Hydrostatic Water Pressure

[Daiji]

This may be a dumb question, but I can't seem to find anything to help me determine an answer so I thought I'd see if maybe someone might could point me in the right direction.

In my industry, we use a column of water in a down tube(open to the atmosphere) to achieve various head pressures. In one of our down tubes, the diameter is 15" by 20' in length and is constricted to 6.75" by a nozzle at the base of the down tube. My question is, if you start out with a certain size diameter in a column of water and then nozzle it down to half that size, how do you determine how much head pressure you have coming out of the nozzle?

 

[francesca]

See this FAQ.

 

[RWF7437]

The static presuure is simply the wieght of the water above. The size, shape and material of the tube have no effect on static pressure. Francesca's answer will lead you to other insights, such as the probable flow, but the pressure is easily determine from the height od the fluid column.

 

[Daiji]

I appreciate yall responding to my question.
Initially, I was under the same understanding as you RWF7437, that the size, shape and material have nothing to do with the pressure except for the height but our results out in the field make me believe that it does have a slight affect on head pressure. What makes this even harder to determine is that there is not an easy way to determine initial flow rate because what we use this column of water for is to invert a liner or flexible tube into an existing drainage pipe or storm drain for rehabilitation. I believe the height of the fluid column is giving me pretty close answer but I am wanting to find exactly what I have if yall have any more suggestions.

 

[cvg]

note that "static" pressure refers to the pressure at zero flow. Once flow begins, there will be friction which causes a pressure drop at the nozzle. The amount of the pressure drop depends on both the velocity of the flow and the geometry of the nozzle.

 

[Daiji]

Your right, but once flow begins I have no way to determine my flow rate or velocity because the liner is hand fed into the down tube. The only information I have is the static pressure and geometry of the nozzle. Do I solve the down tube in separate pieces? Find the flow rate for the 15" tube and use that information to determine what happens in the nozzle?