Honeycomb Seal Effect on Rotor Response to Unbalance

Abstract

High pressure centrifugal compressors are often equipped with honeycomb seal on balance drum in order to optimize rotordynamic stability. In very high pressures applications (>200 bar) the direct stiffness and damping of the honeycomb seal may reach the same order of magnitude of the journal bearings, thus altering the peak frequency and amplification factor of rotor critical speeds as well as their mode shapes. This phenomenon is ultimately due to the density and viscosity of the gas leakage flowing through the seal, and it has a substantial effect on the rotordynamic behavior of the compressor. According to current standards, aerodynamic seal effects are not necessarily included in the calculation of rotor response to unbalance. For high pressure compressors equipped with a honeycomb seal, the associated aerodynamic effects may have major impacts on rotor critical speeds in terms of frequency, amplitude and amplification factor. A procedure for the calculation of rotor response in loaded condition is here proposed, aiming to improve the predictability of the rotordynamic analysis and to provide practical criteria for the evaluation of the outcome. A back-to-back compressor with final discharge pressure of 386 bar is presented as case study; it was tested at full pressure at Authors’ Company facilities in 2013. In this case the stiffening effect of the honeycomb seal is particularly relevant, since it is positioned close to rotor midspan. Test measures show that in loaded condition the 1st critical speed shifts upwards by several thousand rpm, eventually exceeding the Maximum Critical Speed and even the Trip Speed of the compressor.