Cryogenics Section Infrastructure

Introduction

At the Raja Ramanna Centre for Advanced Technology, Cryogenics is recognized as one of the thrust areas, since Cryogenics has a major role to play in both the main fields being pursued at RRCAT viz. accelerators and lasers. Cryogenics is also an essential requirement for conducting experiments with the Synchrotron Radiation Facility at RRCAT in terms of Liquid Nitrogen and Liquid Helium. There are several cryogen plants specifically installed to meet the RRCAT user’s requirement at the center are as follows:

• Helium liquefier with a liquefaction rate of 40 L/hr.
• Helium liquefier with a liquefaction rate of 145 L/h
• Liquid Nitrogen plant with liquefication rate of 40 L/hr.
• Liquid Nitrogen plant with liquefication rate of 20 L/hr.
• Helium gas storage in high pressure equivalent of 10,000 liters of liquid Helium.

1. Liquid Helium Facility

2. Liquid Nitrogen Facility

Average yearly production of liquid cryogens are in the order of 10,000 liters of Liquid Helium and 30,000 liters of liquid Nitrogen.
Important Instructions:

• Specific Guidance for Liquid Helium Users
• Safety and the use of cryogenic liquids
• Hazards related with handling of Cryogenic liquids
• Oxygen Limit in Atmosphere
• First Aid Treatment of Cold Burns
  

Cryogenics facility

Liquid Helium Facility

First Helium Liquefier was commissioned on July 12, 1989. Liquid helium is continuously available since the installation of the plant till date. Till date more than 3,00,000 liters of liquid helium is produced and used by different users. This is mainly due to closely monitored maintenance schedule and breakdowns/ repairs handled in-house.

Figure 7. Linde Make Helium liquefier with a liquefaction rate of 40 L/hr.

Linde Group make Helium liquefier with a liquefaction rate of 145 L/hr.

This larger plant is dedicated to VTS and HTS operations for testing of superconducting cavities in fairly large amount of liquid helium in its super fluid state.

Liquid Nitrogen Facility

Figure 9.Linde make Liquid Nitrogen plant with liquefication rate of 40 L/hr.

First Nitrogen Liquefier was commissioned on July 09, 1989. The major use of liquid nitrogen in RRCAT, among others, is for vacuum application, such as liquid nitrogen traps, cryosorption pumps etc. Many users are also using liquid nitrogen for conducting experiments at low temperature.

Till date more than 8 lakh liters of Liquid nitrogen is produced and supplied to various users. Liquid nitrogen is continuously available since installation of the plant till date.

All the breakdowns and maintenance of both liquid nitrogen and Liquid Helium plants, since the time of commissioning, is being done by staff of cryogenics. This has resulted in uninterrupted availability of both the cryogens to RRCAT users.

Specific Guidance for Liquid Helium Users

Only experienced and properly instructed people should handle liquid helium.

Using Liquid Helium - Special Precautions

To know what precautions to take is to recognize that at 4 Kelvin, all other gases solidify. Therefore, helium systems and Dewars must prevent back flow of air as this constitutes a major safety hazard.
Small volumes of liquid evaporate into large volumes of gas and must be allowed to vent safely, therefore:

• Always connect to helium recovery line and open appropriate vent valves;
• Never leave a Dewar open to atmosphere;
• Use liquid helium only in well ventilated areas;

Dewars open to atmosphere for prolonged periods can cause "ice plugs" causing pressure build-up which can lead to over-pressurization and potential catastrophic failure (explosion).

Liquid Helium - Safe Handling and Usage

• Minimum protection recommended is:
• Cryogenic gloves
• Face visor or safety glasses
• Always inspect the Dewar

High pressure in the Dewar will be indicated by:
- an inflated rubber bladder when the connection valve is opened
OR
- a high pressure reading on the pressure gauge on Dewar (if fitted)
Vent any gas slowly through the helium recovery line if found in this condition.

• Never drop objects into the cryogen liquid

Beware of cold gas and rapidly boiling helium when lowering equipment at ambient temperature into the Dewar. This operation must be carried out slowly to minimize boil off and potential cold vapour burn.

• Never accompany a Dewar in a lift or car (Moving confined space)

A sudden release of vapour in a confined space could be fatal!

• Always use correct syphon and fittings

A vacuum insulated syphon is the only method of transferring helium from a Dewar. It consists of a vacuum shielded tube, which dips below the liquid level in the Dewar. The Dewar is then lightly pressurized which forces the liquid up and out through the syphon tube.

It is important to follow correct order of assembly of a Dewar head siphon, to prevent leaks.

Remove pressure from Dewar and connecting tubes. Remove brass plug and lock nut used to seal Dewar when not in use. Insert syphon 'slowly' and tighten brass flange with the lock nut to seal the "o" ring.

