TUTORIAL
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Design a gas detection system in an industrial environment
One mistake that many tend to commit is to think that installing a gas detection system can be considered the safe installation. This approach is not only dangerous but it is absolutely far from reality. Such a simple action alone cannot make a plant safe; The safety of an installation starts from the project, from the choice of materials, from a good execution of the works and finally from the development of safe operating procedures that foresee trained and competent operators.
It is only through the methodical and obstinate application of these simple rules that a gas detection system can effectively improve the overall safety of an industrial plant. The key role for a fixed gas detection system and that of being a security system.
During normal process activities the detection system should never be activated, but should simply be confined to the sentinel by checking 24 hours on 24 the atmosphere in search of an abnormal event indicating a possible failure of the system or result of an incorrect action that could potentially be dangerous.
Starting from this first consideration, it is possible to determine the second task for the gas detection system, that is to report promptly the presence of the anomalous event, in order to allow a quick action of putting in safety of the plant, before the dispersal of the gases or vapors can lead to serious consequences.
A gas detection system of the type imagined for an industrial plant, should not be used for the control of the process, since this task must be assigned to a process analyzer.
The possibility that the gas detection system can be used routinely by personnel for normal process operations is a very high risk. In a similar eventuality, the risk that an anomalous or potentially dangerous event will not go unnoticed, confusing in the mass of information related to the process is very high.
Sizing
Positioning
Choice of Technology
Approach to the Gas detection system
There is no exact rule or mathematical formula that defines the correct sizing of a gas detection system. It is clear that the greater the size of the detection system, with the same volume to be protected, the more likely to discover a small loss of toxic or flammable gas and less will be the time necessary for the detection. This logic, however, cannot ignore the cost of realizing the system itself and the costs necessary for its maintenance. A compromise between the performance of a system and its cost therefore becomes necessary.
The definition of such a compromise is subjective, it can be based on various factors such as:
- Investment funds available
- The choice of the detection method such as perimeter monitoring using point detectors, positioned with a 20-meter interval.
- A mathematical prediction model for estimating the response time of the system and allowing optimization of the system itself.
- The contribution given by the experience of specialized personnel.
- The analysis of the risk area by area.
In reality, the final decision on sizing the installation to be installed depends on the combination of all these factors.
Evaluation of the area
Every large oil complex or oil / gas plant is one of a kind and even if similar from the production process point of view, every place is so different that in the design phase, for the choice of components and how to operate, it makes necessary, or at least desirable, the advice of plant personnel to decide the extent and location of the detection system.
This becomes even more necessary if the choice for sensor distribution is based on a perimeter matrix or volume solution.
Decide what is essential
As already stated above, the limits imposed by economic and engineering considerations, mean that the fixed sensors are installed in order to detect accumulations of gases that can generate “high risk” situations.
At this point we must first of all give an exact definition to the concept of “High risk”. Although the opinions may differ, a common definition could be: “event capable of producing serious damage to plants or buildings and causing victims or injuries among the population”.
In the petrochemical industry, to define an objective parameter, reference is made to the energy released by the deflagration of a 5 meter diameter gas cloud consisting of a stoichiometric mixture of air and flammable gas. (eg 10% v / v of natural gas with a high concentration of methane)
It should be remembered that an overpressure of about 150 millibars in a normal building is sufficient to cause damage to the structure.
Following this approach, in an area considered to be “High risk” the ideal condition would be to position a gas sensor according to a triangular matrix at every 4 meters.
In addition to the risk for the flammable gases of the many hydrocarbons treated within a petrochemical complex, it is also important to consider the risks due to the presence of toxic gases such as sulfuric acid, in fact when the volumetric concentration of H2S in a gas high pressure exceeds 500 ppm the area must be considered a high risk from a toxic point of view.
Where to place the gas detectors
The factors that should be taken into account when determining the most appropriate locations for detectors can be the following:
Are we examining an indoor place?
Places closed with forced ventilation impose a precise choice for the positioning of the sensors, it is fundamental to carefully evaluate the movements of air, possibly using a smoke to better evaluate the air currents or the possible accumulation points.
Outdoor monitoring involves very different problems, since air movements are mainly determined by weather conditions, wind direction and speed and therefore not very predictable.
