Dynamic design and field adjustments of a large rotary lobe blower system

摘要

To withdraw the CO2 gas from the steel production process, a total of eight large vacuum pump sets have been installed at a steel plant in Saldanha, South Africa. Each vacuum pump set consists of three separate Aerzener rotary piston blowers, also named Roots blowers, which act as three-stage vacuum pumps. The disadvantage of the Roots blower is that they generate pulsations and vibrations, which can lead to fatigue failure, especially at high frequencies. For that reason TNO Science & Industry was ordered to carry out a pulsation and mechanical response analysis. The mechanical response analysis was based on the assumptions that the concrete foundation and some parts of the steel supporting structures were infinitely stiff. Modifications to the original system have been advised to meet the allowable vibration and cyclic stress levels. After commissioning, vibration problems occurred that have led to several fatigue failures of mainly the pipe system and supporting structures. To investigate the actual problems, a field survey has been carried out to determine the present vibration levels and to investigate the cause of the fatigue problems. Another aspect of the survey was to judge the safety and reliability of the system. From the vibration measurements it appeared that some parts of the concrete structure were vibrating considerably. Further more, cracks were observed on some upper parts of the concrete structures, including the anchor bolts' mountings. This was caused by an incorrect fabrication of the concrete (loose grouting parts). The consequence was that the concrete structures were not suitable to restrain the dynamic loads and the assumption of an infinitely stiff foundation was not fulfilled. The first part of the paper describes the pulsation and mechanical response analysis of the original system in the design stage. The second part focuses on the results of the field survey and the field adjustments advised for safely and reliably operation for the long term.