Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Website and was written by Nathan Wells, application engineer, Engineered Polymer Systems Division.
Heavy duty equipment moves industry forward in all climates, from the sunny Caribbean to icy Greenland. Effective, reliable sealing is what allows hydraulic systems in heavy duty equipment to do work, no matter the temperature. Reliable sealing solutions allow cylinders on dump trucks and excavators to move icy, frozen tundra, and allow actuators on subsea valves to operate 5,000 – 20,000 feet below the surface of the ocean. We depend on these seals for our safety and productivity, so a little chilly weather is no reason to call it quits.
What happens to seals at cold temperatures?
Most objects shrink as they get cold, with few exceptions, such as water. This applies to all matter in the universe. Materials shrink at different rates, and this is a measurable property called the Coefficient of Thermal Expansion (CoTE). Thermoset elastomers and thermoplastics shrink roughly 5 times more than metals for a given temperature change. This means at cold temperatures, seals shrink more than their housings, and thus have less “squeeze” to make a tight seal.
To make matters worse, elastomers also harden as the temperature drops. At some temperatures, known for each material as its Glass Transition Temperature (abbreviated ‘Tg’), seals become rock hard and brittle … like glass. We don’t make seals out of glass for a reason; they wouldn’t work. In order to keep seals springy and resilient, we need to specify materials with a Tg below the coldest temperature a system will see.
In very high pressure, low temperature applications, there is one additional concern. Applying pressure to seals effectively raises the Tg of the material by about +1°C per 750 PSI. This is called Pressure-Induced Glass Transition and is the reason high pressure seals fail slightly above their measured Tg.
“But That’s Not Cold”
So, there’s low temp, and then there’s VERY low temp. Parker Hannifin Engineered Polymer System (“EPS”) Division headquarters is located in Salt Lake City, Utah. Winter low temperatures in downtown Salt Lake are typically just below freezing. In this climate, most seal materials function just fine.
Siberia purportedly has the coldest inhabited villages in the world, with temperatures down to -60°C (-76°F). Northern Canada isn’t much warmer. In these regions, an operating hydraulic system can generate enough heat from friction to keep the seals sufficiently warm. However, if hydraulic equipment is unused or stored overnight in such frigid environments, the use of cold-rated seals with a low Tg is critical to preventing leaks in equipment.
For rotating equipment that is idled or shut off in extremely cold temperatures, the lip on a rotary shaft seal can freeze to the shaft when moisture is present at the seal lip/shaft interface. When the shaft starts, the tip of the lip can be ripped or sheared off, leaving a small band of rubber on the shaft (cold temperature seal fracture).
And then there are cryogenic systems, which are entirely different beasts. Liquid nitrogen tanks require seals that can handle the -320°F fluid. At this temperature, all elastomers will be rock solid. So what seal material can handle that? Polytetrafluoroethylene (PTFE).
Pure (unfilled) PTFE, while not considered an elastomer, remains flexible down to -425°F. That’s 35 degrees above Absolute Zero – the coldest possible temperature. The chart in Figure 1 shows the effective ranges for seal materials offered by Parker.
Typical material temperature ratings
After looking at this chart, you might think, “Why doesn’t Parker make all seals out of PTFE and dump the rest?” PTFE is a great seal material, but it comes with its own set of tradeoffs. Hardware manufacturing and seal installation tend to be more complicated with PTFE than with elastomer seals.
Fluorocarbon rubber (FKM), and more recently perfluorinated rubber (FFKM), have traditionally been selected to seal hot temperatures and nasty chemicals. However, they perform poorly in cold. Parker offers special low-temp blends, spanning -40 to 400°F and -40 to 600°F respectively. “Do-everything” materials such as these tend to be more costly than traditional FKM rubber.
Highly saturated nitriles (HNBR) offer higher temperature capability with better wear resistance compared to standard nitrile (NBR). To improve low temperature resilience, Parker has developed HNBR compounds that perform better at low temperatures than most general HNBR compounds.
Special grades of silicone can handle colder temperatures, but their wear properties are so poor Parker EPS does not recommend these for dynamic sealing. Ethylene propylene rubber (EPDM) compounds are also capable of remaining quite flexible at low temperatures, but care must be taken to ensure that the application is compatible. EPDM often has the look and feel of nitrile rubber but reacts to fluids much differently.
Parker polyurethanes (compounds start with a ‘P’ in the table in Fig. 1) are popular because they offer the best all-around balance between low and high-temperature sealing, wear resistance, pressure rating, and cost. Parker’s compound P5065A88 is compounded specifically to be more resilient at low temperatures than most other polyurethane compounds.
In all sealing applications where temperature ratings are a concern, it is important to know that sealing compounds perform their best when they stay well within their temperature range. Applications that push seal compounds to the end of their temperature range may only perform for a short period of time before damage to the seal occurs, or they become too stiff to effectively control leakage. A good rule of thumb for long-lasting seals is to remain within 80% of the compound’s temperature range.
Now that you’ve selected a seal material with a Tg low enough for the cold environment it will be used in, are you done? Make sure other properties such as pressure rating, wear resistance, fluid compatibility, and high temp capability are also adequate for the system. Be aware of tradeoffs when switching materials to avoid causing a problem in another area.
Recommendations on application design and material selection are based on available technical data. They are offered as suggestions only. Each user should make their own tests to determine the suitability for their own specific use.
For more information about selecting the right material for your application, Gallagher Fluid Seals can help. Contact us today to see what material is best for your specific parameters.