The measurement of level and flow is crucial to effective and efficient waste water treatment. Ultrasonic technology promises that and some more.
"I wasn't so much about winning an order than about the opportunity to increase awareness about the benefits of ultrasonic technology in the water/wastewater treatment plants in this region," said David A Hewitt, CoC Process Sensors Manager Middle East, Siemens Automation & Drives. Hewitt was referring to a six month trial deployment of Siemens' ultrasonic level meters at the STP of a public works authority in the GCC.
The meters were used to control and monitor the operation of two pumps in a dry well, in order to prevent spillage of effluent, provide reliable information to the control room on the level status in the pumping station (interfacing with an existing Remote Terminal Unit - RTU), eliminate maintenance issues associated with contact measurement equipment and provide reliable, continuous measurement of level on a 24x7x365 basis.
Siemens installed and commissioned an Echomax Transducer (for monitoring the well) and a HydroRanger 200 level controller (to process the transducer signal and feed the 4-20 mA output to a central control room for continuous level reading) at the site. A third party data logger was installed to monitor and capture data in order to validate the performance of the level controller and enable a comparison with the RTU data. Hewitt said, "Following the installation, we visited the site three times. The unit performed successfully without any spillages and continuously updated the Authority's central data storage with the level status of the station. Due to its non-contact nature, the unit did not require any maintenance."
Hewitt believes when it comes to flow and level measurement in waste water treatment, ultrasonic is the way to go. The single phrase that sums up the benefits of ultrasonic is non-contact. "You have a choice in measuring sewage level, by being in contact with it or not being in contact," he said.
Level devices can be continuous like ultrasonic and radar or point-level like floats and hydrostatic. Contact measurement devices like floats, bubbler and hydrostatic systems are vulnerable to clogging and other problems caused by suspended solids, harsh corrosives, grease and silt in the effluent, and need to be cleaned and replaced often. The higher maintenance costs and repairs associated with contacting devices can exceed any initial savings. Moreover, there is also a high element of risk to personnel going down into wet wells for maintenance of contact equipment.
Hewitt pointed out that ultrasonic meters are amenable to quicker installation and configuration, with three hours being the approximate time it took his team to get the trial site up and running. The unit was also configured on site. "The unit acts as an ultrasonic tape measure. Once the empty distance is programmed in, the unit will base all its calculation from this reference. The empty distance could be from the transducer face to the bottom of the well or wherever you want the end point to be. The unit can be commissioned, whether the well has sewage in or not," said Hewitt.
He went on to sum up the key features of the trial unit: Two power supply choices 110/240 V AC and 12-30 V DC; a choice of two transducers - 0-8 metres and 0-15 metres; completely sealed (IP65) weather resistant housing for the unit; all the transducers are rated to IP 68 i.e. 100% submersible, should an overfill condition occur, Password protection to prevent of unauthorised programming; alphanumerical LCD display and bar graph to indicate the status of the pumping station well and which pump is running; two 4-20 mA output signals and up to six relays that can be programmed for pump sequences, such as duty standby, duty assist or percentage ratio.
Hewitt also used the opportunity to highlight the advantages of ultrasonic over hydrostatic. "Hydrostatic devices can give only 4-20 mA, a floating switch can give only on/off contact. For ultrasonic, it depends on what you want it to do. If you want it to give 4-20 mA, it will; if you only want it to give a contact closure, it will do that too. Moreover, ultrasonic can be configured on-site while hydrostatic devices have to be calibrated in the factory. So if you order them wrong, you are stuck."
The numerous references to pumps brought the discussion to the core application of level measurement, which Hewitt noted, "is primarily a means to the end of efficiently controlling a process using pumps and valves." And the best candidate for that is, of course, ultrasonic level meters because "they can provide the information you need to monitor and control pumps effectively." Hewitt noted that most ultrasonic meters including Siemens' come equipped with relays for simple pump control that facilitate, for instance, economy pumping by scheduling pumping for off peak periods to save on energy costs; monitor pump performance people including pump status, efficiency and total pumped volume so that failing or under-performing pumps can be repaired, replaced or used less. "Where a plant has pumps of varying capacities, you can program the controller to turn the pumps on and off depending on ?ow. You want to engage the high capacity, high energy-consuming pumps only when needed," said Hewitt. Advanced controllers offer a variety of pump control routines and subroutines such as ?xed duty assist and ?xed duty backup; alternate duty assist and alternate duty backup; service ratio duty assist and service ratio duty backup. These routines manage pumps efficiently to save energy. This information enables plants to optimise operations with pump control by level rate of change, service ratios to balance pump run times, pump start delays to reduce power surges, pump run-on or valve ?ushing to clear sediments. The end result is a reduction in scum line build-up and more effective routine maintenance. "The range of control options is extensive with relays and current outputs as standard, but digital communications like Profibus is becoming also more common," observed Hewitt. Advanced systems also enable monitoring of flow trends. By doing that across the entire network, plants can identify and address current and future problems. For example, one of the big challenges over time is that of specifications to meet future population demand. It is common for pumps to be under or over-speci?ed. Trend ?ow monitoring identifies increases in demand for future planning and engineering.
