Nitrogen is an inert gas. This means it can be used for many types of analysis including Liquid Chromatography ' Mass Spectrometry (LC-MS), Sample Evaporation and Evaporative Light Scattering Detectors.
Laboratories typically buy nitrogen through one of three ways.
Dewars supply gas in large quantities, they are typically located outside the lab and the gas is piped inside. This requires a piping infrastructure to be in place which can be an expensive addition to a lab.
Labs worldwide commonly use nitrogen cylinders for gas supply, despite their lower safety and convenience compared to other options.
However, as many lab managers become aware of nitrogen generators and the benefits they bring to the laboratory, they are choosing gas generators to modernize their lab gas supply.
Gas generators continually provide a consistent purity of gas so you know that it can be relied on to enable your instrument to produce accurate results. With nitrogen gas cylinders it is advised to abandon the last 10% of the gas they contain as this can become contaminated with impurities as the cylinder is used. Given how sensitive equipment has become, it is crucial that labs are not being affected by background noise during their analysis, so this consistent purity is vital to ensure clear results every time.
With gas cylinders you don't just have a single payment when you buy nitrogen gas. In addition to paying for the gas itself, you must also rent the cylinder which it is stored in and pay for cylinder deliveries. On top of these costs, the cost of the gas is subject to market influence and frequent price increases are almost inevitable.
In contrast, with a gas generator you have one, single up-front cost and, in cases where labs have a high gas usage, nitrogen gas generators can pay for themselves in as little as 18 months. Other than the purchase cost, the only other cost which will need to be factored into your budget is for annual maintenance of your gas generator. However, maintenance plan prices can usually be locked in for multiple years, making it very easy to factor maintenance costs into your budget.
Nitrogen gas generators are a safe gas supply solution as they produce gas on-demand for your application, never storing an amount of nitrogen gas which could be unsafe in the event of a leak. On the other hand, if a nitrogen gas cylinder, which typically stores around litres of nitrogen, were to be damaged and leak, it could very quickly change the atmosphere within the laboratory, potentially resulting in asphyxiation.
Once a gas generator has been installed it never needs to be moved. As gas cylinders run out, they need to repeatedly be changed over which can be a challenge and safety concern for laboratory staff. Anyone who is required to move or change nitrogen gas cylinders within the lab should have appropriate training in how to safely achieve this which also adds training costs to the lab and additional downtime for the staff.
A nitrogen generator in your lab will generate gas whenever you need it to, at the push of a button. You won't have to worry about your gas supply running out mid-analysis as can happen with cylinders. Moreover, when you choose the gas generator option when you buy nitrogen gas, you are eliminating all of the associated administration that comes with using gas cylinders, such as placing repeated orders, arranging deliveries and switching out cylinders.
A nitrogen gas generator is, undoubtedly, the most sustainable and environmentally friendly method of supplying your lab gas. Helping you to eliminate repeated cylinder deliveries which goes a long way to lowering your labs carbon emissions, a gas generator can supply your lab with nitrogen 24/7 while having a far lower impact on the environment than cylinders or dewars.
At PEAK, we have also tested our gas generators against other nitrogen generators on the market and the results show that a PEAK nitrogen generator offers a more energy efficient nitrogen supply than our competitors. This not only improves your environmental impact, but helps you reduce lab costs in the long run.
The old saying goes something like:
To know someone is to love them.
If people count on you to supply liquid nitrogen or other cryogenic liquids for storing biological samples or processes like Liquid Chromatography Mass Spectrometry (LCMS) and Molecular Beam Epitaxy (MBE), the saying can apply to your relationship with your dewars and liquid cylinders.
Because, most of the time, things are good.
But when things go bad (you unexpectedly run out of liquid, liquid stops flowing, or your floors get water damaged), the more familiar you are with the operations and maintenance of your dewars and liquid cylinders, the faster you can make things better.
This post will reacquaint you with a key piece of equipment ' dewars and liquid cylinders ' so that you can quickly solve problems and resolve issues that keep you away from your numerous #1 priorities.
Many people use the word 'dewar' to describe a 'liquid cylinder', and vice versa. There are some key differences.
Liquid cylinders are pressurized containers specifically designed for cryogenic liquids. Liquid cylinders let you withdraw liquid and/or gas.
A liquid cylinder has valves for filling and dispensing the cryogenic liquid, and a pressure-control valve with a brittle rupture disk as backup protection.
Dewars are non-pressurized vessels, like a Thermos Bottle. They typically have a loose-fitting cap or plug that prevents air and moisture from entering while allowing excess pressure to vent.
