Diesel Fuel Storage and Handling Guide

In depth analysis of the most common causes of contaminants – particulates, water, microbial activity, corrosion, excess of additives, etc. – and some of diesel’s properties making it susceptible to contaminants – waxes, cold temperature operability, viscosity, fuel degradation, etc. – and plausible fixes – specifications, adequate filtration, good housekeeping practices

• Advances in diesel engine technology make the modern diesel engine “one of the most fuel efficient and cleanest burning in the world” but the technologies require fuel that is as free of contaminants as possible – this requires both robust on-board fuel filtration and that the fuel is clean when delivered to the vehicle
• Generation and accumulation of particulate and water in the distribution system that gets transferred to other locations, as well as free water present at bottom of storage tanks and at low points along pipeline which promote microbial activity and increased corrosion and particulate generation are primary sources of contaminants
• Excess of additives may be another source of particulate formation
• Storage tank practices: coat storage tanks with epoxy; screens on vents; equip ASTs with low point sump and drain line with valve to remove water & sediment; tanks should be constructed with adequate water removing capability (e.g. AST cone up tank bottom design); tight covers on fill connections and gauge access points; use of filtration systems and pre- and post-filtration sampling
• Putting particulate contamination in distribution chain into context: if terminal receives 60,000 barrels/wk. into a specific tank and the material contains 10 mg/L particulate, after one year tank would accrue more than 11,000 pounds of external solids; retail storage tank receiving 8,000 gallons/wk. would gain 35 pounds of solids per year
• Water:
  • Water most common contaminant (part of refining process, rain, ship ballast, condensation) – can be dissolved into fuel or suspended as tiny droplets – can lead to increased corrosion/greater microbial activity
  • Water has more pronounced effect on biodiesel b/c more soluble – combines with fatty acid methyl ester (FAME) molecules, changing FAME’s properties; can also lead to hydrolysis of biodiesel (molecule is degraded or broken apart, causing reactions with other compounds to form salts, soaps, or peroxides)
  • Dissolved water not detectable/water droplets make fuel look cloudy
  • Water in vehicle fuel system can lead to corrosion, increased metal wear, ice, plugged filters or screens, increased degradation of fuel, partial hydrolysis of biodiesel (FAME), microbiological growth
• Microbial Growth
  • Bacteria and fungi use biodiesel as food – all require presence of water (but not much – .5 to 3 microns = 3 mm)
  • Most abundant are low molecular weight organic acids which readily react with inorganic anions (chloride, sulfate, nitrate etc.), creating strong inorganic acids (hydrochloric, sulfuric, & nitric), which cause corrosion/degrade fuel quality
  • Microbes living in tank bottom water will produce detergent-like molecules (biosurfactants), which generate an inverted emulsion of water (making fuel molecules more readily available as a source of food)
  • Majority of microbial contamination is within biofilms (complex structures of sticky slimy polymeric substances) – clog filters/screens – biofilms recolonize and grow much faster (days vs. months) than mass microbial contamination
• Organic Compounds – Wax & Thermal/Oxidative Degradation Products
  • Waxes – normal part of diesel fuel, precipitate from fuel during cold weather and plug filters/screens, tested using Cloud Point (temp at which wax becomes visible)
  • Thermal/Oxidative – produced by exposure to oxygen or high engine temps
• Biodiesel Cold Temp Operability
  • Cold flow properties vary depending on fatty acid composition of feedstock (higher viscosity and cloud point lead to more cold flow problems)
  • Impurities (saturated mono-glycerides (diesel) and sterol glucosides (B100)) also affect cold temp performance
    • Sterol glucosides occur naturally in vegetable oils and soluble fats, high melting point and insolubility make them dispersed solid particles which promote crystallization and co-precipitation of other compounds, likely to settle to bottom of storage tanks and vessels, cold temps accelerate process
    • Mono-glycerides problem species for biodiesel blends, have poor solubility in diesel/solubility is temp dependent
• Diesel Degradation
  • Age causes degradation via thermal and oxidative processes to form polymers and acids, promoted by high temps and pressure, oxygen, acids, copper & zinc – causes fuel soluble polymers and fuel-insoluble gums and particulates
• Inorganic Compounds (rust, metals, dirt, gravel, sand, salt, etc.)
  • Grow in tanks or enter fuel supply at some point
• Fuel System Corrosion in Tanks and Engines
  • Carbon steel in tanks and pipes forms iron oxides and iron oxyhydroxides (types of rust) when exposed to water
  • Switch to ULSD and increased use of FAME have led to increased fuel problems, due in part to biodiesel’s greater affinity for water, higher susceptibility to microbial activity, and increased solvency effect on pre-existing deposits
  • Modern high pressure common rail fuel injection systems have tight metal to metal clearances which cannot tolerate even trace amounts of dirt, particulate or corrosion related particles
  • Salt in tank bottoms causes steel corrosion and fuel soap formation – soaps inhibit fuel’s corrosion protection, leading to further corrosion
  • Corrosion inhibitors naturally deplete as fuel passes through infrastructure
• Fuel Additives
  • Additives that are meant to shear in pipelines and don’t can cause filter problems (DRA)
  • Certain additives very reactive with water or sodium chloride and will form soaps
  • Unintended consequences of additives
• Filters & Filtration
  • Small metal to metal control clearances in engines require fine filtration of 4 microns or better to protect systems from metal scoring or other damage
  • Diesel fuel cleanliness should be measured by particle counting (which provides particle size distribution as well as mass)
  • To achieve cleanliness cited in the World Wide Fuel Charter Specification (ISO Code 4406), 10 micron filters or less will likely be required
  • Filters with 95% or better efficiency recommended
• Fuel Specifications
  • Future ASTM diesel specs will likely contain both water and particulate limits
  • Canadian standards limit sediment and water by bottoms, sediment and water (BS&W) to .02 volume percentage maximum vs. .05 volume percentage for ASTM
  • A given fuel will fail the workmanship requirement (“diesel fuel shall be visually free of undissolved water, sediment, and suspended matter”) well before it exceeds the maximum limits of ASTM’s BS&W test
  • Canadian specs use 4 degrees C (39F) as the test temp to conduct visual haze rating for winter diesel fuel and 15C (59F) for summer diesel vs. ASTM specs which do not refer to haze rating as a requirement; in U.S., visual haze rating is typically conducted at 25C (77f) or 21C (70F) year round
• Housekeeping Guidelines
  • Keep tank fill areas/delivery hoses/connectors clean and dry (“white bucket test”)
  • Physically stick tank with water paste before and 30 mins after delivery to verify tank bottom water, monitor frequently (testing both sides or one if tank is tilted) for water & pump out asap
  • Sample tank bottoms monthly for visual clarity and annually for microbiological contamination (culture tests) & treat with biocide if detected
  • Good seals on tank openings, proper ventilation with vent caps in good condition (never share vent line with gasoline tank)
  • Fill wells above grade & plungers inside should be depressed
  • Halt fuel dispensing while filling storage tanks & keep dispenser nozzles clean
  • 10 micron water-absorbing filter (30 not good enough for modern engine), change filter when flow drops to 50-60% flow rate
  • Keep tanks as full and cool as possible (paint ASTs white, bury USTs at lower depths, insulate any tank or line within close proximity of engine or engine exhaust)
  • 95% efficient coalescing 10 micron filters should be considered, particularly where there is truck, rail, pipeline or marine movement
  • Tanks 30,000 gallons or less should be completely lined

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