How To Build A Fuel Quality Program For A Data Center Generator Fleet

The monthly generator exercise went fine. The annual fuel test eleven months ago showed that everything was within specification. Then a utility failure forces the data center onto backup power. One of the generators begins running erratically before shutting down because of low fuel pressure. The problem? A fuel filter clogged with microbial sludge.

The generator itself wasn't the problem. The fuel was.

This is the kind of failure that routine generator maintenance alone may not prevent. Even a well-maintained generator can fail when the fuel supplying it has deteriorated.

According to Uptime Institute's 2025 Annual Outage Analysis, power-related problems remain a leading cause of significant data center outages. For facilities that depend on multiple standby generators, maintaining fuel quality is an essential part of ensuring backup power reliability.

Here's how to build a fuel quality management program that addresses the risks associated with long-term diesel storage.

What Does a Data Center Fuel Quality Program Include?

An effective fuel quality program includes five components:

1. Baseline fuel testing: Establish the condition of the fuel in every bulk storage tank and generator day tank.
2. Preventive fuel treatment: Use appropriate fuel stabilizers and other treatments to help protect stored fuel from deterioration.
3. Scheduled monitoring: Establish regular testing intervals to identify developing problems before they threaten generator reliability.
4. Defined action thresholds: Determine which test results require additional testing, treatment, filtration, or fuel replacement.
5. Comprehensive documentation: Maintain records of fuel condition, treatments, corrective actions, and testing history for every tank.

Many facilities already perform some of these activities. The problem is that individual maintenance activities don't necessarily add up to a comprehensive fuel quality program.

Why Stored Diesel Fuel Needs More Attention

Modern ultra-low sulfur diesel (ULSD) presents challenges for long-term storage. Changes in fuel composition and refining practices, combined with storage conditions, can influence oxidation stability, water contamination, and susceptibility to microbial growth.

Water is particularly important because it creates the conditions that allow bacteria and fungi to grow in stored fuel. Microbial contamination can produce sludge, accelerate corrosion, and plug filters.

Oxidation presents another concern. As diesel fuel ages, oxidation reactions can produce gums, sediments, and other insoluble materials that interfere with fuel system operation.

Under favorable storage conditions, untreated diesel may remain usable for a year or longer. However, its actual storage life depends on the fuel's initial quality, temperature, exposure to water, and other factors. Proper stabilization and ongoing fuel maintenance can significantly extend its useful life.

Consider a data center with a 12,000-gallon generator fuel tank. If the generator operates primarily during monthly exercise periods, much of that fuel could remain in storage for years.

The question isn't simply whether the generator will start today. It's whether the fuel will still perform properly when the facility needs it months or years from now.

NFPA 110 Establishes a Minimum, Not a Complete Program

NFPA 110, the Standard for Emergency and Standby Power Systems, establishes requirements for maintaining emergency power systems, including fuel quality testing. Applicable editions require diesel fuel to be tested at least annually using appropriate ASTM methods.

But annual testing alone provides only a snapshot of fuel condition. Water can enter a tank, microbial contamination can develop, and fuel can deteriorate between annual tests. A comprehensive fuel quality program builds on the applicable NFPA 110 requirements by adding preventive treatment, more frequent monitoring, and clearly defined corrective actions.

What Should You Test Before Treating Anything?

Before establishing a treatment program, you need to know the condition of the fuel already in storage. Start by collecting representative samples from every bulk storage tank and generator day tank. Include bottom samples where water and sediment are most likely to accumulate.

The initial testing should establish the fuel's physical properties, identify contamination, and determine whether corrective treatment or remediation is necessary.

Establish a Baseline Using ASTM Fuel Testing

Bell FTS offers a Mission Critical fuel testing package that includes several ASTM test methods:

• ASTM D86: Distillation characteristics
• ASTM D93: Flash point
• ASTM D130: Copper strip corrosion
• ASTM D287 or D4052: API gravity or density
• ASTM D976: Calculated cetane index
• ASTM D2709: Water and sediment
• ASTM D6304: Water content by Karl Fischer titration
• ASTM D2622: Sulfur content

These tests help determine whether the fuel meets applicable specifications and identify conditions that could compromise generator performance.

Several results deserve particular attention.

