Duplex steam pumps consist of two pumps placed side by side, with the valves of each cylinder deriving its motion through the piston rod of the opposite cylinder. When one piston has nearly completed its stroke, the piston of the second cylinder is put into motion and in turn, effects the slide valve and piston of the first cylinder. This arrangement being quadruple-acting, ensures a constant and steady flow of the pumped material and because one or other of the steam valves is always open, the pump will start on any part of the stroke.
The steam end twin type cylinders are cast integral with cradles. The steam valves are of the slide valve type operating by direct lever connections. The steam pistons are cast iron with spring rings. The valve gear is supported on cast iron cross stand with bearings for the rockshafts.
The liquid end is of the bonnet and valve plate pattern, with the larger sizes having side access doors for suction valves. The valve type depends on the pumped material but can be for example rotative disc type or wing-guided metal valve type. The pistons are cast iron with leather packing with cast iron or bronze rings.
Plain fitted pumps have steel piston rods, cast iron leathered buckets and rotative disc valves. Gunmetal fitted pumps have bronze piston rods, gunmetal lined pump barrels, gunmetal buckets and gunmetal wing valves and seats, with brass springs or disc valves. All iron fitted pumps have steel piston rods, cast iron buckets and cast iron wing valves and seats with steel springs. Hard cast iron removable liners were fitted as an extra.
There are four variants in the 670 duplex range. There is the series 'A', designed for 'General Service' duties such as boiler feeding. Series 'B' which was designed for 'Low Service' applications, series 'C' for 'Large Capacity' and series 'D' for 'High Pressure' applications with piston valve steam chest and mechanical lubricator.
- Series A ranged in sizes from 2½"x1½"x3" up to 10"x7"x10"
- Series B ranged in sizes from 2½"x2"x3" up to 10"x8½"x10"
- Series C ranged in sizes from 12"x9"x12" up to 20"x14"x12"
- Series D were specific to individual applications
Series 'A' and 'B' are the smallest and most suitable for restoration, their specifications are shown below.
|670 Series A||670 Series B|
Gallons / hour
Gallons / hour
There are four Joseph Evans Duplex steam pumps in the collection.
- Duplex horizontal 2½x1½x3, No.26968, 1930, 'Midlands' Gas Works
- Duplex horizontal 4x4x4, No.29798, 1936, East Greenwich Gas Works
- Duplex horizontal 5x4x5, No.39275, 1954, Caerphilly Tar Plant
- Duplex horizontal 5x6x5, No.29112, 1934, St Anthony's Tar Plant
Duplex horizontal 2½x1½x3, No.26968, 1930, 'Midlands' Gas Works
Size: 2½" steam cylinders, 1½" pumping cylinders with 3" stroke.
Series: 670 "A" General Service version, gunmetal fitted.
This is the smallest duplex pump manufactured and is believed to have originated from a gas works in the Midlands area. It is a gunmetal fitted pump, with bronze piston rods, gunmetal lined pump barrels, gunmetal buckets and seats with brass springs and valves. The side arms are telescopic comprising a solid rod sliding within a tube to maintain parallel motion.
This pump was acquired in June 2006, apparently it had been in storage in private hands for approximately 32 years. It was fully restored in August 2006 and runs well on compressed air. It is now on display on open days at Twyford Waterworks near Winchester.
Duplex horizontal 4x4x4, No.29798, 1936, East Greenwich Gas Works
Size: 4" steam cylinders, 4" pumping cylinders with 4" stroke.
Series: 670 "B" Low Service version, iron fitted.
This pump was purchased new in 1936 for the Tar Plant extension being built by Simon Carves at Ordnance Wharf, Metropolitan Gas Works, East Greenwich, SE London. It was installed as one of two crude tar feed pumps for the new No. 12 Wilton tar distillation unit. In 1971 the coke oven batteries and tar distillation plant were closed down and the equipment offered for sale. The Greenwich Dome has since been built on the cleared site, now known as the O2 Arena.
Thomas Ness Limited purchased several pumps and other related equipment from Greenwich for their own tar plants at Avenue, Chesterfield and Caerphilly, South Wales. This pump was purchased by Caerphilly and installed as a tar feed pump on their No. 1 Distillation unit in 1974.
Following closure of the Caerphilly tar works in 1985, the pump was purchased by Alan McEwen along with several others and taken to Yorkshire. For various reasons the pumps remained untouched until 1996 when they were offered for sale again.
The pump was repurchased from Alan McEwen and transferred to Essex in the October. It has since been fully restored and runs well on compressed air. This pump is in storage under cover in Wiltshire.
The other second identical duplex pump, No. 31417 is owned by Norman Heath and is on display at the Museum of Power, Langford, Maldon in Essex.
Duplex horizontal 5x4x5, No.39275, 1954, Caerphilly Tar Plant
Size: 5" steam cylinders, 4" pumping cylinders with 5" stroke.
Series: 670 "A" General Service version, iron fitted.
This pump was purchased new in 1954 for the Tar Plant extension being built by Simon Carves at Caerphilly, effectively doubling the capacity. It was installed as the crash tank pump for the new No. 3 Wilton tar unit. Its duty being to circulate and pump away the pitch drainings from the coils within each furnace following either a planned or emergency shutdown.
Its use was therefore rather infrequent, but when required to pump it often failed to pick up suction, especially when hot. It was then decided to replace the pump with a Lee Howl Tipton No.28695, 6x5x12. The duplex was removed in the early 1980's and placed in storage never to be used again.
The pump was acquired from Caerphilly in 1991, now in storage in Wiltshire where it has been partially stripped ready for restoration.
Duplex horizontal 5x6x5, No.29112, 1934, St Anthony's Tar Plant
Size: 5" steam cylinders,6" pumping cylinders with 5" stroke.
Series: 670 "B" Low Service version, iron fitted.
This pump was purchased by W.C.Holmes & Co Ltd in 1934 for the new coke oven battery at Norwood Coke Ovens, Dunston, Gateshead, Co Durham. Prior to nationalisation, Norwood was part of the Newcastle upon Tyne & Gateshead Gas Company and W.C.Holmes were contracted to build the new gas purification and by-product plant for the new extension, which included crude tar handling.
In the early 1980's Norwood Coke Works closed, leaving Norwood Tar Plant to continue production on the site. During demolition of the coke works all tar pumps were transferred to the adjacent tar plant for future use. Those items surplus to requirements were eventually taken to Caerphilly Tar Plant in South Wales for storage.
The pump was acquired from Caerphilly in 1989 and has been fully restored; it is now in storage in Wiltshire.