EDITOR’S NOTE: When ASTM International was evaluating Bioheat® Fuel blends in the range of B6 to B20 and considering whether to create new standards for those fuels, technical information was at the heart of the process. Interested parties such as the National Oilheat Research Alliance (NORA) and the National Biodiesel Board (NBB) helped to develop data addressing the compatibility of those blends with essential heating oil equipment.
ASTM International approved the proposed grades of B6-to-B20 Bioheat earlier this year, setting the stage for heating oil marketers to sell those fuels with confidence, knowing that the fuel was endorsed by the widely recognized standard-setting authority.
For ASTM to approve the use of B6-to-B20 blends in current heating oil burners and tanks, voting members had to be satisfied that the fuel would be compatible with all materials and components. One group of materials that was closely studied was the elastomers that are used in seals and gaskets in fuel pumps.
Steve Howell, Chairman of the ASTM task force on biodiesel standards within the United States, recently furnished Oil & Energy with a technical summary on elastomer and pump durability in heating oil equipment using Bioheat blends, which was presented during the ASTM proceedings. The following is an edited version of that report’s Technical Summary, which indicates that Bioheat blends at the B6-B20 level are safe for use with the elastomers used in fuel pumps.
Elastomer and Pump Durability of Biodiesel in Heating Oil Equipment
Key Result: Compatibility between biodiesel meeting ASTM 6751 and NBR (nitrile) elastomer seal materials historically used in oil burners is demonstrated to at least the B20 level.
Part 1: Compatibility of Heating Equipment Elastomer Seals and Biodiesel Blends
Thomas Butcher and Rebecca Trojanowski, Brookhaven National Laboratory, April 2014
In this technical summary, a review of prior work done in the U.S. on the elastomer compatibility is provided. Specific compatibility testing of NBR seal materials common to heating oil system with biodiesel/heating oil blends is also presented. The technical data from both short-term UL type testing (Part 1) and extended bench testing (Part 2, below) indicate that current elastomers perform as expected in the existing unmodified equipment base when using B20 and lower biodiesel blends.
Introduction: For an alternative fuel to be used safely in home heating systems compatibility with the elastomeric seal materials in use is required. Seal changes, in the case of a non-compatible fuel are technically feasible but, with some 8 million home oil-fired systems, the requirement of a seal change would represent a significant market acceptance barrier.
In existing heating systems, the dominant seal material is nitrile or NBR, which contains an acrylonitrile monomer that imparts permeation resistance characteristics to a wide variety of solvents and chemicals, and a butadiene component in the polymer that contributes toward the flexibility.
In the process of obtaining a listing approval for a burner for application in this market testing is typically guided by standard UL 296, which incorporates a material compatibility test for elastomeric materials, UL 157. Suitable elastomers are required to retain more than 60 percent of their unconditioned tensile strength, and elongation and volume swell must fall within the range of -1 to + 25 percent.
Previous Elastomer Compatibility Studies: In a study published in 1997, Southwest Research Institute reported on their evaluation of a range of different elastomer types exposed to biodiesel/petroleum blends. Fuels included in this study included Jet Propellant 8 (JP8), B100, low-sulfur diesel fuel, “reference” diesel fuel and blends at the B20 and B30 level. Samples were immersed at 125 ºF for 0, 22, 70, and 694 hours.
In a more recent study, Southwest Research Institute and the National Renewable Energy Laboratory (NREL) evaluated the compatibility of several elastomers including three different types of nitrile in B20 blends and ethanol-diesel blends. The nitrile materials included a general purpose NBR, and high aceto-nitrile content rubber, and a peroxide-cured nitrile rubber. These materials were selected as being typical of materials used in automotive applications. Samples were immersed at 104 ºF for 500 hours.
Tests reported in the early study by Southwest Research Institute for elastomers common to diesel engines showed some effect of the biodiesel blend on the nitrile materials. This included volume swell in the 20 percent range and a reduction in tensile strength as high as 38 percent. These tests were done at much higher temperature and for much longer times than required by UL 157, but the magnitude of property change reported was still within the acceptable range under UL 157, although marginally. The later study reported on by Southwest Research and NREL showed no significant effect of the biodiesel blends on the NBR materials studied, leading to the conclusion “…all of these elastomers appear to be fully compatible with 20 percent biodiesel blends.”
In another potentially relevant study done by Underwriters Laboratories the compatibly of B5 blends with elastomers typically used in oil burner applications was studied in compliance with the UL157 standard. Two specific nitrile materials were included. The study conducted by UL at the B5 blend level also showed no significant effect of the biodiesel blend on the materials tested.
Elastomer Testing with NBR Used in Heating Oil Systems: As part of a new study to evaluate the practical upper limit of biodiesel content in a blend with home heating oil, Brookhaven National Laboratory has completed compatibility tests with NBR at blend levels from 0 to B100. In collaboration with the dominant manufacturer of pumps on legacy oil burners in the U.S., one specific NBR material commonly used in the heating oil industry was identified for evaluation. This is a high aceto-nitrile material used for the critical pump shaft lip seal. Slabs of this material were obtained from the manufacturer for use in these tests. Immersion was done for 670 hours at 125°F, conditions much harsher than that normally used to qualify seals per UL 157.
