MCerts: The reality
Tony Hoyle, director of the Sensors for Water Interest Group (Swig), discusses the impact of flow monitoring regulations on key stakeholders and describes the measures that water companies can take to ensure that their flow monitoring schemes are up to scratch
The Environment Agency’s Monitoring Certification Scheme (MCerts) is designed to ensure that the equipment and systems that are supposed to tell us about the quality and quantity of industrial discharges are doing their job properly.
The scheme is well established for emissions to air, but it is at a relatively early stage of being applied to water. Predictably enough, MCerts have their critics, with end users and instrument suppliers pointing to the inevitable added costs. But MCerts can deliver positive benefits to all the key stakeholders. For water companies and other industrial sectors involved in abstracting water, outflows or discharging effluent, MCerts provide extra confidence that they are keeping on top of their obligations under the Pollution Prevention and Control (PPC) regulations and other relevant legislation. Similarly, the Environment Agency (EA) can be confident that companies are complying with their permits, allowing it to encourage more self-monitoring within the industry.
Even the instrument suppliers – who must bear the brunt of initial cost increases as they apply to have their products certified – will benefit in the long run. By having good engineering practice enshrined in regulatory requirements, responsible practitioners can be sure that they are operating on a level playing field and are not being forced to compete with cowboy operators whose equipment may be cheaper but may not be up to scratch.
Ultimately, of course, it is the environment that will benefit, and that’s good news for all of us. MCerts support the EA’s Modernising Regulation Agenda, which places increasing emphasis on self-monitoring by all potential polluters, including the water industry.
The scheme should help ensure that the industry’s efforts at self-monitoring are transparent and credible. There is a rolling programme to apply MCerts throughout industry. In some industries, that simply means facilities covered by PPC. But for water companies, MCerts is also an issue in any processes that still fall under Integrated Pollution Control, any processes making consented discharges regulated through the Water Resources Act and sites falling under the Urban Wastewater Treatment Directive.
The latest EA advice is that MCerts already applies to the largest WwTWs, with other water industry sites coming under the scheme over the next two years. MCerts takes a two-pronged approach. First, instruments must be tested and certified to verify the quality of the data they deliver. Second, sites must also be inspected in order to check for sound practice.
On the equipment side, the EA obliges the operator of any relevant site to use MCerts-approved instruments where available. Three categories of equipment for monitoring discharges to water are currently covered by the scheme: automatic wastewater samplers, online analysers and flow meters for continuous monitoring. Guidance on the requirements for all three categories is available on the EA website, but this article will concentrate on flow meters.
There are four types of application that require MCerts-approved water and effluent flow meters. For raw water abstraction, meters may only have to measure the total volume passed unless the abstraction licence also limits the rate of abstraction, in which case that will also be required. These meters must be accurate to between 2% and 10%, depending on the water company’s licence.
Effluent discharge monitoring calls for both the total volume and the flow rate to within 8% of the daily volume, while ultraviolet disinfection monitoring requires the flow rate to within 8%. The criteria for industrial processes are unspecified, enabling inspectors and companies to choose the most appropriate measures.
Sira Certification Service is the body responsible for testing and certifying MCerts equipment, but so far it has certified just three flow meters from only two suppliers. So for now it’s a question of “watch this space” as the scheme gathers pace and more manufacturers pay to get their instruments certified.
The certification process has three stages. Laboratory testing determines the performance characteristics of the meter in a controlled environment, while field testing checks the performance of the meters on representative processes and applications. Initial and ongoing surveillance inspections then check that the supplier has the right manufacturing controls in place to deliver reproducable results. It can be a time-consuming process. For manufacturers marketing stack monitoring equipment, for instance, where MCerts is far more established, certification typically takes six to 12 months.
An inspector calls
The other arm of the scheme is the site inspection process. Once again, Sira operates the scheme on behalf of the EA, this time delivering the service through six competing companies. Reports from industry suggest that these inspections have already started to achieve the desired effect, with inspectors spotting and rectifying problems with existing flow monitoring schemes.
For example, they’ve spotted cases where open channel systems have become overgrown and even cases where the flow measurement concrete flumes were installed back to front. Water companies should not be waiting for inspectors to point out problems.
They should be acting now to ensure they are operating best-practice systems using the best available technologies. So, for example, if there is a problem with accuracy, water companies could consider switching from an open-channel system to a more accurate technology, such as a closed-pipe system with a magnetic flowmeter. They may consider this even though there are no MCerts-certified magmeters available yet, because the generic technology is superior.
