Water and oil don’t mix - avoiding fire and explosion risk in drainage

MSA Australia Pty Ltd
Wednesday, 06 April, 2011

The advent of the discovery of oil and consequently all the by-products of oil (fuels, lubricants, solvents) used in modern society has been highlighted as one of the greatest advances of the industrialised period. Unfortunately this white cloud also has a darker lining. When fuel and by-products (hydrocarbons) are spilt, leak or worse still dumped into drainage systems the potential for explosion, fire or poisoning is significant. Contamination can also occur from stormwater runoff where low levels of hydrocarbons aggregate to form a hydrocarbon layer over water.

Fuel and solvent usage is now so widely spread that it is rare that any industrialised drainage system is entirely free of these. Small levels of hydrocarbons although unwanted present minimal issue to workers and equipment. Large levels are a real concern and need to be avoided or at least detected. The issue here is how to detect hydrocarbons before they present a major risk to allow capture and/or prevention of further discharge and protect workers.

Most commonly used industrial detection systems look to detect gases and vapours at critically dangerous explosive (LEL) levels using either an IR (infrared source) or more commonly a catalytic bead arrangement known as a ‘whetstone bridge’. A whetstone bridge effectively burns the gas on one element increasing the element temperature. This in turn increases the bead electrical resistance which is compared to a non-burning reference. Low levels of gas or vapour do not increase the sensor heat enough to measure apart from the background and so these sensors only work if the gas is in higher concentrations. Similarly IR sensors require sufficient gas or vapour to absorb some red signal. The difference between generated light and absorbed light represents the gas concentration. In practical use, sensor size often limits the detectability to high concentration vapours. In many respects these technologies although widely employed can be considered catastrophic detectors which generally respond when large concentrations of gases or vapours are present. They are often set to 10% of LEL to alarm as explosion is considered the risk. This is typically many hundreds of part per million before a response is seen (for example 10% LEL Hexane (fuel) is 110ppm or double the toxic limit). And of course the gas must be present at the sensor, something hard to achieve.

Drainage systems have large volumes and so gases and vapours dilute easily. Even when fuels or solvents enter the system the airspace above will likely have low concentrations below the detection limits of normal detectors. Most fuels and solvents have low vapour pressures. Vapour pressure is an indication of the tendency of a liquid to form a gas or vapour. Even in the heat of summer spilt petrol still only forms a thick vapour about knee high on a petrol station tarmac seen as a hazy cloud. This physical behaviour is a significant issue in trying to detect fuel in cool drainage systems.

So what if you don’t detect fuel? Quite apart from the environmental and toxicity issues there is a risk that failure to detect a fuel layer could lead to an unforseen accident. A possible scenario could be hot work undertaken above an undetected fuel layer could lead to ignition of the fuel from above through sparks. The ignition energy would then heat the fuel resulting in more vapour and ignition with catastrophic outcomes. Fuels and solvents in drains just don’t mix.

In recent times technology has been developed at a commercial level that allows accurate detection of low concentrations of solvents and fuels. The technology is widely employed to monitor toxic levels of solvents and hazardous gases and is capable of highly selective, stable low-maintenance detection in part per billion and parts per million levels (0.0000001% vol). In fact the technology is so accurate it is used to monitor very low level leaks of refrigerants in HVAC systems before the leak becomes either an environmental issue or causes failure of operation.

The patented technology is referred to as Photoacoustic and marries several detection techniques to achieve the noted performance at very low detection limits. The commercial product is made by MSA The Safety Company under the brand of Chemgard and is supported locally by MSA Australia.

Offering a solution by strategic location around high risk areas, MSA Chemgards are capable of detecting many fuels and solvent in drainage before they present a problem. Each Chemgard is equipped with a sample draw system continually monitoring the target airspace, alarm systems and has datalog capability. Chemgard can also highly selectively detect many compounds not detectable through standard detection systems.

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