Investigation of the American Petroleum Institute's Support Stiffness Ratio Threshold Specification
Griffin, David, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Goyne, Christopher, Department of Mechanical and Aerospace Engineering, University of Virginia
In the design phase of developing a rotating machine, it is important to consider the eﬀect that the supporting structure has on the rotordynamic behavior of the system. The American Petroleum Institute’s (API) Standard Paragraphs state that if the support stiﬀness is greater than 3.5 times the bearing oil ﬁlm stiﬀness, the designer
may omit the supports from the rotordynamic analysis. As discussed in this paper, there is concern that machines operating near the second critical speed may not be adequately modeled using the rigid support assumption allowed by API. Due to the suspected inﬂuence of support dynamics on problem machines in industry, this paper investigates the eﬀectiveness of the support to bearing stiﬀness ratio threshold of 3.5. A Jeﬀcott rotor model is used to capture the eﬀect of the support on the separation margin and ampliﬁcation factors of the ﬁrst and second critical speeds. The trends are then validated using numerical models of two case studies. Results show that the API support stiﬀness ratio of 3.5 has limitations as a rigid support threshold and should be used with caution when the second critical speed is near a separation margin boundary. The paper proposes an addendum to the current API speciﬁcation intended to reduce the risk of separation margin encroachment due to a rigid support assumption. Unlike other papers discussing the inﬂuence of ﬂexible supports on rotating machinery, this investigation studies the eﬀects at API’s support stiﬀness ratio of 3.5 and compares them to a rigid support system.
MS (Master of Science)
API standard, stiffness ratio, flexible support, rotordynamics