Aspirating smoke detection in industrial environments

September 2013 Security by Industry Sector, Fire & Safety

Invariably the approach taken by those who design or install detection systems in industrial applications tends to be much the same as that taken when dealing with cleaner environments. Minimal thought is given to the actual site application. There is limited understanding about the facility, the processes that can take place or the environmental conditions, inevitably leading to an unsuitable system being installed.

A thorough site survey of the facility is fundamental to ensure the correct system design – one of the critical steps that should be undertaken when planning an ASD system. Of course this is not always possible, particularly when the project has not yet been constructed. However, if possible, make a site visit to best understand the site conditions. Also get the client thoroughly involved.

The advantage of client participation is that many things are revealed about the site that may not be evident when simply doing a walk around, or designing from drawings.

Some points to consider when conducting a site survey:

* Building/application construction and neighbouring businesses.

* Production/processes and activities undertaken.

* Working operations/changes throughout a given year.

* What takes place, how, when and where.

* What and where are the fire hazards/risk areas of concern.

* Consider the building height and access to detection.

* Note ventilation – whether natural or forced.

Get to know the environment:

* Air flow movement/speed.

* Contamination – dust/smoke/other airborne pollution, chemical contaminants, etc.

* Summer/winter temperatures/stratification/humidity/moisture.

Client involvement:

* Ask questions. Ensure the end-user understands what can be offered – the benefits and limitations of the proposed system.

* Draft a brief letter/report of your findings and intentions.

* Set down details so all parties are clear as to what the system can and can’t do.

Smoke testing and design considerations

Uncommon as it is, smoke testing is the best way to determine the way in which air behaves in a given environment – but it should only be conducted with the approval of the client. If possible have the client present. A greater understanding of the site conditions is possible with smoke testing and early detection effectiveness can hinge on where detection (ASD sampling points) is located.

Since each application is individual, each will have different conditions, building size, shape, roof height, machinery, equipment, and processes, all of which will impact on the way air moves. Is air movement always consistent? What happens when doors or shutters are closed? Does the pattern change if these are left open? The pipe system layout must be harmonious with the air movement. After all, this is what ASD is all about.

Many applications will be straightforward, others more difficult and some will even require special engineering techniques to operate in conjunction with the ASD system. Regardless of the type of environment, to find the correct location of the ASD sample pipe and holes, smoke testing should be done.

Some applications will require a performance-based approach, not necessarily one where there is a need to engage specialist fire consultants to engineer the design, but more simply a practical or common sense view to do smoke testing and give a system a trial run to evaluate its effectiveness. Fire detection systems are not always a mandatory requirement in industrial facilities.

There will be specified applications where meeting prescriptive requirements could be a hurdle if effective detection is to be met. In these situations consult with the appropriate authorities if a prescriptive requirement can adversely impact on the detection capability. While fire system design in the main must comply with the requirements of codes and standards, prescriptive layouts are not always practical or the best way in which to detect fires in certain industrial environments.

Smoke emitters are typically the best way to conduct testing. They provide a good indication of air movement, but the test must be done in several locations to gain the best appreciation of air movement. Regin or Ventax smoke emitters are available in different sizes from air conditioning companies and are ideal for this purpose. Coloured smoke emitters are sometimes used to demonstrate the effect of smoke dilution to the client.

Sampling pipe installation considerations

While the installation of sampling pipe is important in any ASD application, it is perhaps even more so when installing for hostile environments. The sampling pipe network may seem somewhat insignificant to some, and is from time to time subcontracted to inexperienced installers who treat it as just another conduit run.

An ASD pipe network is the foundation of the system and a good layout is critical to the overall performance. Sampling holes in industrial ASD system designs are typically larger than those used in cleaner environments – generally around 3 mm or larger – and performance must be carefully modelled using appropriate software. It is also recommended that holes be countersunk to minimise contamination build-up around the hole. It helps by creating a mini venturi effect, allowing contaminates to be easily drawn back to the detector where they are filtered.

Air flow

Air flow is the very essence by which this technology functions, so why is it that many forget this basic concept?

No matter what type of detector, controls or type of flow sensing is employed within a detector, implementing a few simple measures can result in a more stable system. A well designed pipe network can also help in finding any air flow faults that may be associated with environmental disturbances.

The smoother the air flow within the pipe – and with minimal interruption and friction – the better the system will function. Sampling pipe leading directly into the detector ports should be straight and without interruption, i.e. at least 500 mm of free pipe should be installed between the detector, before any bends or pre-filtration or any other in-line components are added.

This consideration rules out any possible flow turbulence that may be created when air is immediately being drawn into the detector.

Some other simple measures include:

* Ensuring all installed pipe runs are straight without sagging and fixed securely at appropriate intervals. So be it if this means closer spaced fixings than required by local codes or standards.

* Consider expansion and contraction joints – sampling pipe at roof level in metal buildings will inevitably be influenced by building movement.

* Always install long radius sweeping bends to assist flow and minimise contamination build-up at bends. (T joints and elbows, while suitable in clean applications, are not recommended in industrial environments).

* Minimise pipe joins and ensure the correct glue is applied for the type of pipe used … and make sure it is applied sparingly.

* Consider catenary cable where access may be difficult or where obstructions create the need for more bends. Sampling pipe affixed to catenary cable allows the straight installation of pipe.

* Consider hole orientation.

ASD system engineering

There are many ways in which to approach, adapt or enhance the performance of ASD in industrial facilities to ensure the best possible performance. Many system installations are straightforward; others, due to their location, operation and the site conditions, require some level of system engineering, i.e. the use of additional measures to perform reliably within the environment. This task is made considerably easier if the detection equipment is purpose-built for harsh and difficult environments.

My experience over the years has shown that when designing ASD systems for the industrial sector, one must be conscious of the possible pitfalls, often unforeseen by those with little ASD installation experience or those that have only applied ASD in clean environments.

Temperature variations and humidity can create unforeseen issues. Sometimes a detector can only be located in the area where high and low temperature conditions exist. Applications where hot, cold, humid or wet conditions exist may require some system engineering. Protecting the pipe network from temperature extremes may mean installing specific types of pipe. Metal can be used. In fact copper, stainless steel, and other similar pipe has been used successfully.

A protective enclosure may sometimes be required for the detector to guard against process wash downs, steam or any other on-site environmental condition. Detector enclosures may also require cooling, which can be accomplished by simple thermo-electric means. Chemical pre-filters may be needed to minimise the impact of background air contamination before it enters the detector. Whatever the conditions ASD can in most situations be adapted.

The importance of selecting the correct technology will be highlighted in the next issue.

For more information contact Derek Waddell, regional sales manager, Xtralis, +27 (0)82 316 2601 or dwaddell@Xtralis.com





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