The South African Class "25" Condensing Locomotive

by Paul Smith

(25.GIF 46955) Class 25 mainline 4-8-4 condensing, note the tender! (1696x582x2)

The class "25C" was designed by SAR in 1950 for operations in the arid Karroo region of the Cape and in South West Africa where local water supplies were unobtainable and normally aspirated locomotives trailed additional tankers of feed water. The condensing aspects of the motive power were designed and supplied by Messrs. Henschel-Werke of Kassel who had delivered similar locomotives to Argentina, Russia, Iraq and Germany before WWII. Henschel supplied the prototype and 60 condensing tenders ⁱ. Construction of the remaining 89 locomotives and 30 condensing tenders was entrusted to the North British Locomotive Company of Glasgow. An additional 50 non- condensing units (class "25NC") ⁱⁱ were built by the two companies, (39 Henschel, 11 North British) bringing the total for the class to 140 ⁱⁱⁱ. The total order was supplied between 1953 and spring of 1955.

Apart from the special water saving design, the units represented leading edge technology of the day, incorporating a cast steel frame with integral cylinders from an American company ^iv , roller bearings throughout, a mechanical stoker, automated mechanical lubrication and syphonic brick arch tubes. Class "25"s were noted for their free steaming, low rolling resistance and a photographically pleasing black cloud whenever the char ejector was running. At high speed, the class "25"s demonstrated a pronounced and unique "gait" resulting from their large roller bearing connecting rods. At least one instance was recorded of a class "25" "blowing away in the wind" while its crew was absent from the footplate (although the General Manager holding the discipline hearing was not convinced of the explanation).

Water savings of 85% (and coal savings of 15%) were accomplished by passing exhaust steam from the cylinders through a turbine to drive a smokebox blower, which replaced the blast pipe effect of a normal locomotive. Subsequently, the exhaust steam passed via large 16" pipe on the boiler side fitted with a flexible joint to another turbine in the long tender roof. The second turbine drove five sets of roof-mounted air fans by means of flexible shafts. Both these turbines operated on a very low energy saturated steam supply ^v.

#3404 Elsabe

The tender sides contained rows of condenser units (similar to automotive radiators) ^vi through which the fans drew air. Steam entered the condensers at ~400 F and was condensed by air flow to ~200 F. The fans and associated turbine were designed to draw sufficient air (~800Ib per second) to cool waste steam at a rate of 25 tons per hour under the extreme conditions found in the Karroo. ^vii The whole tender frame was a cast tank holding the condensate before its return to the boiler through two turbine feed pumps directly connected to the tank. Great care was taken to allow both the condenser units and the integrated reserve water tank to move with thermal expansion. Adequate distribution of the exhaust steam into all the condensers was also a design concern. The fan turbine was installed part way down the tender roof to facilitate shorter power shafts and to make the unit easy to remove for servicing. Oil was removed from the waste steam by a volute separator and from the water by sponge filters in the tender tank. Erratically high oil levels in the feed water proved to be a major problem for these locomotives and a vortex water separator, which could be used to discharge emulsion-laden water was installed in the main steam pipe. Efforts were made to recover as much water as possible. Even the exhaust from the feed pumps, vacuum and electrical generators and the first set of safety valves was reprocessed in the tender. The only steam loss was from the second level safety valves, the reverser, the stoker spreader plate, the ash box sprinkler and the train heating system. The design of the condensing equipment presented a complex set of parameters to ensure adequate cooling under the most adverse conditions while maintaining the restricted SAR loading profile and providing the required output power. As examples: Both the blower turbine and the tender fan turbine required by-pass valves to regulate draft within limits required.

Probably the largest condenser locomotives ever built, the Class "25C" suffered from the sorts of technical difficulties we associate today with advanced supersonic aircraft and Space Shuttle tiles.

The demanding road profile, high speed running on long grades and long standing times with freight loads resulting in firing difficulties and frequent water carry over into the cylinders. This led to high oil levels in the condensate and damage to the turbine blades. Cracked and fractured blower turbine blades affected locomotive availability in the early years but a redesign of the blade mounting and reducing water carry over solved these problems. An innovative design allowed individual turbine blades to be replaced in a running shed by removing a simple pin.

The blower blades suffered damage from the high quantity of char; in turn a result of mechanical stoking and the unusual equipment in the smokebox. These problems were overcome by extensive scientific investigations by SAR and the suppliers, improved operating technique and a reduction in the speed of the blower blades. The blower unit was redesigned so that the blades exposed to char damage could be removed from the smokebox without disturbing the gearbox. ^viii Smokebox redesign was necessary to divert as much char as possible from the blower blades into a trough from which it could be ejected up the chimney by means of a steam jet. ^ix

In many respects, the class "25C" represented the zenith of steam locomotive efficiency in the western world and could even be observed double headed on long consists across the Karroo in the 1960s. The units were used as prime motive power on the famed "Blue Train" in the 1960s and 1970s.

Although the "25C"s gave twenty plus years of excellent service (by which time, their main bearings had probably just warmed up), they were eventually converted to non-condensers with ugly water tanks welded in the tenders and reduced to secondary traffic by the SAR diesel and electric adoption programmes. The need for special crew training and maintenance facilities for such a small group of locomotives proved their undoing. The long overhang of the condensing tender prevented widespread use of the type on other routes.

At least one unit is preserved in South Africa and one was returned to the U.K. despite local objections over the lack of a suitable 3'6" running track ^x . Another unit was rebuilt by SAR as the class "26" or Red Devil secondary combustion experimental locomotive in the 1980s which was designed to exploit the availability of low cost coal reserves in South Africa. [still running - webmaster]

The high pitched whine of a condenser heading the Blue Train will probably never be heard over the Karroo again.

Footnotes

i. The Condensing tenders carried 5,450 gallons of water.

ii. The NC tender carried 10,500 gallons of water.

iii. Basic data: Boiler pressure 225 psi; Grate area 70 sq. ft.; TE: 45,360 Ib.

iv. General Steel Castings Corporation

v. Less than 14psi.

vi. Supplied by GEA, Bochum.

vii. An ambient temperature ranging from 25 F to 110 F and an altitude of 4,500ft

viii. Every 30,000 miles

ix. Presumably in unpopulated areas.

x. The fate of this locomotive is unknown to the author.