Re: Balanced Throttle

Imagine you want to open and control a 4" diameter opening for 150# of steam or air pressure. If you remember your math, some type of cover over the hole would be 12.56 square inches of valve surface times 150 = 1884 pounds of force acting on the face of the valve, which means you would need to overcome that force to open the valve. As size and pressure increase, so will the force on the valve that you will have to overcome to open it.

The balanced throttle valves I've dealt with are a hollow piston with a seating surface at the top and bottom, with the cylinders connected to a chamber between the seats. When you open the throttle, the linkage mechanism lifts the valve up. This allows the steam or air on the outside of the valve to flow past the seats into the valve and to the cylinders. Because the steam or air pressure is acting equally in all directions, the valve is "balanced" and only requires the force to overcome the friction of the linkage and weight of the valve spool.

Slide valves in steam chests also have to be "balanced", otherwise the pressure acting on top of the slide valve would quickly grind it into the seat, if the valve gear could move it. On the top of the slide valve, there will be balancing strips around the edge of the valve that seal the top of it against the top of the valve chest. This prevents steam (or air) pressure from acting against most of the top surface of the slide valve. They are engineered to provide a reasonable amount of force to keep the valve seated. Piston valves are similar to throttle valves, and are a naturally balanced design.

Locomotives with front end throttles use a series of poppet valves. (thinks of the exhaust and intake valves on your car's engine.) These open and close in sequence as the throttle linkage rotates a cam under them. I don't recall if these are a balanced design or the individual valves were small enough that the mechanism could overcome the force acting on them.

Air brake valves are extensively engineered and work on the principle of various pressures and their resulting forces. Their slides valves have to take into consideration differing pressures acting on top or under the valves, and how much force is needed to move the slide. I know of at least one design that to change the sensitivity would actually add air pressure to a chamber underneath the slide valve to reduce the force acting on it and make it easier for the piston to move.

When #12 arrived at the Loop, it was equipped with a strange butterfly type throttle valve. I guess this would also be considered a balanced valve the pressure would be acting equally on the half of the valve that rotates away versus the half that opens against the pressure.