In MMD 130520, under the title "Building a Transparent Player Piano
Action Model",  John Tuttle delves into the design parameters of
valves.  His observations and wondering concerning bleed size, area
ratios, and other valve characteristics is something I have long
studied.  I too, am still searching for some of the solutions that
he seeks.

Taking his questions in the order that he posted, here are my first
comments, which may not answer entirely his questions.

> 1. Does the square area of the pouch have to be bigger than the
> square area of the valve seat?

To move the valve to the active position, the working area of the
pouch must generate enough force to overcome the force that the
atmosphere is placing on the valve.  All parts of the valve except
the area covering the opening in the inside valve seat are subject to
atmospheric pressure.

The opening in the valve seat has what we call stack vacuum, but in
reality, it is a partial vacuum, and is  something less than normal
sea level pressure of 14.7 pounds per square inch (psi).  It is this
imbalance in pressure, called differential pressure, but commonly
called stack vacuum, that makes the system work.  This stack vacuum
acting on the valve must be overcome by the pouch to move the valve.

Additionally, if the valve moves vertically and moving it to the on
position is upward motion, the weight of the valve must also be
overcome by the pouch.  Standard Action valves move horizontally, and
thus there is no weight factor in that design.

The diameter of the pouch has an affect on its ability to move the
valve.  It is the lifter disk attached to the pouch, covering a portion
of the pouch area, that transmits power to the valve that makes the
valve work.  There are some forces that we can overlook for the moment,
but it is the lifter disk that must be larger in diameter than the
inside valve seat hole that makes the valve function.

To illustrate this concept, think of a balloon.  It is easy to push the
point of your finger into the balloon and displace the surface.   It is
much harder to displace the surface the same amount using the palm of
your hand.  The power of the inflated balloon is much more effective
against the larger surface of the palm, but not nearly as effective
against the finger.   This is the concept of the pouch lifter disk.

Another illustration of the pouch power is the old fashioned game of
blanket toss.  While four or more people stretch the blanket holding a
person, each player exerts only a portion of the energy needed to lift
the person in the blanket.  Here, the people holding represent the
pouch at its glue joint around the perimeter of the pouch well.

We can also think of the pouch as a piston to approximate its power.
But since the pouch will "blossom" we don't get all the power and we
must realize it is an imperfect piston.  The lifter disk helps correct
the "blossom" factor.

When John asks about the pouch size in relation to the "valve seat",
that really isn't the issue.   The issue is pouch _lifter disk_ versus
the _opening_ or the hole of the inside valve seat.  But the valve seat
does play a role.

If the valve diameter is large and the seat is large, when the valve
first starts to lift, the area in which the valve and seat overlap will
be subject to a lower pressure as atmosphere starts flowing into the
partially open inside valve hole.  This creates a larger area of low
pressure acting on the valve.  It can be so strong that the pouch power
is overwhelmed, and the valve never fully opens.

Some designers minimized this by using grommets as inside valve seats,
thus reducing the seat area to a minimum.  When grommets are not used,
a successful design will minimize the area of overlap.  Additionally,
the valve facing surface becomes a factor in the overlap area.  Very
smooth facings will produce laminar flow characteristics while fuzzy
suede surfaces will have turbulent flow.  The turbulent flow is better
as it can't produce low pressure areas that hinder valve movement.

This is probably more comment on John's first question than most people
care to hear.  I didn't give any firm data, just general concepts.

Valve design is not a simple exercise.  I'd like to hear other
comments, and I will get around to the rest of his questions later.

Bob Taylor
Missouri