Construction
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 valve is lifted onto its seat by steam pressure and falls away under gravity when steam is shut off. This allows air to be drawn freely through it. Alternatively, the valve rests on its seat under gravity and is lifted by inward air flow, which can give rise to a characteristic rattle when a locomotive is coasting.
Read more about this topic: Snifting Valve
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