Combination vacuum relief valve and pressure relief valve

A combination vacuum relief valve and pressure relief valve assembly for use in a tank comprising:

• cover plate support means extending upwardly from fixed structure on a railway tank car;
• resilient means biasing a top cover plate into engaged position with said support means;
• said cover plate being movable between a closed position engaging said support means and preventing fluid communication between the tank and the outside atmosphere, and an open position wherein said cover plate moves upwardly against the bias of said resilient means, and wherein fluid communication is permitted between the tank and the outside atmosphere;
• a vacuum relief valve mounted horizontally and spaced below said top cover plate;
• means biasing said valve into closed position engaging a valve seat located below said top cover plate and below said valve;
• said valve in said closed position closing communication between the inside of the tank and the outside atmosphere, and in open position allowing communication between the tank and the outside atmosphere;
• a plurality of first passageways providing fluid communication between the lower surface of the valve and the outside atmosphere;
• a plurality of second passageways spaced from said first passageways providing fluid communication between the inside of the tank and the upper surface of said valve when said valve is in closed position;
• said first and second passageways being located below said top cover plate;
• the cross sectional area of said first passageways throughout their length being at least substantially equal to the cross sectional area of said valve and the cross sectional area of said second passageways through their length being at least substantially equal to the cross sectional area of said valve;
• whereby when the pressure inside the tank becomes less than atmospheric pressure, atmospheric pressure will cause the valve to move upwardly away from its seat and allow air to enter the tank and whereby when the pressure in the atmosphere and in the tank are substantially equalized, the valve will return to the seated position;
• and whereby for a given available valve cross sectional area the volume of air flow into the tank from the atmosphere is maximized for a given pressure drop from the atmosphere into the container.