Cooling Innovations for the Oil & Gas Industry
Chemical Stresses – How to Cope With Varying Water quality
- Multiple sourcing of makeup water (bore well, river, RO pretreated, recycled wastewater, etc.) for cooling towers will create a lot of variability in the water quality based on the changes in mix ratio.
- Scarce supply of water often forces the utilities department to operate with reduced blow-down and very high cycles of concentration, which increases the ionic load in the water and also increases the Holding Time Index (HTI). The HTI is a measure of the time needed for any species in the cooling system to fall to half its original concentration. Longer holding times will stress the dispersant polymers and cause reversion of most polyphosphates to orthophosphate.
- Stringent discharge regulations have driven many refineries and petrochemical plants to look for recycling options of wastewater. When used as cooling water makeup, the biological oxygen demand (BOD) and chemical oxygen demand (COD) levels remain on the brink of the threshold limits.
Disadvantages of Conventional Treatment Methods
Can conventional chemical treatment methods detect and respond to these highly dynamic and variable stress factors? Perhaps not. Conventional chemical treatment operates on one-off survey based control parameters. The chemical dosing equipment is designed for metering fixed stroke and frequency. The treatment activities have to be monitored and maintained through ‘grab’ sample tests, residuals or on-line analyzers. At best, residuals are restored to ‘precalculated’ set points. Often, change is made for a single, ‘out of spec’ parameter and not considering all the KPIs or operational conditions holistically. Thus, these systems are not intelligent enough to respond to any unusual system stress. (see Fig 2).
Dynamic Stress Management – How it's Done
By thoroughly understanding the dynamic stress conditions in various process plants (through years of plant data analyses and bench scale and pilot scale simulation studies), Nalco Researchers have developed the 3D TRASAR technology, a holistic cooling water management system based on the three key principles – detect-determine-deliver. This new technology continuously measures key parameters related to system stress, e.g. pH variability, process contamination, changes to makeup water sources and quality, process changes and heat load variation. It detects upset conditions and takes automatic corrective actions. It controls scale, corrosion and microbiological fouling through precise system monitoring and control.