The old saying really 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 you 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, yet allows 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 help you see the forest from the trees, take a look a the diagram below to become familiar with the important parts of your liquid cylinders.
The Pressure Gauge is probably the one you will look at first and refer to most frequently. This gauge indicates gas pressure inside the inner tank.
Since cryogenic liquids are actually liquefied gases, pressure within the tank will constantly increase as the laws of physics transform the cold liquid into warmer gas. Fortunately, 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 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 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 a 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 come into action when the pressure reaches 100 40 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 the 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 deliveries gas by drawing liquid through the Vaporizer Circuit. The Economizer Regulator should have a set pressure 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 withdrawal tube, and out the Liquid-use Valve.
Liquid withdrawal should be done at low pressure to prevent flash losses. During transfer, if 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.
In the center of the tank there is a Liquid Contents Gauge. This can be a float-type gauge that provides an approximate indication of the tanks contents.
If you want more accurate measurements, try a gauge that uses differential pressure of determine liquid levels. These modern displays also contain graphical digital displays to give you precise measurements. Plus they 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 levels of key cylinders.
Because the Pressure Building Vaporizer contains cold liquid, it cools the outer tank and it is perfectly normal for frost to form on the outside of the cylinder. During a prolonged high-draw, the gas-withdrawal temperature to fall considerably and the outside of the cylinder will be very heavily frosted.
This frost eventually turns to water that can damage flooring, as well as 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: