Can electrical explosions occur in a vacuum?
Explosion protection and operational safety
For this reason, the European Union has passed the Atex product directive 2014/34 / EU. It deals with devices and protective systems for the intended use in potentially explosive areas. The guideline also contains the rules for placing products on the market that are used in potentially explosive areas and also applies to non-electrical devices.
The Atex directive at a glance
The basis for the implementation of the Atex directive on the products is a comprehensive risk analysis. This mainly consists of the assessment of the ignition hazards according to the standard EN 13463-1. The manufacturer of a non-electrical device is obliged to assess whether one or more of the following ignition sources can occur in the device:
- Hot surfaces
- Flames and hot gases (including hot particles)
- Mechanically generated sparks
- Electrical ignition sources
- Stray electrical currents
- Static electricity
- Electromagnetic waves
- Ionizing radiation
- Adiabatic compression and pressure waves
- Exothermic reactions
The ignition sources listed in EN 1127-1 are potential ignition sources. One possible source of ignition that a device possesses is called a device-related ignition source. As part of the ignition risk assessment, it is considered whether the device-related ignition sources represent potential ignition sources and the probability under which they become effective. So-called effective ignition sources cause an explosion in interaction with oxygen and the combustible material. After assessing the ignition hazards, the manufacturer must take suitable measures to prevent the occurrence of effective ignition sources. There are various categories for assessing the security level of a device that define when and to what extent the respective device must be secure.
Category 3: The safety of the device must be guaranteed in normal operation. In the areas of application of this category, an explosive atmosphere is not to be expected, or only for a short time.
Category 2: The safety of the device must be guaranteed if an expected malfunction occurs. In the areas of application of this category, it is to be expected that an explosive atmosphere will occasionally occur.
Category 1: The safety of the device must be guaranteed if two independent, expected faults occur or if a rare fault occurs. In the areas of application of this category, it is to be expected that an explosive atmosphere is constantly or frequently present.
Solutions for the Atex area
The Atex product division of Pfeiffer Vacuum was created in compliance with the strict requirements of Directive 2014/34 / EU. It includes the magnetically coupled rotary vane pump Duo 11 Atex and the magnetically coupled Roots pump Okta Atex. The company developed both pumps for processes in potentially explosive environments or for pumping explosive gases in accordance with the ATEX directive: The rotary vane pump enables a pumping speed of 9 m3 / h at 50 Hz and 10.5 m3 / h at 60 Hz; their Atex approval applies to both the inside and outside of the pump. The unit corresponds to device category 3G and temperature class T4. All gases and vapors up to and including explosion group IIC can be conveyed. The pump is hermetically sealed, which avoids the so-called zone entrainment of explosive gases. The concept of zone entrainment describes the risk that explosive mixtures can escape from the pump through leaks, for example. As a result, the inner explosion zone is carried to the outside, where the operator of the system usually does not expect explosive mixtures. The risk potential is therefore correspondingly increased. The manufacturer has installed an explosion-proof three-phase motor to drive the pump, which makes the pump operate with low vibration and noise. In addition, it is equipped with a non-contact magnetic coupling, which means that there is no need for shaft sealing rings, as are required for other rotary vane pumps. This also enables additional safety, which is important when used in potentially explosive areas: Since the shaft sealing rings are missing, no media can penetrate from the inside of the pump through defective shaft sealing rings. These are usually used in systems that work in potentially explosive environments in order to keep the effects as low as possible in the event of an explosion.
No seal - no problem
The manufacturer has also focused on the aspects of the Atex directive as well as the manufacturer's obligations for risk analysis and assessment of ignition hazards in the further development of its Roots pump, which now combines a magnetic coupling with category 2 or 3 explosion protection for the first time. However, when designing vacuum pumps, potential ignition sources cannot be completely avoided. For example, due to the heat of compression that occurs in practically every pump, it cannot be prevented that the surfaces of the pump heat up during operation. The designers had therefore set themselves the task of designing the potential ignition sources that could not be excluded in such a way that they do not become effective; For example, the surface temperature must not exceed the limit values of the respective temperature class. For this purpose, the Roots pump has a temperature sensor that enables the limit values to be adhered to through continuous measurements. In addition, the units are equipped with a housing that can withstand internal pressure of up to 1,600 kPa. An aspect that the manufacturer tests for every pump before delivery. By installing the magnetic coupling, the devices are also hermetically sealed - the maximum leakage rate is 1 x 10-6 Pa m3 / s - and, like the company's Atex rotary vane pumps, prevent zone entrainment. At the same time, there is no need for shaft seals, which represent weak points in the event of pressure surges and are very maintenance-intensive. Another efficient side effect: The contactless operation of the Roots enables technically dry operation. Air cooling requires significantly less energy than water cooling, which significantly reduces operating costs for companies.
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