hydraulic head in control structure

[sportsnut44]

On a typical flow splitter manhole with a weir on a drainage system, the water quality flowrate is sent to a pipe connected to a treatment train, and flows in excess of the water quality flowrate are sent through another pipe that acts as a high flow bypass. The low flow pipe for water quality flowrate is on the same side of the weir as the inlet pipe. The high flow bypass pipe outlet is on the other side of the weir. The weir is a simple concrete baffle wall.

If a turned down elbow is used on the low flow outlet pipe with an orifice on end of the turned down elbow, is the hydraulic head from the water surface elevation (or top of weir) to the center of the horizontal outlet pipe or to the bottom of the turned down elbow where the orifice is located? Text in design documents say the hydraulic head is to the "center of the orifice" but I believe that statement is meant for when there is an outlet pipe without a turned down elbow with orifice. Can someone please help jog my memory and clarify this for me? Thank you.

 

[cvg]

top of weir

 

[sportsnut44]

Yes, top of weir, but measured to what when the low flow outlet is a turned down elbow with an orifice plate on the bottom of the elbow? Is it measured from the top of weir to the bottom of the turned down elbow where the horizontal orifice plate is located, or to the invert of the pipe outlet, center of pipe outlet or top of pipe outlet? Since the pipe outlet is going to an oil water separator where where when flowing the water level will be at or near the top of pipe inlet and outlet of the oil water separator, I'm thinking maybe the hydraulic head is the distance from the top of weir to the top of outlet pipe.

 

[cvg]

I assume the orifice is controlling the flow, so head is measured to the center of the orifice. If some other piping or fitting is controlling the amount of flow, than your orifice would be ineffective.

 

[sportsnut44]

Thanks for you responses cvg. Follow-up question...if the orifice is controlling flow to the outlet, but the orifice is submerged, would the hydraulic head be the difference between the top of weir and the water level in the outlet pipe (water level in outlet pipe is a result of the flow allowed by the orifice), i.e. the water level in the outlet pipe essentially being a tail water elevation?

 

[cvg]

well again, same question - is the orifice controlling the flow? or is the inlet pipe or is the outlet pipe? If this is simply a submerged orifice, than use delta h.