In today’s world, bigger often is seen as better. Intuitively, when making a purchase or when specifying a product, people often will pick the next largest size because it is assumed that it will result in better performance. However, in the case of cryogenic piping, that may be a bad decision. Oversized cryogenic piping increases system cooldown time, transfer time and cryogenic liquid usage.
Pipe size is particularly important in situations where there is low flow vs. high flow in cryogenic piping. For example, a cryogenic piping company was asked to troubleshoot a 50' long system with cryogenic freezers. The company found that reducing the line size cut the calculated cryogenic liquid delivery time to the freezer by 13 minutes, which represented a 64 percent improvement in liquid delivery time. In the original installation, the cryobiological freezers occasionally would alarm due to the extended time until cryogenic liquid was delivered. This was primarily because the original pipe supplier had specified standard 1" dia. cryogenic pipe instead of taking into account the system flow and how long it would take for the pipe to cool. In this case, the larger pipe size — which they thought was a safety margin — was the wrong choice.
Common Problems with Cryogenic Piping
With cryogenic liquids, very low temperatures are needed to keep the fluid in a liquid state. When a cryogenic liquid such as liquid nitrogen, liquid oxygen or even a warmer fluid such as liquid carbon dioxide (CO2) starts warming up, it naturally gets above the boiling point, creating vapor in the equipment, line or tank.
Traditional fluids such as water or glycol do not typically encounter this situation. Typically, they are at a steady state naturally at normal conditions. When piping water, for example, the liquid will not get above its boiling or vaporization point unless the product is heated. And in many cases, the pressure in the system (or the fluid mixture) raises the boiling point, so that is not an issue.
This is not so in cryogenic piping. If the liquid is left to sit at room temperature, it will naturally convert to a vapor. This vapor needs a place to exit even when the unit is not in operation. Vacuum-jacketed piping extends that time significantly — reducing heat ingress by a factor of 20 over foam-covered pipe — but it still happens.
Cryogenic piping is designed with safety-relief valves before and after every shutoff point to prevent overpressurization. Thus, in no-flow or slow-flow situations, the liquid will convert to a gas and:
• Escape through the use point.
• Be expelled from the safety-relief valve.
• Leave through another venting apparatus like a gas vent in the system.
One problem even with vacuum-insulated cryogenic piping is the naturally occurring heat leak. Larger diameter pipe allows even more heat into the liquid. In fact, there can be a point in low-flow systems where the heat influx evaporates the cryogenic liquid to the point where it never reaches the exit. Thomas Flynn describes this as “The Case of Zero Delivery.” And this point obviously would be reached more quickly with higher heat-leak piping such as foam-covered copper pipe.
Another problem occurs when cryogenic pipe is not used for an extended period of time. Some of the cryogenic liquid “warms” and becomes a gas, which in turn warms the pipe itself. The metallic surfaces of the piping must be cooled down again to prevent a phase change in the liquid cryogen. When fluid starts coming back into the piping, it will not be pure liquid until after the inner pipe’s temperature is reduced again.
That length of time depends on the thermal mass in contact with the fluid. One “secret” in the piping business is to use a thinner wall pipe to reduce the cooldown time. However, using thinner piping can affect pipe strength and durability — especially if using flexible cryogenic piping — so someone with engineering expertise should be consulted regarding cryogenic pipe sizing.
The line diameter also impacts the velocity of the liquid through the pipe. A higher velocity will clean vapor out more quickly. With a smaller line diameter, there is a reduced chance of laminar flow near the liquid/metal boundary, thus the pipe or tubing will be cooled down faster.
Sizing Cryogenic Piping Tips
Several factors should be considered when determining the correct sizing for cryogenic pipe.
• Required System Flow. This is determined by how much liquid is needed at the end point(s) and the equipment duty cycle. Some equipment like storage freezers and environmental chambers have a considerable amount of off/on time when in use.
• Supplied System Pressure. Obviously, this variable has an impact on both the end-use pressure and the quantity of liquid delivered. Supply pressure requirements to operate can vary depending on the supply tank and its condition.
• End-Use Equipment Pressure Requirements. The end-use equipment may have a minimum and maximum pressure requirements to operate correctly, and these needs should be taken into account.
• System Restriction Points. Inline filters, elbows and elevation changes also will create changes that affect how much liquid is delivered to the end point.
Tank supply pressure varies depending on the tank used and its current state of fill. Smaller liquid cylinders, sometimes call mini-bulk tanks or VGLs, can be set for different pressures, starting at a relatively low 22 psi and going up to more than 300 psi. Often the tank pressure is dependent upon the cryogenic liquid supplier and the connections used on the tank.
Larger storage vessels — usually more permanent tanks — may be susceptible to pressure variation due to liquid height in the tank. Most of these tanks control the gas pressure on top of the liquid using special circuits called pressure builder and economizer circuits. Because these control the gas pressure, the weight of the internal liquid will change the output pressure. A good rule of thumb is 0.5 psi per foot of liquid height. Thus, the actual delivered output pressure (and flow) can change depending upon the fullness level of the tank.
One solution that may be used to ensure a proper supply of cryogenic liquid at the use point is a gas vent (see January/February 2012, Process Cooling, page 11). But, the use of gas vents — sometimes called “keep-fulls” in the industry — comes at a cost. They will cause the system to use more liquid cryogen. Because many operators do not track their liquid cryogen usage, this may be a hidden cost. It also can be a problem for systems that use smaller liquid cylinders because tank switch-out will be needed more often due to the additional cryogen usage.
You can begin to see that simple tables or even rule-of-thumb guidelines, while good starting points for sizing, can cause problems in applications. Sometimes, the cryogenic system that delivers the most liquid may not be the best. An experienced cryogenic engineer will take these factors into the design of a cryogenic piping system to make sure the system delivers cryogenic liquid as intended.
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