FEATURE FLOW & LEVEL CONTROL SAVINGS ARE ON THE RADAR
A tricky level measurement application at an EfW plant has been solved with the use of a 80 GHz contactless liquid level radar from VEGA Controls, reducing the amount of water used in the process
C
ory Riverside Energy is a waste to energy plant East of London. With a
population approaching nine million people, London has an appetite for energy and keeping the lights on in the face of increasing demand is a major challenge for the city today. It produces around 22 million tonnes of waste every year – enough to fill the largest skyscraper at Canary Wharf every eight days. The site in Belvedere makes better use of
this waste, utilising it to help provide London with a safe, secure, affordable and sustainable energy supply. Waste materials, that would otherwise have gone to landfill, are used as a feedstock to generate electricity. This facility generates c.525,000 MWh of electricity each year from processing around 750,000 tonnes of waste. What’s more, the River Thames is used as a ‘green highway’ to move the waste from the centre of the city to the facility on their fleet of tugs and barges, removing around 100,000 truck movements a year from London’s roads. Creating energy from waste creates a
hot ash residue that needs cooling quickly. This is achieved by quenching it in water via an ‘ash expeller’ system before stockpiling for disposal. Each system sits under one of the three main combustion chambers, and the waste ash drops into a water filled hopper below. The quenched ash is then scraped out by a ram and stockpiled for loading out and disposal/ recycling.
The water used in the ash quenching process needs a constant level to be maintained through replenishment of either recycled or fresh mains supplies. This is monitored in a small balance tank
off the main chamber. The optimum solution is to use recycled and recovered plant water, which reduces costs and environmental impact. So, could a simple level probe be used to control the water? It’s actually more complicated, as the water contains many contaminants from the ash, readily adhering and building up on any surface and the ‘ideal water level’ control band is only over a few mm. The temperature runs about 50°C, which produces a lot of condensation and sublimation onto the sensor. The total measuring range is only around 15cm, there can also be regular foaming on the surface, carrying ash residue and leaving behind deposits. A site engineer said: “The ram movement in the ash expeller also plays a part, this ram is used to keep the ash moving through the expeller. One of the main issues we have is that the ram movement causes the water level to fluctuate rapidly and it encourages foaming, the old level device often couldn’t keep up or lose the signal and caused it to overflow.” If the probe isn’t reading or working correctly, the automatic safe ‘default’ is to
Top, right: A VEGAPULS 64 80 GHz radar mounted on the quench tank (cover removed), which is working reliably
Above: Diagram showing an ash quenching system
keep fresh water continuously running into the ash expeller system. This results in excess water overflowing out to effluent treatment, incurring costs. The original plant was supplied with
‘Guided Wave Radars’ for the level control, which work well in extreme conditions, but can struggle to work reliably over very short ranges. They suffered both with signal loss and ‘run time’ error, caused by the issues previously mentioned, and resulting in the water replenishment system running an ‘emergency top-up’ using fresh mains water almost continually. Ongoing maintenance and cleaning of the devices was high too. So when a new 80 GHz contactless liquid
The before and after trend of the water valve operation shows the benefits of Guided Wave Radars.
The top trend shows the very high rate of mains water top up valve operations before the guided wave radar system, and the lower graph shows how water is used only when needed since the contactless 80 Ghz level radar was fitted
22 SEPTEMBER 2017 | PROCESS & CONTROL
level radar was launched and demonstrated it could work with mm resolution in a small vessel like a 250ml water bottle, Cory engineers were interested. Especially as it was claimed it could also cope with condensation and build up on the sensor face. After a successful on site demonstration test with a battery-powered unit, a trial device was installed on an existing nozzle. The unit worked well, but it was soon discovered that sublimation deposits carried by foam were blocking the mounting nozzle. This was easy to monitor and diagnose via the echo curve available in the VEGATOOLS Bluetooth App. As a result, a small low pressure purge was introduced inside the nozzle, which has cured the problem. Even though there is still some condensation and sublimation of the sensor face, it copes with this. The result is a reduction in the use of
mains water, saving both operational costs and environmental impact. The radar has been installed and working reliably for a year and the other two ash expellers are also scheduled to be fitted with the devices. The radars are also being used in other applications, including measuring through plastic chemical tanks elsewhere on the site.
VEGA Controls
www.vega.com/uk
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