Elstree to St John's Wood Cable Tunnel Ventilation

The mechanical and electrical contract for the Elstree to St Johns Wood Cable Tunnel was awarded to Naston in December 2001. The tunnel - 21km long and 3m in diameter was constructed to carry 400kV power cables to cater for London's increasing power demands. The major elements of the L2.7million project include ventilation of the tunnel; ventilation of the escape routes; tunnel dewatering; fire detection, electrical control, switchgear and UPS. The equipment is installed in the seven ventilation shafts ranging in depth from 10m to 40m. The multistage project was successfully completed in November 2004.

POWER CABLE TUNNEL VENTILATION

A computerised tunnel airflow modelling exercise was carried out in order to design a ventilation system suitable to remove the heat generated by the 400kV cables. The final design provided for variable speed, axial flow fans (duty/standby) with 175kW motors, capable of displacing up to 49.2 cubic metres per second installed in 4 of the 7 shafts.

In order to reduce the fan noise levels to 39dBA, the fans are mounted in acoustic enclosures standing 14m high installed in the shaft. Smoke dampers were provided in the fan enclosures and shaft headhouses so that the tunnel could be completely sealed in the event of an emergency.

STAIRCASE VENTILATION

In order to provide a safe means of escape from the tunnel in emergency conditions, the staircases located in the shafts are pressurised with fresh air supplied from a duty / standby fan system located in the headhouses. The associated fire-rated ductwork offers a 2 hours integrity in the event of a fire. Pressure relief vents located in the staircase walls maintain the required 50Pa internal air pressure to allow effective sealing whilst still being able to open the doors.

Any ground water seeping into the tunnel is removed by pumps located at the base of each shaft. Progressive cavity pumps were selected to avoid emulsifying any oil contaminating the water. The ground water is pumped through oil interceptors with oil detectors before being discharged in accordance with the Environment Agency. A dry falling main has also been provided at each shaft to allow fire fighting in the tunnel without man entry.

Low voltage Motor Control Centres are located within the switchrooms of each of the shaft headhouses.

Each MCC provides variable speed motor control for the tunnel air extract fans (ranging from 54 to 175kW) employing ABB "active front-end filtered" inverters. Automatic Smoke Damper controls are associated within each fan.

Each main head house also has a second MCC, which is "backed-up" via a three phase UPS to provide VSD control for the duty/standby stair pressurisation fans.

A Fire and Smoke Alarm System is installed at each of the shaft headhouses. Multiple zoned areas covering the shaft base, shaft staircase and switchroom are monitored by a central Fire and Smoke Control Panel. Each zone utilises VESDA early warning smoke detector units in conjunction with combined smoke/heat sensors.