ENSURE ALL PRESSURE HAS BEEN REMOVED BEFORE PERFORMING THIS OPERATION

• Always transfer liquid slowly: To prevent thermal shock and to avoid high pressure build-up (back pressure). This also uses the helium most efficiently.
• Never pressurize the Dewar with gas other than helium

Pressurization with a bladder is sufficient for most purposes and this is obtained by squeezing the bladder to create a slight over-pressure.
Do not use external regulated supplies unless competent to do so

• Pre-cooling equipment

Any liquid nitrogen used to pre-cool liquid helium space in cryostats must be fully removed prior to adding liquid helium.

• Purging

Purging of syphons and cryogenic equipment for liquid helium service should only be done with dry liquid helium gas.

• Always thaw equipment with hot air

This is by far the quickest and safest method

Liquid Helium - Transfer Efficiency

• Syphon and cryostat: Ensure good vacuum is maintained in both
• Syphon and cryostat cool-down: Complete this operation slowly to prevent thermal shock or high back-pressure
• Boil-off due to pressurization gas (external supplies only): Slowly apply the helium pressurization gas as it is hot compared to the liquid
• Depressurization loss: Do not pressurize the Dewar more than is necessary to perform the transfer and always try to fill in one operation.

Ice Plugs in Dewar Necks

On rare occasions an ice plug may form in the neck of the Dewar. This must be dealt with quickly since pressure build-up is potentially dangerous. Take action as follows:
If you discover a Dewar which has been left open to atmosphere for a period of time (e.g. via syphon entry port, helium recovery valve or bladder pressurization valve):

• Probe the inside of the Dewar with helium dipstick to establish if it is clear and able to vent.
• Report the event to your Supervisor,
• If the Dewar is blocked or partially blocked:
  • Clear laboratory of all personnel
  • Inform your immediate Supervisor

Safety and the use of Cryogenic Liquids - Guidance

• Protective Clothing

Personal protective equipment (PPE) must be worn when handling cryogens. However, it is only there to prevent against accidental spillage, splashes, contact with cold surfaces and explosion risks.

PPE IS NOT DESIGNED TO WITHSTAND IMMERSION IN OR PROLONGED CONTACT WITH CRYOGENIC LIQUIDS!

The following equipment must be worn when handling cryogenic materials:

Face shield - protect the users face and eyes against splashes

Gloves - must conform to BS EN 511 (Cold Protection). The gloves should either have been specifically designed for cryogenic handling with ribbed cuffs to prevent splashing into the glove or be loose fitting gauntlets that can easily be removed. The material should be rough to give good grip while handling and not increase the chance of spillage.

Aprons/Overalls - avoid woven materials if possible, if they are used it is essential to ensure they do not become saturated with cold liquid. Fastenings should be at the side or back and there should be no pockets that liquid could get trapped in.

Shoes - should be top-sealed. Never wear boots open from top helping flow of liquid inside the boots or open sandals, which offer no protection in the event of spillage.

General - Sleeves and trousers should be worn outside gloves and boots. All metallic jewellery should be removed to prevent liquid becoming trapped under them.

• Tips on handling cryogenic liquids - DO IT WITH CARE!
  • Ensure the vessel is dry and pour cryogenic liquids slowly into the receiving vessel to minimise splashing, spillage and thermal shock to the vessel;
  • Use tongs when placing objects into or removing them from cryogenic liquids
  • Avoid use of wide-necked, shallow vessels to prevent excessive evaporation and the possibility of oxygen enrichment
  • Whenever removing cryogenic liquids from pressurised vacuum insulated vessels (PVIVs) carry out a safety check.
  • Use an appropriate rod for checking the level of the cryogenic liquid in a Dewar
  • When removing cell-line cages from storage use a hook to locate the handle and raise the cage
  • Never overfill Dewars

Disposal
Care needs to be taken when disposing of cryogenic liquids

DO NOT pour cryogenic liquids down the sink - they will crack waste pipes causing potentially dangerous leaks
DO NOT store cryogenic substances or allow them to vaporise in enclosed areas, including:
Fridges, Cold Rooms, Sealed Rooms and Basements
DO ensure that the area in which the cryogenic liquid is left to vaporise is well ventilated

Hazards related with handling of Cryogenic liquids

Cold burns, frostbite and hypothermia

• Contact of the skin with cryogenic liquids (or even cold gas) can cause severe cryogenic burns; the tissue damage that results is similar to that caused by frost bite or thermal burns. While the cold itself can reduce the feeling of pain, the subsequent thawing of tissue can cause intense pain.
• Contact with non-insulated parts or equipment or vessels containing cryogenic liquids can produce similar damage. Unprotected parts of the skin may stick to low-temperature surfaces and flesh may be torn upon removal.
• Inhalation of cold vapour can cause damage to the lungs and may trigger an asthma attack in susceptible individuals.
• Hypothermia is risk due to the low temperatures arising from the proximity of cryogenic liquids. Risk is dependent upon the length of exposure, the atmospheric temperature and the individual; those exposed for prolonged periods should be warmly clothed.
• The low viscosity of cryogenic liquids means that they will penetrate woven or other porous clothing materials much faster than, for example, water.

Oxygen deficiency and asphyxiation

Whilst not toxic themselves (excepting CO2 which is mildly toxic), the cryogenic gases are capable of causing asphyxiation by displacing the air necessary to support life.

A reduction in atmospheric oxygen results in loss of mental alertness and distortion of judgement and performance. This occurs in a relatively short time period and without the person being aware it is happening.

An oxygen shift as low as 3% below 20.9% (normal air concentration) is potentially dangerous and atmospheres containing less than 10% oxygen can be fatal.

Upon evaporation, the volume of cryogenic liquid expands approximately 700 - 900 times its volume in the gaseous form. If this occurs in a room that is inadequately ventilated, atmospheric oxygen will be displaced. This will result in the oxygen content of the air being reduced to such an extent that it will not sustain life.

The onset of oxygen deficiency problems is often not apparent to the individual involved as there are few warning signs. In going to assist unconscious colleagues, rescuers themselves are often overcome by the lack of oxygen, resulting in further fatalities.

Oxygen Enrichment
Although itself not flammable oxygen, when present in higher concentrations, can significantly increase the chance of fire or an explosion.

The boiling point of oxygen is above those of nitrogen and helium. In closed systems (such as cold traps cooled with liquid nitrogen) these liquids can cause oxygen to condense on their surface (resulting in a bluish liquid on the surface). This can lead to the ignition of normally non-combustible materials and the flammability limits of flammable gases and vapours are widened. Oil and grease may spontaneously ignite and as such should not be used where oxygen enrichment may occur.

Pressurisation and Explosion

Cryogenic liquids vaporise with a volume change ratio of 700-900 and can thus cause violent changes in pressure, particularly if this occurs in a confined space. This in turn can result in an explosion. Vent systems must be in place to allow gas to escape from confined spaces. Pressurisation can occur due to the following:

• Ice forming on the venting tube, plugging it and preventing gas release;
• Damaged equipment resulting in cryogenic fluids leaking into small areas. Upon vaporisation the cryogenic liquid vaporises and causes pressure build up;
• Loss of vacuum inside a cryostat or Dewar;
• If a liquid helium-cooled super-conducting magnet "quenches" (changes spontaneously from a super-conducting state to a normal state);
• Liquid nitrogen having permeated through sealed cryo-tubes containing samples which then return to room temperature;
• Direct contact of the cryogenic liquid with water in a tube results in rapid vaporisation of the cryogenic liquid and can cause the tube to explode.

Damage to Equipment

The very cold temperatures of cryogenic liquids can damage equipment and materials, which can result in danger.

• Spilled liquid nitrogen can crack tiles and damage flooring such as vinyl;
• Rubber tubing may become brittle and crack during use;
• Condensation of water around electrical cables may result in an electrical shock hazard.

Oxygen Limit in Atmosphere

Physiological Effects and Recommended Exposure Limits of Nitrogen
16 - 25 % of oxygen by volume is Acceptable. Asphyxia due to oxygen deficiency is often rapid with no prior warning to the victim. A general indication of what is liable to happen is given in the table below but it should be appreciated that the reactions of some individuals can be very different from those shown.

First Aid Treatment of Cold Burns

Flush the affected areas of skin with large quantities of tap water. Do not apply any form of direct heat e.g. Hot Water, Room Heaters etc.
If medical attention is not immediately available, arrange for the casualty to be transported to hospital without delay.
While waiting for transport:

• Loosen any restrictive clothing.
• Continue to flush the affected areas of skin with large quantities of tap water
• Protect frozen parts with bulky, dry, sterile dressings. Do not apply it too tightly so as to cause restriction of blood circulation.
• Keep the patient warm and at rest.
• Ensure Ambulance crew or hospital is advised of details of accident and first aid treatment already administered.
• Smoking & Alcoholic beverages should be avoided as they reduce the blood supply to the affected parts.

Liberally flood affected area with tap water.
DO NOT RUB or MASSAGE the affected parts. Rubbing will result additional tissue damage.
NEVER USE DRY HEAT and Temperatures in excess of 46oC will superimpose a burn upon the frozen tissue.
Frozen tissues are painless and appear waxy. They become painful, swollen and pone to infections when thawed.
Anoxia

• If someone becomes dizzy or loses consciousness while you are there move them and yourself to a well-ventilated area immediately;
• If breathing stops apply artificial respiration;
• Keep casualty warm and at rest;
• If deemed necessary call for an ambulance