Using Purge and Pressurisation to Prevent Explosions

By Sean Clarke, Epsilon Technical Services Limited

Introduction

We have mentioned in previous articles that there are many types of protection concept. One of the simplest concepts to understand and apply to almost any type of apparatus is purging.

Purge, pressurisation and Ex p or EEx p are all terms used to refer to this concept. The concept works on the principle of keeping the flammable substance away from the source of ignition and ensuring the surface temperature of the purged enclosure is non-incendive.

OVERVIEW

The technique of pressurising and purging enclosures of electrical apparatus is to prevent the ingress of a flammable atmosphere. Purging is a widely accepted protection concept for explosion protection. It is accepted world-wide (using European Standards, NFPA or IEC Standards) and is relatively straightforward to comprehend. Explosion protection is achieved by keeping the potentially explosive atmosphere away from any source of ignition (thermal or electrical). The potentially ignition capable apparatus is mounted inside an enclosure, the enclosure is then pressurised to a positive pressure relative to the atmospheric pressure (a positive pressure of 0.5mbar is all that is required).

As long as this positive pressure is maintained, no gas (or even dust) will be able to enter the enclosure, hence the internal equipment can not be exposed to a potentially explosive gas. There is however a chance that an explosive gas mixture may have entered the enclosure prior to the positive pressure being achieved. To ensure that the enclosure is pressurised with a non-explosive gas (i.e. Air or Nitrogen) the enclosure is 'purged' to flush out the existing contents and ensure that all areas of the enclosure contain only the purging gas (purging of internal dusts have not yet been considered). It normally takes between 5 and 10 volume changes to ensure that the enclosure is 'purged'. (In Europe the first edition purge standard defined five air changes as a minimum, in North America the minimum is defined as 10 air changes)

It is a condition of certification for Zone 1 equipment that power can not be applied to the equipment until the 'purging' (a specified flow of purging gas for a specified time) has been completed. To ensure continuity in the effect of the purging, the maximum leakage rate for the enclosure is also specified. When the purging has been completed, power can still not be applied until the specified positive pressure (at least 0.5mbar) has been achieved.

In the event of a failure to complete the purging cycle (drop in flow or incomplete duration) or if the enclosure pressure drops below the specified positive pressure, power to the equipment shall be removed (for Zone 1) or an alarm indication shall be given (Zone 2).

In the event either of these conditions, the entire purging cycle starts again with the full purge time duration. The control of the automatic purging and pressurisation is normally by a 'Purge Control Unit' (PCU). The PCU is required to measure flow and pressure, and must fail-safe in all conditions.

The enclosure that houses the equipment to be purged must have sufficient physical integrity to withstand impacts and overpressures. The enclosure should also be designed to facilitate the free flow of air. As enclosure integrity is required to a level of IP40 (no holes greater than 1mm), any non-metallic material must be tested for durability and longevity (against effects of heat and light etc.).External considerations, such as the surface temperature of the equipment or static from plastic parts, must be considered. To ensure incandescent particles can not be vented from the equipment, a spark arrestor must be fitted (or the vented gas must be ducted to a safe area). This technique is virtually unlimited particularly in physical size or power rating of the apparatus being protected.

Purge control Units

Purge control units must be able to measure and act on the following information: The operations performed by the PCU must be 'failsafe' by virtue of test and assessment with one fault (i.e. a valve failing), this is even more prevalent with the advent of the ATEX Directive. Failure modes of components must be considered; even relays can fail open or short-circuit (normally open relays can 'arc-weld' in to a closed position). Purging timers must always re-set to zero if they are interrupted during the purging cycle or after a purge failure. There are two basic types of control:

Constant Flow (CF)- The air flow for the purging and pressurisation stages are the same. The flow is left as a constant after it is set, and power is applied after a set period of time.

Leakage Compensation (LC) After purging, the air flow is reduced to a figure just above the leakage level to maintain the pressurisation. The PCU is required to switch from an initial high flow rate (often referred to as fast purge) to a much lower flow rate on completion of the purge time. CF systems are simpler to design, but are more expensive (in air or nitrogen) to run. There are other examples of hybrid systems (CF/LC) but in reality they are just variations on the two basic types.

PCU's are normally either pneumatic or electrical. If the PCU is mounted in the safe area only the operation will require verifying (unless it contains intrinsically safe outputs). PCU's mounted in the potentially explosive atmosphere will require certifying both as safety systems and as potential ignition sources (although sources of ignition from pneumatic systems were not considered until the ATEX Directive).

