Instruction Manual

3
Operation
Filling — It is not necessary to remove the LWD unit
from the Dewar for filling. You may fill the Dewar
by pressure-transfer of liquid nitrogen through the
LIQUID valve of the LWD.
WARNING: Over pressurization of the Dewar
could result in serious bodily injury or property
damage. Never exceed the 10 psig (0.7 bar/69kPa)
relief valve pressure setting of the LWD.
To fill, remove the spout assembly and connect a
22 psig (1.52 bar/152kPa) or lower pressure liquid
supply cylinder to the withdrawal device using a
liquid nitrogen transfer hose with female CGA 295
connections on both ends.
WARNING: The transfer hose must be used with a
pressure relief valve to prevent excessive pressure
build-up in the hose when the valves are closed
at both ends of the hose. If liquid nitrogen is
trapped between the valves of the liquid cylinder
and the LWD, it has no escape path as it warms
and expands. The resulting pressure can cause the
hose to rupture and may cause personal injury. The
current model LWD is equipped with a pressure
relief valve. Previous versions of the LWD must be
used with a transfer hose equipped with a pressure
relief valve.
Once the transfer hose is connected, open the LWD
VENT valve first and then open the LWD LIQUID
valve. Then slowly open the liquid valve on the supply
cylinder until liquid flow is established. Adjust the
VENT valve to maintain a back pressure of 8 to 10
psig (0.6 bar/55 kPa to 0.7 bar/69 kPa) in the Dewar.
When liquid sputters from the VENT valve, quickly
close the liquid valve on the supply cylinder, next the
LWD LIQUID valve and then the LWD VENT valve.
WARNING: Closely monitor Dewar pressure
throughout the fill process. Never let the pressure
exceed 10 psig (0.7 bar/69 kPa). If the pressure
approaches 10 psig, immediately fully open the
LWD VENT valve and/or close the LWD LIQUID
valve as necessary to control the pressure in the
Dewar.
Loosen the hose connection slowly and allow all
pressure and any remaining liquid to dissipate before
completely disconnecting the hose.
The primary relief valve, set at 10 psig (0.7 bar/69
kPa), maintains the normal operating pressure of
the LWD. A secondary relief valve, set at 15 psig
(1.0 bar/103 kPa), prevents hazardous pressure rise
in the event of primary relief valve malfunction or if
its capacity is exceeded. Never replace either of these
parts with higher value relief valves; increasing the
pressure in the Dewar beyond 15 psig (1.0 bar/103
kPa) may burst the Dewar and cause personal injury/
property damage.
To fill the Dewar by pouring liquid from another
container, (This is not the recommended filling
method) first open the VENT valve to release pressure.
Then, loosen the band clamp, unsnap the safety
cable, and remove the LWD unit. When replacing the
LWD on the filled Dewar, leave the VENT valve open
until the flanges are securely clamped. To pressurize
the Dewar for liquid withdrawal, refer to “Pressure
Building” below.
Note: Be sure flanges surfaces are warm, clean
and dry. The “O”-ring must be cleaned and lightly
lubricated before replacing the LWD.
Liquid Withdrawal — when the liquid valve is
opened, pressure in the Dewar forces liquid to go
up through the withdrawal tube and out through the
valve. The liquid nitrogen can be dispensed through
the spout or through the transfer hose connected to
the liquid outlet.
Pressure Building — If the Dewar is filled properly,
liquid nitrogen evaporation will normally maintain
a pressure sufficient to sustain liquid withdrawal.
However, if pressure is lost, bubbling dry nitrogen
gas through the liquid will restore proper operating
pressure. This adds heat to the liquid, causes
evaporation, and increases pressure. Connect a
pressure-regulated.(8 to 10 psig 0.6 bar/55 kPa to 0.7
bar/69 kPa) source of dry nitrogen gas to the LIQUID
connection. Open the LIQUID valve and adjust the
VENT valve to maintain a back pressure of 8 to 10
psig (0.6 bar/55 kPa to 0.7 bar/69 kPa) while the gas
bubbles up through the liquid. After several minutes,
close the valves and read the pressure gauge. If the
pressure does not remain above the 5 psig (0.3 bar/34
kPa), resume the gas low until the liquid has warmed
enough to provide normal operating pressure.
External pressurization can be applied while
withdrawing liquid from the Dewar by connecting a
supply of nitrogen gas, regulated to a pressure of 8 to
10 psig (0.6 bar/55 kPa to 0.7 bar/69 kPa), to the vent
valve connection.