40 metres under

It has been a remarkable achievement, partly the result of over-engineering with the initial tunnel construction and its associated waste water pumping and management plant, but also due to substantial improvements in water quality sensing technology since the tunnel opening in 1994. False alarms Looking at the infrastructure, the arrangement for managing seeping ground and sea water in the three tunnels under the sea (two rail, and a central service and escape tunnel at positive pressure) involved gravity movement to six very large drainage stations. There, water was continuously quality monitored to protect the enormous pumps (capable of almost 1,000 m3/hr each) and pipes, sited in the service tunnel, from corrosive attack. However, when water quality was found to be consistently good and the Dover treatment works was closed, the quality systems took on an additional role - ensuring environmental standards prior to water being pumped out to sea, under a discharge consent from the Environment Agency. All of which was fine, except that the existing monitoring system suffered from a number of problems, mostly caused by blockages in the sample pipes passing water for testing to the sensors and resulting in alarms. Kevin Rivers, senior maintenance and engineering (M&E) technician at Eurotunnel, says it was a nightmare: 'There are strict procedures before an M&E team can enter the tunnel to investigate an alarm. That, coupled with the amount of time it takes to drive to the monitoring equipment, meant false alarms were very costly.' So Eurotunnel called in water analysis specialist Hach Lange to help it to develop a more reliable system. Initial observations led the engineering team to conclude that a flow-though holding tank at each of the drainage stations would resolve the problems with blockages. Also, using large bore pipes would make it far easier to remove sediment. Most important, though, new sensor technology would be used for quality monitoring to improve accuracy and reduce the requirement for recalibration. Re-engineering is now complete and the plant commissioned, with each of the six flow-through tanks equipped with new sensors for conductivity, turbidity, dissolved oxygen, pH, redox and temperature. In operation, data is collected and transferred to a PLC, programmed to alarm on prescribed quality limits. If an alarm is raised, all water is automatically diverted to one of three vast underground sumps, preventing discharge and preserving the plant. The monitoring system returns to normal once water quality levels leave the alarm condition, but the water remains in quarantine until tested and passed as fit to be allowed back into the pipeline, or designated for removal by bowsers. Rivers says the new system is not only more reliable, but requires less maintenance. 'We estimate that the new system requires about one quarter of the maintenance previously necessary. One of the reasons is the new dissolved oxygen sensor [based on luminescence technology], which doesn't require recalibration.' It's also easier to use: 'The water quality monitoring sensors are connected to SC100 controllers, which are ?plug and play'. All you do is tap the serial number into the controller and it starts to monitor automatically. And, in order to prevent sensor fouling, we have fitted a compressed air system that automatically cleans the sensor heads.' Pointers - If blockages are a problem in sample lines for water testing, consider large bore pipes and/or flow-through drainage tanks - It's worth investigating new technologies: sensors based on luminescence, for example, need no calibration and offer excellent accuracy and reliability - An automatic compressed air system can make all the difference when it comes to looking after sensor heads that are liable to fouling