What is the difference between the Hydraulic Grade and Energy Grade 'Structure Loss' calculation option?

Applies To 
Product(s): SewerGEMS, CivilStorm, StormCAD
Version(s): CONNECT Edition, V8i
Area:  Modeling
Original Author: Mark Pachlhofer, Bentley Technical Support Group

Problem

What is the difference between the Hydraulic Grade (HG) and Energy Grade (EG) 'Structure Loss' calculation option when using the GVF Rational Solver? Which is correct to use?

Solution

If you choose the EG option for the structure loss mode the headloss across the structure is the EGL (In) - EGL (Out) and the HGL is then calculated based off the EGL. On the other hand if you choose the HG option the headloss across the structure is the HGL (In) - HGL (Out) and the EGL is then calculated based off the HGL. 

We offer the two options because some engineers prefer that the headloss corresponds to the change in depth across the structure, while others prefer the headloss correspond to the change in energy. Note that when you choose the HG as your option the EGL at any given point is computed by adding the velocity head to the hydraulic grade and because of this energy discontinuities may occur, such as when a pipe size decreases in the downstream direction or a pipe slope increases. If you choose the EG as your option you can see the opposite effect where there could be discontinuities in the HGL. Flow discontinuities can also be responsible for energy discontinuities, since a structure is analyzed based on a different system time than a pipe, a direct comparison of energy grades is not reasonable. 

For the HEC-22 2nd edition it's fine to use either the HG or the EG option, although technically speaking the EG option is more correct. This is because the head loss by definition is the energy loss.  When using the HEC-22 3rd edition, however, an engineer should only use the EG option for a structure loss method because HEC-22 specifically states that only the EG should be used when computing structure loss. 

See Also

Hydraulic grade discontinuity at steep pipes or across nodes

What are the differences between the HEC-22 2nd edition and the HEC-22 3rd edition? 

  • Created by
  • When: Tue, Apr 14 2015 1:41 PM
  • Last revision by Bentley Colleague
  • When: Thu, Jun 14 2018 4:23 PM
  • Revisions: 10
  • Comments: 0
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