In a nutshell, the Ex p concept (aka pressurisation) is an explosion protection method used in hazardous areas to prevent the ignition of explosive atmospheres. The principle behind Ex p is to maintain a protective gas, usually clean air or sometimes inert gas, inside an enclosure at a pressure higher than the surrounding atmosphere. This overpressure prevents the entry of any flammable gases, vapours, or dust into the enclosure, thus avoiding the risk of an internal explosion.

Ex d, on the other hand, is designed to contain any explosion, so the Ex p strategy could be said to be its reverse. You prevent the ignition source from triggering explosions in the first place.

Making It Work

In essence, there are 2 types of Ex p systems, systems with internal leakage and systems without internal leakage.

Systems without internal leakage

For systems without internal hazard sources (combustibles), compensation is generally only required for leakage loss. Before a system can be started up, it must be flushed with clean lunch or inert gas by 5 times its volume.

Depending on the equipment to be placed in the enclosure and the zone in which the system is to be placed, there are 3 possible solutions for Ex p without internal leakage.

Ex pxb is a type where industrial equipment (not ATEX) is put in the enclosure and brought to category 2 (EPL b) and is suitable for use in Zone 1. Here, the system must be switched off when the overpressure is lost.

Ex pyb is a type in which category 3 (EPL c) equipment is enclosed and brought to category 2 (EPL b) and is suitable for use in Zone 1.

Ex pzc is a type where industrial equipment (not ATEX) is enclosed and taken to category 3 (EPL c) and suitable for use in Zone 2. This only requires alarms to be raised when the overpressure is lost.

For more information about equipment categories we recommend having a look at our blog article.

Systems with internal leakage

For systems with an internal hazard source, there is a requirement for a constant flow of clean air or inert gas. This flow is calculated so that the maximum leakage that can take place in the enclosure does not reach the LEL value of the combustible substance. Before a system can be started up, it must be purged with 5 times the volume of the enclosure with clean lunch or inert gas. If the air supply is lost, the system must be switched off.
Special attention is drawn here to restrictions, pressure regulators and flame dover in the sample lines so as not to exceed the maximum determined internal overpressure.

Where It Is Used

This Ex p concept is commonly used in industries such as oil and gas, chemical processing, and other sectors where explosive atmospheres are a risk, including airborne dusts.  It is particularly useful for protecting electrical and electronic equipment that could not otherwise be used in hazardous areas due to the risk of sparking or generating excessive heat.  Equipment certified as Ex p is typically labeled with the Ex p marking.  This is governed by international standards such as IEC 60079-2, which outlines the requirements for pressurized enclosures used in hazardous locations.

Real Life Example: Ex-M Hoeflon C6

An example of an Ex p certified product is the EX-M Hoeflon C6. The EX-M Hoeflon C6 crane shown in the image has been modified to meet Ex P certification, meaning it can be used safely in potentially explosive environments. This modification includes a specially designed cabinet and control system that maintain overpressure, preventing the ingress of explosive gases or dust. Using an external gas cylinder, overpressure is created and maintained in the cabinet. This prevents sparks or heat coming from the crane’s electronic components from contacting hazardous atmospheres, ensuring the safety of operations in high-risk industrial environments such as chemical plants and oil refineries.

Requirements

IEC 60079-13 covers pressurised rooms or entire buildings and stipulates that internal pressures must be no less than 25 Pascals (2.5 mm on a water gauge) above ambient, and pressurised areas must be accessed by airlocks. The IEC requires 50 Pa for Zone 1 locations. It further states that all such areas must have and exhaust velocity of at least 30% of a cubic metre per second when all air passages are opened simultaneously.

Advantages

Ex d enclosures are robust, strong, sturdy, or any number of other adjectives.  Ex p enclosures, on the other hand, are just as reliable and robust, but made of much lighter materials.  In contrast, two units of the same capacity would have significantly different weights and costs.  Whereas an Ex d enclosure would mass (for example) 180 kgs, an Ex p would be just 22% of the mass at a mere 40 kgs.

Sheet metal cabinets made just 2 mm thick cost less than 8-10 mm thick cast iron, or heavy steel enclosures that can resist explosions.  By preventing the problem from occurring you save significantly on material costs, and sheer weight, but with an equal safety margin and track record.

Indeed, Ex p cabinets can utilise Plexiglass™/Perspex™ panels so you can see what is going on inside.  This is much harder to achieve in an Ex d cabinet!  And, of course, the air exchange keeps cabinets temperatures below the gas-ignition point.

Disadvantages

These enclosures require maintenance to preserve internal pressure and cabinet integrity.  There are additional costs such as supplying neutral gasses like nitrogen (if required) or operating pumps to provide cabinet pressures with plain air.  It also takes a certain amount of time to purge equipment during start-up, but this is usually in parallel with other activities of preparation, so it may not have any impact.

Cabinets must be organised in such a way that internal components won’t provide a “pocket” where dangerous gases could collect and fail to be purged properly.  Good cabinet design avoids this.

The Takeaway

Ex p enclosures, typically lighter and less expensive than Ex d enclosures, offer reliable protection while using less material and allowing for features like transparent panels for monitoring.  Your choice will depend on your specific needs and both systems can be combined to complement each other.

While Ex p enclosures require some initial setup, such as purging and maintaining internal pressure, these activities often run parallel with other operational tasks, minimizing downtime.  The ability to prevent an explosion from occurring provides significant safety benefits and operational continuity.