handling Facilities standard (29 CFR 1910.272), which remains in effect.

This standard, other OSHA standards such as Emergency Action Plans (29 CFR 1910.38), and updated industry consensus standards all played an important role in reducing the occurrence of explosions in this industry, as well as mitigating their effects.

The lessons learned in the grain industry can be applied to other industries producing, generating, or using combustible dust.

Cooper Crouse-Hinds publish this report on their website, in relation to the supply of their equipment approved for use in such hazardous areas, particularly relating to the ATEX137 and DSEAR regulations recently introduced.