Impeller rotation, vortex shedding, secondary flows or a combination of these phenomena can lead to the generation of acoustic waves in the compressor cascade causing dynamic pressure loading on the impeller. When the eigenfrequency and eigenmode shape of the acoustic mode match with the structural ones of the impeller, high fatigue stresses and vibrations occur, which can lead to structural failure. It is well known that cavities enclosing shrouded impellers may strongly amplify the acoustic excitation of the impeller by means of Tyler-Sofrin modes; however, little knowledge is available about the physics of flow-induced noise and resonance mechanisms. In this research, a Lattice Boltzmann Method based approach is employed to predict the origin and amplitude of pressure loading responsible for the strong impeller trailing edge vibrations measured in experiments. The results reveal that this is caused by the acoustic mode generated from the interaction of upstream vane wakes with the impeller that is reflected by the return channel vanes. This research highlights the importance of accounting for aeroacoustic mechanisms in the design of centrifugal compressor stages and paves the way towards the numerical assessment of unsteady flow and resonance phenomena.

This paper describes the results of experimental work and simulations of pressure surges resulting from fast closing valves and activation of an emergency release coupling (ERC) in ship-to-ship LNG transfer. The first part presents the results of an ambient water flow test with a fast closing 4-inch full bore valve in combination with a 4-inch flexible multi-composite hose. High amplitude pressure surges up to 4 times the working pressure occur upstream of the valve. Pressure surge amplitudes strongly depend on valve closing time and flow rate. The propagation speed in the upstream hose is lowered considerably due to the hose flexibility. Furthermore the temporarily deformation of the hose results in loss of energy and acoustic damping of the pressure surge amplitudes. The measurement results are compared to the results of modeling. Acoustic damping and propagation speed of sound show a non-linear behavior of the hose, due to a varying internal pressure, which is not captured in the present modeling techniques. The second part of the paper describes the results of a full scale flow test on an 8-inch ERC (KLAW make) for LNG ship-to-ship transfer (Figure 1). The ERC in the test is located downstream of an 8-inch 15-meter long multi-composite flexible hose. The test is intended to investigate operational and mechanical integrity, spill rate and the impact of pressure transients and shock loads on the ERC including upstream hose and downstream piping. Upon hydraulic activation the ERC separates in two parts, each provided with a fast closing butterfly valve. The complete operation takes about 0.2 s and the actual valve closure duration is about 0.02 s resulting in a negligible spill of less than 0.002 m 3 . Pressure transients from 34 to 100 bar, over 10 times the operating pressure, occur at the maximum flow rate of 1000 m 3 /hr, resulting in shock loads and high acceleration levels. The mechanical integrity remains intact even after four successive releases at the maximum flow rate of 1000 m 3 /hr. The results contribute to the certification of the ERC by Lloyds B.V. Further development in modeling is necessary to obtain a validated model for the calculation of pressure surges due to emergency shutdown in a LNG transfer.