Prometheus’ gift of fire
12 July 2012
Legend has it that Prometheus brought fire to mortals after stealing it from the Olympian gods. Today we owe most of our industrial development to this very element, which has shaped human civilization. However, if Prometheus lived in present times, he would have offered the gift of fire with some safety instructions. Mike Fikuart, managing director of fire protection specialist Industrial Design, explains the differences between various fire and gas detection methods available.
Mitigating gas and fire risk with calibration: an engineer makes changes to a gas detection panel
When looking at toxic and flammable gas detection systems in the food sector, the most important thing to understand is what the detection limits are and what they actually mean. For instance, the lower explosive limit (LEL) is defined by any concentration of gas (calculated as air volume) which can potentially ignite and support the flame. As a consequence, anything below the LEL threshold will not support the flame and thus will not present imminent danger.
In the gas detection industry, knowing the LEL levels is crucial, as you do not want the gas concentration to exceed them. For example, over 50% LEL means a highly flammable gas concentration that may ignite. It is important to bear in mind the fact that the actual concentration of gas in the air depends on the type of gas detected; concentrations of over 4.4-5% by volume are at high risk of ignition.
When dealing with toxic and flammable gases, even a percentage of less than 50% gas in air, which would mean a 2.5% concentration, needs to be acted upon, even if it does not present an immediate ignition danger. For this reason, whenever making a gas detection plan, events are rated on different risk levels - 50% would be our upper alarm level and, depending on the application, we normally have two alarm levels. A concentration of 15-20% gas in air would be common for a lower alarm level.
This is mainly because even the lower level of gas leaks should be taken seriously and considered a first warning sign. This allows the safety representative to evacuate the area and investigate the cause of the alarm. If things go even further, let’s say to 50% LEL, machinery and equipment needs to be shut down so that all potential ignition sources are excluded.
In the above scenario we are using gas detection in a safety capacity. Gas concentration is being measured for the safety of people, the plant and the products stored onsite.
Fixed and portable calibration
Calibration is an important part of a successful fire prevention strategy. It enables better and accurate readings and it can make the difference between life and death in some cases. This is why we use two types of calibration systems: fixed systems which can be mounted on a wall on site and portable devices which are usually handheld and can be moved from location to location.
In terms of the actual calibration process, one must always make sure the detector is working to optimum standards. The best way to guarantee this is by exposing the gas detector to the various gas concentrations.
There are several factors that determine the frequency of calibration: operating processes, the frequency with which gas is exposed to the detector and the stability of the detector. Different manufacturers will give you different indications on how to calibrate your detector. We choose to calibrate our equipment four times a year, corresponding to the changing of the seasons and inherent temperature modifications.
Bump testing is a quick and easy way of establishing that your device is working. This is traditionally done on portable devices and it basically means exposing the detector to gas to make sure it’s functional. Bump testing is not to be confused with calibration, as the latter is a more in depth analysis that checks the accuracy of the readings taken by the detector.
Fire Detection
There are a number of different technologies used to detect the presence of fire. They normally look for smoke, heat temperature, infra red radiation from flames or early warning signs from gas detection as in the petroleum industry where methane can be ignited.
The cause of false alarms is not the detectors themselves but the overall design and understanding of standards. You have to look for different effects of the same cause. Ionisation detectors such as those found in hotels, offices and accommodation areas ionise a gap between two sensors where smoke particles accumulate. If an ionisation detector is used in a clean room, it will be perfect, but in a dusty environment, it will be triggered more often.
Optical detection is another option, where smoke obscuring infra red light going from one sensor to another will trigger an alarm. Optical detectors are more stable but will not work as quickly as ionisation detectors. In contrast, ionisation detectors can’t differentiate between steam and smoke.
Heat detectors are perfect for smoky and dusty environments and work in two ways: rate of temperature rise and threshold. They may be electronic or fusible links that melt from temperatures as low as 60 degrees Celsius and, as a result, are often used above kitchen cooker hoods or in hotel laundry chutes.
Fire extinguishing systems are under a regulatory obligation to apply a process called ‘double knock’ verification, where two independent alarms are triggered. The extinguishing medium can be water and/or mist, dry powder, carbon dioxide (CO2) or other such inert gas like argonite.
Industrial Design uses a water mist system on gas turbines in Libya because they will not put personnel within the enclosure at risk of suffocation like CO2 would. Another method is hypoxic extinction, which reduces the amount of oxygen in the air below the 15 percent required to support a flame, but still allows people to work for a limited period of time.
The food industry in practice
In the food and beverage industry, we often come across packaging machinery where they use heat based shrink wrapping and carton sealing processes, which have been known to cause fires. Product can get jammed on the conveyor and, because heat is applied in the chamber, the jammed item will burst into flames. In addition mechanical faults, such as bearings running hot, can generate sufficient heat to cause a fire in a device such as a polythene film packaging machine.
In these applications we would normally use ultra violet or infra-red detection to provide a very rapid alert; required because of the quick burning process. The extinguishing system would probably be carbon dioxide applied using the local application method to extinguish the fire and cool the residue.
Another issue in food plants is deep fat frying; which is of course a common process applied to everything from potato chips and vegetables to meats and fish. The problems here are caused when the oil is raised above its flash point or when ultra heated product that has been immersed in oil for an extended period is exposed to the air. The latter often happens during cleaning, when the oil is emptied out of the vat and the product residue left in the bottom spontaneously combusts.
Here we recommend the use of heat detectors, to detect the rise in temperature, and carbon dioxide for extinguishing. This suffocates the fire by preventing oxygen from supporting re-ignition as well as cooling the combustion source to below its spontaneous combustion level. As an alternative to carbon dioxide, we also consider water mist. The water turns to steam during discharge and combines with the by-products of combustion to create a fire suffocating blanket. This also provides a cooling effect, thus reducing the oil’s temperature.
Conclusion
It is important to undertake site surveys of existing systems and new installations for gas and fire protection requirements. These may be for the assessment of existing systems to evaluate their maintainability or need for upgrade, or for compiling detailed specifications of the requirements of a system which can then be released for tender. An independent systems integrator should be vendor neutral and provide a solution using the most appropriate technologies for the application.
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