User Manual

Fire Safety

Building Technologies 049_Archives_and_Libraries_A6

V10271399_a_en.doc

Fire Safety & Security Products 03.2009

All inert gas systems typically develop extinguishing agent concentrations of 40%

to 50% by volume. This means that the extinguishant occupies after the discharge

40% to 50% of the volume of the enclosure.

To vent the air displaced by

the extinguishant and to pre-

vent excess pressure from

being built up which would

damage the enclosure, the

volume of air displaced =

the volume of extinguishant

discharged

The size of the pressure

relief vent is calculated as a

FREE VENT AREA. This is

based on the maximum

pressure which can be with-

stood by the weakest part of

the enclosure. The weaker

the enclosure, the bigger the

required free vent area, the

stronger the enclosure, the

smaller the free vent area

Fig. 23 Vent area

NOTE:

The free vent area given by the hydraulic flow calculation program is not

the

physical size of the hole required to be made in the wall!

Consider:

– Vent efficiency (typically 45% to 85%)

– External louvres and back-flaps

– Any ductwork in between.

All these listed above and many other restrictions will reduce the free area m

given in the calculation program, the vent size must be increased to compensate

and guarantee the correct pressure relief during discharge phase.

For a vent of 250mm x 250mm, the free

vent area @ 100% efficiency = 0.25 x 0.25

= 0.0625 m2

At an operating pressure of 300 Pa this flap

type is 38% efficient

Free vent area at 300 Pa = 0.0625 x 0.38 =

0.0238 m2

Fig. 24 Pressure relief