From the PMA archive. This article retains its original publication date; product availability, specifications and contact arrangements may have changed.
Many industries require cooling water to transfer heat from the process out of the system. The cooling water must be then be cooled down and discharged before being replaced with fresh water.
Oil from heat exchanger leakages in cooling water causes at best the fouling of the packing leading to inefficient cooling of the water or at worse a potential shutdown of the plant. Oil and grease in the discharge water can also mean a hefty fine from the Environment Agency (EA).
Measuring ‘oil’ in water is a challenge. Whilst regulators may define a consent in terms of ‘oil’ permitted to be discharged, that oil can be made up of many different hydrocarbons, some saturated (aliphatic hydrocarbons) and some unsaturated (aromatic hydrocarbons).
Different measurement methods are more or less sensitive to different types of hydrocarbons. Added to that complication is that the hydrocarbons are in a media (water), often not dissolved and spread evenly throughout it. Some hydrocarbons will float on the top of the water, some will stick to other particles or a sample container, and some will stay as insoluble droplets floating in the water.
Why use fluorescence as a method for online monitoring of cooling water?
There is no perfect solution to oil in water measurement. Without the ability to carry out an extraction, continuous monitoring such as that required in the cooling water of a power plant is even more challenging. But UV fluorescence methods have some distinct advantages over alternative oil in water monitoring techniques.
- Sensitivity
UV Florescence methods are highly sensitive to aromatic hydrocarbons such as that found in refined oils used as lubricants or in power generators. This enables detection of leakages into the cooling water down at the parts per million (ppm) level.
- Detects both dissolved and insoluble hydrocarbons
With UV fluorescence, it doesn’t matter what form the aromatic hydrocarbon is in. Dissolved or not, the hydrocarbon will fluoresce leading to its detection.
- Specificity
Using the flexibility of different excitation and emission filters affords us, we can be highly selective as to the type of hydrocarbon that is to be detected.
- Reduced turbidity effects
Suspended solids don’t tend to fluoresce, so are handily excluded from any UV fluorescence measurement.
Advantages and Disadvantages of different oil in water methods
| Method | Advantages | Disadvantages |
|---|---|---|
| Lab Analysis | Standard method for determination of oil and grease in water | Complex procedure requiring skilled analyst and a representative sample using solvent extraction (usually hexane as per EPA method 1664A or pentane as per ISO 9377-2) |
| Nephlometric Turbidity | Cost effective | Challenging to distinguish between suspended solids and oil/grease |
| absorbance | Well known on line method | Challenging to distinguish between organic matter and oil/grease |
| Visible Fluorescence | Various online systems setups available | Challenging to distinguish between organic matter and oil/grease, low sensitivity to polyaromatic hydrocarbons (PAH) |
| UV Fluorescence | Highly sensitive to PAH | Requires calibration for specific application, not sensitive to aliphatic hydrocarbons |
Infrared methods for oil in water have been utilised for many years but the method does not lend itself to online monitors easily due to the requirement for a solvent extraction, thus are not included in the above table.
4100 series
Online monitoring for trace amounts of oil in cooling water in power plants has often been carried out using the 4100 series from Turner Designs Hydrocarbon Instruments. This was due to its reputation as the best available technology for providing on-line analysis with high accuracy, low maintenance, with built-in alarm system and alarm relay capability.
120 series
With the introduction of the TD 120 monitor, the same best available technology is now available at a more economical price and specifically designed for measuring trace oil concentrations in cooling water applications. With a simpler interface for operators and simpler calibration procedure, the TD 120 brings together low maintenance, simple operation and the latest UV fluorescence technology.
