FAQ for the topic clearance measurement in confined space – Chapter 1

Poisonous or explosive hazardous substances are amongst the most frequent causes of accidents associated with work in confined spaces and containers. A correctly and carefully performed clearance measurement before passing is therefore one of the most important safety measures and also essential for the risk assessment that is carried out whenever a confined space or container is entered.

Basic knowledge and professional tips

Clearance measurement is one of the most demanding tasks that can be carried out with mobile gas detectors – this ranges from risk assessment to performing the measurement and evaluating the results. The person responsible for performing a clearance measurement must have in-depth expert knowledge about the properties of the various hazardous substances, handling of the instruments, specific features of the respective plant and much more. Trainers at Dräger Academy know best what matters in practical application. During their seminars, they learn directly about the challenges and problems their participants encounter in practice. The most frequently asked questions in these seminars and what Dräger trainers reply is presented in the following

1. Working in confined spaces and containers: when exactly should clearance measurement take place?

Shortly before operation – and this means: immediately before operation starts. If you decide to have lunch after having performed a clearance measurement and to release the space or container for passing without performing another clearance measurement, you could encounter a nasty surprise: Environmental factors such as temperature and ventilation can change the atmosphere immediately. Observe the following, in particular during standstills: The preset alarm threshold does not necessarily correspond to the Workplace Exposure Limit (WEL). As working hours tend to be longer during standstills, reduction factors are often considered. These factors equalise the differences between the WEL temporal reference values and the actual working hours. A WEL refers to a total exposure time of 8 hours per day. However, usual working hours are 12 hours during plant standstills. In these cases, the alarm threshold of the gas detectors must therefore be lower than the WEL.

*TIP: If there is the possibility that works could interrupted or delayed after clearance measurement, posittion a mobile gas detector or (even better) a mobile area monitor such as the Draeger X-Zone in a representative location of the container. If the atmosphere has changed and limits have been exceeded during absence, the instrument will report alarm.

2. How often do function tests have to be performed?

Most manufacturers recommend to check the battery life, alarm function and display of their instruments before every use. In practice, international regulations apply as well as regulations that may vary from company to company. In Germany, for example, employer’s liability insurance associations demand to check every mobile gas detector every working day. ”Every working day, could be interpreted as every third shift”, says Dräger Trainer Florian Mehlis. “Actually, I would not take and use a colleague‘s device that has already been in use for 16 hours without prior testing.” The following systematic procedure has become common practice: Testing all available instruments prior to every shift and then storing them in a box which is accessible to every colleague.

By the way: Of course, an instrument may be switched off and later on again after a bump test, for example, if long distances have to be travelled to the site of work.

3. Why does a zero point adjustment have to be performed in a fresh air environment?

To determine the measurement reference point of the gas detector, it is necessary to calibrate the zero point. This ensures that the indicated values correspond to the actually existing gas concentration. To adjust the zero point correctly, calibration should be performed in an environment that really contains zero percent of the hazardous substance – ideally, in a fresh air environment. If an instrument is used for the first time, another factor must be considered. The “original factory settings” of a gas detector are determined under very specific conditions. For example, Dräger calibrates its sensors in Lübeck, Germany, at an air pressure of 1,013.25 hP and a room temperature of 20°C. A significantly lower air pressure – for example at high altitudes – or an ambient temperature deviating significantly changes the physical reference points and thus can influence the measurement results.

4. How can representative measuring points be determined?

If methane is to be detected and the gas sample is taken from the bottom of the container, it should become obvious to everyone that the risk of an explosion is still present. Methane is a light gas that very quickly mixes with the ambient air. The methane cloud tends to rise to the top. The gas concentration on the bottom of the container does not indicate how explosive the atmosphere is. If hydrogen sulphide is to be detected in a container, a gas sample taken from the upper part of the container is not reliable: With a molar mass of 34 g/mol, H2S is significantly heavier than air (29 g/mol) and thus sinks to the bottom. Both examples show: Measurements taken from the wrong location can in some cases lead to death. As a rule of thumb: Light gases quickly mix with air, the volume of a cloud increases rapidly and the cloud rises to the top. Measurements in the open atmosphere should therefore be performed close to the leak. Increases in concentration take place in the upper parts of containers. Heavy gases flow on the bottom like liquids, pass obstacles or stick to them, barely mix with the ambient air and have a high range. Measurement should be performed in the flow area on the bottom. However, the molar mass and the physical properties of the hazardous substances to be expected are only two aspects important for defining adequate measuring points. The following aspects must also be considered:

– Type and shape of the container or confined space: Almost no tank is in a 100 percent even position. Heavy gases accumulate where the bottom is low, light gases accumulate at the highest position. Bulges, installations etc. must also be considered.

– Temperature: If gases are heated – for example, because the sun has been shining on a tank for hours – molecules begin to move faster, whereby the speed of the diffusion (mixture with the ambient air) increases.

– Ventilation: Air currents change the position and concentration of gas clouds. Also important: The container in which works are carried out, cannot always be separated from the pipelines. In this case, it must be determined whether gas can flow in and additional suitable protective measures must be taken, for example, with regard to the personal protective equipment of the staff.

5. How is it possible to determine whether a gas is heavier or lighter than air?

For example, by comparing the molar mass of the compound with that of air (29 g/mol). The molar mass of the compound is calculated by the sum of the molar masses of the elements and by multiplying them by their index numbers. The relative atomic mass of each element can be found in the periodic table under the written out name of the element.

6. Which oxygen value is dangerous for human beings?

* Important: The oxygen value is not reliable. And oxygen content of 20.9 percent does not mean that air is free of hazardous substance!

The natural oxygen content in the air is 20.9%. It becomes dangerous for humans only when the content drops below 17%. Why is a slightly reduced value of 20.9% oxygen alarming in free-swimming?

Because in an atmosphere with a slightly reduced oxygen content, the threshold values for explosive and hazardous substances may have already been exceeded. Air consists of four fifths nitrogen and one fifth oxygen (the exact proportions are shown in the info box). If an inert gas is released into this mixture, not only will the oxygen content be reduced by the displacement, but also the nitrogen content – in fact, by four times. If, for example, 10 vol. % helium is released, oxygen concentration will be reduced by 2 vol. % and nitrogen concentration will be reduced by 8 vol. %. Let us explain what this means by looking at it in reverse order: Suppose a gas detector measures an oxygen content of 20.5 vol. % in a container. The released gas has not only displaced 0.4 vol. % oxygen, but also 1.6 vol. % nitrogen – thus, a total of 2.0 vol. % of the unwanted substance is in the atmosphere. This is roughly equivalent to 20,000 ppm – a deadly concentration with regard to almost all hazardous substances.

As a rule of thumb: 5 vol. % carrier gas reduces the oxygen content by 1 vol. % in a confined space. 1 vol. % of a concentration is equivalent to 10,000 ppm.

To be countinued…

Source: Draeger Global

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