I’m still trying to figure out the working of the Grazebrook engine’s valve gear, up to the point where I can present pictures of the salient features that I understand, and offer them up to the panel for further exploration.
In the meantime, Peter S has drawn my attention to information in a superb book,
A Treatise on the Steam Engine by John Farey, printed 1827.
I’ve only had a quick look so far, but rather than sit on it, I’d like to share some of the information.
I was puzzled about how the blowing pressure was regulated at the near-constant pressure demanded by the blast furnace. John Farey describes the development of various methods. They involve maintaining a reservoir of air at constant pressure, the control of the engine's work rate being a completely separate matter.
One was, in essence, an early gasometer. It was a large capacity cylinder containing a heavy leather-sealed free-floating piston. The bottom of the cylinder was connected to the pipe running from the blowing cylinder to the furnace. The floating piston ensured that the air pressure was constant, and the cylinder provided the capacity to maintain the required steady air flow, bearing in mind that the method was applied to single-acting blowing tubs, so their was no delivery when the tub was inhaling air.
In fact the pressure wasn’t absolutely constant, because of the deadband resulting from inertia and friction at the piston.
A deadweight relief valve was provided on the air regulator. Some examples had a wooden spring which came into action when the piston approached the top of its stroke.
The piston stickage problem was overcome by the much simpler
water regulator.
Imagine a very large cistern containing water, with a large inverted iron box lowered into it so that it is partially immersed. The box is open at the bottom, while the top plate is connected to the furnace's air main by a vertical pipe. The box serves the role of the regulating cylinder (gasometer) described above, the water doing the job of the floating piston. No friction or inertia complications. Some risk of water getting into the furnace, though. Potentially disastrous!
Descriptions start here:-
A treatise on the steam engine : historical, practical, and descriptive
It is sobering to consider that these systems were being developed in the late 1700s, an era when men were wearing three-cornered hats, and the fastest way of travelling was by horse.
Looking at the Black Country archive photos, the Grazebrook engine evidently had the floating piston arrangement, complete with a deadweight relief valve. Relatively small (the tub being
double acting):-
Black Country History