Radar level sensor keeps chemicals safe

14 January 2018

Control Engineering Europe finds out how radar level sensor is offering a safe solution for chemical storage for a UK-based waste-to-energy plant.

Cory Riverside Energy is a waste to energy plant, near London in the UK, which utilises waste to provide a safe, secure, affordable and sustainable energy supply. As one of the largest operations of its kind in the UK, the facility generates around 525,000 MWh of electricity each year from processing 750,000 tonnes of waste.

The energy from waste combustion process presents many challenges – from bulk handling of the incoming waste streams to the monitoring and control of the outputs to the environment.

At various stages chemicals are also used on the purified water systems. These chemicals are stored in tanks at strategic points across the plant, in one such area there are four polypropylene plastic chemical tanks containing Sodium Hydroxide (Caustic Soda) 32% and Hydrochloric acid (32%) - each product utilising a ground floor bulk storage vessel and a smaller ‘day’ tank’ on an upper floor, used for the process-critical dosing and neutralisation.

Accurately monitoring the level in these tanks became a headache for engineers at the plant. All of the tanks had been originally supplied fitted with low-cost ‘back pressure’ level systems that failed due to corrosion and build up. With fumes and vapours escaping through the housings, they were unreliable, inaccurate and unsafe. 

Sensors used in such applications need to be properly specified. It needs to be made of the right materials and of the right construction. During fitting or removal, both the sensor and process also require containment, isolation and decontamination, as well as requiring comprehensive PPE for those undertaking the task. This often results in mandatory closing off an area of the plant while any equipment installation or removal is taking place on the vessel. These necessary health and safety procedures incur extra safety risks and cost. 

Stored chemicals – especially those that are highly acid or alkali – can leave residues and can give off give off vapours and fumes which can result in a hazardous environment for personnel.

At Cory, these chemical products needed to be careful monitored and accurately measured to remove any risk of overfill, and ensure the process has ample supply. 

“We wanted to find a new level system that would avoid any repeat of the issues experienced with the original sensors, ideally eliminating process contact and any risks experienced with the previous technology,” explained a Cory instrument engineer. VEGA proposed the use of its contactless radar sensors, mounted above and outside the vessels looking through the vessel top to measure the liquid level inside. 

Contactless measurement
For liquid level measurement of chemicals a sensor generally requires a process connection into the vessel to get its ‘sensing part’ to connect into the process – even when using a top mounted ‘non-contact’ device – it could be a rod, cable or diaphragm. To measure any process variable completely, without contact, therefore provides many benefits; from longevity of operation, protection from chemicals and the process, and increased safety through reduction or elimination of exposure to harmful substances. Radar has the ability to measure the level of a liquid through an opaque plastic vessel top or window on a nozzle, completely from the outside. 

More companies today are specifying plastic vessels and tanks for chemical and bulk liquid storage. They are quicker to make, are generally more cost-effective and have good chemical resistance. They can also offer a longer lifespan and lower maintenance than equivalent steel painted, lined or coated vessels. Many even come with ‘integrated bunds’ for overflow or overspill protection. Plastic IBC’s are now also one of the most common vessels for bulk transport of intermediate quantities of liquid. Radar technology can be used to measure liquid level through the top of all these vessel types. 

How does it do it? 
Radar uses microwave technology so is able to transmit signals through plastic and other non-conductive materials (like glass and ceramic) and reflect back off a liquid level inside. With a good dynamic range (sensitivity) it can also deal with any condensate or sublimation on the inside of the vessel roof. 

Using 80 GHz radar offers several benefits. Firstly, the higher focusing means it achieves good penetration through the plastic vessel roof, it also offers more mounting position options and is less likely to pick up any unwanted false signals. It also offers wide measuring range capabilities, from very small to very tall vessels, up to 30m high. It also offers high dynamic sensitivity to handle condensation and build up caused by fumes or sublimation from the products inside the vessel. It can even measure low reflectivity, oil-based hydrocarbons in some applications. 

Installation best practice 
Installing the radars above the tank on a suitable bracket perpendicular to the liquid surface is essential. A small gap between the sensor and the vessel top is vital. A sloping roof-top is ideal for a microwave sensor to look through, as any unwanted signal reflected back by the roof is deflected away from the radar. If this is not possible and the roof is flat, such as with an IBC, the sensitivity of the radar should easily be able to overcome any reflection. If a tank is outside, a cover is necessary to stop snow forming directly beneath the sensor and the sensor needs to be positioned so that water does not pool directly beneath the sensor. However, rainfall down a sloping roof tank situated outside should not affect sensor performance.

Cory Riverside Energy now has its first radar successfully working through the roof of the bulk chemical tank, mounted on simple ‘unistrut’ frame and a VEGA-supplied radar bracket that attaches to them. “The sensors are simple to use and the Bluetooth communication for set up via smart device App or with a PC and PACTware makes it so much easier,” said the Cory engineer.

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