BURST & CONTAINMENT: ENSURING TURBOCHARGER SAFETY
“What would happen if one of the wheels in my turbo were to burst at speed? Would the housings be strong and resilient enough to contain all the pieces?”
Garrett – Advancing Motion says in motorsports, the likelihood of a compressor or turbine wheel coming apart can become very real, very quickly. As the limits of today’s turbos are reached, they are subjected to higher speeds, hotter temperatures and severe duty cycles.
New wheels, housings and complete turbos are constantly being developed to outperform the previous generations. The temptation can be great to ignore safety and go for broke with the lightest wheels and thinnest housings possible, to get out every last bit of performance from the turbo and the race vehicle.
But if a turbo manufacturer focuses on performance only and does not qualify their products to contain both compressor and turbine wheel bursts, then there is no guarantee that the high-velocity wheel fragments will remain inside the housings if the turbo is intentionally or accidentally pushed past its safe operating limits.
Two major types of wheel burst: blade and hub
A blade burst occurs when the centrifugal force at speed acts to pull the blades off the central hub and overcomes the mechanical strength of the root sections connecting the individual blades to the hub. Under these conditions if a blade root is too weak it could leave the hub as easily as a petal is plucked from a flower.
Hub burst, on the other hand, is the extreme case wherein the main hub that the blades are attached to reaches its ultimate strength limit and breaks into two, three or more large pieces through the centreline of the wheel. During a Garrett burst containment housing qualification test the hub is intentionally weakened to cause a worst-case scenario burst. Even though a hub burst is a rare occurrence, Garrett explains it qualifies its aftermarket and OE production turbochargers for the containment of a worst-case-scenario wheel burst.
What is containment?
During a wheel burst, whether turbine or compressor, blade or hub, the housing the wheel spins in is the main barrier between the flying debris and the outside world. Containment is the ability of a turbine or compressor housing to successfully absorb the ballistic energy released during a burst and radially contain the fragments within the confines of the housing. The housing should also contain damage caused by loose wheels that have not burst, such as if the central shaft is overloaded and breaks or if the weld joint between the turbine and shaft fails.
Garrett Burst and Containment Qualification and Testing
Every new Garrett aftermarket and original equipment turbocharger is qualified to contain compressor and turbine wheel bursts. But how is this verified? Garrett explains: when a new wheel is designed, testing of unmodified parts is conducted to determine the “natural” burst speed of the wheel. Turbochargers are built and instrumented with speed and temperature sensors and installed in a specially constructed containment stand that supplies hot compressed air to the turbine. The turbocharger is brought up to speed and temperature for a dwell period, and then intentionally and immediately sped up well beyond the qualified speed limits. Turbo shaft speed is recorded and a successful test will empirically reveal the natural burst speed.
When a new Garrett housing is developed, it is qualified in conjunction with the specific compressor or turbine wheel running inside of it. New combinations of existing wheels and housings are also qualified. If the housing in question is a compressor housing, the back plate is included as part of the qualified combination since it forms one wall of the diffuser (throat) and wheel fragments pass through this channel on their way to the containment shroud.
For completing a housing qualification, the first step is a detailed analysis of the wheel and housing to calculate the target burst speed, which is set at a specific percentage of the wheel’s inherent natural burst speed. The burst energy at this speed is determined and if the result is higher than previously qualified for the housing family, a physical test must be conducted.
Whether a compressor or turbine housing test, the wheel is prepared in order to force a worst-case-scenario hub burst and do the most damage possible to the housing. The test is carried out in the same dedicated burst test cell and the turbocharger is again instrumented with speed and temperature sensors. After a dwell period, the speed is ramped up and the burst is almost instantaneous. The turbo is allowed to cool down and then examined.
A “verification shroud” made from thin aluminium sheet metal surrounds the housing during the test as a witness to the event. If this shroud is pierced in any way, the housing has failed the test and is not qualified. The housing will also fail if there is any separation from the turbocharger at the attachment joint or if there are any holes or material missing from the containment band area.
If the housing burst containment test was successful, the turbocharger is still not necessarily qualified as metallurgical analysis is required to verify material specifications after a successful test to show that the actual housing and compressor back plate (if applicable) that survived the burst were indeed made from the exact material alloys that were specified by engineering.
Turbochargers are generally very reliable, but even the highest quality turbos may fail when subjected to excessive abuse. A wheel burst is the extreme example of turbo failure but is a contingency that must be planned for with utmost care.
Using a non-burst containment qualified turbocharger presents a very real risk to the end user and their surroundings, especially in racing situations. Garrett performance aftermarket, replacement aftermarket and original equipment turbochargers are all qualified to successfully contain a wheel burst, as long as the specific combination of Garrett wheels and housings intended for the turbo remains unaltered. Using a genuine Garrett turbocharger for highway, street or racing use ensures a catastrophic wheel burst will be successfully contained by a high-quality housing that was designed and qualified from the ground up to do so, says Garrett.