Each STATE of the system is defined in response to the inputs of the monitoring devices. The states are required to be unique. The logical conditions for the occupation of each state are required to be uniquely defined by BOOLEAN logical expressions. All possible combinations of input conditions are required to be shown in a table. For maintenance purposes it is necessary to work on the apparatus within the enclosure while the purge is off and the enclosure is open. Naturally this is done under a gas clearance or hot work permit for safety but there are other safety implications which have to be taken into account. Once the enclosure has been opened the pressure switch will, via the control unit, cause the power to be isolated from the enclosure. Naturally power must be on so some means are required to by pass this function of the control system. One way of achieving this is to wire a key-operated switch in parallel with the pressure switch. It must be ensured that the purge cycle reinitiates before power is applied again.

Standards and Certification

The methodology for designing and testing a purged and pressurised enclosure has been defined in many Standards and specific industry codes of practice. The two most commonly used Standards are:

3.1 CENELEC Purge Standard

The standard in Europe for the last 20 years has been the CENELEC standard EN 50016: 1977 which in turn is based on a part of IEC-79 published in 1975. The significant features of this standard are: A replacement standard for what has become known as 'the first edition' purge standard was issued in 1996. This is now the standard that is used for certification.

BS EN 50016:1996

The new standard for pressurised apparatus is far more substantial than the 'first edition' and contains more mandatory requirements. The key changes are: This 'Second Edition' Standard is currently being used for European Certification, although the vast majority of certified purged equipment is only certified against the requirements of the 'First Edition'. Both types of certification will exist in parallel until June 31st 2003, when the ATEX Directive will become mandatory.

Zoning Requirements

The European pressurisation standard does not indicate the zone of use of the purged apparatus, this is defined in the 'Code of Practice'.Purge can be used in Zone 1 or Zone 2.

Zone 1: Power must be automatically removed if pressurisation fails. Zone 2: An alarm must be raised (as a minimum) if pressurisation fails.

Most commercially available purge control units have options for 'alarm' and 'alarm and trip' so that the user can select the appropriate measures.

North America Requirements

The NFPA (National Fire Protection Association) 496 standard which was produced in the USA recognised three levels of complexity according to the risk and the objective.

Type X purging was intended for Division 1 locations to change the interior to non-hazardous.

Type Y purging was intended for Division 1 locations to change the interior to Division 2.

Type Z purging was intended for Division 2 locations to change interior to non-hazardous.

Type X purging requires that if the enclosure pressure is lost, the supply is automatically disconnected on loss of purge pressure and a re-purge is required before the supply is restored. (Zone 1 CENELEC)

Type Y purging does not require supply disconnection on loss of pressure but the equipment in the purged enclosure had to be suitable for division 2. (This is now possible under the ATEX Directive for Zone 1 CENELEC)

Type Z purging, because of the lower level of risk in division 2, required only an indication of loss of purged pressure. (Zone 2 CENELEC)

Practical implementation

Enclosures

A standard IP54 enclosure may not be suitable for use as a pressurised enclosure because the sealing is in the wrong direction. The standard enclosure is designed to prevent the external environment entering the enclosure which means that they are generally unsuitable for retaining internal pressure.

For this reason enclosures from suppliers of pressurised systems are not generally the same as general-purpose enclosures. Also, the enclosure must be able to maintain the pressure, on large enclosures the unit can be seen to deform even with relatively low pressures. Additional hinges and cover bolts may be required when the pressure is acting on a large surface area. Plastic parts (e.g. switches) should not penetrate the housing walls. Plastic parts may be used externally if, when the plastic is removed, metal parts remain that provide an ingress protection rating of IP40 (no objects greater than 1 mm can penetrate the enclosure). No live (or potentially live) parts should be exposed outside of the purged area. As switches are normally sealed devices that contain sparking contacts, it is preferable to either use certified switches or mount the switches inside the purged enclosure. Plastic ducting should not be used if the plastic part failing does not create a fail-safe condition, e.g. on the secondary purge system.

Supply and Return of Purging Medium

Special precautions are needed where the pressurisation method uses a fan or blower. It is generally undesirable to put the fan in a hazardous area because this means that the ducting, which extends to a non-hazardous area, will be below atmospheric pressure and, unless it is completely leak tight, may draw in flammable gases. The other advantages of putting the fan in the non- hazardous area is that it does not need to be suitable for use in hazardous area itself and the ducting is under positive pressure which prevents ingress of flammable gas.

The use of compressed air is the normal method of supplying purge air, it must be noted that several purged enclosure on one supply line may drop the operational pressure to below working levels for the pneumatic logic, if such a system is used.

The exhausted air from a purged enclosure may contain small particles that have been heated by the internal sources of the enclosure. To prevent these particles being vented into the potentially explosive atmosphere the following methods are used:

Pressurisation

The minimum pressure required is 0.5 mbar (50 Pa or 0.2" w.g.) and this should be achieved with the lowest possible flow of pressurising gas. The pressure measurement has to at least raise an alarm if the pressure falling below this level so that the working pressure will be above this. The enclosure has be tested to prove it will withstand 1.5 times its normal working pressure (minimum 200 Pa) for 2 minutes without distortion, a figure of 10 mbar is not uncommon as a working pressure.

