Interesting editorial from Peter Mundy explaining how to protect an Hazardous Area (ATEX) with ASD systems.

1. Protection of
hazardous
areas
using High Sensitivity,
Early Warning Aspirating
Smoke Detection
A petrochemical site. Pic courtesy of Vision Systems (Europe) Ltd
a 20 hole system with equal quantities
SUMMARY
By Peter Massingberd-Mundy, of air entering each sampling point. To
ost Fire Engineers, Consultants
M and Fire Prevention Officers
know what an aspirating smoke
detection (ASD) system is and can
M.Eng, C.Eng, M.IMechE
Technical Product
Manager – Europe
have an effective sampling point sensi-
tivity of 4%/m – which is widely con-
sidered to be normal for a standard
point detector – the central detector
describe a centralised smoke detector Vision Systems – VESDA must have a Fire Alarm threshold of
with a fan or aspirator drawing air Vision House, Mark Road 0.2%/m.
samples from the protected area through Hemel Hempstead, This illustration shows how an aspi-
holes in a 25mm pipe running above the HP2 7BW England rating system with multiple holes must
protected area or across the air intake have a high sensitivity detector to
grille of an Air Handling Unit (AHU). match the performance of the generic
What many may not appreciate is the benefits and features that match the point detectors. However, one of the
increasing diversity of applications for price you pay. Naturally the fan or aspi- unique benefits of an aspirating system
which these detection systems are rator used to transport the samples into is its ability to detect lower concentra-
being used and the major benefits they and along the sampling pipe is sized to tions of smoke than a normal point
can offer. reflect the maximum pipe runs (which detector when smoke enters more than
This article highlights the major can be up to 200m) but more impor- one sampling point. In this case the
benefits of high sensitivity ASD systems tantly, the sensitivity of the central smoke concentration at the central
and then concentrates on its applica- detector is key to its performance detector is less diluted. As a result,
tion in high risk hazardous environ- capability. Aspirating Systems have a natural
ments particularly with regard to ASD systems can have many holes in ability to detect diffused smoke – the
meeting the imminent requirements of the sampling network. In order to have more diffused the smoke is the more
the ATEX Directives. confidence that each hole has at least sampling holes it enters. This is known
the equivalent performance to a point as the cumulative effect.
HIGH SENSITIVITY ASD SYSTEMS detector the scenario of smoke only To illustrate with figures: Consider
In their simplest form ASD systems are entering one hole must be considered. the 20 hole system. If 2%/m smoke
easy to understand; they continually In this case all other holes will be draw- enters 2 holes then the detector with
draw samples of air from the equip- ing clean samples and the sensitivity of its sensitivity set at 0.2%/m will declare
ment or area requiring protection and the central detector must be sufficient an alarm. If 1%/m smoke enters 4 holes
assess these samples for the presence of to account for this dilution. the 0.2%/m detector will similarly
smoke. Many variations exist with Illustrating this with figures: consider declare an alarm. And so on.
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2. control. Depending on the particular
application they may be required to
prepare for the full alarm condition.
For example, procedures in an office
might direct them to close windows,
shut down PCs, and warn other person-
nel in the area that a fire Alarm is
imminent. The underlying message is
“take action”. In an IT or industrial
application the Action alarm may start
to initiate back up procedures or shut
down machinery.
After these vital early warnings (that
are only available on top range ASD
detectors) there is the Fire condition.
This is generally connected as an Alarm
input to the central panel and is often
considered to be equivalent to the
standard point detector response. On
some ASD systems that have a wide
Fire Growth Curve.
sensitivity range a fourth alarm level is
While this cumulative effect is an “thermal event” first time but after a also available that can be interfaced
important feature of all ASD systems it number of incidences the warning into automatic suppressions systems.
does not in itself provide the early might be traced, say, to an electric Such detectors can therefore provide
detection capabilities on which the heater. Depending on the particular multiple alarm outputs that can initiate
good reputation of high sensitivity ASD site the source of the alarm can be the appropriate response.
