Pressurized enclosures guarantee safe, cost-effective protection

R. STAHL offers control and monitoring systems that are protected by the pressurized enclosure type of protection in three basic versions. These feature a standardized design and always consist of an Ex p control unit, Ex p pressure monitors and Ex p purging valves with digital and proportional control. Based on time intervals and pressure analyses, the purging valves automatically control and transport ignition protection gases, such as compressed air, into the inside of the Ex p enclosure. Additionally, a temperature monitoring and control system is integrated inside the enclosure to provide adequate cooling. In doing so, the systems cover requirements for extremely small (10 litre volume) to extremely large Ex p cabinets with volumes of 4000 litres or more.

Typical Ex p pressurized enclosures are operated in three separate steps:

1. Preparation phase: Compressed air or inert gases are transported into the Ex p enclosure via the purging valve and an overpressure is generated. Once this is achieved, the "pre-purging" phase begins; this is regulated via the Ex p control unit.
2. Pre-purging phase: Before starting up the equipment located inside the cabinet, any explosive mixture that may be present must be removed from the enclosure. The purging valve opens once the preset internal enclosure pressure has been reached. Next, thorough purging is performed using ignition protection gas that flows through the additional opening in the air outlet valve. These air outlet valves (pressure monitors) are positioned in different locations in order to guarantee controlled flow behavior. The extremely short purge times at very low pressure loads (0.002 bar) are particularly advantageous. This enables the wall thickness of the cabinet walls to be reduced to less than 2 mm. This, in turn, lowers costs. Once the pre-purge phase is complete, the purging valve closes and the purge flow stops. The internal enclosure pressure returns to the preset value and the air outlet valve closes.
3. Operating phase: Normal operation begins and the control unit monitors and regulates the internal enclosure pressure in order to prevent explosive gases or dusts from entering the enclosure. The purging valve automatically compensates for any leakage losses.

The Ex p system can be used in gas hazardous areas in Zones 1 and 2, as well as in dust hazardous areas in Zones 21 and 22. If Ex-Zone 1 or 21 (Ex pxb) is present, the installed Ex p system must be fail-safe if a single fault occurs. This is generally achieved by using an Ex p system that is designed to be redundant. For Ex pyb applications in Ex-Zone 1 or 21, an alternative version is available in which all installed equipment is certified for at least Ex-Zone 2. However, equipment in Ex-Zone 2 poses a reduced hazard due to sparks and temperatures. In this case, therefore a simple monitoring system is sufficient. If the system is used in Ex-Zone 2 or 22 (Ex pzc), a simple assessment of the Ex p system is also sufficient.

The Ex p degree of protection is extremely robust, lasts a long time (service lives spanning 15 years are not uncommon) and, above all, offers a range of advantages when it comes to practical handling. This means that commercially available Ex e enclosures, as well as smaller and larger standard enclosures, can be used. Furthermore, the enclosures are significantly lighter. For instance, a typical Ex d enclosure weighs 180 kilograms; in contrast, a corresponding Ex p enclosure weighs just 40 kilograms. The bushings leading from inside the enclosure to the outside also do not need to feature the Ex d degree of protection. Ex e cable glands can be used, even for pipes and hoses.

It is important to ensure that the components installed in the enclosure do not overheat – as a result, air conditioning is required in around three quarters of all applications. This is primarily due to the fact that the cabinets are now used in a range of climate zones, including very warm zones. The cabinets are suitable for use in a wide temperature range (from -30 to +60 °C). Air conditioning can be provided by any suitable additional air conditioning equipment, or through the use of eddy current air coolers, water cooling or purge air cooling.

The simplest way to get started with the Ex p degree of protection is to use prefabricated units. In this case, it is even possible to incorporate your own devices. Another option is to provide a U-certified Ex p empty enclosure. When doing so, we recommend selecting an enclosure with standard dimensions. Yet another option is to produce or commission an empty enclosure, purchase a suitable Ex p system and combine the components yourself.

In this case, when obtaining overall approval from a test body, qualified expert or approved company, the "semi-finished overall product" must be subjected to all required inspections. During this process, it is helpful if the empty enclosure used with the Ex p system is U-certified equipment. If the empty enclosure is not U-certified equipment, the entire surface must also be checked in accordance with the requirements of Ex p standard IEC 60079-2.

If the internal pressure causes a door or covering to open on its own if the lock is moved, the following measures are intended to prevent the user from being injured:

• Using multiple locks, so that the enclosure pressure escapes slowly before all locks are opened, or
• Using a lock with two positions, which enables the pressure to vent safely when the enclosure is opened, or
• Limiting the maximum internal pressure to more than 2.5 kPa.
• The pressurized enclosure, as well as any conductors and their connectors, must pass the maximum overpressure inspection.

If an increased pressure that causes the enclosure, conductors or connecting units to deform may be present during operation, a safety system that limits the maximum internal overpressure to a specific value must be integrated, in order to prevent deformation from occurring.

In Ex-Zones 1 and 21, it must also be ensured that in the event of pressure loss, the Ex pxb cabinet does not pose any additional hazards. In this respect, automatic disconnection from the power supply has proven to be a useful solution because it is easy for users. If automatic disconnection from the power supply is not implemented, a risk assessment must be performed for the safety equipment and their effects in the event of a response. In Ex-Zones 2 and 22, by contrast, an alarm system such as a signaling device, horn or monitoring system in the control room, etc. is sufficient.

Ex p systems offer clear advantages – these solutions save space and are easy to connect. Thanks to the range of basic sizes offered by R. STAHL, users can find a suitable solution that offers plenty of space, even for additional components. Ex p systems feature improved flow technology, which significantly reduces the necessary supply of compressed air and lowers energy costs. With these systems, users can rest assured that they are on the safe side when it comes to both explosion protection and expenditure.