GWR Standard Boilers - A Beginner's Guide

"The modern Locomotive Question is principally a matter of boiler". G.J. Churchward, 1906

This article principally uses the GWR classification system for boilers, originated under Churchward, which divided boilers into classes based on the physical size of the boiler and thus which locomotives it will fit. This system included a two letter code, which the GWR called a prefix. The first letter indicated the boiler class, and thus the principle dimensions of the boiler. The second letter can be regarded as a subclass, and in some cases there were twenty or more of these. Differences between subclasses would include superheat (presence and degree), water feed type and location and firebox type, as well as subtler things like minor variations in dimensions and the presence or absence of brackets to locate the boiler or tanks, which would have an influence on exactly which locos a boiler could be fitted to. The classes were sometimes referred to by the first letter of the prefix, but they also had names, usually either a standard number, or else that of a class associated with the boiler.

Another system which will be encountered is that used in the RCTS volumes. This complex system enables classification of the appearance of boilers, and is usually one or more letters followed by a number, which describe firebox type and dome location (or absence). The RCTS volumes further allocate boilers to over 100 "Groups", which are boilers of substantially similar type. Note that the two systems have very little overlap. RCTS describes boilers of several different types as being R3 or D4 and within their 100 odd groups some groups contain boilers of more than one GWR prefix and vice versa.

General Development

It's worth noting that Broad gauge and Narrow (=Standard) gauge locomotives were, in the middle of the 19thC, designed by different designers and built at different workshops. There seems to have been little interchange between the two.

Gooch

When Gooch was in charge at Paddington/Swindon he seems to have largely delegated responsibility for standard gauge affairs to Joseph Armstrong at Wolverhampton, producing only a few designs for the narrow gauge. These had quite standardised boilers, typically domeless with raised round topped or (earlier) dome shaped fireboxes.

The Armstrongs and Dean.

Joseph Armstrong moved to Swindon when he was appointed to replace Gooch, and his brother George Armstrong took over at Wolverhampton and maintained his own line of development. Boilers with domes and without raised fireboxes appeared with Joseph Armstrong's 360 class goods type of 1866. These were followed by the numerous 0-6-0 Standard Goods and the Sir Daniel 2-2-2s, which had, as it turned out, the first two of what were to become standard boiler classes - boilers based on those for these classes were in use 70 years later. Under the Armstrongs and William Dean one sees these parallel boilers without raised fireboxes as being the dominant type. Domes moved forward and back along the boilers and pressures increased, but the external style was generally reasonably consistent. Other boilers introduced in this era that became widely used were on the Dean Goods and Metro classes.

Churchward

Churchward transformed GWR boilers. His work started under Dean, but during that era one can see a definite progression, worked out it would seem mainly with the 4-4-0s like the Badminton class, and then the early 4-6-0s. What became the standard tapered GWR boilers started out with parallel barrels with a considerably raised belpaire firebox - perhaps 8 inches above the boiler. Some of these had domes and some did not. The taper came later, strongly influenced, it seems, by the Brooks Locomotive Works of the USA. Having noted the Wolverhampton ancestry of narrow gauge design its perhaps appropriate to comment that significantly raised round-top fireboxes were very much a broad gauge feature.

At the same time there was also much development of the earlier parallel boiler types. The development of these is much less obvious to the uneducated eye, but it is clear that there was a great deal of work with all sorts of variations in tube layout and the like. Although various domeless types were tried, in the end it appears it was decided that, with room to make them of generous size as there was with the smaller locomotives, the increase in steam space and freedom from priming provided by a large dome at a moderate cost in weight was justified.

Weight was a significant design factor. The Standard 2 and Standard 4 were variations on a theme to keep the weight, and thus axle loading and route availability within bounds on some classes, but in general Churchward locomotives were within the available limits. However post WW1 schemes were drafted for uprating 2800, 2900 and 4000 classes with the Std 7 boiler. These would most likely have been very potent locomotives, but the enlarged Saint and Star designs could not be managed at all within the required weight limits, and the 2800 would have had reduced route availability.

