Gravity Fed Valves on Fermenters

[Brewmegoodbeer]

I am pondering on the science of gravity fed valves we have on our fermenters and bottling buckets. When opening the valve, the height of the liquid above the valve creates pressure to which this pressure pushes the liquid out of the valve. My question is where is the liquid coming from that is coming out of the valve? is it being drawn from the top or is it being uniformly drawn from everywhere around the valve. For example: picture a valve on the bottom of a fermenter. 1 inch below the valve is compacted trub and everything above that is liquid. Would the liquid being drawn into the valve via gravity affect the trub on the bottom as well, or would the liquid being draw into the valve be coming from above the valve?

 

[doomy86]

For example: picture a valve on the bottom of a fermenter. 1 inch below the valve is compacted trub and everything above that is liquid. Would the liquid being drawn into the valve via gravity affect the trub on the bottom as well, or would the liquid being draw into the valve be coming from above the valve?

In my experience it won't. As long as the trub is covered by liquid and the fermentor is kept fairly stable the sediment will not be disturbed. You can see the same effect when filling a glas from a bottle with lots of trub.

 

[ajdelange]

Pressure is really a tensor. It depends on the direction you are considering but in an isotropic uniform medium like beer you can assume that the pressure is the same in every direction at a given head. For a 1/2 wide valve inlet aperture the pressure at the top of the valve is 1/2"*density_of_beer lower than at the bottom. If the beer stands 10" above the valve this is 5%. If it stands 20" high it is 2.5% etc. Thus beer will be drawn into the bottom of the aperture a bit faster than at the top and the chances are that any trub close to the bottom of the valve will be disturbed unless you throttle back enough to prevent that in the critical area.