[VAC] Re: Water Pressure

[John Anderson <janderson@xxxxxxxx>]

What a great topic!  Wouldn't an Airstream trailer and all its systems make a
great high school physics lab?  Actually, I'm a bit nervous about wandering off
topic and facing inevitable (and probably appropriate) chastisement.

The principle behind all of this is, like James Clark said, flow in a fluid
system is proportional to (the square root of) the change in pressure resulting
from any flow restriction in the system, such as an orifice, throttled valve,
pipe elbow, or even the pipe itself.  Of course, there are multipliers based on
the size and characteristics of the orifice and of the fluid.  But all we really
need to know is that when flow ceases in a filled system, there is no longer any
pressure drop (other than associated with changes in elevation combined with the
force of gravity, etc., as Cruiser Bob notes).  So we need a variable orifice
(regulator) that is smart and quick enough to keep up with our irregular habits
of water use and keep our tubes intact.

I actually never gave much thought to excessive park water pressure until one
day I hooked up in a new park and saw water streaming out a point on the bottom
of my trailer as soon as I opened the hose bib.  It took me a while to figure
out that it was my water heater relief valve lifting, from the high park water
pressure.  So there is an argument for not plugging your water heater relief!

One problem with multiple regulators is that they all have a time function,
i.e., they are designed to work fairly fast, but they don't respond
instantaneously to changes in pressure.  So with two in a system, they can force
each other into an oscillation that would spit out no more than a few dribbles
of water at a time.

John