In these cases it may be advantageous to use linear optical detectors at least for those substances for which this technology is applicable.
What are the possible danger centers?
Every interruption along the pipes that carry the product, joints, flanges, valves, etc. they constitute a potential leakage point, but also pumps, compressors, pressure reducers, pressure gauges and all those parts of the system subjected to large temperature excursions and high levels of vibration are to be considered as risky points.
Pipes or parts of plant covered or protected to contain any spills, may increase the danger conditions in case of loss of product, in these cases it is good practice to provide a detection point near the point where the devices are collected.
Risk of high pressure vapor loss?
Gas leaks from valves and flanges where the pressure can be of 2-3 bar form clouds of gas / air that spread in a turbulent way in the surrounding environment, exhausted the effect of jet due to pressure, the gas cloud reaches its neutral density will start to move according to the prevailing air motion.
A completely opposite behavior occurs in the case of dispersion of vapor clouds formed by evaporating or liquids having a low evaporation point, for example liquid propane and butane.
These vapor clouds respect the laws of density and flow like concentrated, heavy clouds, influenced by the topography of the place and the prevailing air motion
Under these conditions, the most dangerous points are the manhole covers, the sewers and in general from all those spaces where the gas, heavier than air, can infiltrate and create an accumulation situation that can sometimes occur in areas very far from where the loss occurred.
These events, much less evident, may in many circumstances involve significantly higher explosion risks than high pressure joints.
The latter in fact tend to dilute quickly, dispersing into the environment thanks to the prevailing air motion.
The movements and the direction of the air are an important factor in both scenarios, for this reason the possibility of knowing wind direction and intensity could be a determining factor for the prevention of accidents.
For this purpose the installation of a local weather station or more simply a wind hose is certainly desirable.
Which technology to choose
Sensors for Catalytic Flammable Gases
For decades, the most used sensor for the detection of flammable gases is certainly the catalytic sensor or better known as a pellistor, and today it can be considered a battle horse for this type of application. The fundamental problem to keep in mind when using this technology is that the catalyst can ‘poison’ if it is exposed to particular substances such as silicones, halogenates, etc. These substances tend to inhibit the effect of the catalyst by considerably reducing sensor sensitivity. Although the modern pellistors are much more resistant, with regard to this effect, the problem remains that this technology does not allow to determine with certainty the efficiency of the sensor.
For this reason, when using catalytic sensors it is good practice to plan a maintenance program in which the sensors are tested regularly with the use of test gases.
Sensors for flammable infrared gas
This technology is based on the absorption of a light beam in the infrared band and represents an alternative to pellistors for the detection of flammable gases. This technology offers a potentially long life, a good response time and the ability to perform an effective self-diagnosis of the sensor. The principle of operation of these sensors exploits the fact that many hydrocarbons absorb infrared radiation in the 2.3 or 3.3 micron band, corresponding to the harmonic or fundamental frequency of vibration of the C-H bond. The sensor measures the amount of radiation absorbed at these wavelengths by a volume of gas and this absorption is directly related to the amount of hydrocarbon present.
Over the past 20 years, this technology has been increasingly refined, currently available ‘Open Pat’ (LOS) linear devices that use a modulated infrared source and allowing a range of up to 150 meters between emitter and receiver.
Electro-chemical cells
These devices function as a self-powered micro fuel cell, the gas through a diffusion membrane reaches a gas-permeable electrode immersed in an acidic electrolyte generating a weak electric current. The great limitation for these sensors is the operating temperature, which for these devices can not exceed 40 ° C. Therefore these sensors can only be used in environmental conditions with moderate climates and low humidity levels.
Semiconductor sensors
Semiconductor sensors can be a good alternative to electrochemical cells. These sensors working at high internal temperatures can therefore be used without problems even in environmental conditions where the temperature can even exceed 40 ° C, are mechanically very robust and have a very long working life.
In conclusion
When a new gas detection system is to be built or an existing installation has to be adapted, financial considerations will necessarily lead to compromise choices, it is important at this stage to determine the essential requirements for the system by critically assessing the risk factors in order to optimize the choices on the number of sensors to be used and on their positioning.