View of Lion's regulator from above. The regulator handle is in the closed position, extending to the right. As the lever is pushed to the left, the quadrant causes the regulator rod to be drawn back, opening the plug valve.In the butterfly regulator, the regulator rod merely turned a two-lobed valve across the port face of the main steam pipe which was provided with two wedge-shaped steam ports. With the regulator closed, the valve lobes covered the steam ports, preventing steam from entering the main steam pipe. As the regulator was turned, the steam ports would be progressively uncovered, allowing steam to flow. Illustration of a typical dome-mounted vertical slide-valve regulator
As boiler pressures and output power increased, the widely-used slide valve regulator was developed, often mounted vertically in the steam dome, as shown above. The main steam pipe terminates in a casting provided with a vertical, machined port-face provided with two more-or-less rectangular steam ports. Two flat valves lie over the port face, one on top of the other. Guides are provided which allow the two valves to move up and down independently. The outer valve (the 'pilot' or 'first' valve) connects via a link to a crank at the end of the regulator rod, so that moving the regulator handle pushes the valve up and down. The inner valve (the 'main' or 'second' valve) is also connected to the link but via a slot, such that, on progressively opening the regulator, the first valve is the first to move and the second valve only moves once the first valve is open. A series of ports in the two valves are arranged so that a moderate amount of steam is allowed to flow as the first valve is opened: once the main valve starts to open, a much larger port opening is achieved. This two-stage arrangement is intended to give the driver more accurate control over the flow of steam.In this type of slide-valve regulator, the pressure of steam in the boiler is pressing the both main and pilot valves against the port face, preventing steam from leaking into the main steam pipe when the regulator is closed. But this pressure also makes it hard for the driver to move the regulator, as the pressure prevents either valve from sliding. Novices are frequently surprised at the effort required to initially open the first port. Once steam is flowing, the resistance will be somewhat reduced, but the required effort is still significant. After the pilot valve is fully open, further movement of the regulator handle will start to open the larger, main valve and considerable force may be required. Some locomotives are provided with two regulator handles to operate the regulator rod. This is partly for the convenience of the driver, for instance when shunting, partly so that the driver and fireman can co-operate in opening the main valve. As boiler pressures continued to increase and larger locomotives appeared, the problem of adjusting the regulator became greater. In his larger locomotives, Churchward produced an excellent design of slide valve regulator, using horizontal valves fitted in the smokebox. Perhaps most importantly, he also provided an oil line from the sight feed lubricator directly to the regulator valve. Despite claims made by other designers, the only engines I've worked on which could be said to have "fingertip control" of the regulator were Great Western designs.
So, how about closing a regulator? Even in first valve, the flowing steam tends to interfere with getting the valve to slide closed. The regulator usually needs to be smartly brought against the stop to ensure complete closure. If second valve was open, the problem is increased. It's particularly difficult if second valve was only partly open as it then very hard to get both valves back to their proper, closed position. What usually happens is that the steam is cut off, but with the pilot valve slightly displaced from its normal, closed position. This is detectable from the cab, as the regulator handle may be reluctant to sit hard against the 'Shut' stop on the quadrant, because the misplaced second valve pushes against the regulator linkage and prevents the regulator rod from being moved to the correct, closed position. When the regulator is next re-opened, second valve is immediately partly open and an unexpectedly large steam flow may be obtained. This can be dangerous, for instance, whilst shunting, which is why it is so important for all drivers to have a thorough understanding of their machines. Once a regulator is partly open on second valve, the only reliable technique for full closure is to briefly open the regulator to the fully open position ('putting the handle in the roof') and then very smartly bring it to the fully closed position. After this treatment, when the regulator is re-opened it will open on the pilot valve.
There's more about Regulators in part 2.
(Only took me 9 years to complete part 2 which is why the blog is called 'occasional').
[Link to part 2 added: 11-Sep-2016]