hydraulic grade line 1

[Thuba]

Good day, l have a hydraulic grade line, but l dont understand why it deeps down where there a valve, but pressure loss in a valve in not that much.

 

[LittleInch]

Incorrect.

Pressure loss in a valve can be very high.

What this is showing is that to remain in tight flow regime, you need to hold a high back pressure.

Is not clear why there are two control valves, one whet shown and one at the terminal. Normally you would only have one control valve at the terminal.

 

[bimr]

The valve at the end of the open channel flow segment also makes no sense. If you have open channel flow, a slide gate is used, not a valve. Valves are used on closed conduits.

 

[Snickster]

This is my interpretation.

Pipe flows full upstream of the peak. Pump has just enough discharge head to get the flow over the peak. Hydraulic pressure curve is sum of pump discharge pressure plus static head plus velocity head minus friction losses. Once flow reaches peak all head is in static pressure head plus velocity head with pressure head zero.

Flow on right side of peak static head is converted to velocity head and any loss in hydraulic head is due to friction losses. I interpret the dashed line as that above the dashed line there is open channel flow and below the dashed line there is full pipe flow. The valve keeps enough back pressure to develop full pipe flow upstream of the valve where the dashed line intersects the pipe. Note that the valve is a control valve and this is what it is controlling.

As LI indicated the pressure drop is across the valve and is the delta P shown in the diagram and equal to the pressure drop induced by the valve to get full flow upstream. Downstream of the valve the head is the sum of pressure head plus static head plus velocity head. Static and pressure head is converted to friction and velocity head is lost when the fluid hits the ground so at "D" total head is zero.

 

[Thuba]

Incorrect.

Pressure loss in a valve can be very high.

What this is showing is that to remain in tight flow regime, you need to hold a high back pressure.

Is not clear why there are two control valves, one whet shown and one at the terminal. Normally you would only have one control valve at the terminal.

Ok noted, thank you

 

[Thuba]

The valve at the end of the open channel flow segment also makes no sense. If you have open channel flow, a slide gate is used, not a valve. Valves are used on closed conduits.

its a conduit/pipe with slack flow.

 

[Thuba]

This is my interpretation.

Pipe flows full upstream of the peak. Pump has just enough discharge head to get the flow over the peak. Hydraulic pressure curve is sum of pump discharge pressure plus static head plus velocity head minus friction losses. Once flow reaches peak all head is in static pressure head plus velocity head with pressure head zero.

Flow on right side of peak static head is converted to velocity head and any loss in hydraulic head is due to friction losses. I interpret the dashed line as that above the dashed line there is open channel flow and below the dashed line there is full pipe flow. The valve keeps enough back pressure to develop full pipe flow upstream of the valve where the dashed line intersects the pipe. Note that the valve is a control valve and this is what it is controlling.

As LI indicated the pressure drop is across the valve and is the delta P shown in the diagram and equal to the pressure drop induced by the valve to get full flow upstream. Downstream of the valve the head is the sum of pressure head plus static head plus velocity head. Static and pressure head is converted to friction and velocity head is lost when the fluid hits the ground so at "D" total head is zero.

Thank you for available information. Does is it mean the dashed lined is hydraulic pressure curve for open channel flow?

 

[swazimatt]

that is not a standard gate valve symbol and looks similar to a PRV valve symbol i have seen before (backed up by the significant HGL drop)

you need the high initial head to pump over the peak, but that could mean you get excessive pressure at discharge, hence the PRV. With that head loss i would think you would only get slack flow without the valve - the valve will back up the flow and remove the slack flow IMO

 

[Snickster]

Thank you for available information. Does is it mean the dashed lined is hydraulic pressure curve for open channel flow?

I don't think so. I think it just represents where full pipe flow exists. Above the dashed line is open channel flow just as it indicates in the diagram. I believe the pressure curve is the thick line as shown. Up to the valve the energy of the fluid is still the entire static head at the peak plus velocity head minus some head loss due to friction so hydraulic curve is still relatively high. With slack flow all static head at the peak is converted to velocity head as it falls down the pipe with losses due to friction making the hydraulic line drop slightly.

On the upstream side of the valve you still have most of the energy available as you had at the peak just minus friction losses. Then right at the valve there is high delta P across the valve = vertical drop of thick dark line. Then thick dark horizontal line from there is continuation of hydraulic line. I think the vertical dark line between the top of the valve to where the thick dark line goes horizontal at the valve is an error and should be erased.

"D" is where the hydraulic grade is at the end of pipe which is due to velocity plus static height above the datum point (which is horizontal line at very bottom, then goes to zero at "B" when fluid hits the ground.

 

[GD2]

Thank you for available information. Does is it mean the dashed lined is hydraulic pressure curve for open channel flow?

It essentially shows the water surface elevation. It's open to atmosphere, hence there will be no pressure head.
The bold line HGL is surely for the conduit flow, i.e through the pipeline.