Businesses storing DERV in bulk need to consider more than how long the fuel has been in the tank. Its composition and the conditions within the storage system can both affect fuel quality over time.
Standard DERV supplied in the UK generally meets BS EN 590 and may contain up to 7% Fatty Acid Methyl Ester (FAME) by volume. FAME increases the fuel’s renewable content, but it can also make stored DERV more sensitive to water, oxygen and extended storage.
This is particularly relevant to businesses with low-throughput tanks, seasonal machinery, standby generators or other equipment that remains unused for long periods.
This guide explains why FAME affects DERV storage and the practical steps businesses can take to protect their fuel.
What is FAME in DERV?
FAME stands for Fatty Acid Methyl Ester. It is a type of biodiesel produced from renewable feedstocks such as vegetable oils, used cooking oils and animal fats.
When blended with conventional diesel, FAME contributes to the fuel’s renewable content. DERV containing FAME is suitable for most diesel road vehicles, but appropriate fuel management becomes particularly important when it is stored in bulk for long or uncertain periods.
For more information about DERV and other commercial fuels, read our guide to diesel grades.
Why can FAME affect stored DERV?
DERV does not remain unchanged while it is stored. Its condition can be affected by oxygen, water, temperature, tank cleanliness, fuel turnover and storage duration.
FAME can make stored DERV more sensitive to some of these factors.
Water and moisture
FAME has a greater affinity for water than conventional fossil diesel. Consequently, DERV containing FAME can dissolve and retain more moisture.
Water can also enter a commercial storage tank through:
- Condensation
- Damaged seals or caps
- Poorly maintained vents
- Rainwater ingress
- Contaminated equipment
- Inadequate tank maintenance
Depending on the quantity and conditions, water may remain dissolved in the fuel, become suspended as small droplets or settle as free water at the bottom of the tank.
Free water is particularly important because it creates conditions in which microorganisms can proliferate. Water can also contribute to corrosion and affect the quality of the stored DERV.
Oxidation
DERV reacts gradually with oxygen during storage. This process is known as oxidation.
FAME is generally more susceptible to oxidation than conventional fossil diesel. Heat, exposure to air, certain metals and extended storage can accelerate the process.
As oxidation progresses, the fuel may become more acidic and form gums, sediment and other insoluble products. These materials can accumulate in the tank or enter the fuel system, where they may restrict fuel flow and block filters.
Oxidation stability can differ between fuel batches. The delivery date alone cannot therefore confirm whether stored DERV remains suitable for use.
Microbial contamination
Bacteria, yeasts and moulds can proliferate when free water is present, particularly at the boundary between the fuel and water. This contamination is commonly called diesel bug.
FAME does not automatically cause diesel bug. However, DERV containing FAME can be more susceptible to microbial growth when the necessary conditions—especially free water—are present.
Microbial growth can produce biomass, sludge and acidic by-products. If left untreated, contamination may:
- Block filters and fuel lines
- Restrict fuel flow
- Degrade fuel quality
- Contribute to tank and component corrosion
- Affect vehicle or machinery reliability
- Cause recurring operational problems
Read our guide to fuel contamination in commercial storage for more information about contamination types and warning signs.
Does stored DERV have a fixed shelf life?
There is no single expiry date that applies to every delivery of DERV.
Its condition during storage depends on several connected factors, including:
- The initial quality and condition of the fuel
- The amount of dissolved or free water present
- Tank cleanliness and integrity
- Exposure to oxygen
- Storage temperature and temperature fluctuations
- Fuel turnover
- Previous contamination in the tank
- The condition of filters and dispensing equipment
- Whether older and newer fuel have repeatedly been mixed
DERV stored in a clean, dry and well maintained tank with regular turnover is generally less exposed to storage related problems than fuel left unused in a poorly maintained system.
Businesses should not judge fuel condition solely by its delivery date. The condition of the DERV and its storage environment must also be considered.
Where the age, quality or storage history of the fuel is uncertain, sampling and testing may be needed before it is used.
Which businesses should pay particular attention?
FAME related storage considerations are most relevant where DERV turnover is low or equipment is used intermittently.
This may include:
- Haulage businesses with low-throughput bulk tanks
- Farms storing DERV for road using agricultural vehicles
- Construction businesses with equipment used only on particular projects
- Sites operating diesel powered standby generators
- Businesses holding emergency or contingency fuel
- Operations with tanks larger than their typical requirements
- Businesses that repeatedly top up tanks without fully turning over the existing stock
- Equipment that remains unused for extended periods
Standby equipment requires particular care. A generator may run infrequently, but its fuel must remain suitable when the equipment is needed.
