A. A piston valve is a cylindrical spool-shaped valve constructed with
packing rings much the same as the steam piston that moves through the
cylinder, except that a piston valve is double or composed of two
pistons connected by center rod or spool working in a bushing of equal
diameter. Steam and exhaust ports are cut through this bushing; steam
ports to the cylinder and exhaust port to the exhaust pipe. There is
also a steam port for live steam from the boiler. As the pressure on
this valve is equal in both directions it is practically balanced.
27. Q. What is a balanced slide valve? How is it balanced, and why? For
what purpose is the hole drilled through the top of the valve?
A. One in which the steam pressure on the top and bottom of the valve is
nearly equalized. This is done by protecting a portion of the top of the
valve from the steam pressure. It is usually balanced by strips held
against the pressure or balance plate by one or more springs. This is
done to prevent live steam from getting on top of valve and thus relieve
the valve from the top pressure which would cause excessive friction
between the bottom of the valve and its seat. The hole through the top
is to allow any steam which might leak by the strips to pass into the
exhaust, so pressure could not accumulate on the top of the valve, also
to equalize the exhaust pressure between the top of the valve and
exhaust cavity as well as to assist in lubricating the balance plate.