Chemical tanks on radar
28 Jan 2008
When Cheltenham-based Premiere Products installed a new 40,000-litre polypropylene (PP) tank for processing and storing detergents, it found that the ultrasonic level measuring system included with the tank could not cope with the foamy nature of the detergents.
This was a particular problem when the products were being recirculated, resulting in spurious and inaccurate level readings at Premiere, which has an annual capacity of 9,500,000 litres of chemicals for cleaning and maintenance products applications.
To tackle the problem, device company Hycontrol, of Redditch, developed a solution based on its VF7 series TDR radar level measuring system. This involved installing the device in a plastics tube mounted to the outside of the tank instead of installing the stainless steel waveguide cable inside the tank in the traditional way.
As PP has a low dielectric, the instrument’s microwaves pass straight through the tank wall; the system operating as if the tank doesn’t exist. The level readings from the VF7 are converted to volume and displayed on an HYC3600 digital indicator mounted in an IP65 enclosure adjacent to the tank. The devices offer accuracy of better than ±3mm over a ten-metre measuring range and have a maximum measuring range of 35 metres, claims Hycontrol.
The VF instruments work on the principle of Time Domain Reflectometry, which sends low-power electromagnetic pulses along the instrument’s wave guide. Upon reaching the surface of the vessel contents, the pulses are reflected back with a signal strength that is dependent on the dielectric constant of the medium being measured.
The instrument measures the time between the emission and the reception of the signal, converting this into a 4-20mA output current proportional to level. Any weak or spurious signals are analysed and ignored by the electronics. TDR technology is unaffected by pressure, temperature and density variations or process factors such as dust, foam, vapour and agitated and boiling surfaces.
Duvel Moortgat is replacing the pH sensors for monitoring wort passing to the fermentation vessels at its brewery in Puurs, Belgium. The company’s old sensors had become unreliable and were failing after only a few weeks due to a cleaning cycle, which uses cleaning fluids at 85-90ºC.
The replacement pH sensors — Rosemount Analytical Model 3300HT PERpH-X units — can tolerate temperatures up to 145ºC and are equipped with a special bulb that improves sensor speed of response and resistance to thermal shocks. A double junction reference electrode on the sensor, meanwhile, uses an outer gel-based reference that resists the pumping action produced by temperature and pressure changes.
With planned maintenance, Duvel Moortgat expects to save direct costs of over Euro 1,200/year per wort monitoring point, as well as achieve efficiencies from improved reliability and reduced failure alarms. The company has also recently opted to install the Rosemount sensors on a new line as part of a major expansion at the brewery.
