Parallel operation of centrifugal blowers

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Our company’s wastewater treatment system is equipped with three multi-stage centrifugal blowers; usually two are in use while one serves as a backup. The flow rate is 150 cubic meters per minute, and the outlet pressure is 69.6 kPa. Due to insufficient dissolved oxygen, another unit with a flow rate of 80 cubic meters per minute and the same pressure of 69.6 KPA was recently repaired and put back into use. However, after running the smaller blower for two hours, it was unable to reach sufficient operating speed; the outlet check valve could not be opened either. The larger blowers had to be turned off for the current on the smaller blower to increase, but this resulted in exceeding the rated current level, and moreover, no aeration took place in the biological treatment tank. Here’s the question: Can multi-stage centrifugal blowers with different flow rates but the same outlet air pressure not be used in parallel? Additionally, some colleagues suspect that the outlet check valves installed on smaller blowers are the same as those on larger ones, resulting in excessive resistance Should the DN300 one be replaced with a DN250 outlet for a smaller fan?