Toxic sludge, aggressive chemicals & flammable gases means work in water treatment plants, sewer systems, tanks or pipelines can be dangerous.
Its typical odour revives childhood memories of swimming floats, headers off the 10-metre dive board and water slides, but this same odour puts an employee or safety engineer in the industry on alert. We are referring to chlorine, Cl2 for short.
The most prominent property of chlorine is its reactivity: it links with many elements, even at normal temperature. Chlorine is present in numerous organic and inorganic compounds from harmless chlorides, such as sodium chloride (table salt) through to a highly toxic dioxide, a chlorocarbon.
Chlorine gas itself is an everyday and fundamentally controllable risk in the industry if its presence is reliably detected. But this is not that easy…
Their high reactivity means that chlorine molecules are looking to link with other elements, even if they then diffuse in the direction of a Cl2 sensor.
‘’The problem with chlorine gas is that it quickly gets caught on device surfaces, on the diaphragm of a gas detector for example, on the bump test adapter or in the valves of a calibration station’’ explains Ulf Ostermann, Sensor Expert at Dräger. The consequence: it simply takes much longer for the gas molecules to reach the sensor, regardless of whether a personal gas detector, clearance measurement or a functional test is used. In particular, chlorine molecules like to accumulate on the insides of hoses. As a result, many devices are not approved for chlorine clearance measurements.
A reason for the accumulation is contamination on the devices or hoses. ‘’Anyone working in the industry knows a mobile gas detector is never completely clean’’, says Ostermann. ‘’Slight to severe contamination is impossible to avoid, depending on the environment in which it is used. Deposits are formed by particles, vapours or even just the skin of the user’’. Chlorine molecules immediately get caught if a surface displays traces of grease. Chlorine also particularly likes to accumulate on soot particles.
Even a device that is fastidiously wiped down with a moist cloth still displays minuscule deposits with which the chlorine molecules would react. Even the extensive use of cleaning agents does not improve the situation.
Countering the inertia of chlorine gas during detection with a rapid sensor is much more effective. Chlorine sensors have significant differences in their response times, the products available on the market range from 30 to 120 seconds and more. The DrägerSensor® XXS for chlorine has a t-90 time of 30 seconds, 4 times faster than many other chlorine sensors. The expert Ostermann explains the practical implication of a sensor that is 4 times faster than another: ‘’Sensor speed is a safety issue for safety engineers and gas analysts in a plant environment: faster response times means more time to react, more reliability when deciding on the safety precautions and in an emergency fewer injured people and a smaller amount of damage’’.
Faster means safer – this equation would be emphasized for a hazardous substance such as hydrogen sulphide. Ultimately, a few seconds can be the difference between life and death in the event of exposure to H2S, but how important is the sensor speed for chlorine? Ulf Ostermann explains: ‘’H2S is obviously a special case because it is impossible to smell in dangerous concentrations. In contrast, chlorine can be directly identified by its odour in all concentrations. This means that, in the event of unexpected exposure, e.g. due to a leak, your own nose would provide a warning even if the sensor did not respond. But: the odour alone does not indicate whether a concentration is within the workplace threshold range or 20 times over this limit. This makes a significant difference after a few breaths. Let‘s not forget: this is about the health and safety of employees’’.
And sensor speed is also an economic issue. The quicker the response time, the shorter the test duration, the lower the working time and the test gas consumption. ‘’In everyday practice there is a huge difference between whether a chlorine sensor takes 30 seconds to respond during a bump test, or two minutes’’, stresses Ostermann. ‘’Every industry specialist can easily calculate what this one-and-a-half-minute shorter response time means for their equipment and frequency of use over a year’’.
www.draeger.com
