OPERATING
INSTRUCTIONS FOR MODEL CMSH SERIES
MULTISHIELDED LIQUID HELIUM CONTAINERS
Introduction
The Cryofab
CMSH Series liquid helium dewars are designed for minimum
loss storage, transportation and dispensing of liquid helium.
The CMSH container employs state of the art vapor cooled multishield
technology in conjunction with superinsulation to achieve
the highest efficiencies.
The CMSH
Series is ruggedly constructed and simple to use, however,
the extremely low temperature of liquid helium requires the
use of cryogenic fluids. The low latent heat of vaporization,
characteristic of liquid helium, can cause unnecessary transfer
losses to occur unless established procedures are closely
followed.
Precautions
General
precautions regarding liquefied gases are covered in the pamphlet
attached to the container, however, the unique properties
of liquid helium require that even further precautions be
taken. Liquid helium is a colorless, nontoxic liquid with
a density of one-eighth that of water, inert under all temperature
and pressure conditions. At atmospheric pressure it has a
temperature of approximately -452°F (132 degrees colder
than the freezing point of liquid nitrogen). It is the only
substance known that remains liquid under ordinary pressure
at temperatures close to absolute zero.
Air liquefies
and solidifies readily when exposed to the extremely low temperature
of liquid helium. For this reason, the fill and vent ports
of storage containers should be kept closed at all times (except
during filling) to prevent blockages from forming in the exit
passage and a resultant pressure buildup.
Moving
and Handling
CMSH Series
cylinders should not be subjected to rough handling either
full or empty. Bumping, jostling or excessive agitation of
any filled liquid helium container will cause abnormal evaporation
and may result in increased flow of gas through relief devices.
The CMSH
container is designed for use in the vertical position and
should not be laid on its side. If a container must be lifted,
use a forklift or similar device beneath the base or hoist
by means of the lifting slots in the halo ring struts. Do
not attempt to lift by the handles or by means of slings around
the shell.
Measurement
of Contents
Normal
practice requires that weighing the container and then subtracting
the stamped tare weight should determine the amount of helium
in a container. Scale accuracy should be checked often and
the scale serviced as often as necessary to maintain required
accuracy. In order to obtain the greatest accuracy remember
to correct for the vapor remaining over the liquid, which
can be a significant portion of the observed net weight, especially
when the dewar is near empty.
A Flutter
Tube may be used only to give a rough estimate of the liquid
contents. This method can be expedient but somewhat wasteful.
If the
dewar is equipped with a Superconducting Level Indicating
System, please refer to the manufacturer's instructions included
in the documentation package. The height of the liquid observed
may be converted to a volumetric equivalent by using the calibration
chart attached to the dewar.
If the
dewar is equipped with a differential pressure type liquid
level gauge the contents can be read directly in liters. These
readings can, however, suffer from small inaccuracies inherent
in this approach.
Pre-cooling
the Cylinder
The pre-cooling
operation is especially important to limit the post-fill flashoff
to a reasonable amount. All warm vessels should be pre-cooled
to -300°F or less using one of the methods below. If the
container is already below -300°F the pre-cooling operation
may be omitted.
a) Pre-cool
by filling the container at least halfway with liquid nitrogen.
Let the cylinder stand for approximately 1-2 days. Remove
the liquid nitrogen and purge with gaseous helium according
to the procedure below, prior to filling.
b) Pre-cool
with -320°F or colder helium vapor. For best results,
introduce approximately 50 CFH into the bottom of the cylinder
while exhausting through the vent valve. Approximately 12
to 16 hours of pre-cooling is required prior to filling with
helium.
c) Pre-cool
by adding approximately 25 liters of liquid nitrogen to the
cylinder and then introduce 25 CFH of gaseous nitrogen through
a dip tube extending to the bottom of the cylinder. The warm
gaseous nitrogen will bubble through the liquid nitrogen,
producing -320°F gas. Exhaust the gas through the vent
valve. About 16 hours are required for pre-cooling. Remove
any remaining liquid nitrogen and purge the container with
gaseous helium according to the procedure below, prior to
filling.
Purging
the Cylinder
To purge
the container of residual nitrogen or other gases prior to
filling proceed as follows:
| 1. |
Check
the valve position: Liquid closed, vent closed and isolation
open. |
| 2. |
Connect
a regulated helium purge gas source to the vent connection
on cylinder. |
| 3. |
Open
the liquid withdrawal valve and insert a tube to near
the bottom of the reservoir. |
| 4. |
Open
gaseous helium source valve and the vent valve on the
cylinder and adjust the flow to approximately 1 SCFM.
