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Operational aspects of experimental liquid metal facilities 131

sensors. An in-line filter could be installed on a dedicated filling line to capture any
unwanted solid oxides or impurities.
Facility cleaning and drying before filling with LBE for the first time is no doubt
important. It is best to foresee sufficient draining to remove water after hydrostatic
testing. Further drying could be done by using the heat tracing while pulling vacuum
to the system. Vacuum connections are commonly installed in every cover gas com-
partment of a facility. This typically includes the expansion tank, commonly the
highest point on the loop, and the drain tank, commonly the lowest part of loop.
The location of the vacuum connections should always prevent any inflow of process
media (LBE/water) into the gas supply system.

3.4.3 Gas conditioning sequence (inerting)

Gas conditioning is a transition sequence procedure required to purge the facility with
an inert gas to effectively reduce the oxygen concentration in the system before
heating. This procedure serves to reduce the possibility of oxidation when the system
is preheated. This gas conditioning sequence should be completed before any
preheating or filling sequence is activated. An easily implemented precaution is to
system interlock the preheating sequence, thereby requiring the gas conditioning
sequence to have been completed before any preheating is done. Between runs, a facil-
ity can be maintained under some overpressure of an inert gas. In this case, gas con-
ditioning may not be required. Note that the gas conditioning sequence can also serve
as a means of leak testing. Monitoring the residual gas pressure over time can help to
identify potential loss of facility leak tightness. However, beware of pressure changes
due to temperature change.
There are numerous methods of purging systems with inert gas, such as pressure
purging and vacuum purging. Vacuum purging is particularly well suited for systems
with several regions of dead space (Kinsley, 2001), provided that the systems (vessels,
piping, and components) are designed to withstand vacuum. Vacuum purging also
consumes smaller volumes of inert gas for the same resulting oxygen concentration.
Vacuum purging involves evacuating the air from the facility using a vacuum pump,
followed by flushing the facility with an inert gas. The process (pulling a vacuum and
flushing with inert gas) is repeated until the desired concentration of oxygen is
reached. The oxygen concentration x after k purge cycles (vacuum and flush) is given
by Eq. 3.4.1:
k

P L
x k ¼ x 0 (3.4.1)
P H
where x 0 is the original oxygen concentration and P L and P H are the highest and lowest
pressures in the cycle. Gas conditioning at the SCK CEN LBE facilities typically
l
involves three vacuum purging cycles, the so-called triple flushing.