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 (6.5 m diameter) and heat-exchangers resulted in leakages and subsequent shut downs 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 50 m/s, which is rather high considering the speed of sound in the gas of 195 m/s. Furthermore separation of the flow from the pipewall occurs at the outlets of a flashing vessel, which resulted in estimated flow velocities of over 100 m/s in the vena contracta.
This paper provides an overview of the analysis done by a one-dimensional acoustical and a mechanical finite element model.
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 from 70 to 36%.
The modification with a perforation ratio of 50% 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 in October 1996 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.