DIESEL FUEL STORAGE AND HANDLING

Fuel transported on board makes its way from the point of refining to storage tanks before being transported either by land or by barges. in this process the contaminants in the lines/ tanks/ any previous residue form an integral part of the fuel supplied on board , added to the time they were stored before being supplied. This results in a supply of contaminants , followed by aging of fuel depending upon the time which it is stored.

Fuel contaminants come in numerous shapes, sizes and chemistry. They include water, “dirt”, microbial growth, solids composed of organic salts, soaps, fuel degradation products, polymers, oxides and silicates. Regardless of their chemistry and origin, contaminants can contribute to engine deposit formation, erosion-wear and filter plugging. 
Bio- diesel blending into diesel also poses another set of problems if handling is not taken care of.

Additives

Fuel additives are an important part of finished fuel and are used to convert unfinished fuel into usable fuel as well as to help keep fuel stable
 From the moment that fuel components are made and blended at the refinery, additives are incorporated into their manufacture, distribution/storage and end use. Additives injected into diesel may include, but are not limited to, anti-oxidants, corrosion inhibitor, cold flow improvers, lubricity improvers, cetane improvers, and conductivity improvers. Refinery fuel additives give refiners more flexibility in manufacturing fuel: corrosion inhibitors give fuel anti-rusting protection, anti-oxidants keep fuel from degrading by improving their oxidation stability, and low cetane stocks can be improved with cetane improver. Other additives such as cold flow improvers are added at refineries to allow diesel fuel to meet important pipeline entrance specifications and vehicle cold operability requirements.

Drag-reducing additives (DRA) are typically long chain polymers added during pipeline transportation to allow fuel to travel through these systems faster, thereby increasing pipeline throughput. 

Cold flow improvers may also be added at the terminal, usually during the winter season. These additives keep wax crystals—formed as fuel temperature is lowered—from agglomerating into larger, filter blocking crystals.

It is important to note that some fuel additives are more reactive than others, especially acidic additives (e.g., fatty acids such as mono acid lubricity improver or certain types of corrosion inhibitors) that can react with caustic water or other sources of cations such as sodium chloride salt to form soaps. When soaps form, additive protection can be lessened. Instead of providing corrosion or lubricity protection, the reacted product becomes inert, but is still capable of blocking filters and causing injection system deposits.

Water is the most common contaminant in fuel systems , the ingress may be traced to the following 
 -Part of the refining process
- Rain, as ship ballast water or as condensation in storage tanks and equipment
. Water can be dissolved in the fuel or suspended as tiny droplets. The amount of water that will dissolve in any given fuel depends on both the composition and temperature of the fuel. In some systems, removal of accumulated water may be sporadic. A build-up of water in a tank can lead to increased corrosion and the potential for greater microbial activity.(Sulphur present in fuel is a natural biocide, however due to Ultra low sulphur diesel oil production , biocides have to be introduced post refining stage)

If biodiesel is present in the fuel, water can have a more pronounced effect on the diesel fuel. Water is more soluble in a biodiesel blend as compared to a fuel with no biodiesel. Dissolved water in biodiesel can hydrate (or add water molecules to) the fatty acid methyl ester (FAME) molecules. The properties of hydrated FAME are different from those of non-hydrated FAME. Dissolved water in biodiesel, usually in combination with other contaminants, can also lead to hydrolysis of biodiesel, where the molecule is degraded or broken apart, which can allow further reaction with other compounds to form salts, soaps or peroxides.

Water is also a good solvent for inorganic salts and can contain dissolved acids or other contaminants that can harm the engine. Free water in storage or vehicle tanks can accumulate salts and polar compounds over time and severely impact fuel injection and engine operation

Fuel system corrosion can be controlled with the proper combination of preventative maintenance, proper housekeeping and biocide use where microbial growth has been found. Much of the fuel related corrosion problems are caused by the presence of water. It is common to find water in any fuel storage system no matter how good housekeeping practices are. Water can lead to a whole host of problems including corrosion.

