Comprehensive testing of water glycol fluids on common seal materials reveals temperature-related issues.
By: James Simpson and Eric Bucci – Trelleborg Sealing Solutions
The polyglycol is a water-soluble polymer thickener that can be formulated to cover a wide range of viscosities. The resulting viscosity-temperature properties give water glycols good low-temperature cold-start pump wear protection and minimized cavitation. The additive package imparts corrosion resistance, metal passivation, seal and hose compatibility, oxidation resistance, antimicrobial properties, antifoaming agents and antiwear properties. Water glycol fluids also have better thermal transfer properties than other fire-resistant fluids. 
Why use water-based hydraulic fluids?
Water-based hydraulic fluids are widely used in oil and gas, mining, hot-rolling mills, and similar applications where the potential for fire could cause catastrophic consequences. They are also replacing traditional oil-based HLP fluids in applications where environmental regulations must be observed. As a result, they have become more prevalent in many applications within offshore energy production as a means of protecting people, the environment, and resources. Low- viscosity versions operate more effectively than oil-based HLP fluids over long distances, and the low compressibility of water gives faster response times.
The International Standards Organization (ISO) classifies fire-resistant, water-based hydraulic fluids into four categories:
- HFAE, which includes oil-in-water emulsions, typically with more than 80% water content
- HFAS, which are synthetic aqueous fluids, typically containing more than 80% water
- HFB, which are water-in-oil emulsions typically containing more than 40% water
HFC (also known as glycol solutions, polyalkylene glycol solutions and water glycols), which include water polymer solutions, typically containing more than 35% water
Figure 1. Relatively small changes in the properties of Trelleborg’s Turcon PTFE seals were seen when tested with water glycol fluids.
HFC fluids are the most common hydrous, fire-resistant hydraulic fluids because they have the best fire resistance and hydraulic properties. They are also used wherever hydraulic fluid escaping under high pressure can ignite on contact with hot materials. At temperatures above 600°C, these fluids should not ignite or continue to burn. They can be used at ambient temperatures of –20 to 65°C and up to working pressures of 250 bar.
The fire resistant and environmentally friendly qualities of HFC fluids make them suitable for use in offshore installations, whether on surface equipment such as motion compensation cylinders or on subsea equipment when used as a control fluid to operate valves and blow-out preventers. The fire-resistant properties mean greater fire safety, offering more time to initiate fire-fighting measures and bring people to safety in the event of an accident.
What else should you consider?
Chart 1 – Hardness and volume change of seal materials in Oceanic HW 740 R
As they are mostly made of water, HFC fluids have vastly different lubrication properties as compared to oil-based fluids. In hydraulic fluids, the interrelation between viscosity and temperature is described by the viscosity index (VI). HFC hydraulic fluids have a better viscosity temperature behavior than HLP mineral oil. In HFA hydraulic fluids, the dependency of the viscosity on the temperature is negligible. The differing viscosity temperature behavior should be taken into consideration when selecting hydraulic fluid for the required temperature range.
Owing to high vapor pressure, in comparison to a similar HLP mineral oil, the maximum operating temperature when working with fire-resistant, water-containing hydraulic fluids must be limited. Reservoir temperatures above 50 °C must be prevented in open systems because they can lead to serious water loss and accelerate the aging process in the hydraulic fluid. Furthermore, in HFC hydraulic fluids, water losses that are too high can lead to both an increase in viscosity and a reduction in fire-resistant properties. The minimum operating temperature for HFA hydraulic fluids is 5°C. HFC hydraulic fluids respond very well at low temperatures and have a lower pour point compared to HLP mineral oils.
Chart 2 – Tensile strength and elongation change of seal materials in Oceanic HW 740 R.
It’s critical that the hydraulic fluid does not negatively affect the materials used in the components within any system that uses HFC fluids. Compatibility with coatings, seals, hoses, metals and plastics should be observed to prolong the service life and integrity of equipment.
Oil and gas hydraulic applications often involve demanding dynamic movements. For instance, sealing systems in offshore motion compensation cylinders can be subjected to significant wear due to long strokes. Compared to oil-based HLP media, the water base of HFC fluids typically produces different reactions within the traditional sealing materials used in these applications. The different reactions can range from lower lubricity relative to dynamic sealing elements, to corrosion concerns of supporting metal hardware, to compatibility with sealing polymers.