systems is founded. This reputation is removed or the alert threshold can
built on the utilisation of very sensitive be permanently adjusted to compen- APPLICATION OF ASD IN HIGH RISK AREAS
central detectors to give alarm warn- sate for this regular occurrence. The reputation of High sensitivity ASD
ings that are significantly earlier than If the smoke condition continues to systems originates in the protection of
conventional detectors – even when escalate then a second stage warning is the high risk, high value environments
smoke is only entering one hole. It generated – this is the Action Alarm. of Telecom switches and Electronic
should be noted that the discussion Generally this is configured to raise a Data processing areas. These areas typi-
and illustrations thus far relate to the pre-alarm warning on the central panel, cally have large Air Handling Units for
ALARM condition. For true early warn- the CIE (Control and Indicating Equip- cooling the electronic equipment and,
ing what matters is not that the ASD ment). Local personnel might be due to the dilution of any smoke by the
system can match and in many cases expected to inform the relevant author- large airflows; effective smoke detection
exceed the performance of a point ity if they have identified the source is only practical using high sensitivity
detector – what really matters is that it and ask for the unit to be isolated systems. Over the years, the technology
can provide a warning as soon as any temporarily until the cause is under has been applied successfully in many
abnormal conditions are discernable.
In this context, Early Warning should
not be confused with Pre-alarm. Pre-
alarm provides an indication that an
ALARM condition is approaching. Early
Warning provides an indication that
normal conditions are no longer present
– something unusual is happening.
This is best illustrated by considering
the multiple alarm outputs available on
the top range ASD detectors.
MULTIPLE ALARM THRESHOLDS
First level Early Warning alarm Alerts
key personnel to a potential problem –
perhaps the on site security staff are
informed of the condition and/or the
local room occupants are alerted to the
unusual conditions. The response to
this Alert warning might be that local
personnel stop an unapproved activity
(e.g. soldering) thereby avoiding an
unnecessary evacuation of the premises.
They might not find the source of the Hazardous Area Detectors. Pic courtesy of TEPG
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3. ATEX 95, the “equipment” Directive (94/9/EC) specifies the Essential
Health and Safety Requirements for equipment that may be used in an
explosive atmosphere.
It places requirement on manufacturers of such equipment both in terms of
performance and product quality, which was not included in the majority of
the national requirements that preceded it.
The essential safety requirements for equipment covered by the directive
include requirements for the selection of materials, marking, user instructions
and design and construction. Typically manufacturers will use the appropriate
European and/or international standards to demonstrate their compliance
with many of these essential requirements.
The directive defines a number of categories, which correspond to the
classification of the hazardous areas in which the equipment is to be
deployed. However, it is important to realise that there is no direct relationship
defined in the directive between the area zoning and the category of
equipment, which must be used.
The full list of ATEX categories are as follows (there is a sub-division which
separates equipment for use in mines from all other equipment) where the
VESDA Exd product image. Pic courtesy final column provides a link to the zoning classification.
of Vision Systems (Europe) Ltd
Group Category Locations Comments Zone Classification common
other areas where business continuity is in Europe
critical and/or where risks and potential (Based on IEC 60079-10)
losses are high. Examples include ware- I M1 Mines capable of functioning
housing, cold stores, industrial processes, safely in the presence
n/a
cable tunnels, production machinery, of an explosive
atmosphere
wet benches and prison cells.
One specific example of the growing I M2 Mines must be de-energised
when an explosive n/a
adoption of ASD systems is in haz- atmosphere is present
ardous areas. Generally these areas by
II 1 Other protection assured in Explosive atmosphere
their very nature fall into the category the event of two Zone 0 will be present
of high risk and they are often critical independent failures continuously
to the business continuity. PetroChem II 2 Other protection assured Explosive atmosphere
plants have many hazardous areas; in the event of will be present some
foreseeable failures Zone 1 of the time (e.g. due to
warehouses storing solvents and alco-
hol also have classified areas, which operational reasons)
require specialist fire detection prod- II 3 Other protection assured Explosive atmosphere
during normal Zone 2 may be present (e.g.
ucts. Essentially these are areas where
operation in the event of a fault)
explosive mixtures of gases or vapors
can accumulate which if ignited would
cause an explosion. Equipment intended for use in the higher risk categories is required to under-
go independent certification by a Notified Body (an independent and suitable
ASD IN HAZARDOUS AREAS authorized certification body). Equipment for lower risk categories may be self-
Within Europe there have historically certified by the manufacturer in order to affix a CE mark to the product.
