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Extending pH sensor life in steelworks effluent

Emerson Process Management : 10 December, 2004  (Application Story)
Emerson has collaborated with engineers from the Corus steelworks at Port Talbot in the development of a novel installation approach for standard pH electrodes, improving their performance in effluent.
Emerson has collaborated with engineers from the Corus steelworks at Port Talbot in the development of a novel installation approach for standard pH electrodes. This extends the service life of the site standard pH sensor, the Rosemount Analytical 396P, when monitoring the steelworks effluent tanks, where previously the metal and sulphide ions had very quickly caused electrode poisoning. The approach enables Corus to avoid the need for special electrode designs for this duty, and standardise on the single 396P sensor model across the whole site, reducing cost and spares inventory.

Steelworks effluent tanks have a particularly high loading of metal and sulphide ions, which are notorious for poisoning the reference electrodes used within a pH sensor, resulting in premature sensor failure. The reference electrode is typically constructed from a porous material, which is immersed in electrolyte, and produces a known stable reference against which to measure the voltage from the sensor. Penetration of this electrolyte by the sulphide or metal ions causes poisoning of the reference electrode, and invalidates the pH measurement.

Since the poisoning ions reach the primary reference element by a diffusion process through the cell, Emerson and Corus engineers decided to apply a positive pressure on the electrolyte externally, from the 'dry' side of the sensor. By filling the dip pipe supporting the pH sensor with electrolyte to above the level of the tank surface, the hydrostatic pressure generated provides the necessary positive pressure conditions to oppose the diffusion of the poisoning ions.

The systems have now been operating for nearly a year and Corus engineers report that the standard pH electrode life has been extended from as low as a few days in certain conditions to typically 5 months. Whereas the effluent tank monitoring was frequently inaccurate or unserviceable, they now have good performance from all of their sensors, and lifetimes equivalent to the rest of the site. They do not have to top up the reference reservoir during operation, because the volume of electrolyte held in the dip pipe is large: and keeping maintenance routines relatively simple has enabled an improvement in effluent standards.

Gary O'Neill, Rosemount Analytical Sales Manager for Emerson Process Management, commented: 'We have used pressurised, flowing reference electrodes for pH measurement on difficult applications before, but usually with a complex sensor on a pipeline. This simple approach suits the nature of the installation on an effluent tank, and demonstrates how Emerson can adapt technology to suit the customer requirement.'

The effluent pH measurement is used to enhance the efficiency of the Corus water treatment plant by controlling the addition of lime for neutralisation prior to final treatment. The output signal from the pH meter is connected to a standard SCADA system which performs dosing control.
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