Also known as water glycols, HFC fluids are the most commonly used fire-resistant hydraulic fluids. Because of their higher ignition temperature, they are increasingly being used in place of mineral oils with corrosion resistance and anti-wear inhibitors, or HLP media, in hydraulic oil and gas applications with a high fire risk. Over time, various sealing materials have been tested for use with HLP fluids, and a range of traditional materials exists for use with HLP fluids. However, because of the different chemical makeups of HFC and HLP fluids, it cannot be assumed that seal performance will be the same.
In tests undertaken by one seal manufacturer, sealing materials usually used with HLP media showed significantly higher wear with HFC fluids. Choosing the right sealing material is further complicated by the variation of water content in HFC fluids. A research program has identified alternative sealing materials that give better performance and extended life in HFC fluids.
Unique HFC Fluids
Looking at other common pressure fluids for hydraulic systems, HFC fluids have a significantly higher ignition temperature because of their composition of glycol and water. This makes them suitable or even mandatory for applications with increased fire risk.
The water content of HFC fluids varies between 35 and 50 percent, and the tribological properties of the fluids change depending on water content. This leads to a fundamental problem in specifying a suitable sealing system that will meet length-of-life performance criteria. Oil and gas hydraulic applications often involve dynamic movements. For instance, in the case of compensation cylinders and due to long strokes, sealing systems can be subject to significant wear. Compared with oil-based HLP media, the water base of HFC fluids offers limited resistance to wear. Because oil and gas applications increasingly use HFC fluids and length of life needs to be extended, the tests considered the optimum sealing material for HFC fluids.
The seal manufacturer performed tests on a number of sealing materials to investigate the various influences and their effects on sealing systems in HFC fluids with regard to friction, wear resistance and leakage. The HFC fluid chosen for the tests has been proven in critical oil and gas applications.
For rod seal tests, the seal design was held constant across all materials tested, which included proprietary polytetrafluoroethylene (PTFE)-based materials with various fillers, proprietary polyethylene and proprietary polyurethane. The amount of reduction in the radial cross section of the seal ring (w-measure) and the change in weight were used to determine the wear rates.
Two test benches were used in the tests: one for wear measurement and another for frictional force. In position 1/4, the pressure as a function of the piston rod direction was reversed as would occur in a rod seal in a differential cylinder. Position 2/3 reflects the rod seal behavior in a plunger cylinder.
Figure 2 shows loss of radial cross section of the seal ring (w-measure) in HFC fluid in two positions. Positions 2/3 and 1/4 are affected differently in terms of pressure in relation to the direction of movement of the piston rod. This leads to a difference in the lubricant film thickness, which has a substantial effect on the wear rates. The PTFE-based materials show a greater deviation of wear dimensions. These relationships were confirmed when looking at weight loss with dimension of the sealing profile contour. The two materials that demonstrated the least weight loss—and therefore wear—were a premium-grade, ultra-high-molecular-weight polyethylene and a polyurethane. Leakage from the cylinder was measured, and measurable leakage tended to arise at position 1/4.
The test outcomes show strong influences in the behavior of the seals related to friction and wear. Because of the significant fluctuation of the mixing ratios (water/glycol) in HFC fluids and in operation, reliable behavior of sealing systems can only be predicted to a limited extent.
To ensure seal life and performance in hydraulic oil and gas applications, it is important to know the type of hydraulic fluids in service and, using test results and experience, match the correct sealing material to the hydraulic fluid. Despite the successful use of PTFE-based materials with HFC fluids, alternative sealing materials can offer better wear resistance and extended life.