Hydraulic Analysis

[gman348]

Here is my situation:
Everything I can find in my hydraulics books from college show that losses are added to the Total Energy Grade, EGL. Then the velocity head is subtracted from that value to obtain the hydraulic grade elevation. This, in turn, produces a sloping HGL across a structure if there is a change in pipe diameter. This is the way I have been analyzing storm drains since I got out of college. HEC-22 shows that losses are added to the downstream HGL then the pipe velocity head is added to obtain the upstream EGL. So the HGL and EGL slopes are not parallel in a run of pipe. Therefore, the velocity in that pipe is now dynamic.

Has anyone ever encountered this situation? What is the correct approach for summing head losses in a system? Am I just missing something?

 

[BRIS]

Are we discussing full or part full pipe flow.

 

[gman348]

My topic is for pressure flow, and should somewhat be the same for free flow.

 

[BRIS]

I am not sure I understand your problem. For full pipe flow the HGL is always sloping and is parallel to the EGL. At a change in diameter there is a step in both the HGL and EGL. The step in the EGL is equal to the head loss across the structure. The HGL is derived from the EGL - V^2/2g (SI units). (This is assuming by HGL we mean the EGL - velocity head.

HEC-22 may add the losses at the end of a section for convenience - in general we are interested only in the total loss and flow.

For free surface flow the HGL is only parallel to the EGL when the flow is at normal depth. Structures cause backwaters and drawdown which can have affects over long lengths where HGL is not parallel to EGL.

Brian

 

[gman348]

Bris, you are correct. That is the way I was taught but this is what I ran into:

First, I will paraphrase from HEC22;

at a structure, ie manhole, add the losses to the downstream energy grade then subtract the outlet pipe vepocity head to get HGL. Next step is to add pipe losses to HGL then add the upstream pipe velocity head to get the upstream EGL.

When you do this then work backwards the EGL at the outlet of the upstream pipe can be lower than the one calculated at the upstream end of the structure. This is the problem i ran into. I normally add all losses to EGL then subtract the velocity head to get the HGL. If you run up the system using the EGL you will get different values of HGL at the upstream end of structures.

I was just wondering if anyone else has run into this?