In this video, we will examine the features of functions of a liquid cylinder, or dewar, as well as how to read and understand the various valves and gauges on the unit. Dewars are constructed of a stainless steel inner vessel surrounded by an outer shell and come in various sizes and pressures for your application. They are designed to insulate the cryogenic liquid inside to slow the phase change, or in other words, to reduce the time it takes to turn into a gas. Most common sizes included 160 Liter, 180 Liter, 196 Liter and 230 Liter liquid capacity. Most common pressure ratings of 22psi, 230psi and 350psi.
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The float gauge, sometimes also referred to as the liquid contents gauge, is seen here in the center of the dewar. It indicates the amount of liquid remaining in the vessel. The float gauge on this dewar shows “E” for empty and “F” for full. In addition to the horizontal gauge seen on this dewar, you may encounter a vertical gauge, with a pin that moves vertically to indicate the volume of the liquid contents. Liquid cylinders are now being manufactured to include the Rotarex C-Stic. This digital gauge provides product level measurement accuracies of plus or minus 1%, and includes outputs for telemetry.
Next is the safety relief valve. When the pressure in the dewar reaches the psi set on the safety relief valve, it opens and releases the excess pressure. Dewars can be equipped with one or two safety relief valves. When two valves are present, you can set two different pressures and select which valve is activated at the time of filling. This allows you to use the dewar at different pressures based on your application. If the internal pressure of the tank exceeds the set level, you will hear a hissing sound indicating that the pressure is being released or vented. Do not be alarmed by this noise, this means the valve is working as intended.
The pressure building circuit is used to ensure the pressure inside the tank remains within the desired parameters for your application. When your downstream application is drawing a high volume of product, the pressure inside the tank will decrease. Turning this hand wheel to the left allows the liquid inside the dewar to travel to the coils that run between the inner and out vessels, which raises the temperature of the product and converts it from liquid to gas. This, in turn, builds pressure inside the dewar. The pressure builder will continue to build pressure until it reaches the set point on the pressure building regulator. When the pressure building circuit is activated, you will see frost rings around the outside of the dewar, this is completely normal operation. .
This is the vent valve, indicated by the tag labeled “vent” and the tamper-resistant collar. This valve is primarily used during the filling process. You don’t need to open and close this valve during normal operation, but if the pressure in your dewar becomes too high, the vent valve can be used to reduce the tank pressure. You can open the valve by slowly turning this hand wheel until you hear gas escaping. Keep in mind, this is a loud and abrupt operation. It is also important to note that venting gas will displace oxygen and poses a risk of asphyxiation, therefore the vent valve should never be opened when the dewar is in a confined space. .
The gas use valve is indicated with a tag labeled “Gas Use” and a tamper-resistant collar. This is where gas product is extracted from the dewar to your downstream application, with pressure being controlled through an attached regulator. You will get the best performance out of your dewar if you use a liquid cylinder regulator on this valve. General Air recommends the Harris Model 330 single stage liquid cylinder gaseous regulator or the Victor ELC4 Edge liquid cylinder regulator. While pulling product from the gas use valve, it is normal to see frost accumulation on the dewar. Frost on the top of the unit is typically seen after it is filled. Frost can also be seen covering the sides of the dewar, which is normal during or after use or with high rates of withdrawal. If you notice the dewar accumulating frost when not in use, this could be an indication of a leak in the gas use line or loss of vacuum within the tank and requires further inspection..
The liquid use valve is indicated with a tag labeled “Liquid” and a tamper-resistant collar. If you intend to use liquid product in your downstream application, the safety relief valve should be set at 22 psi, meaning the internal pressure of the tank will stay at or below 22 psi. When drawing liquid from this port, it is critical to use a cryogenically rated transfer hose and a phase separator. The phase separator slows the stream of liquid coming from the hose, reducing the risk of splashing allowing better control of liquid flow. Other applications require direct connection of the transfer hose to an appliance rated for cryogenic use. Because cryogenic liquid is hundreds of degrees below freezing, it is important to wear proper PPE when handling liquid gas. This typically includes a faceshield, safety glasses, insulated gloves, closed-toed shoes, and a nylon apron. You should also ensure that you are in a well-ventilated area when dispensing liquid gas from a dewar into any cryogenic container.
The best way to move your dewar is with a hand cart specifically designed for transporting dewars. The oval holes seen here on each upright around the ring of the dewar are intended for safely moving the unit. Because a full dewar can weigh anywhere from 600-800 pounds, it is critical to follow proper safety protocol to avoid injury. When sliding the dewar into place, hook the locating pin into the opening and pull the cart backwards, resting it on the wheels of the cart. The pin keeps the dewar on the cart and prevents it from rolling to the left or right. Next, secure the strap or chain, and you are ready to transport your dewar.
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Additionally, an overhead crane can be used to lift a dewar off the ground. It is important to use at least two of the cutouts when securing the hooks so that the strap is centered over the tank. Never try to lift a dewar with the hand ring.
A cryogenic storage dewar (or simply dewar) is a specialised type of vacuum flask used for storing cryogens (such as liquid nitrogen or liquid helium), whose boiling points are much lower than room temperature. It is named after inventor James Dewar, who developed it for his own work. They are commonly used in low-temperature physics and chemistry.
Cryogenic storage dewars can range widely in size and may take several different forms, including open buckets, flasks with loose-fitting stoppers, and self-pressurising tanks. All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the contents boil away. Precautions are taken in the design of dewars to safely manage the gas which is released as the liquid slowly boils.
The simplest dewars allow the gas to escape either through an open top or past a loose-fitting stopper. More sophisticated dewars trap the gas above the liquid, and hold it at high pressure. This increases the boiling point of the liquid, allowing it to be stored for extended periods. Excessive vapour pressure is released automatically through safety valves. Dewars are also designed to be resistant to any sort of puncture to preserve the contents, as cryogens are costly to produce, and some (like helium) are in limited global supply.
The method of decanting liquid from a dewar depends upon its design. Simple dewars may be tilted, to pour liquid from the neck. Self-pressurising designs use the pressure of the gas in the top of the dewar to force the liquid upward through a pipe leading to the neck.
Selected expansion ratios CryogenicCryogens present several safety hazards, and their storage vessels are designed to reduce the associated risk.
Firstly, no dewar can provide perfect thermal insulation and the cryogenic liquid slowly boils away, which yields an enormous quantity of gas. This is known as the liquid nitrogen evaporation rate. In dewars with an open top, the gas simply escapes into the surrounding area. However, very high pressures can build up inside sealed dewars, and precautions are taken to minimise the risk of explosion. One or more pressure-relief valves allow gas to vent away from the dewar whenever the pressure becomes excessive. In an incident in at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically and exploded.[3]
Secondly, if a dewar is left open to the air for extended periods, atmospheric chemicals can condense or freeze on contact with the cryogenic material. This can introduce contaminants. If these materials freeze, for example, water vapor becoming ice, they can block the openings, leading to pressure buildup and the risk of an explosion.
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