Nitrogen Hazards

On facilities nitrogen can be used for many reasons. The hazards that will be discussed in this article relate to those from temporary facilities such as nitrogen bottles or the use of liquified nitrogen which is vapourised to be used.

So what activities could be covered?

Nitrogen is an inert gas which does not react readily with many compounds. It is relatively abundant and easy to extract from the atmosphere. It can be transported in pressurised bottles (normally about 220 barg) which could be transported in smaller vehicles. Nitrogen can also be liquified and then transported in this way. This requires specialised cryogenic tanks and in this form the nitrogen has a limited life as it absorbs heat from the surroundings boiling off.

When restarting facilities after large shutdowns the units are normally filled with air. This can be ok if the fluids which will be reintroduced into this equipment will not degrade when in contact with oxygen or will not create a flammable atmosphere. However for many applications this is a real concern which needs to be actively managed on preparation to restart. The most cost effective way of achieving this is to ensure that the air is forced out with a higher pressure gas which is inert. This is often nitrogen. If the volumes required are small, then nitrogen bottles will be used, with larger volumes liquid nitrogen would be required. This is routed to the equipment through a temporary connection and vented through another connection either directly to atmosphere or through the equipment vent system.

When restarting facilities which would normally contain flammable or toxic fluids it is important to ensure that the system is leak tight before introducing that fluid. This can be done by completing a pressure test. This is when nitrogen gas is used to increase the pressure to design pressure and the rate of decay is monitored. If there is a leak then it will become evident through this process. Nitrogen is often used with a liquid (for example water) to increase the pressure in the test equipment. Alternatively it can be used as the test medium although this is generally used as a last resort as failure of a leak test at high pressure is far more hazardous when the equipment is filled with gas rather than liquid.

Introducing one hazard by managing another

The above all sounds reasonable. Nitrogen makes up most of the air that we breathe, we can harvest it and transport it and it prevents explosions or degradation of product. Used properly nitrogen can be just what you need to make your facility safe however there are key hazards to manage through the use of the substance. Many times as this equipment is temporary, the hazards are not fully assessed or it is assumed that the vendor or temporary equipment will manage any hazards effectively.

Top Event: Equipment Damage

Equipment damage is the top event which is to be avoided in the use of nitrogen for temporary activities. This damage could either be on the temporary equipment itself which would lead most likely to nitrogen release local to the equipment damage either in the liquid or gaseous form depending on the source and the location of release. Alternatively the damage could be to the permanent equipment which could result in a nitrogen release at the time of damage or worse a release of the hazardous fluid on start up if the damage is not identified.

Causes of Equipment Damage

When using nitrogen there are two main causes of equipment damage:

1. Overpressure - The pressure in the system downstream of the nitrogen source increases above its design pressure leading to equipment damage
2. Low Temperature Embrittlement - The equipment downstream of the nitrogen source can be exposed to low temperature in one of two ways. The first is called white line where the liquid nitrogen is not fully vapourised and it remains in liquid form in the part of the equipment which is not designed to have low temperature nitrogen, this then causes embrittlement and loss of containment. The second way in which equipment downstream of the source can be exposed to low temperature is through high differential pressure leading to Joule-Thompson cooling. This is most common in the use of nitrogen bottles.

Consequences

There are a few different scenarios which could materialise from the above causes.

1. Over-pressurisation - If the system pressure is increased above its design pressure it all depends on how much by. If it is 1.1 to 1.5 times design pressure it will likely result in flange leak. If it is greater than 1.5 times design pressure then there is a good chance there will be a rupture. The consequences of this can be two fold. The first is that there will be flying debris which could impact people and cause injury or fatality. The second is there is a localised large release of nitrogen which could result in a depleted oxygen environment. This could result in asphyxiation.
2. Low Temperature Embrittlement (Liquid Nitrogen Break Through) - This will most likely result in damage to temporary equipment downstream of the vapouriser. This is sometimes called "white-line". This can lead to rapid equipment damage and depending on the proximity of the permanent equipment it can cause damage in addition to the temporary equipment. This release can cause large quantities of cold nitrogen to be released which can cause a dense cloud which can result in asphyxiation. If the liquid nitrogen comes into contact with structural steel this can cause damage to the structure. This can be very costly to repair and in the worst case could cause failure which could lead to collapse and injury or fatality
3. Low Temperature Embrittlement (Joule-Thompson) - This will most likely result in damage to temporary equipment downstream of the pressure control valve. In most cases this will be insufficient to cause damage however it depends on the flow rate of the nitrogen and it is worth checking on your system.

Barriers

So surely the equipment from the vendor will have safeguards to protect my equipment? Well not actually. In fairness everyones equipment is different and the vendor doesn't know your specific requirements. They will be focussed on protection of their equipment. Therefore if there is a relief valve on the nitrogen pump it will be set at the design pressure of the pump which could be in excess of 400 barg. If the equipment downstream of the vapouriser is low cost there may be no protection for white-line required in view of the vendor as they can carry spares. Finally as the operation they are going to perform is manual there is often a high reliance on human barriers which may not actually be acceptable for the risk at your facility.

What Now?

Hopefully now you are more aware of some of the risks associated with the use of this temporary equipment at your site and you can go and check to see if there are sufficient safeguards.

If you want a checklist to take with you to allow you to evaluate the adequacy of the safeguards then check out my website.

Disclaimer: My articles are based on my engineering experience in two major operators over 11 years. They do not reflect the processes of either and are not endorsed by either nor the Safety and Reliability Society or the Chemical Industry Association (CIA). If you find an error in the text please feel free to correct me as I too am prone to human error (although I like to think at a lower than average frequency). The intent of these articles are to educate. From beginners to experienced engineers. I hope that through the article or reference material you all get something from it. If you find the content too basic or too advanced again get in touch. Any improvement suggestions welcome.