brither valve with flame arrestor

If you check it at night, don't use a match.

Seriously, have a competent technician check it.

What is the liquid? Often water will condense and drip/spray when the vent is blowing. Depending on the size of your tank, some venting in and out is common as the temperature changes or the volume of fuel in the tank changes.

They are designed to hold in a pressure higher than the vapor pressure of the fluid in the tank. At night the tank will cool and vapors above the fuel will shrink causing the pressure to drop and the vacuum will open the valve to equalize the pressure. During the day as the sun heats the tank, pressure increases until it opens the valve and again releases the excess pressure.

I have often seen water spray from the vents during the day. The phenomenon is similar to the water droplets that form inside a half full water bottle even if left in the sun. If the water bottle were large enough and pressure accumulated inside it would spray the droplets near the opening as it is opened.

You should have the vent inspected if it is venting constantly and vapors are always seen exiting the vent.

we have the tank of EC ( ethile carbonate )

Concept should be the same, but your condensation on the inner surfaces may not be water. Test the liquid and if it is product you may have to redesign the vent system with something to trap the liquid.