Re: Balanced Throttle

Most every steam engine out there has some sort of balanced throttle. The idea is to get the steam pressure to assist in opening the throttle valve otherwise you have boiler pressure pushing down on the throttle valve and it would be very difficult to open and adjust.

Most throttles use a double seated spool valve arrangement, sort of like a piston valve above the cylinders. The valve has 2 seats, the lower one is smaller than the upper one. When the valve is opened, steam comes in both from the top and the bottom, then heads down the dry pipe to the valve chests. Steam is pushing in both directions on the valve spool which gives a balancing effect on the valve.

As both seats face the same direction, on one seat (usually the top one) steam is pushing the valve down and closed, while on the bottom seat seat, steam is pushing up and open, creating the balance effect. Generally the top seat has a larger surface area than the bottom seat which creates an "underbalance" that will make the throttle easier to close than open - and keep the throttle closed when unattended.

There are other throttle valves out there that in addition to the balanced seats, used a "pilot valve" in thier design that upon cracking the throttle open, allowed a small amount of steam to enter the dry pipe before the main valve seats opened, assisting in the balance effort.

FWIW, 490's had notoriously bad balance on thier throttles, being very difficult to open and adjust. The 497 had it's throttle valve changed when in service on the D&S. A home made throttle was designed and fabricated by Mark Yeamans. It uses a pilot valve and a single seat main valve. It works very well.

Then there are mulitple valve front end throttles, but I won't go into that.