Microbial Contamination with water

Aerobic microbes require a source of free oxygen to grow. In contrast, anaerobic microbes will grow only in the complete absence of free oxygen. 
A third broad category—facultative anaerobes—will live aerobically when oxygen is present and anaerobically when it is not. Like all other life forms, all microbes require the presence of water. Water collects in low points of fuel systems and can become stagnant. This is often where microbiological contamination will occur.
These microbial organisms do not require large amounts of water. They typically range from 0.5 to 3 microns in size and are therefore generally less than 1/1000 the size of a 3 mm thick film of water. This explains why small spots of condensation on the walls of a tank are sufficient to support growth. Viable organisms are found in the air, soil, and even in the fuel, so it is virtually impossible to keep a fuel system sterile without the use of biocides. The effects of these organisms can vary by the type of organism and the volume or mass of organisms present in the system.
The most abundant metabolites produced by microbial communities are low molecular weight organic acids. Although these organic acids are not as aggressive as strong inorganic acids, they can readily react with inorganic anions such as chloride, sulfate, and nitrate to produce strong inorganic acids (hydrochloric, sulfuric and nitric, respectively).
 These strong inorganic acids can cause corrosion and degrade fuel quality. Moreover, microbes living in the tank bottom water commonly produce detergent-like molecules (i.e., biosurfactants). These biosurfactants generate an inverted emulsion of water in the fuel, distributing water into the fuel-phase and making fuel molecules more readily available as a source of food for the microbes.
The vast majority of microbial contamination in fuel systems is within biofilms. Biofilms are complex structures of sticky, slimy polymeric substances that provide a protective habitat for microbes growing within them. Moreover, biofilms are typically in a state of dynamic stability in which masses of cells are sloughed off, forming biomass clusters as new biofilm is generated.
These microorganism clusters, or flocs, can plug filters or screens or other small orifices in the fuel system. Historically, the fungus Hormoconus resinae was found to be an especially proficient filter/screen-plugging agent. 
Once a fuel system has been disinfected the process of re-colonization and biofilm population restoration can occur in a matter of days. However, it can take considerably more time for the microbial contamination to achieve a mass sufficient to become detrimental to the fuel product or tank
in fuel systems the process takes several months to a year, depending on fuel grade, fuel turnover rate and conditions within the fuel system

Wax and Other Organic Compounds

Problem-causing organic compounds are usually of two types: waxes, which are a normal part of diesel fuel, and thermal and oxidative degradation products, which are produced by exposure to oxygen during storage or produced from exposure to high engine temperatures.

Wax can precipitate from the fuel during cold weather and plug filters, screens, and even sharp bends in fuel lines. The most widely utilized specification test to measure this fuel property is the Cloud Point (ASTM D2500), defined as the temperature at which wax particles become visible. The Cloud Point requirement is usually seasonally adjusted and will normally be higher in the summer and lower in the winter.

In addition to cloud point, Cold Filter Plugging Point (CFPP – ASTM D6371), Low Temperature Flow Test (LTFT – ASTM D4539), and Pour Point (ASTM D97) are also utilized to characterize a fuel’s cold temperature performance. 

Biodiesel may also affect cold flow properties and operations in part due to higher viscosity, higher cloud point, or the inclusion of other compounds that adversely affect cold flow operationsThe cold flow properties of any biodiesel being blended must be understood, especially when biodiesel is blended into a diesel basestock at terminal locations where options to correct the cold temperature performance may be limited.

There are two different cold temperature performance concerns: handling and operability. Handling, including filterability, is the ability to store, blend and pump B100 or the resulting diesel blend. Operability refers to the use of the diesel blend in a vehicle’s fueling system.

As shown in Table 1, the cold flow properties of biodiesel vary with the fatty acid composition of the feedstock. Typical values for biodiesels produced from various feedstocks are shown in the table. However some biodiesel is produced from multiple biodiesel feedstocks.

 Typical handling properties for biodiesel manufactured from different feedstocks

Feedstock	Cloud Point (oC)	CFPP (oC)	Viscosity (mm2/s at 40oC)
Canola/Rapeseed	-3	-13	4.4
Sunflower	+3	-3	4.4
Soybean	+1	-4	4.0
Coconut	0	-4	2.7
Palm	13	12	4.6
Tallow	16	14	4.8

The second factor influencing cold temperature performance of biodiesel is impurities which can promote the formation of precipitates and deposits. 

Precipitation from B100 is thought to be associated with the presence of sterol glucosides while precipitation from diesel blends is more often linked to the presence of saturated mono-glycerides.