Top Feed

The location of the water feed into the boiler has a significant effect on the life of the boiler. Boilers tended to pit in the area of the water feed, and dissolved solids would be precipitated as scale on the barrel, affecting the heat transfer.

In the mid Victorian period many locations for the boiler feed were tried, but clack boxes (valves) on the side of the barrel were a common option. Towards the end of the Dean era a location one each side of the back plate of the boiler became preferred. The arrangement is obvious on photographs of Steam Museum's Dean Goods. This arrangement persisted on some classes as late as the 1930s.

After a brief and unsuccessful flirtation with feed on the underside of the boiler Churchward decided on his own arrangement of top feed, inspired by American practice. In the GWR implementation top feed meant that boiler water was introduced through the top of the boiler into the steam space, and delivered onto trays. These trays sloped down to the front of the boiler barrel where they had notched ends so that the water fell onto the main water in the barrel in a series of fine streams. The trays tended to catch the scale and it could be washed out from there.

Superheating

In the 20thC the major development was superheating. A "saturated" boiler normally has all the fire tubes of around the same size, on the GWR typically between 1⅝" and 2". In a superheated boiler a number of rows of the small tubes are replaced by fewer rows of fewer large tubes, on the GWR typically a bit over 5". Into these large tubes are inserted superheater "elements" (Units in GWR terminology). These are usually tubes in a U shaped arrangement, about the length of the barrel, and depending on the design, on the GWR there were usually 4 or 6 of them in each large tube. After the steam leaves the main boiler space it passes through the regulator and then these tubes on the way to the cylinders. Steam in the main boiler will not heat to above the boiling point of water because of the presence of liquid water, but there is no liquid water in the superheater and the temperature of the steam may be considerably increased, which increases both the power and efficiency of the locomotive.

The primary drawback to the increased steam temperature is lubrication. The high temperatures carbonise the oil, more oil is required to maintain adequate lubrication and the steam passages, valves and pistons become coated with carbon. After early experiments with a high degree of superheat Churchward opted to use only moderate superheat. Superheat, although much tested on various pannier tank classes, was eventually rejected for the smaller classes used extensively for shunting. I understand that it appreciably reduces the speed of response to the regulator which makes shunting more difficult.

Collett

Collett immediately ran into the problems with weight that had prevented Churchward from upgrading Stars and Saints with Standard 7 boilers. The Castle's Standard 8 boiler was effectively a Standard 7 with a smaller diameter barrel, intermediate between the Standard 1 and the Standard 7, and brought the locomotive right up to the weight limit. The King's Standard 12 was an enlarged Standard 7 up to a new weight limit for a limited number of routes. The Manor brought another problem since it required a higher capacity boiler (and especially a larger firebox and grate area) than the Standard 4 of the 4300 it was replacing. However the locos needed to be much lighter their sisters, the Granges with Standard 1 boilers, so the all new Standard 14 had to be designed. It came between a Standard 2 and a Standard 1 in all the main dimensions. Collett also introduced high tensile strength nickel alloy steel to build some boilers, mainly Standard 2 for 2-6-2 tanks, of higher pressure but no extra weight.

Collett's other problem was several hundred absorbed locomotives with what must have seemed like several hundred designs of boiler. GWR maintenance practice required a stock of spare boilers, and clearly this would be impractical. So three of the standard boiler designs were modified, mainly with different fireboxes, and these were widely used on rebuilds of absorbed locomotives that could not be readily adapted for existing standard boiler designs. The Standard 9 and 11 were based on Dean era parallel boilers, but with longer fireboxes, and the Standard 10, which was later used on new GWR 0-6-0 and 0-6-0T designs, was a short taper boiler based on the Standard 2/3.

From 1925 a significant design change was introduced on non-superheated belpaire boilers. A pair of 5⅛in. flue tubes were installed in the top corners of the firebox tube plate. This concept, which appears to have been unique to the GWR, was intended to reduce a tendency for the tube plate to crack at this point. This would have brought a slight reduction in boiler efficiency, but an improvement in boiler life. This development was introduced on replacement boilers for 517 class 0-4-2Ts in 1924, extended to 850 and 2021 boiler replacements in 1925, and then included in the boilers for subsequent new classes with parallel saturated boilers, notably 57xx pannier tanks and 48xx 0-4-2Ts. The design wasn't used on unsaturated Standard 10 taper boilers.

