50
7500 Series Manual
7 Determining Purging Time
To make sure the enclosure is safe from the hazardous atmosphere, the inside of the enclosure has to be
free of the hazardous atmosphere and pressurized before the equipment inside can be powered.
The first step in this process is to get rid of the hazardous atmosphere within the enclosure.
For a dust atmosphere, the inside of the enclosure must be cleaned out and then pressurized. Because most
vents on a pressurized enclosure have a spark arrestor, purging is not the method used. The dust must be
cleaned out manually or with a vacuum that is rated for the area. Alternatively, it must be cleaned out in a
non-hazardous area.
For gas atmospheres, the enclosure is purged by introducing a flow of protective gas (compressed air, or
Inert gas) through the enclosure to make it safe. Depending on the standards that are being used to evaluate
the effectiveness of the purging operation, the volume of protective gas through the enclosure determines
the amount of time for purging. The exchange of protective gas is related to the volume of the enclosure, the
number of exchanges, and the flow rate through the enclosure.
Below is an equation for determining the purging time:
(number of volume exchange) x (volume of the enclosure) / flow rate = purging time
The number of volume exchange depends on the item being purged and the standard it is being evaluated:
Number of exchanged Class/Division (NFPA 496) Zone (60079-2)
4 X n/a
5 n/a X
10 (motors) X X
Example:
P3 = 2.6 inch H
2
O 6.5 mbar
Vent = EPV-7500-...-02
table for P3: EPV-7500-...-02,
see chapter 4.2.2
Enclosure volume = 10 ft
3
282 liters
Flow Rate from P3 (see table) 11.3 scfm 320 liters/min
NEC (class/division) 4 volume exch. 4 volume exch.
Zone (ATEX, IECEx) 5 volume exch. 5 volume exch.
NFPA: 4 x 10 ft
3
/ 11.3 scfm = 3.6 min 4 x 282 liters / 320 l/min = 3.6 min
Zone: 5 x 10 ft³ / 11.3 scfm = 4.5 min 5 x 282 liters / 320 l/min = 4.5 min
Motors: 10 x 10 ft³ / 11.3 scfm = 8.9 min 10 x 282 liters / 320 l/min = 8.9 min
The 7500 control unit has a purge timer and is user-selectable through the menu.
The purge timer is activated when the enclosure pressure goes above P3. The pressure must always be
above P3 for the timer to continue until it counts down to 000:00. If the enclosure pressure drops below P3 for
any amount of time, then the timer is reset to its starting value and will not start counting down until pressure
is above P3.
The flow rate for P3 value can be found on the tables for vent flow in chapter 4.2.2. The flow rate for P3
depends on the EPV-7500 vent that is being used.
The more the enclosure leaks pressure, the higher the flow rate into the enclosure required to achieve the P3
threshold.
To Next Page
Loading...
Need help?
General
