Snifting valve
Encyclopedia
A snifting valve is an automatic anti-vacuum valve used in a steam locomotive
when coasting. The word Snift imitates the sound made by the valve.
When the driver shuts off the steam to the cylinders
of a steam locomotive while it is in motion, the moving pistons could create a partial vacuum in the cylinders. This would give rise to two problems. Firstly, the pumping action would absorb energy and prevent the engine from coasting freely. Secondly, when the exhaust valve opened, soot and cinders from the smokebox
could be sucked down the exhaust pipe and into the valve chest or cylinder, causing damage. (The exhaust is open to the smokebox because in normal running the exhaust steam is sent through the blastpipe
to draw the fire and eject the combustion products from the chimney.) These problems are avoided by using snifting valves to allow air to be drawn into the cylinder. On railways which did not use snifting valves, drivers were instructed to keep the regulator
slightly open when coasting to avoid the creation of a vacuum.
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.
, the snifting valves are mounted directly on the cylinders or steam chests
. The drawback of this arrangement is that they admit cold air which cools the cylinders and leads to condensation when the steam is turned on again.
If a superheater is fitted, one or two snifting valves may be mounted on the "wet" side of the superheater header. This causes air to be drawn through the superheater and heated so that it keeps the cylinders warm. Different railways had different ideas about the merits of drawing air through the superheater elements when coasting.
used them very widely. The London, Midland and Scottish Railway
did not use them on new locomotives but inherited some locomotives with snifting valves from constituent companies. The Southern Railway
used them for a while but later removed them.
The probable explanation for this diversity is that snifting valves were useful as long as steam temperatures were relatively low. As locomotive development proceeded, larger superheaters were fitted and steam temperatures increased. When the locomotive was coasting, the air passing from the superheater to the cylinders became so hot that it oxidised the cylinder oil and interfered with lubrication. At this point, most railways decided to abandon snifting valves.
and hydraulic ram
s. They are not generally used in later stationary steam engines, road engines, or marine engines.
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
when coasting. The word Snift imitates the sound made by the valve.
When the driver shuts off the steam to the cylinders
Cylinder (steam locomotive)
The cylinders of a steam locomotive are the components that convert the power stored in the steam into motion.Cylinders may be arranged in several different ways.-Early locomotives:...
of a steam locomotive while it is in motion, the moving pistons could create a partial vacuum in the cylinders. This would give rise to two problems. Firstly, the pumping action would absorb energy and prevent the engine from coasting freely. Secondly, when the exhaust valve opened, soot and cinders from the smokebox
Smokebox
A smokebox is one of the major basic parts of a Steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the chimney .To assist...
could be sucked down the exhaust pipe and into the valve chest or cylinder, causing damage. (The exhaust is open to the smokebox because in normal running the exhaust steam is sent through the blastpipe
Blastpipe
The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.- History :...
to draw the fire and eject the combustion products from the chimney.) These problems are avoided by using snifting valves to allow air to be drawn into the cylinder. On railways which did not use snifting valves, drivers were instructed to keep the regulator
Steam locomotive components
A listing of the components found on typical steam locomotives.center|720px|Schematic steam locomotiveGuide to steam locomotive components .A listing of the components found on typical steam...
slightly open when coasting to avoid the creation of a vacuum.
Construction
The valve is usually either a flat disc or a poppet valvePoppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
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.
Location
Where there is no superheaterSuperheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...
, the snifting valves are mounted directly on the cylinders or steam chests
Steam locomotive components
A listing of the components found on typical steam locomotives.center|720px|Schematic steam locomotiveGuide to steam locomotive components .A listing of the components found on typical steam...
. The drawback of this arrangement is that they admit cold air which cools the cylinders and leads to condensation when the steam is turned on again.
If a superheater is fitted, one or two snifting valves may be mounted on the "wet" side of the superheater header. This causes air to be drawn through the superheater and heated so that it keeps the cylinders warm. Different railways had different ideas about the merits of drawing air through the superheater elements when coasting.
UK railways
UK railway companies did not agree on the merits of snifting valves. The London and North Eastern RailwayLondon and North Eastern Railway
The London and North Eastern Railway was the second-largest of the "Big Four" railway companies created by the Railways Act 1921 in Britain...
used them very widely. The London, Midland and Scottish Railway
London, Midland and Scottish Railway
The London Midland and Scottish Railway was a British railway company. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railway companies into just four...
did not use them on new locomotives but inherited some locomotives with snifting valves from constituent companies. The Southern Railway
Southern Railway (Great Britain)
The Southern Railway was a British railway company established in the 1923 Grouping. It linked London with the Channel ports, South West England, South coast resorts and Kent...
used them for a while but later removed them.
The probable explanation for this diversity is that snifting valves were useful as long as steam temperatures were relatively low. As locomotive development proceeded, larger superheaters were fitted and steam temperatures increased. When the locomotive was coasting, the air passing from the superheater to the cylinders became so hot that it oxidised the cylinder oil and interfered with lubrication. At this point, most railways decided to abandon snifting valves.
Other uses
Snifting valves were also used in atmospheric steam enginesNewcomen steam engine
The atmospheric engine invented by Thomas Newcomen in 1712, today referred to as a Newcomen steam engine , was the first practical device to harness the power of steam to produce mechanical work. Newcomen engines were used throughout Britain and Europe, principally to pump water out of mines,...
and hydraulic ram
Hydraulic ram
A hydraulic ram, or hydram, is a cyclic water pump powered by hydropower. It functions as a hydraulic transformer that takes in water at one "hydraulic head" and flow-rate, and outputs water at a higher hydraulic-head and lower flow-rate...
s. They are not generally used in later stationary steam engines, road engines, or marine engines.