Introduction

The training for the handling of cryogenic fluids (including liquid helium and liquid nitrogen) consists of a theoretical test and a practical assessment:

The theoretical aspect involves reviewing the information provided on this page and in the Moodle course on cryogenics, followed by completion of the quiz within the course. A 90% pass mark is required.

The practical aspect involves training to ensure that you follow the correct procedures when filling dewars, or other types of vessels. You must demonstrate your competence following training. The local trainers are:

(Millburn House - Dr Andy Howes).

Trained competent persons are added to the list of permitted users. This is maintained by Health & Safety Officer and posted on the wall near the filling point in the workshop lobby. If you are not on the register then contact the person named on the register for demonstration and training.

Safe handling of cryogenic fluids

Cryogenic fluids are extremely cold and will cause cold burns/frostbite if they come into contact with skin.

Depletion of oxygen levels to dangerous levels can easily occur in an enclosed space if cryogens are venting or boiling off. Calculate the risk of depletion using the oxygen depletion calculator. An oxygen depletion calculation must be performed for every space where cryogenic fluids are in use, as part of the risk assessment process.

It is mandatory that handling and use of any cryogenic fluid is carried out under the rules stated below.

Liquid Nitrogen Filling Points

Millburn House - Adjacent to the storage tank, within the protective cage

MAS - By the rear door and goods lift.

Physics Building - (i) In the Workshop lobby and (ii) In the Loading Bay near the storage tank

Safety notes

(1)The enclosed area in the Physics workshop lobby is large and sufficiently well ventilated to minimise depletion of oxygen levels; however, a safety device is fitted to the delivery pipework to shut off the flow if a reduction in oxygen levels is detected by the system. This will also sound a local alarm and activate a flashing beacon.

(2)The filling points at MAS and Millburn House are both outside and therefore naturally ventilated. Take special care in winter to avoid slippery conditions.

General Filling Precautions (all sites)

• Locate the dewar so that it does not pose an obstruction
• Before filling, check that the delivery pipe is secure and the vessel is ready to receive fluid.
• Use the gloves provided.
  Note: Gloves can be a cause of accidents if they become filled with liquid. Ensure that it is not possible for this to occur.
• Use a face mask when filling a system at high level, or above shoulder height, or there is any risk of liquid coming into contact with your face.
• The decanting pressure is designed to be low (less than 2 Barg) to prevent splashing and too rapid transfer of the liquid. However, TAKE CARE to OPEN VALVES SLOWLY, in case the pressure is greater than expected.
• Dewars and cryogenic vessels must not be left unattended during a fill. If you have to leave during a fill either stop the process or ask someone else to look after the filling for you
• On completion of the fill, ensure that the pipes are stored away safely to prevent accidental contact with other people.

Using the Workshop lobby filling point:

• Familiarise yourself with the procedure to follow if the alarm sounds. This is posted close by the filling point.

• The silenced exhaust system must be treated with caution; the pipe becomes extremely cold during use and MUST be handled using the gloves provided.
• The exhaust must not be allowed to spray liquid. This can occur when the Dewar is close to being full.

Using the Physics yard point:

• Before filling, check that the decant pipe is either secure, if the container is a closed Dewar, or that the sintered metal spray device is clear of ice and debris.
• On completion of the fill, ensure that the decant pipe is placed away safely on the framework to prevent accidental contact.

Using the Millburn House filling point:

• You will need to know the code for the padlock in order to access the protective cage around the storage tank
• If the screwed fittings are seized with ice, use the hot air gun to thaw them out. The gun is supplied through an RCD device to make it electrically safe to use in a wet environment.

Using the MAS filling point:

• Take care moving large and heavy dewars over the threshold strips and when on the platform surface.
• You will need your university identity card to open the door back into the building.

Transporting full dewars

• Large, full, dewars are heavy, have a high centre of gravity and a narrow wheelbase, so they are at risk of toppling over on slopes and whilst negotiating ridges and gaps in floors.
• Move them slowly and consider the floor surface in front while moving.
• Ensure that there are no obstacles or people in front of the dewars whilst moving.
• If you need to use a goods lift DO NOT TRAVEL in it with the dewar or vessel. Call the lift, place a warning notice on the inner gate, or use tape to barrier the entrance to the lift (to ensure that nobody enters the lift at any point in its journey), then call the lift from the floor you require.
• Do not use passenger lifts for transporting any cryogenic fluids.

Decanting

• If you are unfamiliar with the type of dewar and its controls then find someone to help you.
• Do not use valves and other devices unless you are sure how to operate them safely.

Helium dewars

Liquid helium is transported in dewars from the Departmental Helium Liquefier Plant and from delivery by BOC.

Helium dewars have controls and valves to enable the pressures to be raised to decant the helium into the receiving vessel.

IT IS EXTREMELY DANGEROUS TO OPERATE THESE VALVES WITHOUT DEMONSTRATION AND TRAINING.

Transportation

Take care when moving the vessels. Use the same guidance as for nitrogen dewars, written above.

Decanting

• Use only the correct tubing and piping.
• Take care with very cold pipes after decanting has been completed.
• Do not leave the decanting procedure unattended.
• Ensure that all valves are returned to the correct positions after the decant has been completed, this is to ensure that vents and safety devices do not become blocked with ice and thus lead to dangerous pressures building up in the Dewars.
• Ensure that your delivery pipe-work is secure and the vessel to be filled is ready.
• Use gloves if there is any risk of liquid splashing or coming into contact with skin.
• Use a facemask if a system at high level, or above shoulder height, is being filled, and/or there is any risk of liquid coming into contact with your face.
• Although it is normally safe to decant on your own it is very good practice that another person is aware and can be called on for assistance. This is especially true for helium transfers when using step ladders.

In some circumstances, such as when filling large vessels or magnets, it may be necessary to use ladders to reach the decant points. In these cases ENSURE that someone is able to assist you in the event of difficulty with the procedure.