6 Purge Gas and Operation of Control System (Gas, Vapour and Mist Risks)

Installation of pressurized apparatus 555



established to be above the 50N/m2 minimum and below the maximum

acceptable pressure specified for the enclosure. The purge timing may begin

when all of these conditions are met.

If during the purge time the flow of protective gas falls below the

minimum figure specified, the flow detection device should initiate a cessation of purge and purging should not again begin until the purge criteria

are re-established. The purge timing should then begin again from zero.

Likewise if the pressure in the enclosure falls below the minimum specified figure or rises above the maximum permitted overpressure the purge

should be shut down and restarted only when the correct conditions have

been re-established. Additionally, in the case of overpressurization of the

enclosure or ducting, the enclosure (and, if necessary, the ducting separately) should be vented or its supply of gas removed to prevent damage.

The electrical circuits within the enclosure must remain isolated from

all supplies and other unprotected electrical circuits (circuits not otherwise

explosion protected) during this purge period.

Normal operation



When the purge has been completed and the time delay relay or other timing

device indicates this to be so then, provided the flow and pressure conditions

remain within those acceptable, the outlet of the enclosure is closed (in the

case of pressurization with leakage compensation)or reduced in size (in the

case of pressurization with continuous flow of protective gas). At this time

provided the internal pressure remains within its limits and, in the case of

continuous flow, the flowrate is above the minimum specified for normal

operation, the electrical circuits within the enclosure may be energized and

the other energized intelligence connected circuits (such as control and intelligence transmission circuits which were isolated) may be reconnected.

Actions on faults in the system



If the pressure inside the enclosure falls below the minimum of 50N/m2

or the flow, in the case of enclosures with continuous flow of protective

gas, falls below its acceptable minimum, the monitoring devices should

identify this.

In all cases (both where the enclosure does not have flammable

materials fed into it and where it does have limited normal and any

abnormal release) the action required on this identification is given in

Table 19.5. The alarm-only requirement for apparatus suitable for Zone 2

without isolation refers only to Zone 2 compatible apparatus (i.e., Type

'N' apparatus and suitably selected industrial non-sparking apparatus) and

not to all non-sparking apparatus. It is also only of limited term and if

the pressurization cannot be restored in a short time (say four hours) then

isolation must take place. In this case it can be automatic or manual rather

than automatic only as is the case with immediate isolation requirements.



556 Electrical installations in hazardous areas



Table 1 . Alarm and shut-down requirements for pressur95

ized enclosures without internal releases

Classification of

area of



installation



Zone 1

Zone 2



Enclosure contains

apparatus

Suitable for use

in Zone 2 without

pressurization



Enclosure contains

apparatus not

suitable for use

in Zone 2 without

pressurization



Alarm (Note)

No action



Alarm and isolation

Alarm (Note)



Note: Only valid i the short term. If situation persists then isolation will

n

be required.



Where it is not possible to isolate apparatus in situations where isolation

(either immediately or delayed) is necessary it is not acceptable to waive

this requirement. Thus some more reliable pressurization method, such as

the use of main and standby pressurization supplies, must be used with

isolation not necessary unless both supplies fail. If this is not possible then

the technique of pressurization is not possible.



19.7 Multiple enclosures

There is nothing in principal to prevent several enclosures from being pressurized from the same source provided that no action in respect of any one

of the enclosures results in any other enclosures in the group exceeding any

of the pressure/ flow requirements applied to them. This normally precludes

serial connection of enclosures and is subject to the following requirements:

1. Opening of any door or cover in any one of the enclosures is preceded by

manual or automatic isolation of that particular enclosure. (It is recommended that this action be automatic wherever it is possible, usually

executed by a switch which senses the opening of the door as soon as

opening is started thus ensuring isolation occurs before the door is fully

open). If there is residual heat in the enclosure it will be marked with

the minimum time before which opening can commence and in such

circumstances isolation will need to be manual.

2. The pressures and flow rates in the other enclosures in the group are not

affected to the extent that they exceed the appropriate limits and their

parameters continue to be monitored effectively.

3. When any enclosure in the group has been de-pressurized it must go

through its entire purge cycle after pressurization is restored and before

the electrical equipment enclosed is re-energized and other unprotected

electrical circuits reconnected.



