There is a lot happening on the top of a liquid cylinder. Understanding each component and its function assists in the safe and effective operation of the container.
The pressure gauge is a popular reference point, indicating gas pressure inside the inner tank. Pressure within the tank will constantly increase as the laws of physics transform the cold liquid into warmer gas. This pressure assists the withdrawal of liquid or gas from the cylinder. In most cases, the pressure inside the tank must be artificially maintained, hence the pressure building circuit.
Opening the pressure building valve draws liquid through 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 a rise in pressure.
After pressure has been built inside the liquid cylinder, you can draw gas 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. 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 application to complete the vaporizer circuit. Often a single-stage regulator is attached directly to the gas use valve to reduce the supply pressure to match the application’s requirements.
If the liquid cylinder sits dormant for several days, pressure will continue to rise at a rate of 30 psi per day when heat permeates the inner tank. This heat vaporizes the liquid and causes the pressure to slowly rise. The pressure builds up to the pressure control valve, the valve opens, and gas is vented to the atmosphere. This is why liquid cylinders often have a “use it or lose it” design, and are not intended to hold cryogenic liquid for extended periods of time like a microbulk or bulk tank.
To minimize losses from this venting, the cylinders have an economizer circuit. The regulator allows gas from the top of the cylinder to flow through the internal vaporizer out of the gas use valve to your application. This reduces pressure in the inner tank and minimizes losses from venting. When pressure normalizes, the economizer regulator closes and the cylinder delivers 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 relief valve is mounted on the same stem as the pressure gauge. Often set to open at 230 psi, the pressure relief valve works in conjunction with an inner container bursting disk. As a secondary relief device, there is also a vacuum bursting disk on the outer tank to protect the space between the inner and outer tanks from high pressure.
To withdraw liquid from your liquid cylinder, first close the pressure building and gas use valves. Then open the liquid valve to allow head pressure in the tank head to force liquid up the withdrawal tube, and out of the container.
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. 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 technician can adjust the pressure building regulator.
In the center of the cylinder is a contents gauge. This can be a float-type gauge that provides an approximate indication of the tank’s contents. If you need to know exactly how much liquid is in your cylinder, use a digital gauge.
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 outside of the cylinder will be very heavily frosted. As frost turns to water, make sure your liquid cylinder is not in a location where flooring can be damaged. Many customers find comfort in a drip pan.
For onsite consultation or support, contact us. When multiple liquid cylinders are required, ask us about microbulk. Our gas experts will explore the pros and cons of each mode with you and your team.
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Source: Taylor-Wharton, US Tank, WestAir
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.
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.
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