Laboratory dewars have wide-mouthed openings and do not have lids or covers. Laboratories primarily use these small containers for temporary storage.
The remainder of this post focuses on Liquid Cylinder operations.
To become familiar with the important parts of your liquid cylinders, take a look a the diagram below:
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The Pressure Gauge is probably the one you will look at first and refer to most frequently. It indicates gas pressure inside the inner tank.
Since cryogenic liquids are actually liquefied gases, the pressure within the tank will constantly increase as the laws of physics transform the cold liquid into warmer gas. This pressure will help you withdraw the liquid or gas from your cylinder.
But for most applications, the pressure inside the tank must be artificially maintained. A Pressure Building Circuit can automatically do that.
Opening the Pressure Building Valve located at the top of the tank takes liquid from a line that runs from the bottom of the inner tank and passes it through the Pressure Building Coil attached to the inside wall of the outer tank.
As the liquid passes through the Coil, it is vaporized by the heat of the outer tank. The resulting gas is fed through the Pressure Building Valve and Pressure Building Regulator into the inner tank, causing the pressure to rise.
When the pressure has been built, you can draw gas from your cylinder by opening the Gas-use Valve. Opening this valve lets the pressure in the tank force liquid up a withdrawal line, and then down into a vaporizer coil. Once again, heat is conducted through the outer tank walls to the vaporizer.
As the liquid moves through the coil, it is vaporized by this heat. The resulting warm gas flows up through the Gas-use Valve out to the user system to complete the Vaporizer Circuit.
Generally, a single-stage regulator is attached directly to the Gas-use Valve to reduce the supply pressure to match your application's requirements.
If you don't use the cylinder for several days, pressure will continue to rise at a rate of 30 psi per day because a small amount of heat will leak into the inner tank.
This heat vaporizes a small amount of liquid and causes the pressure to slowly rise. The pressure may build up to the design of your Pressure Control Valve. The valve will then open and vent gas to the atmosphere.
To minimize losses from this venting, the cylinders have an Economizer Circuit. The Economizer Circuit comes into action when the pressure reaches 140 psi.
At this point, the regulator allows gas from the top of the tank to flow through the internal vaporizer out of the Gas-use Valve to your target system. This reduces pressure in the inner tank and minimizes losses from venting.
When pressure normalizes, the Economizer Regulator closes and the cylinder then delivers gas by drawing liquid through the Vaporizer Circuit. The Economizer Regulator should have a set pressure of 15 psi higher than the Pressure Building Regulator.
The Pressure Control Valve is mounted on the same stem as the Pressure Gauge. Often set to open at 230 psi, the Pressure Control Valve works in conjunction with a Rupture Disc in the inner tank.
As a secondary relief device, there is also a Burst Disc on the outer tank to protect the space between the inner and outer tanks from high pressure.
To withdraw liquid from your cylinder, first close the Pressure Building and Gas-use Valves. Then open the Liquid-use Valve to allow head pressure in the tank head to force liquid up to the withdrawal tube and out the Liquid-use Valve.
Liquid withdrawal should be done at low pressure to prevent flash losses. During the transfer, if the pressure in the tank is higher than the normal liquid withdrawal pressure, open the Vent Valve to lower the pressure. Before withdrawing liquid, liquid is typically withdrawn at less than 15 psi.
When filling an open container, if a greater liquid withdrawal pressure or rate is required, a qualified service agent can adjust the Pressure Building Regulator.
A Liquid Contents Gauge is in the center of the tank. This can be a float-type gauge that provides an approximate indication of the tank's contents.
If you want more accurate measurements, try a gauge that uses differential pressure to determine liquid levels. These modern devices also contain graphical digital displays to give you precise measurements. They also often have the intelligence to eliminate the need for lookup charts.
In addition, many of these digital liquid content gauges have telemetry capabilities to make it easier to monitor the levels of key cylinders.
Because the Pressure Building Vaporizer contains cold liquid, it cools the outer tank. It's perfectly normal for frost to form on the outside of the cylinder. During a prolonged high draw, the gas-withdrawal temperature falls considerably, and the outside of the cylinder will be very heavily frosted.
This frost eventually turns to water that can damage flooring or seep into your facility's interstitial space to create more damage to other systems.
A Drip Tray can save you a lot of headaches and hassles. Place your liquid cylinder and/or its vaporizer on a pan or tray to catch the water as the frost evaporates. The higher the tray's lip, the less worry you'll have about water damage.
The more you know about your dewars and liquid cylinders, the less you have to worry about them.
If you think something has gone wrong, just remember:
For more information, please visit Liquid Dewar Cylinder.