API RP 684-2010 pdf download.API Standard Paragraphs Rotordynamic Tutorial: Lateral Critical Speeds, Unbalance Response, Stability, Train Torsionals, and Rotor Balancing.
1.3 STANDARD PARAGRAPHS In order to aid turbomachinery purchasers, the American Petroleum Institute’s Subcommittee on Mechanical Equip- ment has produced a series of specifications that define mechanical acceptance criteria for new rotating equipment. Experience accumulated by turbomachinery purchasers over the past ten years indicates that if the API standards are prop- erly applied, the user can be reasonably assured that the installed unit is fundamentally reliable and will, barring prob- lems with the installation and operator misuse, provide acceptable service over its design life. An integral component of these individual equipment spec- ifications is contained in the API Standard Paragraphs, those specifications that are generally applicable to all types of rotating equipment. The criteria associated with lateral and torsional rotordynamics and balancing have been categorized as standard paragraphs. In rotating equipment specifications published by API (for example, API Standard 617— Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical and Gas Industry Services ) there is a section on rotordynamics and balancing. The backbone of those sections are the standard paragraphs augmented by additional information that is applicable only to the type of unit considered in the standard. The Standard Paragraphs relevant to each section of the document are introduced at the end of each section. Limited comments are made to explain the individual paragraphs. Reference is made to the appropriate discussion in the tutorial to describe the background, justification, or application of the paragraph. The complete text of the Standard Paragraphs is included at the end of the document. 1.4 DEFINITIONS AND REFERENCES Definitions are incorporated into each section of the docu- ment. Due to very large number of references employed, they are identified at the end of each relevant section.
The modes of vibration are important only if there is a source of energy to excite them, like a blow to a tuning fork. The natural frequencies of rotating systems are particularly important because all rotating elements possess finite amounts of unbalance that excite the rotor at the shaft rotation frequency (synchronous frequency) and its multiples. When the synchronous rotor frequency equals the frequency of a rotor natural frequency, the system operates in a state of reso- nance, and the rotor’s response is amplified if the resonance is not critically damped. The unbalance forces in a rotating sys- tem can also excite the natural frequencies of non-rotating elements, including bearing housings, supports, foundations, piping, and the like. Although unbalance is the excitation mechanism of great- est concern in a rotordynamics analysis, unbalance is only one of many possible lateral loading mechanisms. Lateral forces can be applied to rotors by the following sources: impeller aerodynamic loadings, misaligned couplings and bearings, rubs between rotating and stationary components, and so on. A more detailed list of rotor excitation mechanisms of particular interest is found in the API Standard Paragraphs, 18.104.22.168. This subject is discussed in detail in 3.5, as well as scattered in the appropriate sections of this document. The vibration behavior of a rotor can be described with the aid of a simple physical model. Assume that a rotor-bearing system is analogous to the simple mass-spring-damper sys- tem presented in Figure 1-1.
This result is called a forced response analysis and is anal- ogous to the unbalance response analysis performed in rotor- dynamics studies. The amplitude ratio depends upon frequency of the excitation and the damping in the system. Figure 1-2 shows the amplitude ratio versus the excitation frequency. Maximum amplitude ratio is seen where the exci- tation frequency equals the natural frequency of the system. Amplitude ratio also increases as damping decreases with the amplitude becoming infinite at zero damping (a situation which is not physically practical). There is a phase difference between the excitation and response. This phase difference is a function of damping and reaches 90 degrees at the natural frequency. Figure 1-3 shows the phase angle versus excitation frequency. If a transient rather than a sinusoidal excitation excites the system, the actual response normally look like that shown in Figure 1-4.