The studies at Brookhaven National Laboratory showed full compatibility between the NBR material used in common oil burner seals and biodiesel blends up to B100. Figure 1 below, for example, illustrates the effects on volume swell, and results are similar for tensile strength, hardness and compression across more than 670 hours of testing with petrodiesel and all biodiesel blends up to B100.
In an interesting part of the BNL study the effects of elevated acid number on NBR material properties was evaluated. It was shown that acid in numbers well above the specification limits does lead to significant interaction with the NBR materials. In this test, acid number was increased through the addition of decanoic acid, and this effect is illustrated in Figure 2. It is postulated that elevated acid number caused by accelerated testing degradation contributed to observed effects of biodiesel on NBR materials in the earlier reported tests, especially since many of these earlier tests were completed prior to the addition of a stability specification for B100 and other changes to the B100 specifications which were implemented to secure the ASTM approval for biodiesel blends in 2008.
Figure 1. Example results, tests at BNL. Impact of biodiesel content in blend on NBR material volume swell.
Part 2: Impact of Biodiesel Blends on Durability of Oil Burner Pumps
Vic Turk (R. W. Beckett Corp.) and Thomas Butcher (Brookhaven National Laboratory). April 2014
Introduction: A critical component in the fuel system of an oil-fired heating system is the burner pump. This unit performs the following functions:
- Lifting the fuel from underground storage, clearing the fuel line air rapidly during initial operation;
- Creating and regulating the required pressure for proper atomization, typically 100-150 psi.
- Providing a “sharp” turn-on and turn-off of flow to the nozzle to prevent after-drip or injection of fuel under a low-pressure, poor atomization condition.
The dominant manufacturer of the pumps in use in existing equipment in the field is Suntec Industries, with an estimated 90 percent market share for these installed units. This gear-pump includes a NBR lip-seal on the rotating input shaft. Potential leakage of this seal with biodiesel blends has been identified as a high priority area for evaluation in considering higher levels of biodiesel in heating oil.
Detailed bench level compatibility studies using elastomer slab samples provided by the pump manufacturer showed no impact of biodiesel blends up to B100 compared to conventional heating oil. To complement these basic materials studies, a decision was made to undertake long-term, cyclic durability tests with pumps. In the field, these burners and pumps cycle on/off 5,000 to 10,000 times annually, and it was desired to confirm the performance of the seals in actual pump operation.
The pump test was implemented at the Energy Institute of Penn State University with oversight by the industry Bioheat Technical Steering Committee. The pump manufacturer was very involved with the definition of the test setup and evaluation protocol. The methods were based on established methods used to evaluate candidate seal materials.
Pump Tests Conducted: The testing was planned to involve a 5-gallon fuel supply for each pump, set up in a continuous loop with a 5-minute-on/1-minute-off controlled cycling pattern. The piping was arranged without a fuel spray nozzle but the pump developed its full operating pressure each cycle.
In the test program, a key performance measurement parameter was observed seal leakage rate. The project was started in 2010 but upon seeing some of the initial results it was discovered there was some confusion regarding the leakage rates measurements. The measurements were being done differently than those being used by the manufacturer. This was corrected, and all new pumps were installed and the test restarted.
The testing was done with two base fuels – a conventional No. 2 heating oil at 1,500ppm sulfur content and an ultra low sulfur heating oil at 15ppm. Three different biodiesel blend levels were studied for each fuel – B0, B12 and B20. The biodiesel was a commercial blended-feedstock fuel provided by Hero BX. This fuel met all requirements of ASTM D-6751-11. For each fuel blend a total of seven pumps were run in this 7,000-hour test. Quality of all fuels was monitored throughout the project to ensure the fuel had not degraded significantly during the test due to the stressing of the fuel in the test. Acid number was considered the primary criterion for this. High acid numbers were not observed, and thus the test was considered acceptable from that standpoint.
Fuel pump shaft seal observed leakage was a key performance measure and this was monitored on a regular basis. The manufacturer provided a scale from 1 to 4 based on observed leakage. These are all very low leakage rates. For example a No. 2 leak is described as “wet seal with a slight accumulation in the seal cavity area”. A No. 4 (highest) leak is actual fuel running down over the hub face. These leak rates likely would not be noticed in the field. A seal leak metric for the entire set of pumps was based on a weighted-percent-dry metric. The weighting penalizes a leak situation to a greater degree if it occurs early in the 7,000-hour test period.
Key Results: Figure 2 (below) provides a summary of the test results. In this figure the Seal Rating is used – a higher value indicates better performance. The most significant conclusions are:
- Seal performance improves with increasing biodiesel content.
- Seal performance is equivalent at B0 for both 15ppm and 1,500ppm sulfur fuels.
- Seal performance is better with 1,500ppm sulfur fuel than with the ULSD fuel at the same biodiesel level.
Two pumps “bound-up” in the 4,600-5,000 hour time frame. These were both at the B12 blend level and both base fuels were involved. Other than this occurrence no operational problems were observed. Following these tests the pumps were all shipped to Brookhaven National Laboratory for internal inspection. No unusual conditions or fuel related issues were noted from the inspection. Thus, while both the seizures were with B12, it is not believed they were fuel-related.