Design Considerations for Cryogenic Tank and Pipe Insulation
Heat flow is proportional to the difference in temperature between two points. The bigger the difference, the faster heat moves from hot to cold. That’s why tanks and pipes used in cryogenic systems must be properly insulated.
“Proper insulation” means more than just adding foam. The extreme temperatures in cryogenic tanks and pipes demand an engineered solution.
This blog post discusses the main points to consider when designing or specifying cryogenic tank and pipe insulation. First though, is an overview of cryogenics and where they are used.
Gas Liquefaction
Gases take up less space and are easier to move and store in liquid form. However, many common gases boil at extremely low temperatures. Methane for example only becomes a liquid at -258.6° F ( -161.5° C) and helium remains a gas right down to -452.1° F (-269.0° C).
Achieving these temperatures involves using cryogenic coolers. These can achieve temperatures below -238° F (-150° C).
Cryogenics has a wide range of industrial and medical applications. Besides storage and transportation of liquefied natural gas (LNG), uses include fast freezing of food, creating superconducting magnets, and preserving medical samples and tissues.
Insulation for Cryogenic Tanks and Pipes
Insulation serves two functions. First, it prevents fluid inside the tank or pipe from absorbing heat from the surroundings. This would be an efficiency issue because the fluid would need to be cooled even lower if it was going to warm up. In addition, if cryogenic liquid transitions to gas, this could cause flow problems and potentially be a safety hazard.
The second issue is condensation. Condensation occurs when ambient air comes into contact with a cold surface. This lowers the temperature of the air, which causes water vapor in the air to turn to liquid. In a cryogenic system, the extreme temperatures involved would result in a great deal of condensation, which could cause corrosion of the pipes, valves, and tanks.
Most cryogenic insulation systems have two components: the insulating material itself and an outer jacket. (More complex multi-layer insulation systems may play a bigger role in the market in the future.)
The insulating material is either cellular glass or polyisocyanurate (PIR) foam. The jacket can be formed from stainless steel, although aluminum is more common. A vapor retarder is often applied to the inside of the jacket to keep the insulation completely dry. Once insulation gets wet, ice can form and all insulating functions will be lost.
Cellular glass has an upper temperature limit of around 900°F, compressive strength is around 100 psi, and its insulating “K” value is 40 mW/m-K. In comparison, PIR goes to 300°F, has a compressive strength of 22 to 44 psi, and a “K” value of 23 mW/m-K.
Design Considerations for Cryogenic Tank Insulation & Cryogenic Pipe Insulation
Cryogenic pipes and tanks need insulation to prevent fluid from warming and possibly boiling. They also need it to prevent the damage that would result from condensation.
However, the challenge is greater for pipes as they have a higher ratio of surface area to volume than tanks. Thus, while what follows applies to cryogenic tank insulation, the focus will be on cryogenic pipe insulation.
The main points to consider during the design of a cryogenic insulating solution are:
- Operating environment
- Insulation thickness/space available for insulation
- Inclusion of a vapor-retarding layer or material
- Type of jacketing to use
- Compressive strength required
Operating Environment
There are several factors to consider regarding the placement of insulation. The most important of these is temperature. However, rather than looking at average highs, determine what the highest temperature could be. Insulation designed for this will function in all other temperatures, but if designed for the average high, extreme conditions could require a process shutdown.
Two other important factors are humidity and wind speeds. Humidity contributes to condensation, so you need to know the worst case. With wind speed, low/no wind conditions are worse as these increase humidity.
Insulation Thickness/Space Available
Increasing the thickness of insulation reduces the rate at which heat flows. However, it also makes tanks, pipes, and valves bulkier so they take up more space. Pipes with thicker insulating jackets also need larger diameter supports, which adds cost.
Switching from cellular glass to PIR reduces the thickness required to achieve a given level of insulation. However, there can also be cost implications, so check on the benefits of reducing insulation thickness before deciding.
Vapor-Retarding Layers or Material
It’s essential to keep moisture on the outside of the insulation. If it gets in, it’s likely to severely degrade the insulating performance, and will probably cause corrosion of the pipe or tank.
Some jacketing systems include an inner polymer film that protects against moisture. Consider whether this will be enough or if more is needed. Also, look at how the jacketing is sealed against moisture ingress.
Type of Jacketing to Use
Stainless steel is strong and corrosion-resistant. However, it’s also expensive and difficult to cut and form. Aluminum is less expensive and easier to shape but lacks the strength of stainless.
Deciding between them will come down to the budget and how much strength is needed. For example, if people might need to climb up on the jacketing, stainless will resist damage better.
Compressive Strength Required
If the insulating jacket is likely to be trodden on, bumped into, or have loads applied, it may be prudent to go with higher strength cellular glass. However, the tradeoff is that this will result in larger diameter insulated pipes. This may lead you to specify a stainless jacket over PIR insulation.
Consult Professionals With Cryogenics Insulation Experience
The extreme temperatures involved make it imperative to engineer a proper insulation solution for cryogenic systems. Failing to do so will lead to wasted energy, life-shortening corrosion, and possibly also safety hazards.
You have several choices when specifying or selecting cryogenic tank and pipe insulation. This article has covered the main factors and considerations, but we encourage you to click below for help or to get a cryogenic tank and pipe insulation quote now.