LCC Quick Notes, A Common Misconception in Calculating Turbine Actuator Flange Loads
At some point in the course of an electric actuator upgrade for steam turbine valves the question of secure mounting and proper bolt sizing of the actuator mounting usually arises. In these instances a common misconception of bolted flanges often surfaces. This quick note discusses the correct bolt stress analysis technique for an actuator mounting flange operating under tensile load.
Referring to the figure below, when calculating the required bolting of a flange to be subjected to a tension force "F", the sum of the flange bolt stress under prescribed torque for the bolts should provide a healthy safety factor above "F". The misconception that often is suffered in reviewing the selection is the erroneous tendency to then recalculate the stress of the bolting as the torqued stretch PLUS the distribution load of "F". This stems from viewing the bolt clamping force and the load as aligned rather than opposed.
In fact, the applied load "F" causes NO ADDITIONAL bolt stress as long as the flange gap remains zero. Think of this in light of the bolts cannot be additionally stretched without relative motion between the flange and base. Of course, no gap can occur as long as the clamping force exceeds the load.
The failure to properly understand the flange forces in this type of case can result in oversized and under-stressed flange bolting which reduces friction clamping between the surfaces which can lead to bending moments, slippage, and fatigue.
Although seemingly a basic mechanical engineering analysis, this has been improperly reviewed in enough cases to cause alarm.
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