Practical design processes

Periodic blade-pass pressure pulsation excitations from the suction and discharge of centrifugal compressors can cause acoustic resonance conditions and associated pipe vibrations upstream and downstream of the compressor. These high frequency pressure excitations are usually created by the translation of non-uniform jet-wake meridional flow of the first or last stage compressor impeller blade passages from the rotating impeller to the stationary suction/discharge piping frame. The excitation frequency of these blade-pass pressure pulses, relative to the stationary frame, is determined by the speed of the compressor multiplied by the number of impeller blades. The potential of these types of vibrations to cause structural damage to the pipe and pipe small bore connections has been well documented.

Although both Helmholtz resonators and quarter wave absorbers are effective in reducing first order singular frequency pressure pulsations, they cannot remove or attenuate higher orders of these periodic excitations. They also effectively shift first order excitations to the second order, and if not carefully designed, can actually create damaging higher frequency acoustic resonances in the upstream/downstream piping. However, a series of two or more progressively placed quarter wave resonators, called a quarter wave comb filter, can be designed to shift excitation frequencies multiple orders higher, such that their pulses superimpose to form a continuous steady waveform.

Case study and parametric optimisation

This case study discusses the practical design of such a comb filter, its installation in the downstream piping near the discharge of the compressor, basic comb filter design guidelines, a step-by-step design process for comb filters, and quantification of its attenuation effectiveness. Although the case study is based on a multi-stage compressor, only pulsations from the last stage in the compressor were analysed and attenuated. Also, the compressor used a vaneless diffuser such that only blade-pass frequency pulsations at the running speed were of relevance.

Operating conditions:

• Speed: 7000 rpm (116 Hz).
• Impeller tip diameter/no. of blades: 10 in. (25.4 cm) 16 blades (no splitters).
• Process gas: natural gas (SG=0.62).
• Discharge pressure/temperature: 80 bara/40°C.
• Discharge pipe diameter: 9 in. (20.3 cm).

Based on these operating conditions, the primary fixed speed blade-pass excitation frequency is 1867 Hz, and the predicted periodic excitations at the compressor discharge are 0.2 bar. This excitation frequency coincides closely to an acoustic two-pipe diameter radial mode in the discharge pipe at 1875 Hz, and a radial pipe resonance would be expected. The excitation pressure trace was conservatively assumed to be a square wave to make sure the comb filter design properly attenuates higher order frequencies …

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Read the article online at: https://www.worldpipelines.com/equipment-and-safety/13032023/practical-design-processes/