Theblastpipe is part of theexhaust system of asteam locomotive that discharges exhaust steam from thecylinders into thesmokebox beneath thechimney in order to increase thedraught through the fire.
The primacy of discovery of the effect of directing the exhaust steam up the chimney as a means of providing draft through the fire is the matter of some controversy, Ahrons (1927) devoting significant attention to this matter. The exhaust from the cylinders on the first steam locomotive – built byRichard Trevithick – was directed up the chimney, and he noted its effect on increasing the draft through the fire at the time. At Wylam,Timothy Hackworth also employed a blastpipe on his earliest locomotives, but it is not clear whether this was an independent discovery or a copy of Trevithick's design. Shortly after Hackworth,George Stephenson also employed the same method, and again it is not clear whether that was an independent discovery or a copy of one of the other engineers.Goldsworthy Gurney was another early exponent, whose claim to primacy was energetically advocated by his daughter Anna Jane.[1][2]
The locomotives at the time employed either a singleflue boiler or a single return flue, with the fire grate at one end of the flue. Because a single flue had to be wide to let the exhaust through, a blastpipe could lift the fire, pulling soot and sparks up the chimney. It was not until the development of the multitubular boiler that a forced draft could be used safely and effectively. The combination of multi-tube boiler and steam blast are often cited as the principal reasons for the high performance ofRocket of 1829 at theRainhill Trials.
Soon after the power of the steam blast was discovered it became apparent that asmokebox was needed beneath the chimney, to provide a space in which the exhaust gases emerging from the boiler tubes can mix with the steam. This had the added advantage of allowing access to collect the ash drawn through the fire tubes by the draught. The blastpipe, from which steam is emitted, was mounted directly beneath the chimney at the bottom of the smokebox.
The steam blast is largely self-regulating: an increase in the rate of steam consumption by the cylinders increases the blast, which increases the draught and hence the temperature of the fire. Modern locomotives are also fitted with ablower, which is a device that releases steam directly into the smokebox for use when a greater draught is needed without a greater volume of steam passing through the cylinders. An example of such situation is when the regulator is closed suddenly, or the train passes through a tunnel. If a single line tunnel is poorly ventilated, a locomotive entering at high speed can cause a rapid compression of the air within the tunnel. This compressed air may enter the chimney with substantial force. This can be extremely dangerous if the firebox door is open at the time. For this reason the blower is often turned on in these situations, to counteract the compression effect.
Little development of the basic principles of smokebox design took place until 1908, when the first comprehensive examination of steam-raising performance was carried out byW.F.M. Goss ofPurdue University. These principles were adopted on theGreat Western Railway byGeorge Jackson Churchward. A later development was the so-calledjumper-top blastpipe which controlled the area of the blastpipe at different steaming rates to maximise efficiency.
The aim of blastpipe modification is to obtain maximum smokebox vacuum with minimumback pressure on the pistons. The simplest modification is adouble chimney with twin blastpipes, but many other arrangements have been tried. Towards the end of the steam era theKylchap exhaust was popular and used on theNigel Gresley-designedMallard. Other designs includeGiesl,Lemaître andLempor blastpipes.