Installation of pressurized apparatus 557

Process gas to



PC(Hi) = High pressure switch (alternative positions shown)

PC(Lo) = Low pressure switch (alternative positions shown)

FIC(Lo) = Low flow switch



Fig. 19.5 Typical pressurization control system. Notes: (1) The control unit should

be mounted in a non-hazardous area or suitable for potentially explosive

atmospheres (e.g., flameproof). (2) All electrical switches and electrically

operated valves to be suitable for hazardous areas (e.g., flameproof or

intrinsically safe).



19.8 Typical pressurization control system

A typical pressurization control system is shown in Fig. 19.5. It will be noted

that the flow monitor device needs to be in the exhaust duct or very close

thereto to ensure that it monitors the flow leaving the enclosure. Where the

exhaust ducting is long it needs to be close to the valve which closes after

purging or the restrictor which controls operational flow to ensure that it

identifies leakage in the ducting. Likewise, the low pressure indicator needs

to identify the pressurization of the exhaust duct as well as the enclosure

where the latter is long.



19.9 Pressurized enclosures in dust risks

Installations should comply with the requirements applied to gas/vapour

risk areas installations in Zone 21 and with requirements for installations

in Zone 1, and those for Zone 22 with Zone 2 requirements. In addition the

ducting used in any installation must ensure that, although the enclosure

may not remain IP6X (for Zone 21) or IP5X for (Zone 22) after pressure

failure when the ducting is no longer pressurized closures are present which



558



Electrical installations in hazardous areas



effectively prevent the ingress of dust by closure of outlet apertures. The

ducts themselves must, with the exception of their outlets, satisfy these

enclosure integrity requirements.



Table 1 . Alarm and shut-down requirements for enclosures with internal

96

sources of release

Internal release

description



Enclosure is in

Zone 1

and contains:

NonZone 2

Protected

protected



Enclosure is in

Zone 2

and contains:

Zone 2

NonProtected

protected



No normal



Alarm



Alarm and



release and

limited

abnormal release

No normal

release and

unlimited

abnormal release

Limited normal

and abnormal

release

Limited normal

release and

unlimited

abnormal release



(Note)



isolate



Alarm

(Note)



Alarm and

Isolate



No action



alarm

(Note)



Alarm and

isolate



Alarm

(Note)



Alarm and

isolate



Alarm

(Note)



Alarm and



Alarm

(Note)



Alarm and

isolate



isolate



No action



Alarm



(Note)



Alarm



(Note)



Note: Although it is permitted to maintain the apparatus energized this is a short term relaxation only and if the pressurization is not quickly reinstated isolation is also, necessary particularly where normal releases occur



19.10 Analyser houses

Analyser Houses have become very common over the last decade or

so. These are small, usually prefabricated, buildings into which several

instruments and analysers are fitted and which are regularly accessed by

personnel. They are common where more sophisticated analytical processes

are carried out, requiring apparatus which is not necessarily appropriate

to outdoor mounting. Analyser houses are pressurized to prevent any

external explosive atmosphere from entering the house thus permitting

the use of normal electrical apparatus. They also, however, normally have

flammable materials fed into analysers within them and these constitute

internal sources of release, and can have an effect on the external hazardous

area by release of explosive atmosphere into that area. There are three

considerations:



Installation of pressurized apparatus 559



1. Purging and subsequent pressurization of the analyser house to exclude

external explosive atmospheres.

2. Purging and subsequent pressurization of the analyser house or the analysers within it to dilute the release of flammable atmospheres within the

analyser house or analysers therein.



3. Purging and pressurization of the analyser house to ensure that it is safe

for personnel to enter.

79.10.1 Pressurization considerations



Exclusion of external atmosphere



For the purposes of external atmosphere, exclusion is only necessary to

follow the procedures for a pressurized enclosure without internal sources

of release. As people may enter this enclosure, however, pressurization

with inert gas is not permitted and thus air must be used. Likewise, it is

preferable to use a fan system rather than compressed air and to utilize a

continuous flow of air rather than leakage compensation. The continuous

flow to satisfy this requirement will normally be five times the analyser

house volume as in other cases (see BS 5345, Part 55).

