16
Pressurization and Startup
f. Find the leak(s), using a leak detector or soap bubbles. (The most
common cause of leakage is an O-ring pinched in a flange.)
g. Correct any leaks that are found. Then repeat the leak test until the
results are satisfactory.
Step 2. Purging the
System
When the system is new, and any time that it has been opened, it must
be purged with dry gas before operation to eliminate moisture.
The dry gas used may be dry cylinder nitrogen or air from a compres-
sor-dehydrator. Shively Labs suggests three volume changes of dry gas
for an "average" system.
There is a pressure relief valve at the top of each four-way power
divider. This valve is set to open at about 10 psig. So, to purge the sys-
tem, it is not necessary to send a worker to the top of the antenna to
open a valve or loosen a flange. Simply raise the internal pressure
enough to open the relief valve. When the purge is complete, lower the
pressure and the valve will close.
Purge your system as follows:
a. Determine how wet the system is. If a system of rigid line carefully
protected from weather and assembled in dry weather is average, a
system exposed to moisture during storage or installation will be rel-
atively wet. New semi-flex transmission line, delivered pressurized
with dry gas, will be relatively dry; used semi-flex will be extremely
wet.
Important
Never apply transmitter power while the antenna is under vac-
uum.
b. If you have any liquid water in your transformer or your transmission
line, use a vacuum pump to dry the transmission line and trans-
former. Apply as much vacuum as you can to the system and hold
the vacuum for 8 hours. This should remove any liquid water. [A vac-
uum pump can be rented or borrowed from a refrigeration contrac-
tor.]
c. Determine the volume of dry gas to use for the purge.
d.
Table 3
shows approximate volumes inside various coax sizes. Add
the length of the antenna to the length of the transmission line to
determine the overall length of the system. You may ignore the vol-
ume inside the radiators.
Summary of Contents for 6017
Page 4: ......
