Connectors ensure safe operation of waste water treatment plant

20 September 2010

Explosion-proof plug-in connectors from Cooper Crouse-Hinds are helping to minimise maintenance time and costs at the Dokhaven waste water treatment plant in Rotterdam. The eXLink connectors ensure that the site’s 12 gas detectors are fully operational at all times, ensuring the safety of the plant, employees and the surrounding environment.

Connectors ensure safe operation of waste water treatment plant

Explosion protection plays a crucial role in waste water treatment. Substances dissolved in waste water can sometimes chemically react, potentially causing an explosive atmosphere. For this reason, gas detectors are installed at Dokhaven, which guarantee the safety of the plant and employees at all times.

Located in the centre of Rotterdam, the Dokhaven waste water treatment plant from waterschap Hollandse Delta is the only underground waste water treatment plant in the Netherlands. The plant treats around 5,500 cubic metres of waste water every hour, processing waste water from almost half a million households.

Unlike conventional waster water treatment plants, Dokhaven has a two-stage waste water purification system with a processing time of 12 hours during dry weather. During the first stage, after any coarse particles have been removed the water is biologically purified, by enriching it with oxygen. This process breaks down the bacteria in the sludge and any contaminants. Next, the remaining sludge is separated and some of this fed back into the biological purification stage. The remaining sludge is taken to a nearby sludge treatment plant.

Next, the constituent parts of the waste water are broken down further in a second purification stage. The nitrate content is reduced again and the sludge is separated. The water – now biologically ‘clean’ – is fed into the Nieuwe Maas River via a run-off channel. This water that is fed into the river is often cleaner than the river water itself.

The complete system is ventilated by an overpressure system. The air pressure in the processing chambers is lower than that in the tunnels, which means that the contaminated air cannot escape uncontrolled, but is fed back into the atmosphere in a controlled manner after being purified in filter systems.

During the first biological treatment stage (when oxygen is added to the waste water) the safe operation of the plant is guaranteed by the 12 LEL (lower explosion limit) gas detectors. These Ex-de explosion-protected devices are protected to IP 66/67 and have a 3-pole connection, 24V DC at 3.5W and a 4-20mA signal output.

The eXLink plug-in connections supplied by Cooper Crouse-Hinds enable these 12 gas detectors to be replaced and serviced by a maintenance engineer without requiring a ‘Hot Work Permit’. eXLink is a range of connectors that enable the connection and disconnection of electrical apparatus in potentially explosive gas (or dust) atmospheres without the use of tools and without an engineer having to isolate the apparatus from the mains or disconnect the terminals.

This plug-in version of the gas detector has several advantages. In the event that an LEL meter is contaminated or faulty, the detector can be isolated electrically and replaced within a very short time – even in a potentially explosive atmosphere, by the eXLink’s simple disconnection procedure. There is no need for an engineer to isolate the apparatus from the circuit or to disconnect it at the terminals.

In the event of a fault with a gas detector and to enable the maintenance of a detector to be carried out with a minimum amount of effort and in the fastest possible time, the detectors use plug-in connections. A connection cable with an explosion-protected eXLink coupler, 4-pole plus PE, is fitted via an M20 Ex-e cable gland. The counterpart to this is a socket with an eXLink inlet, 4-pole plus PE, which is mounted directly adjacent to the gas detector. The gas detector can be connected and disconnected at any time thanks to this IP66/68 protected, EX-e plug and socket connection. The advantages here are the compact dimensions of the eXLink connectors (30mm diameter by 96mm or 60mm in length) and an M20 thread of the inlet, whereby, similar to a cable gland, the latter can be integrated into any type of enclosure.

The rapid replacement of this type of apparatus is of great benefit to the plant, because if the LEL meters are faulty, the pumps are automatically shut down and the complete water treatment process is disrupted. In certain cases, long term delays or disruptions can have serious consequences for the surrounding environment, and so minimising process downtime is critical.

Another key advantage is the higher degree of operational safety. Due to the plug-in connections, manual errors that may occur when disconnecting or connecting electrical equipment are prevented, particularly when work is being carried out under time pressures or when wires are contaminated. Furthermore, longer operating cycles without shutdowns and eliminating manual errors when connecting apparatus effectively improves the overall safety of the installation.
The approach adopted at Dokhaven is of critical importance to the City of Rotterdam. By using eXLink connectors, the LEL meters are fully functional at all times, which is an essential basic requirement for the safe operation of the plant. In a fictitious worst case scenario, the initial situation is a leaking petrol tank, the contents of which spill into the Rotterdam waste water system. Then, because the LEL meters are not functioning correctly, the gas fumes are left undetected. Such a situation may not only have disastrous consequences for the surrounding environment and the City of Rotterdam, but also for the plant itself and the people working there.