Hawksworth

Hawksworth must have had similar weight problems when designing the County Class 4-6-0s. A Standard 8 Castle boiler would presumably have been a possibility, especially if Hawkesworth could have accepted the lower boiler pressure, but tooling was available from building LMS 8F (hence W.A. Stanier designed) boilers for the MOD, and this was used to build a notably shorter and stumpier boiler than the GWR designs.

Collett had followed Churchward's lead in using only moderate superheat even when improved heat resistant lubricants became available with advances in refining technology, particularly in the 1930s. While its an exaggeration to say that GWR locomotives would only steam on best steam welsh coal - the Castles on the exchanges with the LNER and LMS used good quality local coal - they were arguably more vulnerable than some other designs to poor quality coal. While good quality coal was available to the GWR moderate superheat brought no steaming problems. However during and after WW2 the quality of coal available diminished significantly and so higher degrees of superheat were introduced, rather later than on other lines. This came at a cost, as consumption of lubricants increased and high superheat locomotives were normally fitted with mechanically driven lubricators and increased oil flow.

The Boiler Classes

Class	Name	Mainly Fitted to Classes	Boiler Barrel				Firebox		Superheater	Total Heating Surface	Grate Area	Grate type	Smokebox Type	Comments
			Front Diameter	Rear Diameter	Length	Heating Surface	Length	Heating Surface	Heating Surface
V	1393	1393, 1361, 1366	3'9	3'10	8'2	816	3'11	75	0	891	10.7	flat
R	850	850, 16-18 Crane Tank	3'9	3'10	10'0	905	4' 0	76	0	981	11.1	flat		Extinct by end of 1950s
S	517	517	3'9	3'10	10'0		4'6			0	14.4	flat		Classes SA-SD. Built up smokebox. Extinct by 1950s
S	4800	1400/5800	3'9	3'10	10'0	870	4'6	83	0	953	12.8	flat	drum	Classes SS-SV. Modernised S class
U	2021	2021, 3571, some 517	3'9	3'10	10'0	905	5' 0	90	0	995	14.7	flat		Extinct by end of 1950s
Q	Standard 16	1600	3'9	3'10	10'1	877	5' 0	80	0	957	14.9	sloping	drum	Modernised U class
T	Metro	Metro, some large 0-6-0T Classes	4'2	4'3	10'6	1125	5'1	101	0	1226	14.6	flat		Extinct by 1950s
Q	Sir Daniel	Sir Daniel, River, some early 0-6-0T classes	4'2	4'3	11'0	1059	5'4	99	0	1158	15.6	flat		Somewhat interchangeable with O (Standard Goods). Extinct by 1940s.
L	Standard 11	various absorbed	4'2	4'3	10'6	1008	5'6	82	0	1090	16.8	sloping		Modified T class for absorbed locos
F	Standard 21	54/64/74	4'2	4'3	10'6	1004	5'6	82	0	1086	16.8	sloping	drum	Modernised Std 11
M	Bogie Single	30xx, 7,8,9,10	4'2	4'3	11'6	1170	6'4	115	0	1285	20.8	flat		Extinct by 1915
E	Standard 5	4400, 4500, 3900, 4600 small 4-4-2	4'2	4'9	10'6	993	5'10	94	78	1165	16.6	sloping	drum	Early variants were not superheated.
N	Duke	Duke, Barnum, Queen, Cobham, Dukedog 90xx	4'4	4'5	11'0	1029	5'10	114	81	1224	17.2	flat		Earlier variants were not superheated.
P	2301	2301,3521, later large 0-6-0T, 57xx, 97xx	4'4	4'5	10'3	1013	5'4	103	0	1116	15.5	flat		There were very many variants of the P class boiler, with and without superheaters.
O	Standard Goods	Armstrong Goods, some Sir Daniel, some large 0-6-0T	4'4	4'5	11'0	1180	5'4	106	0	1286	15.5	flat		Extinct by 1940s. Somewhat interchangeable with Q. There were a few superheated Class O boilers.
