With the industrial industry’s current emphasis on sustainability and the move to reduce society’s dependence on fossil fuels, today’s gasoline and diesel fuel blends are increasingly diverse. As the number of available ingredients is expanding, the cost and availability of these various blending agents has an effect on the fuel’s composition. Additionally, federal mandates like The Renewable Fuel Standard (RFS) require transportation fuels to contain a minimum amount of fuel from renewable sources. As renewable fuels are produced using a variety of processes, care must be taken when selecting compatible hose materials for transfer and fueling. In loading and unloading operations, corrugated metal hoses are often preferred due to their fire safety, low permeation rates, and rugged resistance to weathering, temperature extremes, and corrosion. As the properties of each fuel varies, it is important to understand when to use flexible metal hoses for fuel transfer, and proper alloy selection for these fuels.
By Frank Caprio, Corporate Trainer and Major Market Specialist, Hose Master
Differences in Renewable Fuels
Before discussing flexible metal hoses for fuel transfer, it is beneficial to review the properties of the different fuel types.
Pure gasoline is a mixture of hydrocarbons produced from crude oil, which are blended to make the desired grade, or octane rating. Although various technologies exist to produce gasoline from sustainable sources, none are commercially viable at present. Instead, gasoline is oxygenated by adding alcohols like ethanol to increase the octane rating and reduce emissions. The increased use of ethanol has led to more frequent reports of corrosion caused by ethanol-eating bacteria. Inspectors can easily identify the presence of these bacteria, because they produce acetic acid as a by-product, which is characterized by a strong vinegar odor. Service stations are particularly vulnerable to this corrosion in the sump area beneath the fuel dispensers. If ethanol vapor permeates into the sump from the underground storage tank, the presence of even trace amounts of water can trigger bacterial growth and spur rapid corrosion of the fuel pumping system.
Diesel fuel has variances in processing methods and feedstock which result in fuels with different chemical compositions, and different challenges.
Petroleum-based diesel fuels continue to experience strong demand in the market as diesel-powered engines are used in a multitude of applications that range from transportation vehicles and construction equipment to power generation. Diesel fuel is less flammable than other fuel types and provides more energy per gallon than gasoline. Unlike gasoline, diesel can also be economically produced from renewable resources, but the properties of these fuels vary depending on the process employed. A brief discussion of diesel production from renewable resources illustrates the variability of this product.
Renewable “Green” Diesel
Green diesel, also called renewable diesel, can be produced via several patented technologies. Most production methods employ a form of processing called hydrotreating, where plant-based oil undergoes a multi-stage conversion process using high-pressure hydrogen to remove the oxygen, water, and other impurities from the oil. Other technologies include thermal processing, catalytic reforming, and biomass-to-liquids conversion through a thermochemical process. Animal or fish fats may also be used as a feedstock. The resulting green diesel is chemically similar to petroleum diesel, although it burns cleaner (due to fewer impurities). Renewable diesel can be blended with petroleum-based diesel in any percentage without effecting engine performance.
The production of biodiesel involves a process called transesterification, which transforms the fatty acids found in oils into biodiesel through a chemical reaction. The production of biodiesel utilizes oil derived from plants or animals, or even used cooking oil. Biodiesel contains oxygen, which is present in both the fatty acids and the alcohol used in production. This makes biodiesel significantly different from green diesel, from which all oxygen is removed during production. The presence of oxygen in biodiesel is problematic for several reasons. First, biodiesel is biodegradable. While this is great for the environment, it also means that biodiesel will degrade over time through oxidation, limiting its long-term storage capabilities unless antioxidants are added at additional cost. The stability of the product varies depending on the feedstock, the production process as well as the storage conditions (air, humidity, UV rays, et al). Second, biodiesel oxidation produces corrosive by-products. This not only causes potential corrosion issues in storage tanks and pumping systems, but also can affect transfer hoses.
Appropriate Alloy Selection
As 300-series stainless steels are resistant to standard gasoline, diesel fuels, and fuels containing ethanol or renewable diesel, they are widely used in the construction of components such as storage tanks, pumps, valves, etc. For biodiesel, many resources still recommend using 304 SS or 316 SS, as most biodiesel contains antioxidants to prevent degradation of the fuel and the formation of corrosive compounds. Due to the complexity of each of the mediums that require transfer and transportation, special consideration should be given to fuel transfer hoses.
To mitigate the risk of hose failure, corrugated metal hoses are typically used for fuel transfer. They are formed from relatively thin-walled tubing, with corrugations introduced that allow the hose to bend. When transferring biodiesel and biodiesel blends, a 316 SS hose should be recommended for added corrosion resistance. Not only does 316 resist mild acids that are produced as biodiesel oxidizes, but it also offers resistance to salt air in seaside environments. If corrosion problems persist, then special alloys such as 276 (UNS N10276) or 625 (UNS N06625) may be required. It is important to note that 300-series stainless steel hoses should not be immersed in salt water, as the chlorides will attack the stainless steel.
Additional considerations are required for non-metallic hose and gasket usage, as chemical compatibility is just one facet of proper material selection. Certain polymers and elastomers should be avoided when transferring biodiesel such as natural rubber and some nitrile compounds. It is best to consult the supplier for proper material recommendations. The other concern when transferring biodiesel and oxygenated fuels is permeation; the gradual penetration of the media through the hose wall. Flexible rubber and composite hoses may experience permeation which can create any number of safety issues including degradation of the hose and the presence of flammable residue on the exterior of the assembly.
Corrugated metal hoses avoid these permeation issues and are resistant to ozone depletion and weathering. Additionally, metal hoses will not melt in the event of a nearby fire. These are some reasons terminal operators prefer corrugated metal hose assemblies for fuel loading and unloading.
Corrugated metal hose assemblies used for the transfer of gasoline and diesel fuels should only be fabricated by qualified welders, as these hoses can be subjected to significant tensile stresses during normal operations. Special techniques can also be utilized when fabricating these assemblies, to help alleviate any crevices at the hose-to-fitting interface where media might collect and exacerbate corrosion. As various internal and external agents can initiate corrosion, the source of any chemical attack must be identified in order to implement the proper solution. An important part of this process is to determine if the corrosion began in the interior or on the exterior of the assembly. Because this can be difficult to verify in the field, it is imperative to have access to a lab that can analyze the assembly or to utilize analysis services provided by the manufacturer (if available), as third-party inspectors may not have adequate expertise regarding flexible hoses, and the analysis can get quite expensive. A far more costly option is treating any corrosion issue as an isolated problem.
A systemic approach to alloy selection and diligent maintenance can improve hose longevity, increase safety, and eliminate corrosion-related failures while reducing costs over time.