Figure 3-9. Vent valve.
(1 ) counterclockwise to open. Close the vent valve
h. Venting. The transformer should be vented to
when pressure gage shows no further drop, ap-
the atmosphere before placed in service if it has been
proximately zero. At higher altitudes, decreased air
pressurized for leak tests or storage, or if the unit has
density reduces transformer cooling efficiency and
been opened and resealed. Venting should take place
lowers bushing arc-over voltages. Lower atmospheric
with the liquid temperature at 25° . If it is necessary to
pressures may require venting of the transformer before
vent at other temperatures, re-vent as soon as the unit
putting transformer in service in order to equalize
returns to 25° . This operation is necessary to
internal and external pressures. If transformer is to be
prevent excessive operating pressures or vacuums.
used at elevations above 3300 ft. (para. 2-19).
To vent the transformer open vent valve (fig. 3-9) by
turning control knob