The MCERTS self-monitoring of flow scheme was introduced by the Environment Agency to govern the regulation of treated flows being discharged back into the environment, typically from sewage treatment sites within the UK. Since the scheme was introduced back in the early 2000’s there has been an overall improvement in flow measurement systems and MCERTS is often used as a benchmark standard for any new flow measurement system. New critical depth flumes, v-notch thin plate weirs and electromagnetic flow systems have been installed throughout the UK to meet the MCERTS standard and as such the end user have a more robust flow measurement system.
However, problems remain, and in nearly all cases this can be traced back to the initial design. MCERTS puts great emphasis on the need for any flow measurement system to be easily maintainable. This is from both an operational point of view but also to allow for routine calibration checks. If the flow measurement system cannot be calibrated or maintained, then you cannot easily know for sure if it is operating correctly.
The classic historical case for this is that of buried electromagnetic flow measurement systems. Techniques such as the Siris PROVALoop mobile flow calibration rig have been introduced to allow periodic calibration checks which provide evidence that the flow meter is operating correctly and that the level of fouling within the sensor tube is to an acceptable low limit. However, these types of tests are often not straightforward and require a good deal of planning. Luckily buried electromagnetic flow measurement systems are becoming a thing of the past. Problems however do remain with flow measurement chambers being designed to the absolute minimum requirements. The 5-diameter upstream approach distance is applied in a lot of designs with no consideration as to how the system can be calibrated. A common method used for a routine calibration of an electromagnetic flow system is to use a clamp on time of flight flow meter on the same section of pipe adjacent to the electromagnetic flow meter. A portable time of flight flow meter will often require a greater approach length than that of the electromagnetic flow sensor, therefore the system should be designed to accommodate the time of flight flow meter in the first instance.
Another example of poor design is that of access and confined spaces. Thin plate weirs (eg: v-notch weirs) and critical depth flumes are gravity type flow measurement systems and are used extensively for accurate flow measurement on sewage treatment works. Access to both the flume and approach section are required to allow sediment deposits to be removed. This is also the case with a thin plate weir system. Often both these types of systems are in confined spaces which makes access for maintenance a challenge. The water industry commonly applies the C&G confined space rules and as such new flow measurement chambers should be designed to allow safe access and egress. Too often we still come across new build NC2 type confined space chambers with flow measurement systems installed which require a 2-person team and tripod. For weekly maintenance this cannot be an acceptable solution for a single site operator.
Designers & consultants have a difficult balancing act. They are tasked with providing a cost-effective flow measurement system that is fully compliant from both a health and safety point of view and an MCERTS view. All too often the resulting Health and Safety requirements will cause a flow measurement system to subsequently fail an MCERTS site inspection. For example, if it is shown that operational personnel are prevented from doing a routine maintenance task. Bolted down covers over a flume or v-notch thin plate weir can often result in an MCERTS failure as a site operator may mention they cannot be safely removed for maintenance. Hinged lockable covers are the obvious solution to this problem but are seldom used in the first instance due to cost.
Luckily help is at hand, SIRIS Environmental Limited provide flow measurement design and consultancy to help ensure regulatory compliance and ensure any new flow measurement system is done properly in the fist place. As we always say when it comes to flow measurement “Do it once Do it right”.