Allow the gas to exhaust from the center tube. |
| 5. |
Purge
with a minimum of eight volumes of pure dry helium gas
if the cylinder was pre-cooled with liquid nitrogen. |
| |
|
| Note: |
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|
| |
A
minimum of 2 volumes of ambient temperature gas should
be used to purge an ambient temperature vessel if nitrogen
pre-cooling was not used. |
| |
|
| 6. |
When
purging is complete remove the dip tube and close the
liquid valve. |
| 7. |
Pressurize
the dewar to 5 psig then close the vent valve and disconnect
the helium gas source. |
| |
|
| An
alternate purging procedure would consist of evacuating
the inner vessel and backfilling with GHe. Proceed as
follows: |
| |
|
| 1. |
Close
all valves and connect a suitable vacuum pump or vacuum
system to the vent connection. |
| 2. |
Open
the vent valve to begin evacuation of the inner vessel.
Pump down to approximately 1 torr. |
| |
|
| Note: |
| |
|
| |
Be
careful not to let the vacuum pump get too cold. If necessary,
insert a coil of copper tubing between the dewar and the
pump to warm the exhaust gas to a suitable temperature. |
| |
|
| 3. |
Close
the vent valve and disconnect the dewar from the vacuum
system. |
| 4. |
Connect
a regulated helium purge gas source to the vent valve. |
| 5. |
Admit
pure dry helium gas to the dewar to a pressure of approximately
5 psig. |
| 6. |
Close
the vent valve and disconnect the helium gas source. |
| |
|
| Filling |
| |
|
| In
order to fill the container the following procedure should
be used: |
| |
|
| 1. |
Close
the isolation valve (if it is not already closed) and
open the vent valve. |
| 2. |
Open
the liquid access port valve and insert an appropriate
fill lance or transfer line leg through one of the nested
couplings provided. For efficient filling the inserted
length should be between 34" and 40". Tighten
the coupling nut to complete the seal around the tube. |
| 3. |
Complete
additional connections between dewar and the liquid helium
supply. Verify that the supply pressure is between 2 and
4 psig. |
| 4. |
Slowly
begin to transfer liquid helium to cool-down the transfer
line and the inner vessel. When the cool-down is complete
the flow may be increased to maximum. |
| 5. |
When
the vessel is full, shut off the helium supply. Remove
the fill tube and close the liquid access port valve.
Replace the plug to keep foreign matter from collecting
in the ball valve. |
| 6. |
If
a recovery system is available, connect the dewar to it
via the vent valve. Otherwise, leave the vent valve open
until excessive exhaust vapor subsides (approximately
2-4 hours) indicating that the dewar has stabilized sufficiently. |
| 7. |
Close
the vent valve and open the isolation valve to establish
the proper operating pressure for the dewar. |
| |
|
| Withdrawal |
| |
|
| Transferring
liquid helium from the container is accomplished by using
the following procedure: |
| |
|
| 1. |
Verify
that the vent valve is closed and close the isolation
valve. |
| 2. |
Remove
the dust plug and insert an appropriate withdrawal lance
or siphon tube through one of the nested couplings above
the liquid access valve. |
| |
|
| Note: |
| |
|
| |
Purge
the transfer line with helium gas for a few seconds prior
to insertion. |
| |
|
| Let
the tube just contact the ball of the liquid access valve
and then open the valve and slowly insert the tube the
rest of the way into the dewar until it reaches the bottom.
This will prevent any loss of pressure from the dewar. |
| |
|
| 3. |
Lift
the tube slightly off the bottom to insure free flow and
tighten the coupling nut to complete the seal around the
tube and hold it in place. |
| 4. |
Liquid
transfer will begin as soon as the transfer line cools
down if there is sufficient pressure in the dewar (2-4
psig). If the transfer system is equipped with a shut
off valve, throttle the flow at first until cool-down
is complete. |
| 5. |
If additional pressurization is required, turn on the
electric Pressure Building System (if so equipped) or
introduce room temperature helium gas at 5 psig through
the vent valve. |
| 6. |
When
transfer is complete stop the liquid flow by closing the
shut off valve or depressurizing the dewar. |
| 7. |
With
the dewar at zero pressure, loosen the coupling nut and
withdraw the siphon tube. Close the liquid access port
valve and replace the dust plug. |
| 8. |
Close
the vent valve and open the isolation valve to return
the dewar to normal operating condition. |
Special
Instructions for Air Shipment
When a
helium container is used for air transport an absolute relief
valve set at 15.2 PSIA is installed in place of the primary
relief valve. This will prevent atmospheric pressure variations
experienced during flight from affecting the contents of the
dewar.
Flowmeter
When a
helium container is to be used as a collector under a liquefier
a flowmeter is provided in order to maintain high performance.
In this type of application the flowmeter functions in lieu
of the primary relief valve. The flowmeter directs some of
the exhaust gas from the container to the helium recovery
system or the low-pressure side of the liquefier. The recommended
flow rate should correspond to the normal boil-off of the
container.
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