W	Badminton	Badminton	4'4	4'5	11'0	1175	6'4	122	0	1297	18.3	flat		Extinct by 1913
J	Standard 9	various absorbed	4'4	4'5	10'3	961	6'6	104	75	1140	19.7	sloping		Modified P class. Ten were constructed without superheaters.
K	Standard 10	2251, 9400, 1500, numerous absorbed	4'5	5'0	10'3	1070	6'0	102	76	1248	17.4	sloping	drum	Shortened Std 2, originally for absorbed classes. There was an unsuperheated version used on the 1500s and most 9400s.
B	Standard 2	33/3400 (Bulldogs) 4100 (Badminton / Atbara / Armstrong), some 26xx, 2221, 3101, 30xx ,56xx, 41/51/8100, numerous absorbed	4'5	5'0	11'0	1145	7'0	122	82	1349	20.4	sloping	drum	Early variants were not superheated.
C	Standard 3	36xx, 3521, 3514, some absorbed, some 33xx	4'5	5'0	10'3	1069	7'0	122	76	1267	20.4	sloping	drum	Early variants were not superheated.
Z	Standard 14	Manor	4'8	5'3	12'6	1286	8'8	140	190	1616	22.1	sloping	drum
D	Standard 4	some 26xx, Cities, Some Badminton, 3150, 3800 County, 4200, 4300, 7200, Collett 3100, Barry 0-6-4	4'11	5'6	11'0	1349	7' 0	129	192	1670	20.6	sloping	drum	Early variants were not superheated.
A	Standard 1	2800, 2900 (Saint), 4000 (Star), 4900 (Hall), 6800 (Grange), 6959 (Mod. Hall)	4'11	5'6	14'10	1687	9'0	155	263	2105	27	part sloping	drum	Early variants were not superheated.
O	Standard 15	County 4-6-0	5'	5'8	12'7	1545	9'9	169	265	1979	28.8	part sloping	drum
HA	Standard 8	4073 (Castle)	5'2	5'9	14'10	1886	10'	164	263	2313	30.1	part sloping	drum	14 large tubes, 201 small tubes, 84 s/h elements in 2 rows (6 elements per large tube)
HB	Standard 8	4073 (Castle)	5'2	5'9	14'10	1856	10'	163	263	2282	29.4	part sloping	drum	14 large tubes, 201 small tubes, 84 s/h elements in 2 rows
HC	Standard 8	4073 (Castle)	5'2	5'9	14'10	1800	10'	163	313	2276	29.4	part sloping	drum	21 large tubes, 170 small tubes, 84 s/h elements in 3 rows (4 elements per large tube)
HD	Standard 8	4073 (Castle)	5'2	5'9	14'10	1670	10'	163	393	2226	29.4	part sloping	drum	28 large tubes, 138 small tubes, 112 elements (4 elements per large tube)
G	Standard 7	4700	5'6	6'0	14'10	2062	10'	170	288	2519	30.3	part sloping	drum	Used only on the 4700 class of 9 locomotives, but considered for use in several design proposals.
W	Standard 12	King	5'6	6'0	16'0	2008	11'6	194	313	2515	34.3	part sloping	drum
F	Standard 6	Great Bear	5'6	6'0	23'0	2597	8'0	159	506	3262	41.8	part sloping	drum	Extinct 1924

This table needs to be read with considerable caution. In particular the heating surfaces varied considerably over the years. Just because boilers were outwardly of the same dimensions and interchangeable doesn't mean that they were the same inside. There were very great differences. Superheated boilers would have much lower heating surface area in the tubes than saturated boilers because the superheater elements were installed in large tubes. Looking at the dimensions over the years there was a definite tendency for the heating surface in the tubes to be reduced, even without changes to the degree of superheating. There seems also to have been a tendency for the size of superheaters to reduce: later versions of the 4 row superheater in Castles had less surface area then earlier ones, but of course still far more than the three row ones.

