One of the critical tests in certifying the use of Volute tanks in accordance with pressure vessel regulations is demonstrating safety in a localized fire. Composite pressure vessels typically have a maximum safe operating temperature of 85 C; if they exceed this they could rupture catastrophically. Hence, they are required to be installed with thermal relief end valves that automatically vent the contents when the ambient temperature exceeds 110 C, which occurs rapidly during a fire. However, this still does not eliminate the potentially dangerous scenario in which a localized fire remote from the valve can go undetected and cause tank rupture. Volute formulated a cost-effective strategy using simulations combined with test data to safely conduct and pass the localized fire test of a pressurized tank in a single attempt.
Volute’s strategy of using simulations combined with test data consisted of a two-step approach:
- Creating a model of an identical, unpressurized tank in a localized fire. They first carried out a “dry run” of the fire test using an identical, unpressurized tank instrumented with thermocouples and heat flux sensors — a safe and inexpensive test. One set of data extracted from this test was used to calibrate heat transfer coefficients used in a transient thermal simulation of the same test. Another set of data was used for model validation.
- Predicting a safe duration for fire exposure of a pressurized tank using the virtual tank developed in step 1. This enabled Volute to assess the risk of a pressurized tank in a fire test (an expensive test to be carried out at a special facility requiring numerous safety precautions) in a cost-effective manner while safely incorporating the thermal mass of 70 MPa pressurized gas. Volute also used simulations to test system design strategies such as use of insulation and the use of a conductive wire as a remote temperature signal mechanism.
The simulation predicted that the safe temperature limit would be reached on the tank 8 minutes after the start of the test. The fire test of a pressurized tank was performed with thermocouples mounted at critical locations identified through simulation, and a switch was installed to trigger venting of the tank in 8 minutes, or when the safe temperature limit was reached at any of those thermocouples.
By developing and validating this simulation, Volute was able to:
- Perform a normally destructive and dangerous test in a safe and controlled manner.
- Use inexpensive field tests to develop a high-fidelity digital twin of an expensive test for validating future design changes.
- Virtually perform the fire test on a full-scale tank (> 50 chambers, which would be even more expensive to test), even though the prototypes have only six chambers.
- Develop the framework to test feasibility of fire protection concepts such as insulation, coatings and remote triggers through simulations instead of expensive field tests.