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Maintenance in an ATEX-environment is already a topic in industry for many years. This newsletter deals with maintenance that requires the opening of a reactor or tank containing flammable liquids. The replacement of a vertical pump (Figure 1) or agitator of a tank/reactor (Figure 2) are hereafter taken as examples. The toxicity of the product and the corresponding danger are not considered here.
Figure 1 : Two examples of a vertical pump on a tank
Emptying the system
If installations containing flammable solvents have to be opened for maintenance purposes, the safest way to avoid dangerous explosive atmospheres is to empty and clean them completely, i.e. to make the installation "chemical-free" before opening it.
It must be taken into account that even the smallest amounts of solvents, which are adsorbed on walls or in linings and may outgas, can lead to the formation of explosive mixtures in a vessel. Solvents that do not mix with water and are therefore not sufficiently washed out by rinsing with water (e.g. carbon disulphide, toluene, hexane) are particularly critical in this respect.
The emptying of a complete tank can also have a major impact on the production in an interconnected system and therefore in some cases cannot be carried out in a practicable way due to unacceptable production losses or massive disturbances in the interconnected system.
There are two reasons why it is difficult to inert the system during maintenance work:
1. During maintenance the plants are by definition open systems, i.e. inerting is rendered ineffective by gas diffusion and tank breathing effects
2. Oxygen can also be displaced by inert gases in areas where work is carried out, which can create an acute risk of suffocation for employees.
Inerting with argon
Inerting a tank with argon at maintenance is a method which is rarely used in the industry, but which presents one main advantage: argon is heavier than air and acts as a blanket. The disadvantages of argon are its fairly high cost, the lack of experience of production and maintenance teams with its use, and that it needs to be stored and used in bottles.
Inerting with nitrogen
Nitrogen is readily available as a standard inert gas in many plants and is therefore significantly less expensive than argon. However, the duration of an effective inertization in an opened reactor or tank is significantly less than with argon.
The graph below (Table 1) shows an experiment in a 20-liter sphere inerted with argon or nitrogen. The upper part of the 20 litre sphere was opened and the evolution of the oxygen concentration in the lower part of the sphere was measured. When inerting with nitrogen, the typical limiting oxygen concentration for organic vapours of 10% was exceeded at the bottom of the sphere within 105 seconds, while for argon the increase in oxygen concentration above the critical threshold took around 100 minutes. The inerting effect of argon in this example lasted more than 50 times longer than with nitrogen. For each physical system, the time it takes to reach a safety-relevant oxygen concentration will vary, depending on its shape, size and surrounding environmental effects.
Table 1: Oxygen ingress in the inerted 20-L sphere in minutes after opening the top of the sphere
Tank breathing effects
When liquids are pumped from open containers, atmospheric oxygen enters and the inert gas is diluted. When liquid is pumped into the tank, the liquid level rises closer to the tank opening and thus closer to the outside atmosphere, accelerating the loss of inertization due to diffusion effects. As a result, liquid transfers, and thus production, must be interrupted during maintenance operations.
In most cases, the inerting methods for an open reactor or tank with nitrogen or argon are not sufficiently reliable for the above-mentioned reasons to allow maintenance work with persons and/or ignition sources without danger. Nevertheless, in special situations or in combination with additional measures, the following methods can reduce the risk to an acceptable level.
To prevent the formation of explosive atmospheres outside the affected containers, ventilation measures could be an interesting choice. The extraction of gases and vapours with temporary devices is very effective if the following points are taken into account:
1. The extraction must take place directly at the vessel opening (extraction at source). The area from which gases and vapours are effectively extracted extends only approximately to a distance from the suction line opening that corresponds to the diameter of the suction line.
2. The density of the gases and vapours must be taken into account when positioning the extraction unit. For most organic vapours, this means that extraction should be located slightly below the container opening or near the floor.
Design change: replacement of the vertical pumps
Predictive design and low-maintenance equipment avoid high costs for additional safety or excessive production downtime during maintenance work. In our example, if horizontal instead of vertical pumps had been installed, there would be no need to open the tank during maintenance and service work. In addition, horizontal pumps are easier to isolate and replace, thus reducing longer production downtime.
The protective measures for maintenance and servicing measures in Ex-zones or with interventions in Ex-zones must be analysed within the framework of a formal work approval system, and the corresponding measures must be implemented.
Possible measures when opening containers with flammable solvents include:
It should be good practice that in-situ generated hazards in special situations, such as during maintenance or repair work, are taken into account already in the planning phase of a new installation.
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