VDOT-ASD-400 Aspiration Detector
Velocity Detection
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VDOT-ASD-400 Aspirating Smoke Detector User’s Manual V1
Sampling Pipework Design
Aspirating system design is inherently simple. It is often possible to achieve good system
performance with very simple installations. There are however a few rules, which must be adhered
to, and these rules are equally applicable to all aspirating systems. The information contained in this
manual is intended as an overview only. For further information please see the complete Velocity
ASD Design Manual.
Considerations
o
Primary Detection Sampling Systems
Are usually arranged to monitor the flow of air movement by the use of pipework and air sampling
points mounted directly in the airflow. This type of system is usually regarded as supplementary to
other forms of detection due to its limited response capability once the air movement ceases.
In such a system when monitoring a single point of supply or extract, its system sensitivity may be
directly related as equal to the sensitivity of the central detector due to the cumulative effect. In the
case of a system monitoring more than one point of supply / extract then the system sensitivity will
only be determined in discussions with the manufacturer or his representative.
Always locate the sampling points in a position to which smoke may reasonably be expected to
travel. This may sound obvious, but, for example, do not expect ceiling mounted sampling points to
operate satisfactorily if air flow prevents the cool smoke from an incipient fire from reaching ceiling
level. In this instance it is usually better to locate the sampling pipes directly in the airflow (for
example in an air conditioning unit air intake). There is no substitute for carrying out smoke tests
prior to installation of pipes to indicate suitable sampling point location.
o
Secondary Detection Sampling Systems
Are arranged such that the air sampling points are sited and spaced as if they are point type smoke
detectors. They can be positioned to satisfy NFPA 72, NFPA 76, BS 5839-1, BS 6266 and local fire
code requirements when the calculated relative sensitivity per air sampling hole equates to a point
detector. See Relative Sensitivity below.
o
Maximum Permissible Transport Time
The time taken for a system to transport a sample from a protected area should not exceed 120
seconds (2 minutes). Transport times in excess of this must be the subject of a variation. Shorter