Water and sediment: ASTM D2709 measures water and sediment in diesel fuel. The ASTM D975 specification establishes a maximum of 0.05% by volume for this measurement.

Water content: ASTM D6304 provides a more sensitive measurement of water content using Karl Fischer titration. Bell FTS uses 200 ppm as an operational threshold for further investigation or corrective action. This is not an ASTM D975 specification limit, and a result above 200 ppm does not necessarily establish that free water is present.

Sulfur content: ASTM D2622 determines sulfur concentration. Diesel classified as S15 ULSD must contain no more than 15 ppm sulfur. Maintaining the proper sulfur specification is particularly important for engines equipped with emissions aftertreatment systems.

For fuel expected to remain in storage for extended periods, additional testing such as ASTM D2274 can help evaluate oxidation stability and the potential formation of insoluble materials.

Include Microbial Testing

Conventional ASTM fuel quality testing doesn't necessarily reveal microbial contamination. That's why a baseline assessment should also include microbial testing.

ATP testing measures adenosine triphosphate, a biological indicator associated with microbial activity. It provides a relatively fast way to evaluate contamination and establish a numerical baseline that can be monitored over time.

In Bell FTS's experience, ATP testing typically costs approximately $50 to $100 per sample, compared with substantially higher costs for more extensive laboratory microbiological analysis.

ATP testing is particularly useful for routine monitoring because changes in results can indicate developing contamination problems. Laboratory culture testing can provide additional information when organism identification or more detailed microbiological investigation is needed.

Sample from the Right Locations

Where you collect a fuel sample matters almost as much as what you test. Water, sediment, and microbial biomass tend to accumulate near the bottom of storage tanks. A sample collected near the top of the fuel may appear clean even when significant contamination exists below.

A Bacon Bomb sampler or equivalent bottom-sampling device can help collect fuel from near the tank floor, where contamination is most likely to accumulate.

Day tanks should also be sampled separately from bulk storage tanks. They may experience different temperatures, turnover rates, and contamination conditions. The objective is to understand the condition of the entire fuel storage and delivery system, not simply the fuel at one convenient sampling point.

How Often Should Data Center Generator Fuel Be Tested?

For mission-critical facilities, we recommend establishing a regular monitoring schedule that goes beyond annual compliance testing. A practical program includes monthly visual and water checks, quarterly microbial testing, and annual comprehensive ASTM testing.

Monthly Fuel Condition Checks

Monthly generator exercise periods provide an opportunity to inspect fuel condition. Depending on tank design and available sampling equipment, maintenance personnel can:

• Inspect a representative fuel sample for cloudiness, discoloration, or visible sediment.
• Check for accumulated water at accessible tank low points.
• Monitor fuel filter restriction indicators.
• Record changes that could indicate developing contamination.

Visible water, cloudy fuel, or an unexplained increase in filter restriction should trigger further investigation rather than waiting for the next scheduled laboratory test.

Quarterly Microbial Testing

Quarterly ATP testing provides a useful balance between monitoring frequency and cost. Microbial growth can accelerate when water is present, particularly in warm storage environments. By comparing ATP results over time, facilities can identify increasing microbial activity before contamination becomes severe.

A single test tells you what conditions look like today. Repeated testing helps reveal whether those conditions are improving or deteriorating. 

Bell FTS offers different monitoring frequencies through its Fuel Secure program:

• Basic: ATP testing twice annually
• Advanced: ATP testing quarterly
• Premium: ATP testing six times annually

Facilities with a history of microbial contamination, persistent water problems, or unusually long fuel storage periods may benefit from more frequent testing.

Annual Comprehensive Testing

At least once annually, perform the fuel quality testing required by the applicable edition of NFPA 110. A more comprehensive annual evaluation can include the full Mission Critical testing package, microbial testing, and an assessment of tank condition. Internal tank inspections should be scheduled based on tank condition, accessibility, applicable requirements, and the facility's maintenance procedures.

When Additional Testing Is Necessary

Scheduled testing shouldn't prevent a facility from responding to changing conditions.

Additional testing may be appropriate following:

• A new fuel delivery, particularly when fuel quality documentation is incomplete or contamination is suspected.
• Flooding, storm damage, or suspected water intrusion.
• Unexpectedly frequent fuel filter replacement.
• An increase in microbial activity.
• Fuel transfer problems or unexplained generator performance issues.