Purging

Effective purging of the enclosure and its contents has to be provided. As a guideline, 5 volume changes are generally sufficient if the enclosure has been designed to a few basic guidelines:
  1. Avoid air traps (pockets)
  2. Avoid 'channelling' of the purging air
  3. Create turbulence
  4. Avoid sealed volumes

Temperature Classification

Since the flammable gas is prevented from entering the enclosure the exterior of the enclosure determines the temperature classification. It is to be noted, however, that internal hot surfaces will remain hot even after the power has been removed. A full assessment of the thermal properties of hot parts of a purged enclosure must be conducted.

Internal Energy Storage

For things such as capacitors in power supplies this often means either waiting until the charge has leaked away before the enclosure is opened or fitting bleed resistors to ensure it happens. Batteries cannot be dealt with in this way and invariably have to be protected by one of the other methods, a draft standard for batteries in purged equipment outlines the methodology:

Pressurised and Purged Enclosure with Internal Source of Release

The primary objective here is to prevent a flammable atmosphere from occurring within the enclosure. This can be achieved either by continuous dilution or by the use of inert gas. If the quantity of the internal source of release can be defined and controlled, dilution can be used. If the quantity of release can not be defined, inert gas must be used. In either case an initial purge is still required; where air is the purging medium the intent is to dilute any flammables below the lower flammable limit. In the case of inert gas the intent is to render the interior of the enclosure non-flammable by the removal of oxygen. For obvious reasons, apparatus capable of igniting a flammable gas must not be located in the dilution zone of a possible release.

Certification and Testing

Pressurised equipment can be 'self certified' for Zone 2. If the pressurised equipment is designed to be used in a Zone 1 atmosphere then it must be certified through a European Notified Body.

The certification process involves an assessment of the sample against the provided drawings (to ensure that the constructional requirements have been met and that the sample is a representative test sample) and a series of tests. The exact nature and type of testing conducted will vary from product to product, but will typically be as indicated below.

Test Standard
Impact test on enclosure EN50 014
Impact test on glass (x3) EN50 014
Thermal shock on glass EN50 014
Purge test (Argon and Helium) EN50 016
Overpressure test EN50 016
Leakage test EN50 016
Low pressure test EN50 014
Temperature rise test EN50 014
Thermal decay test EN50 014
Secondary purge test EN50 016
Dilution test EN50 016

The level of 'impact' for the impact test can vary depending on the material and the risk of impact. Plastic materials require pre-conditioning before impacting, and the impact test is conducted with the plastic parts at high and low temperatures. Plastic parts that penetrate the enclosure may also require resistance to light testing.

Purge Verification Tests

The test is normally conducted by filling the enclosure with representative test gasses (Argon and Helium are used to represent heavy and light explosives gasses) and physically measuring the removal from the enclosure when the inert purge gas is applied. The actual gas that will be present where the equipment will be used can be used, but Argon and Helium are normally used as they cover all possible flammable gas atmospheres.

The test is carried out by positioning small bore tubes in the purge cabinet at positions likely to 'pocket' and measuring the actual content of test gas. Initially the cabinet must be at least 70% full of the test sample to be removed. The test sample is then removed by air purging until acceptable levels (based on the LEL) have been reached.

The time taken to remove the test gasses from the enclosure is referred to as the 'purge time', and will be marked on the certification label and conducted prior to pressurising the enclosure and applying power.

The purge time will be dependent on the rate of flow of purging gas and internal geometry of the enclosure to be purged. By ventilation and a considered approach to the internal configuration, purge times can be greatly reduced (reducing the downtime of equipment before power can be applied). There is not a linear relationship between purge times and purge flow rate, i.e. doubling the air flow will not necessarily half the purge time.

Other types of pressurisation

Static Pressurisation:

Static pressurisation relies on the enclosure being pressurised with an inert gas and having a sealed enclosure to maintain the pressurisation.

Pressurised and Purged Enclosure with People Working Inside (Purged rooms)

Naturally inert gas cannot be used and compressed air is not generally recommended. In addition, emergency facilities for the personnel are required. Lighting and means of escape are of prime importance. The lighting is required under all circumstances and hence must be protected by some other suitable means such as flameproof. Kick-out panels or crash bars on doors usually provide for escape.

Sean Clarke is a specialist ATEX Compliance Engineer with Epsilon Limited, a company which designs, test assesses and certifies equipment for the CE marking and Potentially Explosive Atmosphere certification including the ATEX Directive. Epsilon also offers training for both users and manufacturers of potentially explosive atmospheres equipment. Readers may contact Mr Clarke at the address below for further information or to arrange a free technical meeting.

Epsilon Limited Tel: 01244 541551 Fax: 01244 543888 URL: http://www.epsilon-ltd.com

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