In one configuration the valve is lifted onto its seat by steam pressure and falls away under gravity when steam is shut off .
The valve is usually either a flat disc or a poppet valve which may be either opened or closed by gravity .
In one configuration the disc is lifted onto valve 's seat by steam pressure and falls away under gravity when steam is shut off .
The valve is usually either a flat disc or a poppet valve which may be either opened or closed by gravity .