So how should one go about selecting ultrasonic level measurement systems? Hewitt indicated that customers are concerned mainly about accuracy and reliability. However, accuracy and reliability are seen differently by different people. He continued, "If we take measurement accuracy, when customers state they want to stay within +/-6mm, they mean that irrespective of the time of the day or the season, irrespective of where the level is in their wet wells, they want to know it to within 6mm. For the product designers, it is about testing the product in the laboratory under ideal conditions and applying standard statistical methods to determine accuracy, repeatability, and linearity. We end up with a situation where all the products can 'state' that they meet the required accuracy over range."
Similarly, reliability for the customer is often reliability of the measurement not the product, whereas the product designer takes that requirement at face value and builds in product reliability.
However, measurement devices today are more or less meeting the user expectations on accuracy and reliability fronts. They are expected to work first time out of the box and still be working a year later in the field. The twist in the tale is their perceived measurement reliability over time.
Measurement reliability
Hewitt pointed out that skilled technicians control and carry out tests in the laboratory under ideal conditions. Though the design process may have included extended site trials on carefully selected applications, again these would have been attended by skilled personnel who adjusted and fine tuned the product for best performance. The fact of the matter is that the world inside a wet well is anything but ideal with various influences that affect the return signal. There are internal structures such as pump cases, benching, guide rails, ladders, braces and struts. The process will have filling and emptying flows that obstruct parts of the well and cause turbulence or foam on the water surface. Ambient noise can be a negating influence and so can temperature. The challenge is to ensure reliable and accurate level measurement under such real life conditions.
An ideal situation would be one where the level measuring product can be mounted clear of all obstructions. However, this is often not possible, and even otherwise, the ideal state could be disturbed once the filling and agitation begins. Hewitt noted that when faced with site constraints and obstructions, the product with the narrowest beam angle has the least chance of accidentally hitting an obstruction and seeing a false echo. But if the beam angle is too narrow, agitation of the water can cause loss of echo at the receiver. "Our experience is that the optimum beam angle is between 6 º and 10º," said Hewitt.
While emphasising the importance of careful selection of mounting and optimised beam angle in getting reliable and accurate measurement, he cautioned that at times, even this could fall short, leading to false echoes and unreliable measurements. One solution would be to install products that can be taught the positions of obstructions giving rise to false echoes so that they can be ignored. But this often requires a reasonable degree of input from a skilled commissioning engineer, and once again, there is an assumption of constancy. "For example, a conventional system using false echo mapping may successfully be programmed to allow for a set of ladder rungs. However, what happens as product build up occurs on the rungs. The shape of the echoes in the pattern changes and so does their apparent position. Eventually, false echoes will again start to appear, and measurement becomes unreliable," said Hewitt. A better alternative, he said, would be a system with built in signal-processing software, like Siemens' Sonic Intelligence, that will not only automatically learn an installation but continue to adapt its internal map as things change. Sonic Intelligence is basically a set of algorithms that analyses echo pro?les (a digital picture of the re?ected echo) and learns to ignore obstructions and false echoes. "Sonic Intelligence distils down more than 20 years of accumulated data and experience with level applications," claimed Hewitt, and went on to expand on its benefits, listed below:
Sonic Intelligence automatically optimises itself for both long and short range measurement by varying the ultrasonic pulses which are fired at the surface to be measured. At short range a narrow pulse is used to minimise blanking distance to within 300mm, at long range a wider pulse delivers more power right to the bottom of the well where it is needed maybe 15m away.
In sump applications where submersion shield is a standard protection for the sensor face, Sonic Intelligence can look at the signature of the echo from the air within the shield. This information can be used to turn on additional high-high level alarms.
Turbulence or foam on the water surface can result in echoes being directed everywhere except back to the receiver. In such a situation, many level measuring products quickly revert to "lost echo mode" and fall back to various modes of 'hold last' reading or simple error output. Sonic Intelligence can determine if the surface is agitated or covered in foam by analysing the statistics of the fluctuating signals which manage to return. With the knowledge of how the level was moving before and also which pumps or valves are open, it can continue to track the level even with intermittent measurements.
As the interview drew to a close, the discussion veered around to how ultrasonic technology faring in the Middle East? Hewitt replied that the ratio is 10:1 in favour of hydrostatic. Why so? Tenders continue to specify hydrostatic devices, and engineers prefer to stick with that because going back to the consultant to change the specs to ultrasonic would lead to time delays in closing the bids. Pump suppliers like KSB and Grundfos use hydrostatic over ultrasonic only because the end-user has specified so. Hewitt ascribes this preference to the hydrostatic mindset of the engineers. "It is more than a job for us to change. We have to slowly put something in for evaluation, make sure it proves itself before we are able to get ourselves in," he said. However, the earlier STP story ended on a successful and pleasant note: At the end of the six month trial, Hewitt and his team discovered that the STP had 24 Siemens ultrasonic units fitted on its treatment works. "This technology is not entirely unknown in this region, having made its way here through overseas contractors. But for it to become more widely prevalent, a mindset change is needed," he noted. Hewitt hopes to build ultrasonic's case in general and his company's strength in particular on the back of the million units installed by Siemens globally.
© H2O 2008
