Stirring and mixing in an environment with an explosion risk

Since 1 July 2006 all work places in Europe must comply with the minimum requirements of the 1992/92/EG (ATEX 137) Directive in order to comply with the Working Conditions Act (ARBO-wet in the Netherlands and ARAB in Belgium). The directive gives the minimum requirements for improvement of the health and safety protection of the employees in an environment with an explosion risk.

As a consequence of this legislation users and suppliers of stirrers have been forced to take notice of this subject. A case is described below involving an installation recently supplied to the chemical industry by Typhoon Roertechniek B.V. where the present installation did not comply with the legislation applying so that continuity of production was endangered. PROCESS
The installation - already in use in numerous applications in the chemical industry, the foodstuffs industry and the cosmetics industry - is used for moistening various powders in a highly viscous liquid. An important condition for this specific situation was the possibility of using the system for products with a dry powder content of up to about 70%.
The installation is a combination of two mixing systems consisting of a rapidly moving stirrer and a scraper stirrer. The rapidly moving stirrer has a movable propeller finished to sanitary standard that ensures that the powders are rapidly mixed in. In order to prevent air being absorbed during the process after tipping in the powders the tank is placed under reduced pressure. The particular property of the second element on this stirrer axle is that it has both an axial and a radial driving working so ensuring that the powders become dispersed through the liquid. The easily removed scraper blades prevent the product from sticking both to the bottom and the side walls of the tank. 

Because production at the particular customer is during 8 hours each day at the end of the day the whole installation has to be cleaned using various solvents. This leads to an emphatic focus on the standard of finishing and ease of cleaning. Naturally, the installation is also suitable for use in companies where production takes place in more than one shift.

In the case described the viscous liquid is highly flammable so that the whole process must comply with the ATEX directive.

ZONING
The user is responsible for determining the explosion risk and the degree of this, expressed as the zone, temperature class, gas group (for gases) and the degree of conductivity (for solids). These are defined as follows for the particular case:  

Zoning                         Zone 0 in the tank
                                      Zone 1 outside the tank
Gas group                  IIB
Temperature class   T4

This leads to a coding of II 2/1 G, cb Ex d ia IIB T4 for the system with the safety devices chosen by Typhoon Roertechniek BV. 
 
METHOD OF WORKING
In this case the user uses various portable tanks with a tank capacity of 800 litres. The tank is filled with a highly viscous liquid and various semi-manufactures. After moving these are positioned under the installation and earthed to the earthing station. Both tank and tank content are discharged so that spark formation is impossible. The lifting device is released when the earthing has been carried out properly and the lid with both stirrers can then be placed in position (2 hands operation). As soon as the tank is closed (vat detection) and the barrier liquid pressure, level and temperature are correct, the stirrers can be started.

When the powders are added during the process the pressure in the tank is reduced to prevent air being absorbed. After a certain period of intensive stirring an inert gas (heavier than air) is added after the stirrers have been stopped in order to prevent the end product from coming into contact with the atmosphere as soon as the lifting device opens the tank. The tank is opened after uncoupling it from the earthing station under the installation.

The production time is shortened considerably by using this combined mixing system.

SPECIFIC CHARACTERISTICS OF THE SOLUTION PROVIDED
The whole installation is supplied in turn-key state and naturally with a IIA certificate in accordance with the machine directive. Both the electrical and the mechanical components have been fully tested in accordance with the ATEX directive. 
The operating unit is set up in the zone while the control unit is placed in a separate room. Electrical components are manufactured to be pressure proof and the sensors are intrinsically safe. A great deal of attention has been paid to possible charging of the portable tank by static electricity.   
In extensive testing by EagleBurgmann and TÜV the double working mechanical seal has been approved as suitable for the high revolutions required for the dispersion process. The seal has temperature, level and barrier liquid pressure detection and is therefore suitable for ATEX zone 0.
All plastic parts such as scrapers, guides, cable track etc. are manufactured from antistatic material (ESD). The stirrer elements have a double safety device against coming loose with the portable stirrer element being constructed in such a way that this becomes extra tightly fitted when moving both downwards and when rotating. 

Despite the complexity the installation is very sturdily constructed with a particularly high quality of finishing. In order to be able to guarantee continuity an “Optimaal” service contract has been taken out by the particular customer.