Sterol glucosides (SGs) occur naturally in vegetable oils and fats in a soluble (acylated) form. In biodiesel, however, they may be converted to non-acylated SGs. Due to the higher melting points of these SGs (approximately 240°C) and their relative insolubility in B100 or diesel fuel, SGs can be considered to be dispersed solid particles in the FAME product. Relatively low concentrations (approximately 35 ppm), SGs may promote the formation of aggregates in biodiesel, exacerbating problems caused by saturated mono-glycerides and other known cold-crystallizing components. This may cause the formation of a cloud-like haze in FAME, even at room temperatures.

If SGs are present and have sufficient time in storage, they are likely to settle to the bottom of storage tanks and vessels. Cold temperatures can accelerate this process and increase the likelihood that SGs will act as nucleating agents for larger agglomerates which will also settle to the bottom of storage tanks and vessels. Draw-down of an unstirred tank that results in these agglomerates being pulled into the delivery system could lead to sudden and unexpected operability problems. As a result, SGs in biodiesel may cause filter blockage at temperatures above the cloud point. For this reason, the contribution of SGs to filter blocking should be considered when addressing filter problems with B100 and diesel blends.

Saturated mono-glycerides have been identified as one of the problem species for biodiesel blended diesel fuels. The saturated mono-glyceride content of biodiesel will depend on the manufacturing steps taken to reduce their concentration (e.g., distillation, settling). Mono-glycerides have poor solubility in diesel fuel and their solubility is temperature dependent

Fuel Degradation

As diesel fuel ages, it can degrade via thermal and oxidative processes to form polymers and acids^. This chemical change is promoted by exposure to high temperature and pressure, oxygen, acids, certain metals such as copper and zinc, or combinations of these. These chemical changes (or reactions) normally result in the formation of both fuel soluble polymers and fuel-insoluble materials, such as gums and particulate. Crosslinking of soluble polymers can generate varnishes. Gums can adhere to surfaces, limit fuel flow or clog small clearances in fuel systems causing problems such as injector malfunction or seizures. Particulate can plug fuel filters. Besides polymers, acids can also be generated by fuel oxidation. These acids often consist of mixtures of formic acid and other low molecular weight acids. They are also soluble in fuel and can easily form soap precipitates if reacted with any metal ions present as fuel contamination or generated from metal surface oxidation

Inorganic Compounds

Fuel contamination with sodium, calcium and other metal cations can have many different sources, such as:

·         Residue from ballast water after sea transportation

·         Refinery salt carry over during removal of water from diesel fuel (salt driers)

·         Refinery caustic neutralization

·         Insufficient catalyst removal (sodium hydroxide/methanolate) during biodiesel production

·         Use of alkali metals for hydrogen removal during desulphurization

·         Sodium based corrosion inhibitor additive for pipeline protection

·         De-icing compounds such as sodium and calcium chloride

·         Used lubricating oil blended into fuel

·         Engine oil lubricated fuel pumps

Typical Contaminants and Countermeasures

Contaminant	Carboxylate soap (metal carboxylate soap)
Symptom(s)	High restriction, plugged filters
Field Test Method(s)	Delta-pressure gauge across filter head, if possible
Laboratory Test Method(s)	Flow vs. Restriction Test, ICP ASTM D5185 modified, ASTM D7111 or similar
Source(s)	a)              Water bottoms b)              Acidic additives such as pipeline corrosion inhibitors, some lubricity improvers, etc. c)              Various metals in incoming supply fuel or introduced within the engine can help generate carboxylates d)              Tank ventilation opening arranged in location conducive for contamination entering fuel tank (e.g., spray area of wheels) e)              Carry-over or spills from refinery salt driers
Contamination level that may cause problems	>200 ppm of water to stage 1 filter; metals content goal is <0.1 ppm of Na, K, Ca, Mg or Zn or no trace or detectable amount. Existing test method minimums are 0.21, 0.19, 0.10, 0.10, and 0.09 respectively per ASTM D7111. It is anticipated that future methods will be capable of 0.1 ppm or less. ICP-MS and other non-standard tests are already capable of these lower numbers.
Countermeasures or Corrective Actions	a)              Drain water bottoms and periodic inspection and maintenance of fuel storage, fuel delivery and equipment fuel tanks b)            Work with fuel supplier to ensure compatible corrosion inhibitor usage and that metal-free fuel is provided c)              Use approved detergent/dispersant additives d)              Replacement of galvanized wetted surfaces/components (supply tanks, plumbing, fittings, etc.) to eliminate possible zinc contamination.

