thuba PS 830, Руководство пользователя, Страница: 25 / 72

Manual BVS 19 ATEX E 080 X 21
Edition March 2020 thuba Ltd., CH-4002 Basel
Copyright Switzerland
3.2.2 Pressurized enclosure system with pre-
purging
To pre-purge the housing connect a digital work-
ing 2/2 way solenoid valve (SVD.L) to the termi-
nal 5 and 6 of the PS 830. Also configure the pre-
purging period into the structure menu of the
control device. After pre-purging the valve closes
automatically and the adjustable throttle of the
SVD.L lets a small amount of purging medium
into the housing to hold pressure.
The installer can determine the pre-purging time
once by doing the attenuation test according IEC
60079-2.
As an alternative he can calculate the purging
time without the elaborate attenuation test see
below:
The purging time depends on minimum flow
(Q
min
), free internal volume (V) and the free vol-
ume of the connected pipes (Va). Final the cal-
culated time must be multiplied by 10.
The purging time t
purge
is:
The minimum flow (Q
min
) depends on the mini-
mum pre pressure (P
premin
), the pressure lost at
the valve (P
Valve
), internal pressure of the Ex pzc-
housing (P
Int
) and the nozzle diameter (d). The
pressure lost at the valve (P
Valve
) should not
exceed 500 mbar. The maximum internal pres-
sure of the housing is defined to be less than 20
mbar.
The minimum flow Q
min
can be calculated by:
ρ is the density of the purging medium. The den-
sity of air is ρ =1,2393 kg/m
3
, the density of nitro-
gen is 1,25 kg/m
3
. We calculate the ρ of air for
all gases, because the difference is only 3%.
The installer can calculate the purging time of its