Parkandillick Gallery 3: This page has images of the Middle Chamber which houses the top of the 50″ cylinder, the valve chest and the piston rod rising to the beam above.
A feature of special interest is the arrangement of three valves in the top valve chest or ‘nozzle’. These usually stand in line with the valve admitting steam to the cylinder in the middle. In this engine the ‘equilibrium’ valve, controlling the exit of steam on the return (pumping) stroke, occupies this position.
This makes a convenient arrangement for joining to the equilibrium pipe, which can be seen in front of the cylinder, but is not conducive to thermal efficiency because the live steam impinges on surfaces cooled by the equilibrium steam on its way to the cylinder. (Courtesy of The Trevithick Society)
The engine also has the sole, surviving working example of a St. Austell governor – a ratchet device which may be seen upstairs in the middle chamber.
Its purpose is to sense the length of stroke the engine makes and then to adjust the throttle valve to admit more or less steam to keep the stroke constant.
So this was not a normal fitment on a mine pumping engine. But was used in the clay district to enable the engine driver to leave the engine to operate a steam hoist, supplied from the same boiler.
The invention of the St. Austell governor is credited to local engineer, ‘Jackie’ Menire. Other features include the use of hardwood pins in the valve gear to make it work more quietly. (Courtesy of The Trevithick Society)