The diesel flash point is the minimum temperature at which its vapors can form a flammable mixture with air under specific test conditions.
For No. 2-D S15 (ULSD) diesel, ASTM D975 establishes a minimum flash point of 52 °C (approximately 126 °F), determined using ASTM D93.
What is the diesel flash point and why it matters for your operation
The flash point indicates the temperature at which a product can form a flammable mixture with air under specific test conditions. ASTM D93 uses the Pensky-Martens closed-cup method to determine this property in petroleum products.
For companies that use diesel in generators, fleets, machinery, or storage systems, this parameter provides information that can complement fuel quality monitoring.
Flash point and facility safety
A flash point lower than specified may be a sign that the fuel contains more volatile components than expected. ASTM D93 includes this test, among other uses, to detect possible contamination of relatively non-volatile materials with volatile or flammable substances.
Therefore, a result outside the specification must be investigated before using or accepting the fuel as compliant with applicable requirements. The flash point is a relevant parameter, but it should not be used as the sole measure to determine a facility’s fire risk.
Flash point and engine damage
The flash point is not a direct measure of engine wear or performance. However, an abnormally low result may indicate contamination or the presence of components that are more volatile than expected.
If a deviation is detected, it is advisable to review the result along with other quality parameters and the corresponding specifications before using the fuel.
Requirements for the diesel fuel your company receives: ASTM D93 standard
| Parameter | ASTM D975 (ULSD) Specification | Practical Implications of Non-compliance |
| Flash point (ASTM D93) | Minimum 125.6 °F (52 °C) | A lower value may indicate the presence of more volatile components or possible contamination and requires investigation. |
| Sulfur content | Maximum 15 ppm for S15 | Compliance with this limit is important for equipment and applications subject to specific fuel and emissions requirements. |
| Cetane number | Minimum 40, ASTM D613 | This parameter relates to the fuel’s ignition characteristics. |
| Cetane Index | Minimum 40 where applicable, per ASTM D976/D4737 | This is a calculated value used to estimate certain ignition characteristics of the fuel and must be interpreted according to applicable requirements. |
| Kinematic viscosity (40 °C) | 1.9–4.1 mm²/s, ASTM D445 | A value outside the specification may require further evaluation before using the fuel. |
| Cloud point | According to applicable operational requirements | Its relevance depends on temperature conditions and the application. |
| Water and Sediment | Maximum 0.05% by volumen | Their presence can contribute to storage, filtration, and corrosion problems, among others. |
How to verify the quality of the diesel fuel your company receives
- Request the Certificate of Analysis (CoA). This allows you to review the results of the fuel analyses and compare them with the relevant specifications.
- Conduct a visual inspection upon receipt. An unusual appearance or the visible presence of water or sediment may warrant further evaluation.
- Request a laboratory analysis when it is necessary to verify the flash point. ASTM D93 establishes the test method for determining this property. A field test should not automatically be considered equivalent to the ASTM method.
- Verify the sulfur content when applicable. This is especially important for equipment subject to specific fuel and emissions control requirements.
- Keep a record of deliveries. Record information such as the date, volume, and analysis results to facilitate tracking of the fuel used in your operation.
American Petroleum, a supplier of certified ULSD diesel with complete documentation
For companies that rely on diesel, having information about the fuel’s characteristics makes it easier to track supplies and manage their operations.
American Petroleum offers ULSD diesel with analysis documentation associated with the fuel supplied. Customers can access this information as part of their record-keeping and monitoring of the analyzed parameters.
Additionally, interested customers can request to receive the CoA monthly via info@americanpetroleumpr.com. American Petroleum can set up automated delivery to facilitate access to the most recent documentation.
To learn more about our ULSD diesel supply options or the analysis documentation available for your company, email us at info@americanpetroleumpr.com.
Frequently Asked Questions about flash point and diesel quality
What should be the minimum flash point for ULSD diesel?
For S15 No. 2-D diesel (ULSD), ASTM D975 specifies a minimum flash point of 52 °C (125.6 °F), determined using the ASTM D93 method.
Why might the diesel flash point decrease?
An abnormally low flash point may be related to the presence of more volatile components or fuel contamination. Therefore, a result that falls outside the specification must be investigated to determine its cause.
What is a Certificate of Analysis (CoA), and why should I request one?
The Certificate of Analysis (CoA) presents the results of analyses performed on a sample or product. It allows you to review the analyzed parameters and compare them with the fuel’s specifications.
Can using diesel fuel that is out of specification void my generator’s warranty?
This may depend on the specific terms of the warranty and the manufacturer’s recommendations. If out-of-specification fuel is identified, review the generator’s manual and warranty terms, and keep the results of the analyses performed.
Technical sources:
• ASTM International. ASTM D93, Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester. ASTM International.
• ASTM International. ASTM D975, Standard Specification for Diesel Fuel Oils. ASTM International.
• U.S. Environmental Protection Agency. Diesel Fuel Standards and Rulemakings. U.S. EPA.