Pressurized Enclosure

Name of the Safety Technique : Pressurized Enclosure

Symbol: ‘p’

Guidelines & Specifications : IEC 60079-2

The idea is to create an artificial atmosphere inside the enclosure (housing the potentially ignition generating components) by maintaining a positive internal pressure of protective gas in relation to the surrounding atmosphere, thus preventing any ingress of the surrounding vapors and, where necessary, by supplying the inside of the enclosure with a constant flow of the protective gas which dilutes any combustible mixtures.

The over pressure is maintained either with or without a continuous flow of the protective gas

(Protective Gas : The protective GAS should be non-flammable and INERT. Inert gas means NITROGEN, CARBON DIOXIDE , ARGON or any gas which , when mixed with OXYGEN in the ration 4 parts to 1 part OXYGEN , as found in air, does not make the ignition and flammable properties more stressful.

Air of normal instrument quality is considered acceptable as a protective gas.

The concentration of OXYGEN after filling with inert gas shall be less than 1% by volume .) IEC 60079-2 sec: 9

Common Use: Switching and control cabinets, analyzers, large motors

Protection by Pressurization is sub-divided into three types:

The subdivision are :

These sub divisions are selected based upon the Equipment Protection Level required for the EXTERNAL EXPLOSIVE GAS ATMOSPHERE (Mb, Gb or Gc), whether there is the potential for an internal release and whether the equipment within the pressurized enclosure is ignition-capable . The type of protection then defines the design criteria for the pressurized enclosure and the pressurization system.

px or pxb= use in Zone 1, 2 and Zone 21 ,22

(Definition : Pressurization that reduses the equipment protection level within the pressurized enclosure from Gb to non-Hazardous or Mb to non-hazardous )

The inside space of the enclosure, which is Zone 1 or requires equipment with protection level Mb , in case of Equipment GROUP 1 (Mines susceptible to firedamp), hazardous area, is flushed , purged and pressurised with an inert gas or CLEAN AIR (instrument quality air), keeping the inside pressure positive as compared to the surrounding atmospheric pressure , so that a normal and non-hazardous spare is created and maintained inside , to make way for the installation of Industrial Equipment (electrical or non-electrical) as per the requirement of the involved process.

py or pyb = use in Zone 1, 2 and Zone 21 ,22

(Definition: Pressurization that reduces the equipment protection level within the pressurized enclosure from Gb to Gc)

The existing Zone 1 inside the enclosure is modified to Zone 2 , so that Zone 2 certified equipment may be installed inside the enclosure and then the panel is energised (of course competing the 3 statges or steps of pressureization .

pz or pzc = use in Zone 2 and Zone 22

(Definition: Pressurization that reduces the equipment protection level within the pressurized enclosure from Gc to non-Hazardous )

The Zone 2 space is converted to safe (non-hazardous ) arae so that Industrail equipment may be installed .

The components are installed in an empty enclosure (ATEX U certified) and then this complete assembly is certified by a notifying body. (Many Ex Manufacturers, Original Equipment Manufacturers, claim to have the certifying authority, granted by a certifying body (for example; KEMA) for a certain relevant product range.

The 3 stages of putting a Ex P (protected) panel into service are as follows:

Preparation :

the Inert gas is allowed to flow in to the panel and the internal pressure is allowed to increase.

Purging:

The Internal pressure is increased to a certain limit so as to force open the orifice plates to complete the purging process.

Operation:

The purging process is completed by closing the orifice plates and the panel is put into operation. The positive pressure is maintained by the pressure control unit thus preventing any ingress of outer explosive atmosphere.

Procedure to be followed in case of LOSS of Positive Pressure

Loss of positive pressure , inside the enclosue/panel , requires immediate actions :

In case of pxb : Automatic Switching off the power to the panel and sounding an alarm.

In case of pyb : Automatic Sounding an alarm and switch-off power to the panel

In case of pzc : Automatic Sounding an alarm , power may be kept on for a certain time during which the pressurization should be restored.

Untimately the fault have to be rectified and panel has to be re-energised AFTER COMPLETING THE THREE STEPS OF PRESSURIZATION PROCEDURE

Application

Large Control System
Large Power Panels
Encapsulation of special equipment (Camera , printers etc)
Application with critical temperature behavior

The concept of Pressurized Enclosures forms the bases of Pressurized Rooms , where the pressurization techniques are expanded and elaborated to be applied on specially built rooms and even whole building consisting of a number of rooms.

The corresponding standard is : IEC 60079-13