To give an idea of this I have included the dimensions of the 4 subtypes of Standard 8, fitted only to the Castle Class. Even within those there were variations: for all of them superheater size was later varied, generally reducing slightly with shorter elements.

In general, and with obvious exceptions, I have picked values for sub classes of boilers built in larger numbers during the 1930s, and definitely not those of the post war higher superheat developments.

Boiler Families

Taper boilers

Std 1-4 and 10.

The Standard 4 (D) is a short version of the Standard 1 (A), with both firebox and barrel shorter.

The Standard 2 (B) has very much the same size firebox as the Standard 4, but a smaller diameter barrel of the same length.

The Std 3 (C)was a standard 2 with a shorter barrel.

The Std 10 (K) is a Std 3 with a shorter firebox.

Std 6-8 and 12

The Std 7 (G) had the same diameters as the Std 6 (F - Great Bear), but with a conventional firebox and a much shorter tapered barrel.

The Std 8 (H) is a Std 7 with a smaller diameter barrel and a slightly smaller firebox.

The Std 12 (W - King Class) has the same diamters as a Std 7 with lengthened barrel and firebox.

Parallel boilers

T, Std 11 and Std 21

Std 11 (L) is a T class with a longer firebox, and a Std 21 (F - the code was reused) is a Std 11 with a drum style smokebox

P, N, Std 9

The N class has the same diameter as the P with a longer barrel and firebox

The Std 9 (J) was a P class with a longer firebox

U, V, R, S, Std 16

Std 16 (Q - reused code) is basically a U class with a drum style smokebox.

R,V and S are the same diameter as the U with different firebox and in the case of the V also different barrel lengths.

Boilers used on the 4800/1400 class (SS, ST, SU, SV) were basically an S class with a drum style smokebox.

Boiler Prefixes

This table has been put together from GWR documents, and represents how the GWR saw their boiler stock. It will be noted that the table is incomplete. This is where older boiler types had become extinct by the date the documents were prepared.

