The Sentinel boiler was a design of vertical boiler, fitted to the numerous steam wagons built by the Sentinel Waggon Works. The boiler was carefully designed for use in a steam wagon: it was compact, easy to handle whilst driving, and its maintenance features recognised the problems of poor feedwater quality and the need for it to be maintained by a small operator, rather than a major locomotive works. Although this design was used in most of Sentinel's products, they also produced larger boilers of quite different types for their railway locomotives.

Description

Sentinel boiler, sections (Rankin Kennedy, Modern Engines, Vol V)

Sentinel boilers are vertical, as was common for many designs of steam wagon, so as to reduce the effects of tilting due to hill climbing or uneven roads disturbing the water level. It also provides a compact boiler that leaves adequate space in the cab for the crew, controls and coal bunker, whilst leaving as much as possible of the wagon's overall length available for the useful load. The boiler is a

watertube boiler A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-gen ...

, with these tubes contained within a vertical, cylindrical outer drum. This drum is double-walled and forms a water jacket around the boiler, with a large vertical flue within. The inner flue has a complex cross section. It is stepped in three diameters, tapering towards the top. The central region is square in section, rather than round. The main heating surface is provided by water-tubes in this squared section. These tubes are short, straight and pass between the flat faces of the squared section in a grid pattern. There are eight layers of tubes, four banks of six each way. A space is left in the centre of the water-tube banks for the firing chute. restoration of a Sentinel, photo showing inside the boiler. The firebox is top-fired (and lit) through this chute and there is no side firedoor. The lower part of the water-jacketed barrel surrounds the firebox. The narrow waterspace here encourages rapid steam raising. Firing is simple, with a thick fire relative to its area and fuel simply poured down. Beneath the grate is a water-filled ashpan, to prevent hot embers falling onto the road. Draught is controlled by a damper in the side of the ashpan, between the grate and the water tray. Above the tube nest the water space widens to form an increased reservoir, protecting against tilting. Sentinel's drawings permitted a hill climb gradient of 1 in 6, approximately 9.5°. Modern regulations for buses require a safe tilt of 35°. The narrowed flue in this area is used, where fitted, to house the

superheater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There ...

. An unusual feature of the Sentinel boiler was the "exhaust drying box", a small reheater, in the upper part of the boiler flue, immediately before the blastpipe nozzle. This heated the exhaust steam to avoid it condensing into a visible white plume. It was a requirement of the Highways and Locomotives (Amendment) Act 1878 that engines should "consume their own smoke".

Washout

Sentinel steam waggon, with boiler dismantled for cleaning (Rankin Kennedy, Modern Engines, Vol V)

By the nature of their use, steam wagons were often required to use feedwater that was either dirty or contaminated with dissolved minerals. Untreated, this builds up boiler scale on the tubes and particularly deposits sludge in the lower parts of the boiler. Both of these disturb circulation and risk local overheating and damage, scale also reduces boiler efficiency and wastes fuel. The Sentinel boiler was designed to cope with these problems, and to permit easy

cleaning Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for aesthetic, hygienic, functional, environmental, or safety purposes. ...

of the waterspace. As well as the usual blow-down cock for daily use, the entire boiler could be dismantled easily. The outer shell was in two sections, inner and outer drum, and were joined by a bolted ring joint at top and bottom. Regular servicing (depending on water conditions) was to separate the water-tubes in their drum from the boiler outer shell, so that they could be cleaned. Sludge dropped free on opening the shell and the short, straight tubes could easily be cleaned with brush or scraper. Several other vertical boilers, such as the

Straker Straker is a surname. The surname of Straker was derived from the Old English word 'stracian' an occupational name 'the striker'. Occupational surnames refer directly to the particular trade or occupation followed by the first bearer of the name. ...

, had similar arrangements for lifting their shells off the tube nest. The Sentinel though left the outer shell in place and instead dropped the tube nest ''downwards'' (having first removed the ashpan). This had the advantages that it required simpler lifting gear: the wagon would be raised on ramps or over a pit, the bolts removed and then the tube bank lowered with a block and tackle from a fixed beam, without requiring a mobile crane that could lift it and then move it sideways. Secondly the many pipe connections to the outer shell were left undisturbed, making the operation quicker. Dropping the firebox was not required at every washout and was recommended at intervals of 2 to 12 months, depending on water quality.

Firebox shape

Sentinel boiler, inner shell with spiral watertubes

Sentinel's best-known flue design was the square-section, but at one time they also used a circular corrugated design, with the water-tubes arranged in a

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:Galloway Galloway ( ; sco, Gallowa; la, Gallovidia) is a region in southwestern Scotland comprising the historic counties of Wigtownshire and Kirkcudbrightshire. It is administered as part of the council area of Dumfries and Galloway. A native or ...

of Manchester. When Galloway closed in 1932, Sentinel switched back to their square pattern.

Uses

Steam wagons

The boiler was used throughout Sentinel's range of steam wagons, from the earliest to the last. It was also used for their steam tractors,

buses A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for char ...

and other vehicles. Mann's of Leeds used a derivation of the Sentinel boiler in their "Express" wagon, launched in 1924.

Railway locomotives

This particular boiler design was not widely used for Sentinel's railway locomotives. Narrow-gauge locomotives used it, as did the smaller

standard gauge A standard-gauge railway is a railway with a track gauge of . The standard gauge is also called Stephenson gauge (after George Stephenson), International gauge, UIC gauge, uniform gauge, normal gauge and European gauge in Europe, and SGR in E ...

models. *

LNER Class Y1 The LNER Class Y1 was a class of 0-4-0 geared steam locomotives built by Sentinel Waggon Works for the London and North Eastern Railway and introduced in 1925. They passed into British Railways ownership in 1948 and were numbered 68130-68153 b ...

* LNER railmotors Their larger locomotives used a range of boiler designs, but all with water-tubes.

References

{{steam engine configurations, state=collapsed Sentinel Waggon Works Steam wagon boilers Vertical boilers Water-tube boilers Steam boiler types Steam wagons Steam road vehicles