Mistuning of steam turbine blade packets

There are many steam turbine rotor stages with blades connected together for added reliability, making them much more difficult to excite from alternating forces such as upstream vane wakes and potential flow effects such as partial admission. Procedures are described to be sure a "safe" design considers variations giving mistuned resonant response factors.

Compressor and turbine cantilevered blade mistuning effects on response are described in numerous publications. Mistuning can help in reducing response from flutter (negative aerodynamic damping), but can increase response by a large amount for forced response at resonance. Figures below from Reference 1 shows how mistuning affects response at a resonant frequency of a responsive blade mode giving increased individual blade response near the same frequency. For the design shown the tuned 3-diameter mode would not respond from forces if resonant at six times speed, but the mistuned blades would respond.

 

Tuned Blades with Three-Diameter Mode Shape

Mode Shape with Blade Mistuning

For steam turbines the technique of packeting to intentionally have some blades out-of-phase with excitation forces has been used for decades, as explained in Weaver & Prohl, references 2 & 3. By choosing numbers of blades and vanes per circle along with number of blades per packet phase cancellation of forces can be optimized. This is especially important if there is resonance of fundamental modes that are much more excitable than modes at higher frequencies. However blade packet out-of-phase modes can sometimes cause failures, especially if endurance limit is reduced due to corrosion. Resonant response factor calculation is also summarized in the author’s 2004 tutorial, reference 4. When blade packet mode natural frequency calculations are made using finite element programs for a complete blade disk it is assumed that all blades and shrouds are identical. Thus there are system modes that are in diametral patterns. Some thus have claimed fundamental modes of packeted blade assemblies will not respond as long as the excitation harmonic is above a certain value; e.g. see Reference 5.

This review of mistuning will be for a typical short blade in the front end of a steam turbine, with excitation from upstream nozzle passing frequency. Figure shows FEA (finite element analysis) result for 120 identical blades attached to a rigid disk, shrouded so that there are 24, 5-bladed packets.