Testing HFC fluids
Trelleborg Sealing Solutions and fluid producer MacDermid partnered to investigate the effect of water glycol fluids on common seal materials. In 2015, they developed a series of tests involving seven seal materials, six fluids, and three temperatures.
The MacDermid fluids tested were:
- ERIFON 818 TLP
- COMPENOL
- OCEANIC HW 525 P
- OCEANIC HW 443
- OCEANIC HW 740 R
- OCEANIC XT 900
Trelleborg materials tested were:
- XploR H9T20
- XploR H9T21
- XploR V9T20
- XploR V9T82
- Turcon T05
- Turcon T46
- XploR J9513
Here are the details on the Trelleborg materials tested:
- XploR H9T20: Explosive decompression resistant HNBR
- XploR H9T21: Low-temperature explosive decompression resistant HNBR
- XploR V9T20: Explosive decompression resistant FKM
- XploR V9T82: Low-temperature explosive decompression resistant FKM
- XploR J9513: Explosive decompression resistant FFKM
- Turcon T05: Proprietary filled Polytetrafluorethylene (PTFE) • Turcon T46: Bronze filled PTFE
Chart 3 – Hardness and volume change of seal materials in Oceanic HW 740 R at 135°C.
The tests took place over a 90-day period in 2016 in three Trelleborg labs: Stuttgart, Germany; Tewksbury, England; and Fort Wayne, Ind. Each seal and fluid combination was tested for hardness change, tensile strength change, strain change and volume change. Each seal was photographed before and after the test to document all physical changes.
Trelleborg undertook a series of tests on a number of elastomeric and thermoplastic sealing materials to investigate the effect of immersion in HFC fluids at a range of elevated temperatures up to 200°C.
As an example of the test results, the seal material property data for MacDermid Oceanic HW 740 R after a 90-day exposure at 70°C is shown in Chart 1 and 2, and 90 day exposure at 135°C is shown in Chart 3 and 4. Oceanic HW 740 R is a hybrid HFC fluid for use in subsea production control systems. It does not contain polyglycol, as high viscosity adversely affects hydraulic response time.
Test results
Relative to elastomer materials, testing highlighted that while HNBR (XploR H9T20 & H9T21) exhibits relatively small changes in hardness, volume and strain with Oceanic HW 740 R (and other HFC fluids) up to 70°C, a more pronounced change exists in these properties when the test is performed at temperatures of 135°C. The HNBR property change from 70 to 135°C is expected and typical because rubber materials generally lose properties as temperature increases. The test results for HNBR are well within acceptable seal performance and operational limits and suggest the best combination of compatibility and material property retention at temperatures up to 135°C.
Chart 4 – Tensile strength and strain change of seal materials in Oceanic HW 740 R at 135°C
In applications where temperatures are above 135°C, industry professionals would typically look to FKM (Fluoropolymer) materials to provide a solution. However, our testing illustrates that water-based HFC fluids create a significant change to the properties of FKM at both 70 and 135°C. As a result, FKM would not typically be recommended for any applications involving these fluids.
By comparison, we see relatively small changes in the properties of perfluoroelastomer Isolast J9513 and Turcon PTFE materials. At temperatures above 135°C, these materials offer a potential solution when dealing with high-temperature applications that involve HFC fluids. More and more deep sea wells are reaching temperatures up to 200°C and additional Trelleborg testing has shown Isolast J9513 and Turcon materials retain a significant portion of their material properties in the high-temperature water glycol fluids, such as MacDermid Oceanic XT900.
Testing reveals the importance of fluid type and seal material choice in ensuring optimum seal performance and service life. Traditional sealing materials, such as FKM, often inert in most fluids, exhibit disadvantageous behavior in HFC fluids.
The material compatibilities mentioned here may not automatically result in success. Offshore operators are free to add extra additives—which were not part of the Trelleborg test—to suit their particular application. The extra additives may lead to seal material incompatibilities and could have dramatic adverse effects on sealing materials. Material incompatibilities may lead to an accelerated aging process in the hydraulic fluid and to increased wear and degradation of the sealing components. Each application must be reviewed to optimize the seal materials with the HFC fluid. It is important that seals are proved in specific applications with specific fluids and under actual operating conditions to ensure seal performance and life.
Filed Under: Filtration/Contamination Control, Fluid Power Basics, Fluid Power World Magazine Articles, Sealing