Page 91 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 67

Pump with packed gland or mechanical seal where leakage is of compressed gas
Zone 1 of 1 m radius is present. Zone 2 should be taken from Table 3.2
for a spirally supported gasket. Account should also be taken of the
proximity of the ground to the leak (see Figs. 3.3 and 3.4).

While it is clearly possible to utilize packed gland type seals for all of the
above situations it is recommended that mechanical seals are used, partic-
ularly in the case of gases and liquids above their atmospheric-pressure
boiling points, because of their enhanced performance in respect of wear.
Throttle bushes are also recommended in these cases to give added confi-
dence in maximum-leak rates. Although it is less likely that concentrations
of rotating pumps and compressors will occur to the extent that hazardous
areas will be modified as discussed in the case of valves, it is necessary to
idenhfy this situation if it occurs and treat the hazardous areas created as
for valves.

Moving reciprocating seals

Reciprocating compressors and pumps present a slightly different situation
as the leakage around their pistons is into a crankcase or the chamber behind
the piston. In cases where the crankcase is ventilated the hazardous area is
created at the ventilation exit which may be in a different location, rather
than at the location of compression. Therefore, apart from the hazardous
areas created by piping in the compressor location, a further hazardous area
may be created elsewhere. Where the leakage concerned is in normal oper-
ation, a Zone 1 of lm radius around the point where the leakage reaches
the atmosphere is sufficient but, for abnormal circumstances the releases
from the compressor moving seals, both at the point of normal exhaust to
atmosphere and local to the seal itself should be considered as those for
a spirally supported gasket (see Table 3.2). As a result of the compressor
duty, this type of consideration will result in significant hazardous areas.

3.4 Other practical well-ventilated situations
The previous sections have discussed the performance of leaking flammable
materials from the more classic leak sources such as pipe flanges and
pump/compressor seals, but there are many particular situations which
require specific consideration.

3.4.1 The fixed roof vented stock tank

This type of tank is extremely common on most process plant for the storage
of flammable materials which are normally liquid. Most of these are in well-
ventilated outdoor areas and commonly have free vents to allow escape of
vapour during filling and entry of air during emptymg. They do, however,