Cataract (beam engine)
A cataract was a speed governing device used for early single-acting
The cataract is distinctly different from the centrifugal governor, in that it does not control the speed of the engine's stroke, but rather the timing between strokes.
Operation
The typical installation of a house-built beam engine spanned four floors. The cylinder and the engine driver's usual working position were located in the 'bottom chamber', approximately at ground level. Above this were the 'middle chamber', with the cylinder top cover and 'top nozzle' (the upper valve chest), and above that the 'top chamber' or beam chamber.[3] The cataracts were located in the lowest part of the engine house, in a chamber below the bottom chamber, along with the exhaust pipe. This space was awkward to access and not visited in normal operation.
The
Use of a cataract could allow an engine to be operated at only a third of its ungoverned speed.[8] When pumping load was variable, cataracts could also be connected and disconnected as required, allowing the engine to work at full speed for a period and then stopped in between.[9]
The cataract itself resembled a small plunger pump.[2] It was an iron box in a cistern filled with water, with a plunger or piston set in the top and pressed downwards by a weight. The water within the pump could only escape through a small tap or valve.[10][11] As the plunger gradually fell, its motion was passed upwards by a rocking lever and a rod to the valvegear in the middle chamber. Once the rod had risen sufficiently, this opened the first valve to admit steam into the upper part of the cylinder, beginning a new stroke.[ii]
Once the stroke had begun, the cataract's rocking lever was pushed downwards by the engine. This lifted the plunger, which acted as a suction pump within the cataract to refill the plunger box, through a flap valve from its surrounding cistern.[13] The cistern was kept filled with water by the pump that the engine itself was working.
The water outlet valve was controlled by a rod from the bottom chamber. This was used by the engine's driver to control the working speed, according to the work required.[13]
The cataract's actuating rod also had a screw adjuster, which acted to vary the water injection time (Newcomen) or the phasing between the inlet and exhaust valves (Cornish).[13] This could be used to give a longer and more effective condensation time, if the condensing water supply was warm, as in the Summer. This adjustment appears to have been poorly understood though, and little used by the engine drivers.[10]
Development
Early cataracts
The cataract first appeared on
Watt
Boulton and Watt used the simple tumbling box design of cataract for some years afterwards, to around 1779.[15][16] After this other designs were used, including a water cataract where the same water was used and recycled continuously and also an air cataract using a circular bellows. An air cataract of this type was supplied for the Ale and Cakes Mine. The plunger pump design of cataract had appeared in Cornwall by 1785, but was not Watt's invention.[15]
Later cataracts
The term 'cataract' became a synonym for dashpot, at least where this was associated with steam engines and their governors. They were used as a damping device to avoid over-sensitivity with centrifugal governors.[17]
Cataracts were also used as an over-speed safety device for direct-acting water pumps.[iii] A seesaw or 'differential' lever was placed between the pump's piston rod and a cataract adjusted for the pump's normal working speed. If the pump suddenly accelerated, owing to the pump bursting or similar, the piston would overtake the cataract and the action of the differential lever would then close the pump's steam inlet valve and stop the pump, limiting possible damage.[18]
Open loop control
The cataract, like most regulators, is an example of a
Synchronisation
One advantage of the independent and open loop nature of the cataract's control was that two engines could be adjusted to run in synchronisation, but in
Centrifugal governor
Although the centrifugal governor was already known from its use for water- and windmills, it was not until 1788 when Watt was the first to apply it to a steam engine.[20] This was the 'Lap Engine', an early rotative engine now preserved in the Science Museum, London.
With a rotative engine, it was necessary to control the rate at which an engine moved throughout its stroke, not merely to vary the timing between strokes. This required the use of a
Cornish engines were not amenable to control by a throttle valve, as their operating cycle depended on the condensation time more than a throttled steam supply. Non-rotative beam engines also had no easy means to drive a centrifugal governor. For these reasons the cataract remained in service for as long as the Cornish engine did.[22]
Notes
- ^ Some Cornish winding engines had their valves arranged across different number of arbors, although their basic operation remains the same.
- ^ This was the initiation of a stroke with the Cornish cycle. For a Newcomen engine, the cataract triggered the water injection valve that caused condensation in the cylinder,[12] and thus the beginning of the power stroke.
- ^ These were the type of small reciprocating pump commonly used as boiler feedwater pumps and often described as the 'Weir' type.
References
- ^ Woodall (1975), p. 29.
- ^ a b Evers, Henry (1875). Steam and the Steam Engine: Land and Marine. Glasgow: Williams Collins. pp. 60–61.
- ^ Woodall (1975), pp. 29–30.
- ^ a b (Woodall 1975, pp. 31–33)
- ^ Clark, Daniel Kinnear (1892). "3: A Treatise on Engines and Boilers". The Steam Engine. Vol. II. Blackie & Son. pp. 275–276.
- ^ Farey, John (1827). A treatise on the steam engine: historical, practical, and descriptive. Vol. 1. pp. 188–189.
- ^ Farey (1827), p. 187.
- ^ Clark (1892), p. 339.
- ^ Clark (1892), p. 203.
- ^ ISBN 0903485354.
- ISBN 0-9536523-3-5.
- ^ Clark (1892), p. 169.
- ^ a b c Kelly (2002), p. 56.
- ^ Dickinson & Jenkins (1927), p. 46.
- ^ ISBN 0-903485-92-3.
- ^ Clark (1892), p. 365.
- ^ Clark (1892), pp. 67, 182.
- ^ Clark (1892), p. 281.
- Kew Bridge Steam Museum. Archived from the originalon 2018-09-25. Retrieved 2017-01-12.
- ^ a b (Dickinson & Jenkins 1927, pp. 220–223)
- ^ Woodall (1975), p. 49.
- ^ 'Victoria' Pumping Engine, East London Waterworks, (Clark 1892, pp. 275–276)