Contaminant	Wax
Symptom(s)	cold operation plugging, longer starting time
Field Test Method(s)	Potential field visual inspection (cloudy), warm fuel or filter to confirm/solve wax plugging issue.
Laboratory Test Method(s)	Cloud Point Test (ASTM D2500, ASTM D5773), Cold Filter Plugging Point (D6371), LTFT (ASTM D4539)
Source(s)	Long chain n-paraffin in fuel, use of summer blend fuel in winter
Contamination level that may cause problems	Cloud point, CFPP or LTFT of fuel above ambient operating temperature could cause issues.

Countermeasures or Corrective Actions

Use of: cold flow improver additive (preemptively), suitable winter blend fuel, approved fuel/filter heaters. Replace filters and keep warm.

Contaminant	Soft and hard particles
Symptom(s)	Contaminant cakes on filters, high restriction
Field Test Method(s)	Visual inspection (if particles are visible, fuel cleanliness is above required level), field particle counting
Laboratory Test Method(s)	Laser Optical Particle Count (ASTM D7619)
Source(s)	Water contamination can bring in dirt/rust Dust contaminated supply fuels Application environment (dust ingested thru fuel tank vent) Switch of fuel type can result in fuel tank debris to be loosened and migrate to filters
Contamination level that may cause problems	Dispensed fuel contamination levels above 18/16/13 per ISO 4406 (reference Appendix B)
Countermeasures or Corrective Actions	Pre-filter (bulk tank filtration) Routine tank inspection and cleaning Fuel tank vent filtration Higher capacity filters Multi-stage filtration

Contaminant	Algae, bacteria and fungi
Symptom(s)	Clear, green, black or brown slime, foul smell (hydrogen sulfide), rotten egg smell
Field Test Method(s)	Laboratory analysis
Laboratory Test Method(s) and Guides	ATP Test (ASTM D7463, D7687), ASTM D6469, , standard culture tests
Source(s)	Water in fuel tank bottoms, warm conditions, lack of tank maintenance, long storage and environment
Contamination level that may cause problems	Any non-dissolved water
Countermeasures or Corrective Actions	Drain free water frequently, sloped storage tanks with petcock drains, desiccant tank vent, tank cleanup, biocides should not be used routinely, annual inspection/routine tank maintenance, tank ventilation and filtration

Contaminant	Corrosion products due to water in fuel (Note: water typically does not intrinsically plug filters but can contribute by aiding the other contaminants to develop and collect)
Symptom(s)	Corrosion, water in fuel sensor alerts, water separation canister visually full, plugged filters, frozen fuel lines
Field Test Method(s)	Fuel Suitability Test (ISO 4020 older version) Clear and bright standard for visual check of fuel ASTM D4176 bar chart test
Laboratory Test Method(s)	Steel Corrosion Test (ASTM D665, D7548, NACE TM0172), Karl Fischer test method for water content, (ASTM D6304, E203),
Source(s)	Ship ballasts, ambient conditions including in-tank condensation of water vapor due to temperature cycling or changes, improper fuel tank   ventilation, lack of fuel tank maintenance, fuel supply already contaminated, poor fuel tank construction
Contamination level that may cause problems	>200 ppm to Stage 1 fuel filter worse than NACE B+ (e.g. B, C, D, E rating)
Countermeasures or Corrective Actions	Drain free water frequently, slope storage tanks with petcock drains, desiccant and filtered tank ventilation, minimum annual inspection, consult fuel supplier, upgrade of filtration to larger capacity and higher efficiency

Contaminant	Asphaltenes, carbonaceous particles
Symptom(s)	Premature filter plugging
Field Test Method(s)	Visual inspection
Laboratory Test Method(s)	Test fresh fuel, Thermal Stability (ASTM D6468), PetroOXY Stability (D7545)
Source(s)	Fuel instability from long-term storage (longer than 3 months), erroneous fuel blend
Contamination level that may cause problems	Less than 80% reflectance for D6468 (180 minutes), <60 minutes for D7545
Countermeasures or Corrective Actions	Asphaltene conditioner, fuel stabilizers, anti-oxidants, drain and replace fuel