Taylor-Wharton High Capacity GST Storage Tanks - Manual (Page 26)

26
CASING VACUUM MAINTENANCE
CHECKING VACUUM
Some vacuum deterioration may occur over an extended period of time due to out gassing of
materials within the vacuum space or from leakage. A history of vacuum readings taken over a
period of time can be valuable when evaluating tank performance and scheduling maintenance
work.
To detect vacuum deterioration, periodic measurement of the tank vacuum is recommended. A
thermocouple-type VACUUM PROBE, located on the lower head of the tank, is provided for this
purpose
To check tank vacuum perform the following steps:
1. Remove the protective plastic cap from the gauge tube connector.
2. Connect a Hastings-Raydist Model No. TV-4A or VT-6 Vacuum Gauge to the
VACUUM PROBE.
3. Open the VACUUM PROBE ISOLATION VALVE and wait at least 30 minutes
before taking the vacuum reading.
4. After the vacuum reading is recorded, close the VACUUM PROBE ISOLATION
VALVE, disconnect the Vacuum Gauge, and replace the protective cover on the
VACUUM PROBE connector.
The vacuum reading obtained on a cold tank is initially less than 50 microns (0.05 mm Hg)
absolute; however, gradual deterioration over an extended period of time is normal. A complete
log of vacuum readings, along with dates when they were taken, can be very helpful in evaluating
vacuum performance and scheduling maintenance work.
NOTE: If the tank is empty and warm, vacuum space pressure will tend to be high
because of the release of gases from adsorbent package inside the vacuum
space.
Because re-evacuation is time consuming and usually requires taking the tank out of service, it is
not normally attempted until tank performance becomes unacceptable. Even a relatively high
degree of deterioration can be tolerated in a tank from which high rates of withdrawal are being
made. However, if vacuum deterioration seriously affects tank operation by producing excessive
pressure buildup and high loss rates, contact the factory for information about how to determine and
correct the cause of the trouble.