Elimination of flow-induced pulsations and vibrations in a process installation - A combination of on site measurements, calculations and scale modeling

摘要

The aim of the work described in this paper was to trace and eliminate vibration sources in a low pressure system with high flow velocities. Considerable vibration on the pipe system between a flashing vessel (6.5m diameter) and heat-exchangers resulted in fatigue failure, leakage and subsequent shut down of a chemical production plant. As compressors or pumps were not directly involved in the process, we considered aero-acoustic pulsations as the main cause of the high vibration levels. Analysis of the flow in the pipe system showed that mean flow velocities could rise up to 50m/s, which is rather high considering the speed of sound in the gas of 195ms/s. Furthermore separation of the flow from the pipe wall occurs at the outlets of a flashing vessel, which resulted in estimated flow velocities of over 100m/s in the vena contracta. This paper provides an overview of the analysis done by a one-dimensional acoustical model and scale modeling of part of the piping. The recommended modification consists of a perforated pipe section between the outlets of the vessel and has been tested in a scale model. Tests have been performed with airflow at identical Mach numbers for different configurations with perforation ratios form 70 to 36%. The modification with a perforation ratio of 70% shows a reduction of a factor 5 of the pressure pulsation and vibration levels over the entire frequency range. The net pressure loss for this configuration is similar to that in the original lay-out without the perforated tube. This modification was implemented during a plant shutdown and proved to be very effective. The results are well in line with the predicted levels and on the basis of this success a similar design has been planned for two new installations.