Cylinders and inerting with nitrogen

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Handling and using high pressure cylinders should only be done by trained personnel.

Please note: Cylinders may contain stored energy in the form of pressure that can be very dangerous if not used properly. For instance removing a cylinder valve with pressure in the cylinder could be very dangerous. Damaging a pressurized cylinder in any way could result in injury.

Small requirements for gas are best served with cylinders. A typical cylinder, about 5′ tall, can hold about 230 cubic feet of nitrogen gas, if it is filled to the maximum operating pressure – which can be in the range of 2,200 psi, that is to say, if it is full.  The cylinder will have the maximum allowable pressure stamped on the side near the valve.  Pressure is the prime indicator to tell if a cylinder is full. Temperature is a factor, but not as important, so long as the cylinder is not heated. Note: There are cylinders that are rated for much higher pressure, but they are not typical.

There are many sizes of high pressure cylinders, some small enough to carry under an arm and others that are mounted on 18 wheel trucks. The internal volume of the cylinder and the pressure of the gas (and to some extent the temperature) are the things that are important to determine the number of cubic feet of compressed gas. Cylinders have a very interesting quality – they will hold the pressure and volume virtually forever, regardless of time. Only the use of the gas or a leak is the only reduction. The volume is the biggest limitation.

Nitrogen 12 pack cylinders
Nitrogen 12 pack cylinders

 

For larger requirements, cylinders are put together in racks.

230 scf high pressure cylinder
230 scf high pressure cylinder

 

The “racks” are steel frames with 6 or 12 cylinders clamped together feeding into a single manifold. They often use the 230 cu.ft gas cylinders. So 12 cylinders x 230 cuft = 2,760 cuft or 78 m3. The strong advantage of cylinders is that the nitrogen will stay in them for years. And it will always be ready to go in an instant (if the valves work). However, the cylinder owner usually charges demurage – a rental fee for the cylinders, and that can be expensive over time. That problem can be overcome by purchasing the cylinders. Cylinders should be tested every 5 years and they are date stamped for the last test. Refilling stations may not refill if the test period is beyond the nominal test date.

Liquid nitrogen dewar
Liquid nitrogen dewar

 

Another option for larger quantities is a dewar. Dewar containers can hold liquid nitrogen, argon, oxygen, CO2, etc. A 180 liter dewar may hold 140 cubic meters at 10 bar. The disadvantage of liquid nitrogen is that it is a perishable commodity. It will vaporize and escape from the dewars over a short period of time. It may all be gone in 3 weeks if the dewar is in the hot sun. Portable dewars come in many sizes from the little 1 liter size that the dermatologist might use to 265 liters. Generally when the dewar is very large it is simply referred to as a liquid nitrogen tank or LN2 tank and it can hold thousands of gallons or liters.

Liquid nitrogen tank truck trailers can hold about 500,000 cubic feet, (about 5,000 gallons) and the nitrogen is always escaping due to boil-off, approximately .2% to 5% per day depending on how good the insulation is. Usually they have relatively low pressure tanks, 25 psi is normal. They usually have a transfer pump to move the nitrogen into stationary tanks.

Liquid nitrogen tanks can be stationary on concrete pads (these are the big ones with thousands of gallons)  and they often have a lower loss rate than tank trucks, but they are always venting boil-off gas at about  .2% to 5% per day. It all depends on the vacuum in the vacuum jacket. They must have a very good vacuum to resist the heat and minimize boil off. Pressures can be from 30 psi to 225 psi in these tanks.

PURGIT ISO tank
PURGIT ISO tank

 

ISO tanks can have very low leakage rates. .2% is not unusual. ISO tanks are used for specialized shipping of nitrogen, argon, oxygen and other specialized cryogenic and industrial chemicals. The tank must be matched to the contents. That is, tanks are certified to carry specific cargoes. A nitrogen tank will have fittings that will only connect to other nitrogen tanks.

The liquid nitrogen tanks that I own could be filled with argon, oxygen, or helium it they were cleaned, prepared for the new cargo with new hose connection fittings and new plackards.

