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.
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.
One of the offerings in our arsenal of gas products is a liquid cylinder. Don’t let the name fool you, liquid cylinders provide either gas or liquid product depending on the facility’s need. People refer to liquid cylinders as dewars, liquid cans, or by an individual manufacturer’s designation. On the gas side, any facility using 5 or more large high pressure cylinders per week will realize the benefits. By converting to a liquid cylinder, facilities see less cylinder handling, smaller footprint, and or a product cost savings
Liquid cylinders do not function in the same way high pressure gas cylinders do. For current users or those considering them, the following discusses how they operate.
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Liquid cylinders work like thermos bottles to hold gases in their cryogenic state. The cryogenic state can be as low as -400 degrees Fahrenheit (liquid phase). These specially designed cylinders can only maintain the temperature of its contents for a limited time. As the temperature inside the cylinder eventually climbs above the threshold necessary to keep the gas a liquid, the liquid product will convert back to gas. The gas will then expand to approximately one hundred times the volume of the liquid. As with all gas storage cylinders, liquid cylinders have safety relief devices. The safety relief devices on liquid cylinders are at 22, 230, 350psi, or in limited cases some pressure in between.
Liquid to gas transference generally occurs at a constant rate. If the product drawn out of the cylinder does not equal this conversion rate, the pressure in the tank rises. The pressure rises until the relief device opens. The relief valve will remain open until pressure in the cylinder drops back below the relief devices setting. For this reason, these cylinders operate most efficiently when the product demand placed on them is steady, constant, and in excess of the Natural Evaporation Rate of the cylinder. NER = rate that the liquid in a cylinder converts back to gas as a result of heat migration.
Liquid cylinders, like thermos bottles, have two containers. One is inside the other with a small vacuum and insulated annular space in between. Manufactures build liquid cylinders to contain hundreds of pounds of liquid product. Keeping that in mind, they are considerably more sturdy than your typical thermos bottle. There is also transfer tubing in the annular space to serve specific product demand delivery functions.
There are four basic circuit designs on cylinders that dispense both gas or liquid.
The simplest circuit is the liquid withdrawal circuit, which provides liquid through a tube at the bottom of the inner tank and connects directly to the liquid valve for use. Pressure forces liquid from this circuit up and out of the valve. Maintaining the pressure at no more than 22 psi is optimal.
Liquid from the bottom of the inner tank converts back into gas as it travels through a tube, which coils multiple times around and attaches to the inside of the outer tank. Attaching the gas withdrawal tube to the inside of the outer tank provides sufficient heat transfer to convert the liquid product back into a gas. Internal tank pressure is what drives the liquid through the circuit.
As with the Gas withdrawal circuit – tubing connects to the bottom of the inner tank and coils multiple times around. Gas attaches to the inside of the outer tank, where it converts back into gas and routed to the top of the inner tank. Any time gas is withdrawn from the liquid tank at a rate beyond its natural capacity to convert liquid to gas, the pressure builder circuit can be opened to increase the liquid to gas conversion process. The pressure building regulator and valve controls this.
An economizer regulator is used to evacuate gas out of the top of the tank to the gas withdrawal circuit once the pressure exceeds the setting of the regulator. This circuit only works when the gas withdrawal valve is open, and the customer is withdrawing gas.
The normal contents gauge on a liquid tank is imprecise and should only be used as a rough indicator of a tanks liquid volume. The pressure gauge is not an indication of a tanks product volume either, but rather an indication of the tanks internal pressure.
Where are liquid tanks used? The gas or liquid can be used for countless application. Upon request a General Air representative will conduct an on-site training and safety program. In addition, if at any time you have questions regarding the operation of one of our liquid cylinders, call or our closest store. You will be in touch with someone who can help you.
Liquid cylinders function best and can provide maximum benefits when the delivery demand placed on them is constant and steady.
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