Dilution of internal sources of release



The instruments within the analyser house may release flammable material into the house normally or in fault conditions. This may be liquid or

gas/vapour/mist.

In the case of liquid it is necessary to provide a drain which removes the

liquid from the house expeditiouslyto a collection point such as a blow egg,

and this provides a further aperture as far as pressurization is concerned

which exhausts gas and vapour to the external atmosphere unless exit is

via a closed lute which prevents this.

The releases of flammable gases, vapour or mists released within the analyser house must be diluted to 25 per cent of their lower explosive limit in all

but the dilution areas within the analyser house. These dilution areas need

to be small so that people are not likely to enter them and must not include

any electrical apparatus except that which is protected appropriately for

use in an explosive atmosphere of the appropriate Zone in the absence of

pressurization. (This means that for a normal release only intrinsically safe

apparatus may be used in the dilution area, and for abnormal release only

apparatus suitable for Zone 1 use). Entry of feeds of flammable gases or

vapours must thus be restricted to as low a pressure as possible, and pressures of less than 5 x 104 N/m2 maximum are recommended to reduce the

volume of any leaks. Likewise, the same maximum pressures for liquid lines

inside the house are recommended both to reduce leakage and to prevent

mists forming. Where fast sample loops are necessary to reduce sampling



560 Electrical installations in hazardous areas



times for both liquids and gases/vapours these should terminate outside

the analyser house, and inputs to the house in all cases should have pressure

relief facilities if their pressure could rise above the values recommended

above. Flow restrictors should also be provided where necessary to limit

the flow into the analyser house and hence the release in fault conditions.

Where flammable material in the form of gas or vapour is deliberately

released as a function of the apparatus into which it is fed, it should preferably be exhausted outside the house taking account of its effect on external

area classification. If this is not possible then the apparatus should be separately pressurized with air within the house so that any release into the

house is normally non-explosive.

It is very difficult to make the airflow through the house to dilute areas

of any normal or abnormal release sufficiently small without producing

an airflow which is uncomfortable to personnel. A release of ethylene, for

example, at a rate of 0.015m3/m (250ml/s) would require an airflow of

6m3/m to dilute it to 25 per cent of the lower explosive limit of ethylene

at the exhaust of the house (based on the equations in Chapter 4 for an

aperture of 10-6m3 and a pressure of 5 x l@kg/m2). In the case of an

analyser house the dilution needs to take place in the small area of the house

where the leak occurs to ensure the remainder of the house is gas free. This

would require significant airflow to ensure dilution in that small distance.

This often conflicts with the normal expectation that air movement velocities

in excess of 0.5 m/s are not acceptable indoors which would militate against

the level of airflow required in some circumstances. Where this occurs a

possible solution is to provide airflow outlets close to the sources of release

of sufficient velocity to cause rapid dilution. As the air expands into the

analyser house the flow velocity rapidly reduces to an acceptable level.

Another variation is to provide such local outlets but also provide local

extract so that releases are diluted and removed from the analyser house

locally to the point of release. This latter approach must be treated with

caution as the overall level of operating pressure in the house still needs to

be greater than 50N/m2.

One point which is helpful is that the airflow required inside the analyser

house is not likely to need a velocity of anything like the velocity required

in outdoor situations to achieve rapid dilution as the airflow is controlled

and directed and not random.

The basic method of calculating local airflows, should this route be taken,

is given in BS 5345, Part 5 which gives a method for pressurized apparatus

with an internal source of release. This method is not suitable for calculation

of the total airflow necessary within the house, as the analyser house, will

be much larger than the enclosure of an item of apparatus in relation to the

volume of released gas and thus the dilution performance would not be

the same. If, however, the air provision is local to the leak it will be more

relevant.

Q = F x (A/lOO) x (100/LEL) x S

m3/5

where



Q = airflow needed

F = maximum release rate



m3/s

m3/s



Installation of pressurized apparatus 56 1



A = percentage of gas in release (where the release

is not a mixture of gases this is 100)

LEL = lower explosive limit

S = safety factor (See BS/EN 60079-14)



%



%v/v

4



As already stated, this calculation should be carried out for each

component of the released gas mixture and the results added together

to give the necessary local gas flowrate. It must be remembered that, in

addition to these local air inputs, there must still be an overall input

to the analyser house to ensure that it is effectively purged before any

electrical apparatus is energized in common with all other pressurized

apparatus.