A full storage tank does not necessarily contain usable DERV. Regular inspection and condition monitoring are especially important where the fuel supports business critical or safety critical equipment.
What are the signs of deteriorating DERV?
Changes in stored DERV are not always visible. Nevertheless, several warning signs may indicate that the fuel or tank requires further investigation:
- Cloudy, hazy or unusually dark fuel
- Free water at the bottom of the tank
- Sludge, sediment or debris
- Filters blocking more frequently
- Reduced or inconsistent fuel flow
- Corrosion within the tank or fuel system
- Difficulty starting vehicles, generators or machinery
- Unexpected loss of equipment performance
- Deposits on fuel-system components
- Fuel with an unknown delivery date or storage history
These symptoms can have several possible causes. The DERV and storage system should therefore be assessed before a treatment or remedial process is selected.
How can businesses protect stored DERV?
Good storage management can reduce the likelihood of DERV degradation and help identify developing problems before they cause equipment failure or downtime.
Match deliveries to consumption
Avoid holding substantially more DERV than the business is likely to use during its normal turnover period.
A larger delivery may appear economical or convenient, but excessive stock can remain in storage for longer than expected. Reviewing historic consumption and seasonal demand can help businesses determine more appropriate delivery quantities.
Maintain accurate records
Record fuel deliveries, volumes, inspections, water checks, filter changes and tank maintenance.
Where practicable, manage stock so that older DERV is used before newer deliveries. This can help prevent old fuel and settled contamination from remaining at the bottom of the tank through repeated top-ups.
Accurate records can also reveal changes in filter life, water accumulation and fuel consumption.
Prevent water ingress
Keep fill points, caps, seals, vents and inspection covers in good condition. Inspect the tank and its associated pipework for damage and address any route through which rainwater or moisture could enter.
Businesses should also check appropriate low points for free water and arrange for it to be removed safely when found.
Removing water without identifying and correcting its source may provide only a temporary solution.
Inspect the tank and fuel system
Routine inspections can identify:
- Leaks or structural damage
- Corrosion
- Water accumulation
- Sludge or sediment
- Deteriorated seals and fittings
- Changes in fuel appearance
- Problems with filters or dispensing equipment
Repeated filter blockage may indicate contamination or degradation within the storage tank rather than a defective filter.
Monitor fuel turnover
Businesses should understand how quickly the DERV in each tank is used and replenished.
Low turnover may not be obvious where tanks receive regular partial top-ups. Monitoring volumes delivered and dispensed can help identify fuel that may have remained in the tank for an extended period.
Test DERV where appropriate
Fuel sampling and testing may be advisable where:
- DERV has been stored for an extended period
- The age or origin of the fuel is unknown
- Water has entered the tank
- Filters repeatedly become blocked
- The appearance of the fuel has changed
- The fuel supports operationally critical equipment
- Microbial contamination is suspected
- The tank has a history of contamination
- Fuel performance has become unreliable
Samples must be representative of the suspected problem. Depending on the circumstances, this may require samples from different tank levels or from water and sediment collected at a low point.
Testing can help establish whether the DERV remains suitable and what corrective action may be required.
What should businesses do if contamination is suspected?
Avoid adding a biocide, stabiliser or other treatment before the nature and extent of the problem have been assessed.
The appropriate response will depend on the condition of the DERV, the amount of water or sediment present, the construction of the tank and the fuel’s intended use.
Corrective action may include:
- Removing free water
- Replacing filters
- Filtering or polishing the fuel
- Cleaning the tank and associated pipework
- Applying an appropriate specialist treatment
- Repairing damaged tank components
- Uplifting and replacing heavily degraded fuel
Any treatment should be selected and used in accordance with its manufacturer’s instructions and applicable safety requirements.
Treating the DERV without removing accumulated water, sludge or contamination from the tank may allow the problem to return. The underlying cause—such as water ingress, poor turnover or damaged equipment—must also be addressed.
Protecting DERV in commercial storage
The presence of FAME does not mean that stored DERV will inevitably deteriorate or develop diesel bug. It does, however, make effective fuel housekeeping and water control more important.
Businesses can reduce storage related risks by:
- Matching delivery volumes to consumption
- Maintaining clean, dry and serviceable tanks
- Monitoring fuel turnover
- Checking for free water
- Inspecting filters and dispensing equipment
- Keeping accurate maintenance records
- Sampling and testing DERV when its condition is uncertain
- Investigating recurring problems instead of repeatedly treating the symptoms
If your business stores DERV for extended periods, speak to your local Oilfast depot for guidance on fuel supply, testing and storage management.
Regular checks can help protect fuel quality, reduce unplanned downtime and keep essential vehicles and equipment ready for use.
