The flash point is essentially the lowest temperature of the liquid or semi-solid at which vapors from a test portion combine with air to give a flammable mixture and “flash” when an ignition source is applied. Flash point testing is required in the petroleum industry to characterize fuels and petroleum-based products as part of quality control and to meet safety regulations concerning the transport of these products.
Basics of Pensky-Martens closed-cup flash point testing
Development and history of flash point testing
The discovery of petroleum and the increased use of flammable distillates in the 19th century for lighting and heating in place of animal and vegetable oils led to a large number of explosions and other fire-related accidents.
Legislation, such as the UK Petroleum Act in 1862 and the German Petroleum Regulations in 1882, quickly spread around the world and led to the development of many types of test instruments. The following list shows the dates when the major surviving instruments were in a form more or less recognizable today:
1870 – 1880 Abel closed cup, Pensky-Martens closed cup
1910 – 1920 Tag closed cup, Cleveland open cup
The Pensky-Martens Flash Point Tester was developed in Germany in 1870 by Mr. Martens and was based on the original tester by Mr. Pensky. Its development in 1870 was for flash points well above 100 °C to test lubricating oils and other similar products such as bitumen products.
During the last century the manual Pensky-Martens Closed Cup Tester was improved to make it a fully automatic instrument.
Fig. 2: Timeline of the development from manual to automatic instrument
Why measure the flash point using the Pensky-Martens closed cup tester?
The flash point is defined as the lowest temperature of a liquid or semi-solid at which vapors from a test portion combine with air to give a flammable mixture and then “flash” when an ignition source is applied.
The Pensky-Martens Flash Point Tester consists of a closed-cup test arrangement that contains any vapors produced and essentially simulates the situation in which a potential source of ignition is accidentally introduced into a container. For this test, a test portion is introduced into a cup and a close-fitting lid is fitted to the top of the cup. The cup and test portion are heated and stirred, apertures are then opened in the lid to allow air into the cup and also the ignition source to be dipped into the vapors to test for a flash.
The closed-cup test like the Pensky-Martens predominates in product specification and regulations due to its greater precision and its ability to detect contaminants.
Specifications of Pensky-Martens closed-cup testers
The specifications of Pensky-Martens closed-cup testers are defined in a number of ASTM and EN standards, see the table below.
| Specification | Sample Type | Standard Test Method | Alternative Test Method | Product Reference |
|---|---|---|---|---|
| ASTM D396 | Fuel Oils | ASTM D93 Pensky-Martens | ASTM D56 Tag, D3828 Small Scale, Miniflash | Pensky-Martens, Tag |
| ASTM D975 | Diesel Fuel Oil | ASTM D93, BIS IS1448 :P-21 Pensky-Martens | ASTM D56 Tag, D3828 Small Scale, Miniflash | Pensky‑Martens, Tag |
| ASTM D1655 | Aviation Turbine Fuel | ASTM D56 TAG | ASTM D93 Pensky-Martens, D3828 Small Scale,IP 170 Abel, IP 523 Rapid equilibrium closed cup , BIS IS1448 :P-20 Abel | Pensky-Martens, Tag |
| ASTM D2880 | Gas Turbine Fuel Oils | ASTM D93 Pensky-Martens | ASTM D56 TAG, D3828 Small Scale, Miniflash | Pensky-Martens, Tag |
| ASTM D3699 | Kerosine | ASTM D93 Pensky-Martens | ASTM D56 TAG, D3828 Small Scale, Miniflash | Pensky-Martens, Tag |
| ASTM D6448 | Used Lubricating Oils | ASTM D93 Pensky-Martens | ASTM D56 Tag, D3828 Small Scale | Pensky-Martens, Tag |
| ASTM D6751 | Biodiesel Fuel (B100) Blend Stock for Distillates | ASTM D93 Pensky-Martens, D3828 Small Scale, D6450 | Miniflash, D3828 Small Scale | Pensky-Martens |
| ASTM D6823 | Commercial Boiler Fuels with Used Lubricating Oils | ASTM D93 Pensky-Martens, D3828 Small Scale, D56 Tag | Pensky-Martens | |
| ASTM D7223 | Aviation Certification Fuel | ASTM D93 Pensky-Martens | ASTM D3828 Small Scale | Pensky-Martens, Tag |
| ASTM D7467 | Diesel Fuel Oil, Biodiesel Blends (B6-B20) | ASTM D93 Pensky-Martens | ASTM D56 Tag, D3828 Small Scale | Pensky-Martens, Tag |
| ASTM D7544 | Pyrolysis Liquid Biodiesel | ASTM D93 Pensky-Martens | ASTM D3828 Small Scale | Pensky-Martens |
| EN 590 | Diesel | EN ISO 2719 Pensky-Martens | BIS IS1448 :P-21 Pensky-Martens | Pensky-Martens |
| EN 14214 | FAME | EN ISO 2719 Pensky-Martens ISO 3679 Flash/no flash Rapid Equilibrium (Small Scale) | Pensky-Martens |
How to measure the flash point with the Pensky-Martens closed cup tester
The general procedure for the measurement of a flash point with the Pensky-Martens Flash Point Tester is described in the ASTM standard method D 93 (Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester).
The test method can be summarized as follows: A brass test cup of specified dimensions, filled to the inside mark with test specimen and fitted with a cover of specified dimensions, is heated and the specimen stirred at specified rates, using one of three defined procedures (A, B, or C). An ignition source is directed into the test cup at regular intervals with simultaneous interruption of the stirring until a flash is detected. The flash point is reported as defined as the lowest temperature corrected to a barometric pressure of 101.3 kPa at which application of an ignition source causes the vapors of a specimen of the sample to ignite under specified conditions of test.
ASTM standard methods and procedures
Depending on the product to be tested there are three different procedures in the ASTM standard method which are:
- Procedure A: is applicable to distillate fuels (diesel, kerosine, heating oil, turbine fuels), new lubricating oils, and other homogeneous petroleum not included in the scope of Procedure B or Procedure C.
- Procedure B: is applicable to residual fuel oils, cutback residua, used lubricating oils, mixtures of petroleum liquids with solids, petroleum liquids that tend to form a surface film under test conditions, or petroleum liquids of such kinematic viscosity that they are not uniformly heated under the stirring and heating conditions of Procedure A.
- Procedure C: is applicable to biodiesel (B100). Since a flash point of residual alcohol in biodiesel is difficult to observe by manual flash point techniques, automated apparatus with electronic flash point detection have been found suitable.
Conclusion
Flash point testing was developed in the late 1800s and has been refined and automated over the decades to produce the automatic Pensky-Martens Closed Cup Testers which are on the market today. Closed-cup testing according to Pensky-Martens is a reliable and safe way of determining the flash point of liquid and semi-solid petroleum products.
References
- https://www.astm.org/Standards/D93.htm
- Technical Report ISO/TR 29662 Petroleum products and other liquids - Guidance for flash point testing