Pulsation Acoustics

Paper No. 115

During commissioning of a large FPSO, a dry screw compressor in a flash gas service experienced significant failures related to high axial vibration at 2x rotor speed. There were also excessive pulsation and vibration levels measured at the compressor, discharge silencer and downstream piping at 4x rotor speed (“pocket pass frequency” or PPF). Eventually, fatigue failures occurred within the compressor and in the downstream systems. This paper describes the collaborative effort amongst the designer/operator, user company, equipment OEM and consultants to determine the root cause(s) as well as design and implement solutions.

The remarkable discovery was that the axial rotordynamics and the 3-dimensional acoustics were actually interrelated. Non-linear, time transient axial dynamics simulations were needed as well as advanced 3D acoustic analyses of the compressor discharge system that are well beyond typical analyses performed for API 619 machines. These analyses, along with extensive field measurements, led to the eventual solution. Comprehensive factory testing was a key factor in proving the solution before implementing the fix offshore. Both rotordynamics and acoustical modifications were required to ensure successful, long-term operation of the dry screw compressor.

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