Carbon dioxide is shipped as a liquid in tanks, but CO2 is technically not a cryogenic liquid because it is not cold enough.

Important note: A specialized pressure reducing regulator is necessary to successfully get gas out of a high-pressure cylinder. Most cylinder valves are specific to the cylinder gas. Oxygen takes one particular thread, fuel gas another, nitrogen another and so on. A regulator must have the correct thread to fit the cylinder. Oxygen hoses, regulators, valves must never be used for other gases. There could be a spontaneous fire because almost everything is fuel to oxygen.

The different thread patterns on valves have numbers for identification. For instance, connection #510 is for fuel gas like propane or acetylene. It is also called a POL which, at one time stood, I believe, for Prest-O-Lite who was an early manufacturer.  Thread connection #240 & #660 is ammonia, #320 is for CO2,  #580 is for nitrogen or krypton or helium, #540 is oxygen, and the list goes on….

Oxygen is dangerous because almost everything is fuel to pure oxygen. If a hose or pipe has been used for a oil product and even a trace of hydrocarbon is present when pure oxygen is connected, it can ignite spontaneously, causing an explosion or fire.

When the big fabrication shops are using torches to cut thick steel, the torches use a mixture of oxygen and acetylene. When the cut has started it is sometimes possible to reduce or shut off the flow of acetylene and only use oxygen. Mixing oxygen with a fuel gas is extremely hazardous and will almost certainly result in explosion or fire owing to the rapid oxidization caused by the fuel-O2 mixture. Oxygen equipment must never be used in fuel gas and then back in oxygen without cleaning.

Nitrogen generator
Nitrogen generator

 

An option to cylinder or liquid nitrogen is a nitrogen generator. They separate nitrogen gas and oxygen from air. They are somewhat expensive to purchase and they require an air compressor, but they make nitrogen when you want it, and there is no expense or loss when they are turned off. They are particularly useful in remote areas. The by-product of making nitrogen gas is oxygen gas, and nitrogen generators can be reconfigured to make oxygen gas for use in hospitals and for industrial O2.

Tank Inerting The trick to tank inerting is the proper placement of the supply hose in relationship to the exit location. The gas should be supplied at one end of the tank and it should exit at the other end. Usually it is not that simple because very few tanks have connections on each end. Big tanks, even ones with reasonable connection locations may take 3 or more volumes of nitrogen gas to reduce oxygen to under 8%.

CO2 is much better for tank inerting because the CO2 molecule is heavy and inexpensive compared to other gases. It can displace lighter gases rather than diluting them. Inerting tanks with nitrogen gas can be dangerous for operators or inspectors because of the chance of asphyxiation. A filter mask is NO GOOD where the vapor has reduced oxygen. If there is not enough oxygen then a filter mask is not appropriate. In the case of low oxygen then you must wear a supplied air mask. It is prohibited to work in tanks with less than 19.5% oxygen without supplied air. Supplied air can come from scuba tanks or from an air hose led in from the outside, but this is very dangerous work and must only be attempted by trained personnel.

Some rules for working around inert gases are:

NEVER enter a tank unless it has been tested for oxygen and has more than 19.5%. The most dangerous steel tanks are the ones that are dry and rusty. The rust can absorb the free oxygen thereby making an oxygen deficient atmosphere.  Oxygen meters can be purchased or rented from safety supply stores.

STAY OUT of tanks with low oxygen. Carbon filter masks are no good. Confirm that there is enough oxygen to breathe.

EVERY TIME ventilate every tank with large amounts of outside air using a forced air blower before entering. Other chemicals may be as much of a danger as low oxygen.

NEVER interchange oxygen regulators or hoses, etc. with fuel gas equipment. Always read and understand the material safety data sheets for the gas.

EVERY TIME use an experienced person to handle high pressure cylinders.

Most tanks – even those that have manways or manholes will be ‘enclosed spaces’. There are many rules governing entry into enclosed spaces. If you are considering entering an enclosed space please read and understand the hazards and follow regulations.

PURGIT is a tank degassing contractor that uses condensers chilled with liquid nitrogen. We have experience degassing tanks and inerting tanks with nitrogen, carbon dioxide, etc.