From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV water regulators
Date: Thu, 30 Mar 2006 23:10:02 -0500
Message-ID: <mm8p22lhnc496fv48nvmecbc34f6m2ol2b@4ax.com>

Here's a cross-section of a water pressure regulator:

http://www.keidel.com/mech/pvf/valve-pregulator.htm

The explanation on that page of how it works isn't quite right.

The water inlet is on the left and the outlet on the right.  The valve
that controls the flow is the moveable part at the bottom.  The rubber
seal is the black ring (drawn as a pair of black rectangles) that
presses against the seat in the center of the drawing.  It works thus:

At rest the spring in the top pushes the diaphragm down and along with
it the valve stem and seal.  The valve is open and there is a flow
path between the inlet and the outlet.

The outlet pressure acts on the bottom side of the diaphragm, forcing
it upward as the pressure builds.  As the diaphragm moves upward
against the spring, the valve stem and seat also move upward,
gradually closing off the valve.  When the pressure is sufficient, the
seal presses against the seat and the water flow is stopped.  That is
the way it remains until there is flow on the outlet side.

When water is drawn from the downstream side, the pressure drops, the
spring pushes the diaphragm and valve stem down and the valve opens.
This opening action slows the pressure drop in the outlet.  At some
point the spring pressure and the water pressure on the diaphragm
reach an equilibrium and the motion stops.  The valve is now open
whatever amount it takes to supply sufficient water to feed the
demand.

If the water demand increases, the pressure will drop, the diaphragm
will be forced down some more by the spring and the valve will open
more until the pressure is in equilibrium again.

Note that the diaphragm has to move for each increment of flow. That
means the spring compression and thus the force exerted changes.  As
the diaphragm moves down to open the valve more, the spring pressure
decreases.  This means that the outlet pressure also decreases just a
little.

This is a characteristic of any purely proportional feedback
controller (the valve is open in proportion to the water demand). Over
its range of proportional control (valve closed to valve fully open),
the pressure will vary according to the compression rate of the
spring.  This is why the dead-head pressure will ALWAYS be a little
more than with any amount of water flowing.

Short stout springs such as used in these compact regulators have a
high compression rate and so the pressure varies a lot over the range
of flow control.

More sophisticated industrial pressure regulators use springs up to
several feet long and/or other constant pressure devices (dead weight
- gravity, pneumatic or hydraulic systems) that can limit the pressure
variation to under a PSI for a very wide range of flows.

To more easily visualize this, consider an engine valve spring vs a
screen door spring.  A car valve spring may have 100 pounds of seat
pressure and yet with only a half inch of movement, be up to 350 lbs
or more.  Then think of a long, skinny screen door spring.  It takes
very little more force to extend it to double its rest length than it
does to extend it that first little increment.

There are various techniques to reduce the pressure swing in compact
regulators.  The most common is the dual stage regulator, essentially
two regulators in a row.  The first stage eliminates most of the
effect of supply pressure drop with flow, presenting an almost
constant pressure to the second stage.  The second stage has an
essentially constant input pressure so it can regulate much more
tightly.

Now lets consider the matter of dead-head creep.  Under normal
conditions, the pressure quits rising when the valve stem moves enough
to completely seat the rubber seal.  But suppose something is wrong,
say, the common occurrence of wire-drawing of the brass seat.

Wire drawing is caused by very high velocity material passing across a
malleable material.  Say, by a grain of sand holding the stem a little
open.  Wire-drawing damage looks like a thin wire has been hammered
into the seat, making a tiny radial, hemispherical groove.  It looks
like the cross-section of a wire-drawing die, thus the name.

With a wire-drawn seat, there is still a small flow path even with the
stem fully seated.  One would think that the pressure would gradually
rise to the supply pressure but that doesn't happen because such
faults are anticipated in the design.

The rubber seal is fairly thick and pliable.  Thus, as the pressure
continues to rise after the stem is seated, more and more force is
exerted against the diaphragm and therefore the stem.  The rubber seal
is compressed and the material bulges into the wire-drawn defect,
sealing it off.  At that point the pressure rise stops.  It works the
same way if a bit of sand, steel filing or something similar gets
embedded in the brass seat.  The seal rubber is simply forced around
the defect by the increased pressure.

This is a characteristic of a seat defect - that the pressure will
slowly rise a little for a short time after flow is stopped.  That
pressure increase is the prime indication that it is time to do
maintenance on the regulator.

John

On Thu, 30 Mar 2006 11:21:57 -0800, altar@nospam.net wrote:

>On Thu, 30 Mar 2006 13:17:40 -0500, Neon John <no@never.com> wrote:
>
>>If there are little holes inside the regulator then it is defective or
>>at the end of its life.  The regulator has a rubber seal that seals
>>off bubble-tight.  This pretty developed "technology".
>
>The defective is in my assumer. I assumed that the water pressure was
>regulated by little holes. Your explanation cleared that up for me.
>
>Thanks,
>
>Tom