White Paper on Oxidation of Buna N Flange Gaskets in Diesel Fuel Systems
May 20, 2016
David C. Geisen, PE
VP of Operations
Platt Rogers Company
Background:
The fuel oil industry has utilized Buna-N and Butadiene based gaskets for years as the gasket of choice for flanged connections in diesel fuel oil supply systems. Most fuel oil system construction specifications still call for Buna-N gaskets.
Platt Rogers has witnessed first hand that Buna-N gaskets are susceptible to a chemical attack that oxidizes the Buna-N rubber, in some cases severely. The analysis that follows explores the causes of Buna-N rubber oxidation in diesel fuel systems
Analysis:
The physical appearance of the sample of gaskets examined strongly suggested a chemical oxidation of the elastomer, the initial investigation focused upon what could have caused such an attack. The sample gasket came from two test lines that are normally dead headed and thus flow induced damage of the gaskets was ruled out.
Platt Rogers has used Butadiene based gaskets for many years in fuel systems with no noticeable degradation. This further reinforced the suspicion that some other chemical was introduced into the system. A Bill of Lading of fuel shipments confirmed that at least on that occasion B2 fuel (Biodiesel) was received. The B2 classification means that the fuel is 2% biodiesel and 98% petroleum diesel. Platt Rogers did not confirm every shipment but having confirmed that at least some biodiesel fuel received, and knowing that biodiesel has in the past been linked to elastomer deterioration, Platt Rogers knew it was worthy of further research.
The National Biodiesel Board did an excellent job of compiling research and published a Materials Compatibility paper (1). Their research showed that Butadiene based elastomers are attacked by biodiesel and as such these elastomers are not recommended for use in systems containing biodiesel. There are far better elastomers for biodiesel applications including PTFE and Teflon based elastomers which are rated very high. The National Biodiesel Board downplayed the likelihood of oxidation of Butadiene based elastomers in their report by stating “Biodiesel blends of 20% or less have shown a much smaller effect on these materials. The effects are virtually non-existent in low-level blends such as B2. When handling blends of B20 or less, normal monitoring of hoses and gaskets for leaks is sufficient.”
Any review would be incomplete without utilizing multiple sources of data, both pro and con. The following is a summary of what the most credible of those sources reported.
- National Renewable Energy Laboratory’s (NREL) Biodiesel Handling and Use Guide (4th Edition – dated January 2009), similar to NBB’s assessment, concurs that lower blends minimize most issues associated with nitrile rubber elastomers. However, the NREL guide goes further in citing lengthy storage can result in oxidation and the formation of corrosive elements that can adversely affect vulnerable materials and that the degree of oxidation of biodiesel may be more of a factor that the concentration of the biodiesel. They recommend that unless the fuel is used within a few months, the biodiesel blend should be stabilized with antioxidants to reduce material compatibility issues. They further cite that the presence of oxidizing metals such as copper, bronze, brass, or zinc will speed up the fuel degradation and oxidation. http://www.nrel.gov/transportation/pdfs/43672.pdf
- Cummins evaluated biodiesel for material compatibility and allows their engines and fuel filtration systems to use up to B20 Biodiesel provided all gaskets and seals are composed solely of a Viton or Fluorosilicone material. (https://www.cumminsfiltration.com/pdfs/product_lit/americas_brochures/MB10053.pdf)
- DuPont has published numerous articles regarding the effect of biodiesel on elastomers. Granted they have a vested interest in publicizing how well Viton holds up to biodiesel since they make the product. http://www.dupontelastomers.com/autofocus/a7/af7.asp?article=biofuel
- Brookhaven National Laboratory weighed in on the compatibility issue, specifically to address the use of biodiesel blends in home heating oil systems. Their results concluded that one type of Butadiene based elastomer, NBR, is acceptable for use with biodiesel blends because of its high acrylonitrile monomer which is chemically bonded to the butadiene monomer to form the NBR polymer. https://noraweb.org/wp-content/uploads/2015/01/ELASTOMER-AND-PUMP-DURABILITY-OF-BIODIESEL-IN-HEATING-OIL-EQUIPMENT-PTS-1-2.pdf
Several Elastomer Manufacturers have also gone on record warning that Buna-N is not compatible with the Methyl Oleate that makes up biodiesel.
Platt Rogers also investigated the impact biodiesel may have on other system components. It has long been known that copper and tin alloys are susceptible to corrosive attack by biodiesel fuels. Research has shown that corrosion rates are increased substantially as fuel temperatures rise (2). This can be a concern for fuel systems that are exposed to harsh sunlight or high outdoor temperatures often seen on tank farms and their associated piping.
Copper, tin, and zinc alloys have a catalyst affect at breaking down biodiesel. The molecular strings of biodiesel are not as long as petroleum based hydrocarbons and as such the biodiesel does not age as well and its acid number increases as it is stored and given the chance to break down (3). All of the research done on material compatibility was done in a lab environment where only one variable at a time was evaluated. It stands to reason that as the acid number increases, the ability of the fuel to oxidize elastomers also increases. Hence though the copper, tin, and zinc alloys may be directly harmless to Buna-N, they may cause biodiesel fuel to chemically change to the point that it is more harmful to Buna-N than lab results indicate.
Conclusion:
The root cause for the gasket degradation Platt Rogers observed was the use of Nitrile Butadiene elastomer based gaskets in a system using a biodiesel fuel blend.
Recommendations for Diesel Fuel Systems that may contain Biodiesel:
- Cease using Butadiene based gaskets in diesel fuel systems and as existing gasketed joints are disassembled for maintenance, reassemble the joints using PTFE, Teflon, or Viton based gaskets which are not susceptible to biodiesel chemical attack. Note that this may require approval of a departure from specification the original construction specification. Though there is research that shows that NBR is a suitable elastomer, no quantitative research was found that supports using pure Buna-N in systems that may contain biodiesel in any concentration. Platt Rogers’ position is that we should be installing gaskets that are not susceptible to the system fluid versus a gasket that may be able to tolerate the fluid in very low concentrations.
- Minimize the use of copper, tin, or zinc based alloys in diesel fuel oil systems.
- Minimize the concentration percentage of Biodiesel stored tank farms used for back-up power purposes. We acknowledge the increased availability of biodiesel fuel and the desire to utilize it because of its renewable resource base. However, the long term storage nature of the emergency back-up fuel supply coupled with the tendency for biodiesel to break down in storage more than its petroleum based cousin, biodiesel is better suited for installations where usage is high and therefore inventory turnover in storage tanks is more frequent.
