Problems with high-speed pumps

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I would like to ask everyone: For the high-speed pump manufactured by the 11th Aerospace Research Institute, the medium is wastewater; the temperature is 100 degrees, and the pH level is 7. The outlet pressure is normal when the pump starts operating, but after 10 seconds of operation, the outlet pressure drops to zero. There is also a cracking sound inside the pump, and it is suspected that this is caused by cavitation. After proper venting, the pump was started again under the same operating conditions, and it operated normally without cavitation. I would like to ask everyone: why does cavitation not occur once everything is operating normally? Is it necessary to keep the wastewater temperature below 100 degrees? Additionally, this pump features a double-end face mechanical seal; on the medium side, it uses a conventional mechanical seal (with the stationary ring made of graphite), while the rotating ring is made of silicon carbide (on both sides). On the back side of the medium side, that is, on the side of the gearbox, a bellows-type mechanical seal is used. At the beginning of operation, the flushing water for the seal often becomes turbid (indicating leakage of waste water). During normal operation, when there is no leakage from the seal, the flushing water discharged is clear. May I ask what causes the leakage of the mechanical seal? I believe it’s because the compression amount of the double-faced mechanical seal hasn’t been set properly. Could everyone please advise me on how to determine the appropriate compression amount? If the compression amount is 5mm, should it be set at 2.8mm on the medium side, 1.2mm on the gearbox side, or should it be distributed evenly? Please feel free to share your thoughts.