been different approaches to Ex envi- The actual relationship between the category and the certification require-
ronments within different member ments is as follows:
states. However, with the imminent
mandatory CE marking of Electrical Category Requirement
equipment installed in hazardous areas 1
Product certification and review of quality control system by
under the ATEX directive there is some 2 Notified Body required
harmonization and renewed emphasis (electrical)
on the suitability and application of 2
(non electrical) Self-certification by manufacturer permitted supported by
electrical equipment in such areas. Declaration of Conformity and Technical File
There are many web pages devoted to 3
the full explanation of the ATEX
Directives, of which there are two, as
summarized (see tables, right and over). l Intrinsically safe (Exi) equipment l Pressurized apparatus (Exp) prevents
In summary equipment installed in limits the electrical energy in the explosive mixtures reaching the
hazardous areas must be CE marked devices installed in the area so that potential source of ignition by
and have appropriate Ex rating. There no significant spark or overheating means of a positive pressure within
are a number of alternative approaches can occur, which may ignite an the electrical enclosure.
to Ex ratings (ref. EN50014). explosive atmosphere.
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4. common are intrinsically safe smoke hazardous area (as depicted in Scheme
and heat detectors. Flame detectors A) with a flame arrestor (see illustra-
and linear systems such as pressurized tion) to protect the hazardous area
pipe or temperature cable/fibre are also from possible ignition through the
available for such environments. How- pipe. There is clearly a problem with
ever, none of these technologies can this arrangement in that the ASD sys-
provide the Early Warning smoke tem is exhausting the hazard into the
detection capability of an ASD system. safe area. To overcome this, the exhaust
When deploying ASD systems for the can be piped back to the hazardous
protection of hazardous areas some area through another flame arrestor
installers have previously tried to (Scheme B). However, serious consider-
exploit the “remote sensing” capability ation must be given to the failure
Flame arrester. Pic courtesy of AMAL of such systems by installing the detec- mode of this arrangement in the event
tor in a safe environment and installing of an explosion. In accordance with
l Flameproof enclosures (Exd) contain the electrically passive pipework in the ATEX 137, the behavior of the ASD
ignition sources in such as way that
any ignition of the hazard inside the
enclosure will not be transmitted ATEX 137, the “user” Directive (1999/92/EC), is specifically
to the atmosphere outside the concerned with worker safety and places requirements on employers whose
enclosure. staff may work in an explosive atmosphere
l Increased Safety (Exe) use mechani- It places a number of requirements on employers. The main ones are to:
cal construction safeguards to l assess explosion risks and draw up an explosion protection document
ensure that the apparatus does not (Articles 4 and 8);
contain normally arching or sparking
l prevent and provide protection against explosions (Article 3);
devices, or hot surfaces that might
l implement measures to ensure work in explosive atmospheres can be
cause ignition.
l Oil immersion (Exo), Powder filling
carried out safely (Article 5);
(Exq) and encapsulation (Exm) are l coordinate the implementation of health and safety measures with other
other approaches that may be used. employers (Article 6);
l classify, zone and mark areas where explosive atmospheres may occur
Fire detection products generally use (Article 7).
either Exi or Exd protection. Most

5. does not become a hazard. In Scheme
D the ASD is mounted in the hazardous
area and a remote display is provided to
provide information and interfaces
within the safe area. Clearly Scheme D
is the most appropriate but this does
not preclude the use of other schemes
where appropriate risk assessments are
undertaken and recorded.
When designing the fire protection
arrangements for hazardous areas
the benefits of installing high sensi-
tivity ASD systems are clear. The
ability for these systems to provide
early warning ensure the best
possible protection for these high
risk, high value areas. However,
careful consideration of the explo-
sion risks is essential and simply
installing the ASD system in a safe
area with flame arrestors in the
enclosure must be considered in rela- ASD detector in a flameproof enclosure pipework to the hazard may be
tion to the safety of personnel (and and provide flame arrestors directly into flawed. The preferred approach is
equipment) in the vicinity. Furthermore, the enclosure (as shown in Schemes to install the ASD system in an Exd
the integrity of the pipework must be C&D). In scheme C the detector is enclosure, which can be mounted
given due consideration in the event of mounted in the safe area, which may in the safe area or the hazardous
flame front travelling from the detector be possible as long as the leak rate of area depending on the particular
towards the flame arrestors. the Exd enclosure is sufficiently low to site arrangements.
The best solution is to enclose the ensure that the surrounding atmosphere