Facilities in hurricane-prone regions should also review fuel inventories and fuel quality before storm season. Maintaining adequate fuel reserves is essential, but maximum fill levels must remain consistent with tank designations.

Defined Action Thresholds

Establishing these thresholds in advance eliminates much of the uncertainty that can arise when a laboratory report identifies a potential problem. It also gives facilities managers a documented basis for authorizing corrective work before fuel contamination causes an equipment failure.

The objective is to respond to developing problems while they are still manageable, rather than waiting until contaminated fuel interferes with generator operation.

How Do You Document a Fuel Quality Program for Auditors and Insurers?

A fuel quality program is only as effective as the records supporting it. For every storage tank, maintain a record that includes:

• Tank identification and capacity
• Sampling dates and locations
• Laboratory test results
• Fuel treatment products, dosage rates, and quantities treated
• Fuel polishing and tank cleaning activities
• Corrective actions and follow-up testing
• Scheduled future inspections and testing

These records provide a documented history of fuel condition and the steps taken to maintain it. They can also support compliance reviews, insurance evaluations, and internal maintenance planning.

Track Fuel Quality Trends Over Time

A spreadsheet or electronic maintenance system can provide an effective way to manage fuel quality records. The important thing is to preserve the history of each tank rather than treating every laboratory report as an isolated event.

For example, plotting Karl Fischer water content and ATP microbial results over time can reveal changes that might not be obvious from individual reports. A tank showing steadily increasing water content may have a developing condensation or water intrusion problem. Rising microbial activity may indicate that contamination is becoming established.

Maintaining fuel delivery records and bills of lading can also help trace fuel quality problems to specific deliveries. When samples are submitted to an outside laboratory, documenting sampling procedures and chain of custody provides additional confidence in the results.

These records demonstrate that fuel quality is being actively managed rather than simply tested once a year.

The Cost of Prevention Versus Remediation

Preventive fuel maintenance also makes financial sense. In Bell FTS's service experience, routine testing may cost approximately $500 to $1,500 per tank annually, with preventive chemical treatment adding another $200 to $500. That places the estimated annual cost of a preventive program at approximately $700 to $2,000 per tank, depending on the scope of testing and treatment.

By comparison, correcting an established fuel contamination problem can be substantially more expensive.

Emergency fuel polishing may cost $2,000 to $5,000, while extensive tank cleaning can run $5,000 to $15,000 or more. If the fuel must be replaced, additional costs include fuel removal, disposal, replacement fuel, and associated labor. Those expenses still don't account for the potential consequences of a generator failure during a utility outage.

For a mission-critical data center, the financial consequences of an extended power interruption can greatly exceed the cost of maintaining the fuel. Preventive maintenance provides a predictable annual expense while reducing the likelihood of much larger, unexpected remediation costs.

Fuel Secure: A Structured Approach to Fuel Quality Management

Bell FTS offers its Fuel Secure program to help facilities establish and maintain a consistent fuel quality monitoring schedule. FUel Secure combines comprehensive fuel testing, ATP microbial monitoring, sampling equipment, and reporting designed to support long-term fuel quality management.

It offers different service levels that allow facilities to select monitoring frequencies based on their operational requirements and contamination risks.

Fuel Secure helps data center operators move beyond isolated annual tests toward a documented program that tracks fuel condition over time and identifies problems before they compromise generator reliability.

The following treatment reference summarizes the Bell Performance products and services that may be incorporated into a stored diesel fuel maintenance program.

Treatment reference for stored generator diesel

Start Your Fuel Quality Program with a Baseline Test

If your data center's fuel maintenance records consist primarily of annual laboratory reports and generator filter changes, the best place to start is with a comprehensive baseline evaluation.

Collect representative samples from the facility's bulk storage tanks and day tanks, then test them for critical fuel properties, water contamination, and microbial activity. Those results establish the condition of the fuel today and provide the information needed to develop an appropriate treatment and monitoring schedule.

Bell FTS can help facilities establish that baseline through its Mission Critical testing package and maintain ongoing monitoring through the Fuel Secure program. The goal is straightforward: make sure that when your data center needs backup power, the fuel supplying its generators is ready to perform.