Saturday, August 29, 2009


Originally a throttle valve was a plain slide valve that moved upon a seat in which were ports similar in operation to the steam ports in the valve chests, just smaller in size.

The problem with that kind of valve was that the pressure of the steam on it when closed made it very difficult to open the throttle gradually, or to regulate or adjust it while open.

These difficulties were solved with the invention of the double poppet valve, seen below.

The valve J, has two circular disks, J1 and J2, which cover two corresponding openings in the upper and lower portions of the valve body, which is attached to the dry pipe L.

When the valve is opened, steam flows around the edges of both J1 and J2 equally into the dry pipe, balancing the forces acting on the valve.

Steam pressure from the boiler acts equally on the top of disk J1 and on the bottom of disk J2, making the valve a balanced valve, tending to keep the valve in whatever position the engineer placed it in, open, closed, or any position in between. This made the valve very easy to operate.

However, disk J1 was made slightly larger than disk J2, since - during the assembly of the valve - disk J2 had to be inserted through the opening for disk J1. As a result, there was a bit more pressure on the slightly larger top surface of disk J1 than on the bottom surface of disk J2. This tended to keep the valve closed when the locomotive was not moving. This was a good thing... No engineerless locomotives drifting off down the tracks slowly gathering speed.

The throttle rod K, passed through the rear of the boiler head through assembly K1, called a steam-tight stuffing box, then attached by various linkage to the throttle lever in the locomotive cab.


valve actuator said...

Complex control systems using valves requires an automatic control based input of an actuator. The actuator strokes the valve allowing the valve to be positioned accurately and allowing control over a variety of requirements.

Bob said...

Any additional information and/or explanation is always welcome.