One of the largest Air-cooled steam condensers in our plant having 16 bays & 32 fans of 4.8m diameter, was observed with high vibrations beyond 100 mm/s, resulting in equipment integrity & HSE concerns. Most importantly, 250 MT/hr of steam has to be vented to atmosphere, if the air-cooler is down. This paper presents the alternate approach adopted which resolved vibration issue, enhanced availability & improved steam system efficiency, with simple but robust solution.
Performed meticulous measurement of Aircooler vibration spectrum, which includes frequency, amplitude, velocity & structural natural frequency. As motor solo-run test confirmed no inherent vibration originating from motor, analysis was imperative to find the root cause. Hence, developed ANSYS model of existing Aircooler to simulate different modes & corresponding frequencies. Analysis confirmed that high vibration was a result of resonance between structure natural frequency and motor/blade pass frequency.
The results from detailed study and analysis recommended that in order to eliminate the resonance criteria, number of fan blades has to be changed from 11 to 12 and enormous amount of additional structures / bay to be added to shift the structural frequency away from its natural frequency. Total estimated cost of execution for this option is ~1 million USD for all 16 bays and require aircooler shutdown.
Instead of the above ceremonial solution, a simple and easy to implement, analytical solution was recommended to locally stiffen the motor & gearbox support plates. This would minimize the vibration amplitude, reduce fatigue loading and enhance structural rigidity & integrity. The cost of this option is negligible as the modification does not involve major changes and can be executed with in-house resources. Primarily, this option can be executed without the need for Aircooler shutdown.
The alternate solution has been implemented and the vibration levels have reduced considerably, ensuring aircooler Integrity. Most importantly, this approach has improved aircooler availability and avoided venting of 250T/hr of LP steam, costing ~24K USD/day.
Vibrations in aircoolers not only causes HSE & integrity concerns but their unavailability leads to steam letting off. The detailed analysis and robust solution has not only improved integrity, but also avoided huge structural modification & associated cost, improved availability, eliminated steam letting down and thus enhancing steam system efficiency.