It is generally accepted that closed pipe flow meters achieve calibrated accuracies of between +/-0.2% and +/-0.5% at the manufacturer’s calibration rig, making installed accuracies well within the 8% daily limit. Of course, once the first magnetic flowmeters are certified, water companies wishing to switch technologies in favour of magmeters will have to use MCerts-compliant products, giving instrument manufacturers a strong incentive to get their products certified.
For example, are flow meters installed with the correct runs of straight pipework up and downstream? If not, their readings are unlikely to be accurate. Most flow meters are designed and calibrated to give an accurate reading for a fully developed turbulent flow profile. For this reason, manufacturers usually specify the minimum number of undisturbed up- and downstream pipe diameters needed to ensure that poor flow profiles don’t affect the readings. Some types of meter are more prone to flow disturbances than others. This depends partly on the measurement principle behind the technology. For example, time-of-flight ultrasonic meters measure the time taken for sound waves to travel through the fluid, so they are particularly vulnerable to undeveloped flow profiles and typically require 10 or 15 pipe diameters upstream, depending on the number of measurement tracks. Even so, meter manufacturers have a bag of tricks they can use to minimise any adverse effects. For example, fitting a cone to narrow the pipe diameter at the entrance to a meter can correct minor irregularities in the flow profile. And for applications where the lack of available straight pipework poses a real problem, flow straighteners may also help. Even so, when it comes to uninterrupted pipe runs, the general rule is the longer the better.
Using flow meters that are the wrong size for the application is another classic mistake. It’s a problem that often arises as circumstances change. For example, a meter may originally have been sized to keep track of water flowing to a thirsty industrial manufacturing area that has now been redeveloped as a retail park.
The drop in demand could lead to the meter struggling to measure flows that are below its lower limit for accurate readings. Water companies should have an ongoing right-sizing programme in place to make sure that their measurement equipment remains appropriate for the current situation.
Correct sizing can also pose a problem on the waste side. For example, storms can flood meters with flow rates many times greater than those they might usually experience. If meters are to cope, they need to have the sort of high turndown ratio that makes them accurate over a wide range of flows. For example, standard electromagnetic meters offer typical turndowns of 50:1, while specialist partial-fill electromagnetic meters offer 100:1. In contrast, venturi-flume open channel meters typically manage just 20:1.
Calibration and verification
Reputable instrumentation manufacturers put a great deal of effort into ensuring that their meters are accurate as they leave the factory and quality instruments should be supplied complete with calibration certificates. But the meter must be periodically verified to maintain confidence in the accuracy throughout its service life.
For example, installation damage may not be spotted immediately. Once the meter is in the ground, no one will know if the magnetic circuit has been distorted, if poor earth bonding is a problem, or if the meter is struggling against EMC interference from noisy pump motor cables or other sources of interference.The ultimate check would be to remove the meter and send it away for recalibration using a specialised Ukas calibration rig. However, this is an expensive option and will be completely impractical in many water industry applications where shutting down operations for any length of time may be out of the question.In-situ testing can be carried out, for example, using an insertion probe.
By taking a series of readings across the pipe, a skilled engineer can gauge the overall flow rate but it’s a demanding job. Clamp-on ultrasonic meters can also be used to check meters, but the problem with applying either of these approaches to electromagnetic meters, for example, is that neither technique is as accurate as the meters they’re trying to check.
It’s a bit like trying to check your digital laboratory scales using a set of bathroom scales. A magmeter is accurate to between +/-0.2 and +/-0.5%, while an aquaprobe or clamp-on ultrasonic device only manage between +/-1 and +/-10%.
In recent years, verification tools have gone a long way to alleviate the problem. Leading examples include Calmaster from ABB and Magflo Verificator from Siemens. These rely on manufacturers installing an electronic fingerprint in the meters during calibration. This fingerprint stores information about the magnetic circuit associated with the individual meter.
The original fingerprint can then be checked against the meter’s current performance using verification software and the verification tool. This will highlight any deterioration. This type of electronic verification has grown increasingly popular over the past 10 years because it is more accurate and less expensive than the in-situ alternatives.
Furthermore, future releases of these verification tools look set to offer even more advantages. For example, the use of fuzzy logic will make it possible to carry out predictive and preventive maintenance if a meter ticks all the boxes but by a narrower margin than previously. Even though these sophisticated technological solutions are appealing, it’s vital not to overlook the importance of good housekeeping in maintaining an effective flow-monitoring regime. This is especially important in open-channel weir systems where debris can accumulate all too easily, but other types of installation are by no means exempt.
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