Although toxic matters are not within the scope of this book it is worthy of

note here that the use of the above formula using the maximum acceptable

gas level for toxic risks, instead of the lower explosive limit, will often

ensure proper dilution of toxic elements in any release. This is important

in view of the personnel access although this must be proved in each case

by test or assessment.

There are two approaches to provision of proof that analyser houses are

suitable for their use. The first approach is to carry out dilution tests similar

to those carried out for pressurized apparatus with internal release with the

normal airflow passing through the house and, by doing this, show that

dilution takes place in an acceptably small area which includes no electrical apparatus and is not one where personnel are expected to be present.

Certification/approval is currently optional in this case and a route exists

whereby the user can, by evidence obtained from similar analyser houses,

assess the suitability of the airflow in the analyser house without the need

for testing. In such circumstances, it should be ensured that the evidence of

similar situations is complete and relevant. The use of the equation detailed

above may well form part of such an assessment although the actual positioning of the air duct in each case is important. It should be as close to

the source of release as possible and direct its flow away from electrical

apparatus in the immediate vicinity and from areas where personnel may

be present.

The second method is to divide the house into two sections, one of which

contains the sources of release and the other the majority of the electrical

equipment. Personnel would then only have normal access to the part of the

house with the majority of the electrical equipment. The airflow in the other

part could then be much higher or the few electrical circuits in that part

protected for use in explosive atmospheres. Typical approaches to analyser

house construction are shown in Fig. 19.6 and 19.7.



Personnel protection



The toxic threshold level of many if not most of the flammable gases and

vapours which occur in industry is very much lower than their lower



562 Electrical installations in hazardous areas



Fig. 19.6 One-part analyser house



Main enclosure



Fig. 19.7 Two-part analyser house. Notes: (1) This door must be gastight and

secured closed so that normal access is not possible. (2) Wiring and

sensing devices must be otherwise protected (e.g., flameproof) if they

enter the dilution area. If this is the case, pressurization is not necessary

in this part of the house unless personnel require access. (3) The pressure

in this part of the house must always be higher by 50N/m2 than that of

the other part



explosive limit. If the gases and vapours released are toxic then the approach

should be to apply the criteria which would be necessary in any other

indoor situation where toxic risks occur. These can be applied alongside

the protection against explosion risks but should not dilute them. If it is

not possible to apply both then an alternative solution should be adopted.

As already stated, it may be possible to cover the toxic risk by using the local

airflow to dilute the toxic gas/vapour to below its occupational exposure

limit9 by using the calculation already described.



Installation of pressurized apparatus 563



79.70.2Analyser house construction and protection



It is not really relevant to test analyser houses in the same way as other

pressurized apparatus although the same criteria apply in that they should

be sufficiently strong to prevent damage in service and must provide the

appropriate degree of protection by enclosure required for the risk at their

point of installation with the pressurization off. (IT54for normal outdoor

situations rising to IP65 for such areas where dust risks occur). This does

not, of course, include the air outlet but it should be fitted with some device

to ensure that moisture or dust cannot enter when pressurization is off.

Analyser houses additionally require local alarms to ensure that personnel

inside know if the airflow or pressure falls below the acceptable minimum

but require a delay of around one or two minutes before electrical shutdown to permit people to enter and leave as pressure in the analyser house

and flow in the exit ducting will fall at such times. An override is necessary

to allow maintenance work but this should be secure to ensure it cannot be

inadvertently used.

The problem of personnel being trapped in the analyser house also needs

attention and an emergency exit such as a second door or kick-out panel,

should be fitted at the end of the house remote from the normal door. To

avoid operational problems it is recommended that the second exit be such

that it cannot be used in normal service and both exits should be fitted with

alarms which identify when they are open. Lighting which is protected by

another protection concept needs to be provided for emergencies as does

some secure form of protected communication.

