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

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

Problem

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

Solution

HEC-22 documentation can be found at this location:

Calculations for the 3rd edition are done exclusively with energy grade (EG), not hydraulic grade (HG). 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. 

    1. Plunging Flow

      • In 2nd ed., the largest incoming invert is considered. In 3rd ed. it’s done based on each pipe.

      • The 2nd ed. refers to center line of the outflow pipe whereas the 3rd edition ignores the outflow pipe

      • In the 2nd ed., plunging coefficient is statically declared, in 3rd edition it is a proportion of plunging flow to all incoming flow.

      • In the 3rd ed., there is a ceiling put on maximum plunging depth of 10 * diameter.

      • In the 2nd ed. there’s a comparison of incoming pipe invert to structure invert, in the 3rd it is compare of incoming pipe invert and structure energy level.

    2. Benching

      • New benching classification titled “Improved” (both 2nd and 3rd editions)

      • Benching is interpolated in the 3rd edition and not in the 2nd

      • An “IsSubmerged” condition is defined for a lower threshold in the 3rd

    3. Bend Loss

      • In the 2nd ed. it is folded into an initial headloss term

      • In the 3rd ed. it is a weighted composite loss from each incoming pipe

    4. Initial Headloss

      • In the 2nd ed. it is computed with bend angle and structure equivalent diameter

      • In the 3rd ed. it is an inlet or outlet control that governs this

    5. Entrance and Exit Losses (most significant component of 3rd ed. junction losses)

      • These are transitions between the node structure and the outflow pipe

      • Exit losses are only computed in the 3rd edition

      • 3rd ed. computes each exit loss for each incoming pipe. 2nd ed. lumps entrance, structure, and exit loss considerations into one structure loss result and uniformly applies it as tailwater for all incoming pipes.

    6. Profile Enhancements

      • Addition of Exit, additional, and entrance losses is displayed from pipe to pipe

 

As of the CONNECT Edition (10.00.00.40), with the Explicit (SWMM) solver, there is a separate result field for the Entrance loss component, and all other losses are included in the field "Total additional structure energy losses", along with the total structure energy loss result. These can be seen in the "Results (HEC-22, Third edition)" section of the manhole properties, or in the manhole flextable. With the GVF-Convex and GVF-Rational solvers, there are additional result fields, for all the different components.

See Also

What is the difference between the hydraulic grade and energy grade structure loss calculation option?

  • Created by
  • When: Mon, Dec 14 2015 10:44 AM
  • Last revision by Bentley Colleague
  • When: Wed, May 24 2017 4:45 PM
  • Revisions: 8
  • Comments: 0
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