We frequently visit clients who simply decant (pour) liquid nitrogen from a 25 litre dewar into a smaller working flask by manually tipping the dewar and (hopefully) aiming the liquid into the flask which they have placed on the floor. This process can be made safer by using a tipping trolley.
Several manufacturers of dewars weld studs or trunnions onto the side of the dewar allowing it to be placed into the hooks of a tipping trolley. The dewar can then be tipped in a more controlled manner.
What is a liquid withdrawal device (LWD)?
Many of our clients are unaware of a device called a liquid withdrawal device (LWD) which may be a useful way to manage the safe withdrawal of liquid rather than pouring direct from the dewar.
How an LWD is fitted
The LWD has a hollow dip tube that sits in the liquid inside the dewar. The top of the tube is attached to a manifold that sits on the neck of the dewar and is held in place either by a series of springs or by a clamp that sits around the neck.
When the manifold is secured in place it effectively turns the dewar into a pressurised container.
How an LWD works
The very cold liquid inside the closed dewar will vaporise due to the heat in leak from the warm atmospheric air and will create a head pressure of gaseous nitrogen above the liquid. This head pressure will then push liquid up the hollow tube.
When the dispense lever on top of the manifold is opened the liquid will then flow smoothly along the dispense tube and out through the sintered filter.
Pressure relief devices
The manifold is usually fitted with two spring loaded pressure relief devices which normally operate at 4 psi and 7 psi respectively.
Consequently, all dispense operations are undertaken at relatively low pressure, which reduces any splashing of the liquid and ensures a safer operation. The head pressure inside the dewar can be reduced manually by means of a gas vent valve.
Safety considerations
Prior to fitting a LWD, a risk assessment should be undertaken. The area where the fitting will take place should have sufficient ventilation.
The LWD should only be fitted by operators who have received appropriate training. They must wear suitable personal protective equipment and follow an approved standard operating procedure.
When an LWD may be useful
If you are having issues pouring small volumes of liquid nitrogen then a liquid withdrawal device could be the control measure that you are looking for.
If you have a LWD and would like to know how to use it safely, effectively and correctly then please contact us on 01270 758890 or at admin@gassafeconsultants.co.uk
