Gentlemen,
I confess that I don’t know much about the Peter Brotherhood radial steam engines (or their hydraulic motor counterpart). I know that they were widely used on all sorts of applications, including ships’ steering gear (on these engines, steam admission was controlled by sleeves around the crankshaft at one end - a distributor-type of arrangement). A Brotherhood engine was also used on this crane locomotive:-
The three cylinder engine, used for slewing, is just visible beneath the counterweight. However, although a good number of these crane engines were made for steelworks and shipyards, most seemed to favour two cylinder engines for slewing.
Radial engines were also adopted by the British Navy for torpedoes.
The hydraulic version was widely used, for example for powering capstans.
Back to steam, and the question of compounding, I came across the drawing below, and wonder whether the stepped pistons signify that it was a compound? I don’t know whether the steam could be passed from an HP cylinder directly to an LP, or whether a receiver would be required.
The Brotherhood radial arrangement had advantages of compactness, simplicity, and full enclosure. Being only single-acting was clearly a disadvantage, but, apparently, it greatly reduced the tendency to knock (because the direction of the load on the bearings was always in one direction).
Going back to the rolling mill engine, the supply pressure was only about 50 psi. It developed about 350 HP.