Boiler	Prefix	Description
		Flue tube rows	Superheater	Tube Units (elements)	Blower etc	Firebox	Comments	Fitment	Pressure	Feed
S/1	AH	DR	Superheater	Old type 6 units		B				Top Feed (combined)
S/1	AK	TR	Superheater	Melesco Units		B				Top Feed (combined)
S/1	AL	DR	Superheater	Old type 6 units	outside blower	B				Top Feed (combined)
S/1	AM	DR	Superheater	New type 4 units		B				Top Feed (combined)
S/1	AN	DR	Superheater	New type 4 units	outside blower	B				Top Feed (combined)
S/2	BA	SR	Superheater	Old type 6 units		B				Top Feed (combined)
S/2	BB	SR	Superheater	Old type 6 units		B			225	Top Feed (combined)
S/2	BC	DR	Superheater	Melesco Units		B				Top Feed (combined)
S/2	BD	SR	Superheater	New type 4 units		B				Top Feed (combined)
S/2	BE	SR	Superheater	New type 4 units		B				Top Feed (combined)
S/3	CA	SR	Superheater			B				Top Feed (combined)
S/4	DG	DR	Superheater	Old type 6 units		B			200	Top Feed (combined)
S/4	DH	DR	Superheater	Old type 6 units		B			225	Top Feed (combined)
S/4	DJ	DR	Superheater	New type 4 units		B			200	Top Feed (combined)
S/4	DK	DR	Superheater	New type 4 units		B			225	Top Feed (combined)
S/4	DL	TR	Superheater	Melesco Units		B			225	Top Feed (combined)
S/5	EB	SR	Superheater	Old type 6 units		B				Top Feed (combined)
S/5	EC	SR	Superheater	New type 4 units		B				Top Feed (combined)
S/21	FA	NSOO				B		Pannier tanks	165	Firebox Backplate
S/21	FB	NS				B	Converted S/11		165	Firebox Backplate
S/21	FC	NSOO				B		Pannier tanks	180	Firebox Backplate
S/21	FD	NSOO				B		Pannier tanks	165	Top Feed (separate)
S/21	FE	NS				B	Converted S/11		165	Top Feed (separate)
S/21	FF	NSOO				B		Pannier tanks	180	Top Feed (separate)
S/7	GA	DR	Superheater	Old type 6 units		B				Top Feed (combined)
S/7	GB	DR	Superheater	New type 4 units		B				Top Feed (combined)
S/8	HA	DR	Superheater	Old type 6 units		B				Top Feed (combined)
S/8	HB	TR	Superheater	Melesco Units		B				Top Feed (combined)
S/8	HC	QR	Superheater	Melesco Units		B				Top Feed (combined)
S/8	HD	DR	Superheater	New type 4 units		B				Top Feed (combined)
S/9	JA	SR	Superheater			B		Side Tank Tdr
S/9	JE	NSOO				B		Side Tank Tdr
S/10	KA	SR	Superheater	Old type 6 units		B		Side Tank Tdr Pan		Top Feed (combined)
S/10	KB	NS				B		Pannier tanks		Top Feed (combined)
S/10	KC	SR	Superheater	New type 4 units		B				Top Feed (combined)
S/11	LA	NS				B		Tender
S/11	LB	NS				B		Pannier tanks
ROD	MB	DR	Superheater	Old type 6 units		B				Top Feed (combined)
113th	NA	SR	Superheater	Old type 6 units		B				Firebox Backplate
3200	NB	SR	Superheater	Old type 6 units		B				Firebox Backplate
113th	NC	SR	Superheater	Old type 6 units		B				Top Feed (separate)
3200	ND	SR	Superheater	Old type 6 units		B				Top Feed (separate)
113th	NK	NS				B
S/15	OA	TR	Superheater	Melesco Units		B				Top Feed (combined)
S/15	OB	QR	Superheater	Melesco Units		B				Top Feed (combined)
2301	PA	NS				B		Tender
2301	PD	NS				B		Pannier tanks
2301	PE	SR		None		B
2301	PH	NSOO				B		Pannier tanks	200	Firebox Backplate
2301	PI	SR	Superheater			B		Tender
2301	PJ	NSOO				B	Improved	Pannier tanks	200	Firebox Backplate
2301	PK	NSOO				B		Pannier tanks	200	Top Feed (separate)
2301	PL	NSOO				B	Improved	Pannier tanks	200	Top Feed (separate)
S/16	QA	NSOO				B		Pannier tanks		Top Feed (separate)
850	RA	NS				RT		Saddle Tank
850	RB	NSOO				B		Pannier tanks
850	RF	NS				B		Pannier tanks
4800	SS	NSOO				B		Side tank		Firebox Backplate
4800	ST	NSOO				B	Converted 517			Firebox Backplate
4800	SU	NSOO				B		Side tank		Top Feed (separate)
4800	SV	NSOO				B	Converted 517			Top Feed (separate)
Metro	TL	NS				B		Side tank
2021	UF	NS				B		Pannier tanks		Firebox Backplate
2021	UG	NS				B		Side tank
2021	UH	NSOO				B		Pannier tanks		Firebox Backplate
2021	UI	NSOO				B		Side tank
1393	VA	NS				RT		Saddle Tank	150
1366	VB	NSOO				B		Pannier tanks	165
S/12	WA	DR	Superheater	Old type 6 units		B				Top Feed (combined)
S/12	WB	QR	Superheater	Melesco Units		B				Top Feed (combined)
1101	YC	NS				B		Side tank	170
2194	YF	NS				RT		Saddle Tank
781	YG	NS				RT		Side tank
7	YL	NS				B	Vale of Rheidol	Side tank		Top Feed
1331	YW	NS				RT		Saddle Tank
942/1153	YX	NS				RT		Saddle Tank		Firebox Backplate
S/14	ZA	DR	Superheater	Old type 6 units		B				Top Feed (combined)
S/14	ZB	DR	Superheater	New type 4 units		B				Top Feed (combined)
NS: No Superheater. NSOO: No superheater, two 5⅛ tubes, SR: Single Row of superheater s, DR: Double Row, TR: Three Rows, QR: four rows. B: belpaire firebox, RT: round top firebox