Pic. 34. Expansion tank cap:
1 - exhaust valve spring; 2 - inlet valve; 3 - intake valve spring; 4 - graduation (steam) valve; 5 - brass valve block; 6 - filler neck of the expansion tank.
With a running engine, when the temperature rises sharply or the liquid boils, the pressure in the cooling system rises. This also increases the boiling point, as a result of which the heat transfer of the radiator increases. When the pressure in the system increases, the exhaust valve opens and the vapors exit the tank into the atmosphere. The exhaust valve opening start pressure is not less than 1.1 kgf/cm2.
When the liquid is cooled or drained, its pressure in the system decreases and atmospheric air is sucked in through the inlet valve. The excess pressure of the beginning of the opening of the intake valve is 0.03... 0.13 kgf / cm2. To completely drain the fluid from the system, the plug from the expansion tank must be removed.