Thermal Oil Testing – Differentiating between Open and Closed Cup Methods

Heat transfer systems are used in many applications where the thermal fluid is re-circulated by a pump through a heater so the fluid is heated to a specific temperature range. The system then transfers the heat, typically by radiating it, to perform many applications such as; supplying heat for plastic or rubber molding, petrochemical processes, pulp and paper applications and curing of finished board product to name a few.

Heat transfer systems are often large and mostly ignored systems when talking lubrication in plants. However, they are critical systems in the operations. The nature of their operation makes them subject to severe conditions and lack of maintenance can lead to catastrophic results. With this in mind, determining if the oil is still good for operation requires testing that is more complex and demanding than normal automated lab testing.

Two major tests are the ASTM D92 Cleveland Open Cup (COC) Method and the ASTM 93 Pensky-Martens Closed Cup (PMCC) test. These tests evaluate the fluids flash point using two different methods which often causes confusion between reported value, product data sheet value and customer limits (if applicable). In the ASTM D92 Cleveland Open Cup Method, the oil is heated at a rate of approximately 10°F per minute in an open cup while a flame is passed over the cup every 5 minutes until a flash occurs. In the ASTM D93 Pensky-Martens Closed Cup test, the oil is placed in a closed cup which is opened every 2°F up to 220°F after which it is opened every 5°F. When the cup is opened a flame is passed over the cup. If no flash occurs the cup is closed and the test continues until a flash occurs.

Both methods provide valuable information but significant variation between flash points reported by the two tests is possible. Variances up to 50 - 100°F can occur as the closed cup method can retain higher light end hydrocarbon concentrations that accumulate, resulting in lower temperature flash. Understanding the difference in the test methods and what they report can be a significant factor in determining when the oil is reaching a point where continued use could be detrimental to operational safety.