The silicone seal seals the water tank when the safety cap is tightened. The silicone seal is a wearing part that must be replaced whenever necessary to seal the water tank.
For replacement, a chisel or other tool is needed to dig the old seal out of the groove in the bottom of the safety cap. To install a new seal, press the new seal into the groove in the safety cap.
Recycle the silicone sealant to the mixed waste.