A lot has to do with the materials used in the tubing and the fittings. But that's not all.
In a When Will It Burst? video, we discuss two factors that require you to derate the working pressure of tubing. One of the factors is temperature. Heat and cold can change the performance of some fluid system components. When it comes to Swagelok tube fittings there are a few factors to keep in mind when tubing and fittings are being used at temperatures higher than 100°F (37°C) or below -20°F (-28°C).
Temperature limits of most fitting and tubing materials are listed in ANSI Piping codes, including specific conditions regarding tubing materials and methods of manufacture. Steel, for instance is good up to about 375°F, while aluminum and copper top out at about 400°F. To perform under really high temperatures, it takes something like 304 stainless steel (800°F), alloy C-276 (1000°F), or 316 stainless steel (1000°F).
As the temperature increases, the tensile strength of metal decreases. So the higher the temperature, the lower the allowable working pressure of tubing. Let's take the 316 stainless steel tubing mentioned above. At 400°F the allowable working pressure is only 96 percent of the pressure allowed at 200°F. When the temperature climbs to 1000°F, the allowable working pressure drops to only 76 percent.
To determine allowable working pressure at elevated temperatures, multiply allowable working pressures by the derating factor.
Example: Type 316 stainless steel 1/2 in. OD 3 0.035 in. wall at 1000°F
When the temperature gets high enough, some fluids turn from liquid to gas. In those circumstances, it's important to make sure the tubing has the minimum wall thickness recommended for gas service.
It is particularly important to use the same alloy for both the tube fitting and the tubing so that thermal coefficients are the same. For example, you could run into problems if you put a 316 stainless steel fitting on alloy 400 tubing. As the temperature rises, the stainless steel would increase in size at a greater rate than the alloy tubing. The fitting would become loose on the tubing.
On the other hand, if an alloy 400 fitting is put on stainless tubing and raised to a high temperature, the joint becomes tighter. However, the stainless steel tubing increases in size at a greater rate than the alloy 400 fitting and may yield the fitting, and a leak could result when the temperature comes back down.
Materials are just as important to consider when the system has to work at extremely low temperatures. In general, plastics and elastomer materials usually are not good choices. Plastic components used as sealing members at room temperature will shrink markedly when temperatures are lowered, causing leakage. Many plastics also have some porosity, which allows water absorption. Water will solidify when temperatures are lowered and make the tube or fitting material brittle. Most elastomers harden at low temperatures and may crack.
The most commonly used materials in cryogenic piping systems are aluminum and austenitic stainless steel. Alloy 600, alloy 400 and titanium also work well for some applications. Once again, it's not a good idea to mix materials.
Always use a tube thickness recommended for gas service, even if the fluid is a liquid at ultra-low temperatures. If the system is ever brought up to room temperature, the fluid will become a gas.
We have tube fitting performance product test reports for various temperature conditions including high temperature and low temperature cycling and cryogenic service. These tests will give you an idea of how different Swagelok tube fitting perform perform at temperature extremes.
Please note that the tests were done in a lab, which can't duplicate the variety of actual operating conditions in the field. Temperature ratings depend on application and installation methods, cycle life required, and other variables. You can download the tube fitting performance product test reports here:
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