Getting Started in ASD
Updated: Feb 11, 2020
A quick guide to getting started with Aspirating Smoke Detection (ASD) Products.
What is Aspirating Smoke Detection (ASD)?
Aspirating Smoke Detection or Air Sampling Smoke Detection is a method of detecting the presence of smoke via a network of tubes, known as air sampling pipes. The tubes cover the area to be protected and contain holes drilled at similar locations to where a smoke detector would be placed.
The tubes are connected to an aspirating smoke detector, usually located away from the risk being protected. The aspirating smoke detector incorporates a fan (aspirator) which draws air through the holes in the tube and analysis it for the presence of smoke particles.
ASD provides many benefits over traditional point detection and a single aspirating detector can replace multiple smoke detectors.
Most ASD products allow the fire activation level to be programmed for different densities of smoke. Aspirating Smoke Detectors allow the detection of smoke over a far greater sensitivity range than ordinary detectors; from 100's of times more sensitive to many times less sensitive.
ASD Design Considerations
As with all life safety systems, the design and planning stage of an Aspirating Smoke Detection system is crucial in ensuring its successful operation.
Typically, a sampling hole is placed on a drawing at each location that a standard smoke detector would be placed. The holes are subsequently joined by lines which represent the tube or sampling pipe.
There are limitations to how much pipe can be joined on a single pipe run. Different detectors have different limits, which ultimately dictate which detector should be used for a design.
Take for example a Vesda LaserFOCUS 250 (VLF-250). The VLF-250 allows a maximum linear pipe run of 25m. Perfect for small areas such as lift shafts and small data rooms but not very useful for a large warehouse.
On the other side of the scale is the Vesda VEU, capable of providing four separate pipes up to 100m in length per pipe. Fully networkable, multiple VEU detectors can cover vast areas.
The Design Science
The design and planning stage outlined above requires a little further consideration. The pipe lengths, hole quantities and hole sizes need to be taken into account to create a 'balanced system'. A balanced system aims to ensure that each sampling hole along a pipe responds to the same amount of smoke as every other hole along the pipe.
If a pipe contains 10 sampling holes, the first one may be located as close as 5m from the detector. The last hole may be located as far away as 100m from the detector. An ASD would respond very quickly to smoke introduced into the nearest hole but not so quickly to the hole 100m away. Smoke introduced into the furthest hole may also have to pass a number of holes that are diluting the smoke sample with clean air.
Fortunately, all manufacturers provide a software program that takes all of this into account and calculates parameters such as hole size, detector sensitivity and fan speed for you. Ventec Systems can help with most calculation software products from various manufacturers if you need guidance.
The installation of an ASD system is very straight forward for those experienced in fire alarm installation.
The detector, which resembles a small fire or security panel, is mounted at the required location marked on the drawings.
The pipework, made of ABS, is installed along the routes marked on the drawings and in accordance with the calculations.
The pipework installation is similar to installing PVC electrical conduit. Various sizes exist across manufacturers but it is typically limited to 25mm OD tube or 27mm OD tube.
The tube is installed using saddles or clips that are fixed directly to the building structure. The tube can be run along containment and Ty-Wrapped instead of clipped if required.
Similar fittings to standard conduit, such as 90 & 45 degree bends, couplings, Tee pieces and end caps ease the installation process.
All joints should be cemented with the correct adhesive. The connection of the pipes to the detector should never be glued. All detectors have a method of securing the pipe without cement.
Finally, and before the ASD is switched on, sampling holes of the required size are drilled at the locations detailed on the drawings and calculations.
Various detectors from various manufacturers have different commissioning requirements. Some detectors such as the Astec ASD range require little to no setting up. They are supplied pre-configured and can very nearly be switched on working.
Full commissioning is beyond the scope of the article but certain commissioning tests are generally required, dependant upon local codes and standards.
Here in the UK, two tests are usually carried out. A transport time test and a performance test.
Transport Time Test
This test involves introducing a small amount of smoke into the last sampling hole on each pipe. The time taken for the detector to react is recorded. The detector should respond within a prescribed period of time. The response time should be detailed on the calculation results and checks regulatory compliance along with confirmation that the ASD system performs as expected.
A response from the detector is NOT required to be a fire condition. A visible increase in smoke level by the detector is deemed sufficient. Under real life conditions, smoke would continue to enter the pipework and would result in a fire condition.
A performance test verifies that the ASD system performs as expected to the presence of smoke. It is not sufficient to carry out a transport time test only. If the pipework only has one sampling hole in the end of the pipe, the detector will respond to a transport time test but would be unlikely to detect a real life fire without additional sampling holes.
A performance test varies across environments and standards but essentially involves creating smoke from either a 'hot wire burn', a smoke pellet or in certain circumstances, a real fire.
The test should be carried out at ground level in agreed location(s) within the protected area. The generated smoke is allowed to rise from ground level to the pipework where a further period of time is allowed for the detector to respond.
Hopefully, the above goes some way to introducing Aspirating Smoke Detection. With the right help, guidance and assistance, most fire detection companies can quickly add ASD to their range of products and services. If you would like further information regarding any of the above, please get in touch, we would welcome the opportunity to assist wherever possible.