Because of the internal release the interior of the analyser house should be

considered as Zone 1 in respect of all electrical apparatus installed within

it and remain energized when no airflow is present, and all apparatus not

so protected must be isolated when airflow fails (with the delay already

identified as necessary) even if the enclosure is installed in Zone 2 or 22. A

typical electrical installation is shown in Fig. 19.8.



19.1 1 Pressurized rooms

Pressurization is sometimes used for such things as control rooms where

no internal source of release occurs. These are not enclosure in the sense of

‘pressurized enclosures’ but the protection technique is similar. The basic

requirements for strength, keeping out the dust and environment apply

and, in this case, special attention is necessary to ensure that no excessive

leaks occur at such points as windows and cable inlets. Because such rooms

f

are usually very important to the operation o a plant it is not normally

possible to isolate all electrical equipment therein if the pressurization fails.

This leads to a main and stand-by pressurization system being necessary

to give a very high integrity to the pressurization, together with the need

for an airlock at points of entry to minimize the risk of ingress even if

both pressurization systems fail. Utilization of both of these precautions is



564 Electrical installations in hazardous areas

To pressurization

control unit

Start/stop button

utside but local to house)



G

,

Emergency light (note 1)

Flow switch (Note 2)

and pressure switch



I

I



I

I



I

I

I

I

I



Domestic consumer unit



w

Air inlet

with

heater if

necessary



1z-7------50 V TFR+ Sockets



I



Annlvser home



Fig. 19.8 Typical analyser house electrical installation. Notes: (1) The emergency

light must remain on when pressurization fails and when main electrical

supplies fail. It must be fed from a separate supply and explosion protected

(e.g., flameproof). (2) Pressure and flow switches must be explosion

protected (e.g., flameproof or intrinsically safe). (3) Alarm buzzer must

be explosion protected (e.g flameproof or intrinsically safe)



not intended to allow continuous use of the room without pressurization,

but to allow it to remain in operation long enough to allow a controlled

shut-down of the plant to be undertaken if the pressurization cannot be

re-applied within a short time (e.g., four hours). The use of such a room

without pressurization should be limited to as short a time as possible

(probably not longer than one day) and access should be restricted as far as

possible during that time. Entry points should have warning notices to alert

personnel to the fact that the doors must be kept closed and should be fitted

with alarms which sound if a door is deliberately propped or inadvertently

left open. The alarm need not sound during the period of normal entry.

Initial purging to remove any internal flammables before start-up is again

necessary and should be at least long enough to allow five times the room

volume in air to flow through the room. Pressure detectors and flow detectors similar in position and operation to those in analyser houses should

be fitted.



References

1 NEC



2 CENELEC



National Electrical Code (US). Chapter 5 ‘Special

Occupancies’, Article 500, hazardous (Classified)

Locations.

Centre Eurogeen de Normalization Electrique



Installation of pressurized apparatus 565



3 B 5501

S



Electrical Apparatus for Potentially Explosive Atmospheres. Part 1 (1977).General Requirements. Part 3

(1977). Pressurized Apparatus ‘p‘.



4 BS/EN 50016 (1995) Electrical Apparatus for Potentially Explosive Atmospheres. Pressurized Apparatus ’p’.

5 BS 5345



6 BS/EN 60079



Selection, Installation and Maintenance of Electrical Apparatus for Use in Potentially Explosive

Atmospheres (other than mining applications or

explosive processing and manufacture). Part 1

(1989). General recommendations. Part 5 (1983).

Installation and Maintenance Requirements for Electrical Apparatus Protected by Pressurization ’p’ and

by Continuous Dilution, and for Pressurized Rooms.



Electrical Apparatus for Explosive Gas Atmospheres. Part 14 (1997) Electrical Installations in

Explosive Gas Atmospheres other than mines)

7 BS/EN 50014 (1993) Electrical Apparatus for Potentially Explosive Atmospheres. General Requirements.

8 RoSPA/ICI

Engineering Codes and Regulations, Electrical

Installations in Flammable Atmospheres (1973).

Group C (Electrical) Volume 1.5.

Guidance note from the Health and Safety Executive.

9 EH 40

Occupational Exposure Limits. (Updated Regularly).