LMS Crab, showing cylinder-mounted snifting valves UK railway companies did not agree on the merits of snifting valves. The London and North Eastern Railway used them very widely. 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.
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Peppercorn A1 class Tornado The snifting valve is the small mushroom directly behind the chimney. The square cover beneath is another distinctive LNER feature, covering the end of the superheater header. A snifting valve (sometimes snifter valve) is an automatic anti-vacuum valve used in a steam locomotive when coasting. The word Snift imitates the sound made by the valve.
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The results of these extended tests did not prove entirely satisfactory and all these engines were gradually refitted with standard piston valves and snifting valves.
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The smokebox snifting valves applied to the class by Maunsell were removed by his successor Oliver Bulleid, who also fitted U1 class chimneys to improve the draughting.
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With somewhat different functions, snifting valves were also used in atmospheric steam engines and hydraulic rams. They are not generally used in later stationary steam engines, road engines, or marine engines.
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The snifting valves may be 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.
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The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes this cooling effect is absent, and thus a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives (particularly on the London and North Eastern Railway) were fitted with snifting valves which admitted air to the superheater when the locomotive was coasting. This kept the superheater elements cool and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives.
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The entire U1 class had smokebox snifting valves, a feature previously used on the other Maunsell moguls. The main batch saw the front steps relocated ahead of the cylinders, as opposed to behind on the A890 rebuild, which was another relic of its previous guise as a 2-6-4 tank locomotive. Smoke deflectors were fitted to the whole class from 1933, as with most of the locomotives designed by Maunsell. The smokebox snifting valves were removed by Oliver Bulleid by the end of the Second World War, although the U1 class chimney was used to improve draughting on the other Maunsell moguls.
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876 in August 1932, Class 15CA no. 2852 in March 1933 and finally on Class 15A no. 1961. The results of these extended tests did not prove entirely satisfactory and all these engines were gradually refitted with standard piston valves and snifting valves.
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With both ends of the cylinders now in communication, the use of ordinary by-bass or snifting valves became unnecessary. The first such experiment was carried out on Class 16C no. 851. Further similar experiments were carried out on Class 16B no. 805 in July 1932, Class 16DA no.
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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.
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This opened briefly when steam was first admitted to and non-condensable gas was driven from the cylinder. Its name was derived from the noise it made when it operated to release the air and steam "like a Man snifting with a Cold"."A Course of Experimental Philosophy", John Theophilus Desaguliers, 1744, Vol II p. 474.
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The dry battery for the ignition is under the upper frame tube (in the case of a motor tricycle); ignition timing could be adjusted with a small lever. Side valves were driven via a gear-driven camshaft, with automatic snifting valve activation. The advantage of the design should have been very early (0–5 degrees before top dead centre) opening inlet valve. The cylinder head was removable and bolted with four bolts on the crank case.
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Those overhauled in the last years of steam operations were repainted black. A number of modifications were carried out over their life including the fitting of large mushroom air snifting valves. Several had additional sandboxes and/or rear headlights fitted at various times for working lines where no turning facilities were available. On 5 May 1947, C17 class locomotive 824 left the rails near Camp Mountain on the Dayboro line claiming the lives of 16 people and 38 injured.
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Newcomen found that his first engine would stop working after a while, and eventually discovered that this was due to small amounts of air being admitted to the cylinder with the steam. Water usually contains some dissolved air, and boiling the water released this with the steam. This air could not be condensed by the water spray and gradually accumulated until the engine became "wind logged". To prevent this, a release valve called a "snifting clack" or snifter valve was added near the bottom of the cylinder.
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The hand reversing screw spindle carried a disk, suitably notched with a locking device to ensure that the cam rollers would be in the centre of the selected cams under working conditions. During drifting, the reversing gear is placed in the neutral position where the exhaust valves are held fully open. These conditions were ideal for drifting and superior to any Walschaerts valve gear in admitting steam to the steam chests, even with cylinders equipped with by- pass valves, snifting valves and drifting valves. In service, some trouble was experienced with transmission shaft joints and with worn cams and rollers.
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The SECR and Woolwich batches also received smoke deflectors to prevent drifting smoke from obscuring the driver's vision ahead. The U1 chimneys replaced the standard N class type on the earlier locomotives, which, along with the removal of the piston tail rods on the earlier batches, created a truly standardised appearance. In 1937, Maunsell's replacement Oliver Bulleid saw no need to improve draughting of the class, and spared them from trials with Lemaître multiple-jet blastpipes and wide-diameter chimneys. However, he had Maunsell's smokebox-mounted anti-vacuum snifting valves removed at the end of the Second World War in an effort to reduce maintenance.
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49 Maunsell retired from the Southern Railway in 1937 and his replacement was Oliver Bulleid. Despite making various modifications to the steam circuit on other Maunsell classes, Bulleid saw no reason to make such drastic alterations to the N1 class. As a result, the final set of modifications made under Southern Railway ownership was made after the Second World War and included the removal of superfluous smokebox-mounted anti-vacuum snifting valves and the replacement of the wide-diameter N1 chimney with the U1 type to improve draughting on the prototype; the standard N class chimneys of the other five were also replaced.Haresnape (1983), p.
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When the regulator is opened, steam forces the loose valve bodies against their respective fixed discs, and they then act as units similar to ordinary piston valves. When steam is shut off, the loose valve heads become detached from their respective discs and remain in their idle positions near the centre of the steam chest, while the valve spindle and fixed disks continue their reciprocating motion with the spindle sliding freely through the now stationary loose valve heads, and with the steam and exhaust ports now in communication. With both ends of the cylinders now in communication, the use of ordinary by-bass or snifting valves became unnecessary. Further similar experiments were carried out on Class 5B no.
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40 The first of the production batch emerged from Brighton works in August 1928 and featured a tapered chimney and smokebox snifting valves, both of which were used on the K and N class locomotives. The 20 locomotives of the first production batch were split equally between Brighton and Ashford works, and all were completed by December 1929. The production locomotives had a slightly different profile to the K class rebuilds and featured left-hand drive to improve the visibility of signals from the driver's side of the footplate. They also had higher running plates along the sides of the boiler than the rebuilds, which required smaller wheel splashers to cover the tops of the driving wheels.
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Snifting valves were provided to prevent vacuum formation in the cylinders when the locomotive was stationary, and the outside Walschaerts valve gear incorporated single slide-bars and piston tail rods. Innovations added by Maunsell's team included steam-powered locomotive brakes, locating the boiler water top feed inside a dome-like cover with external clackboxes and water feed pipes mounted on either side, and a new type of superheater that segregated saturated and superheated steam. Maunsell also incorporated a screw reverser to control valve events, which was easier to maintain than the complex steam reverser configuration of previous SECR designs. All components were standardised for interchange with similar locomotive classes to ease maintenance and reduce production costs.
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436 Holcroft's previous employment as an engineer involved with the GWR's 4300 class meant that the N1 used the same Churchward features as the N class to aid standardisation and the manufacture of parts. These included the use of long-travel valves for free running at speed, right- hand drive in the cab and driving wheels. The retention of the 2-6-0 wheel arrangement meant ample accommodation for the N class' tapered boiler with Belpaire firebox.Clarke (2008), p. 40 As with the N class, detail design on the N1 was left to another of Maunsell's assistants, James Clayton, who brought functional Midland Railway influences to the design: the loading gauge-friendly shape of the cab, the tender and the large-diameter smokebox.Bradley (1961), p. 48 The smokebox housed a large superheater, regulator valve and snifting (anti-vacuum) valves.Casserley (1966), p. 35 The N1 also retained Walschaerts valve gear on both outside cylinders.
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