• State Verified Answer
  • Locked Locked
  • Replies 1 reply
  • Subscribers 83 subscribers
  • Views 573 views
  • Users 0 members are here
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

How is the hydraulic grade calculated

Christen Crique
Offline

I would like to know the equation the model uses to calculate the Hydraulic grade. Because I want to calculate the hydraulic grade line for my field results in order to compare it with the model results but I cannot figure out how the model is calculating the HGL. Because from what I found 1 bar is equivalent to 10.199773339984 m of head. So I would then simply multiply the measured  pressures in bar by that number and then ad the result to the elevation. So to test that I took the pressures predicted by the model and calculated the HGL using that method but I get different results then the HGL that the model produces (see the results in the table below). And also the way I calculate the Hydraulic grade I get a hydraulic grade that is higher than the hydraulic grade of my boundary element (a reservoir) which does not make sense since there are no pumps or prv’s downstream of the reservoir.

Hydraulic grade of the boundary element

Pressure from model prediction (bar)

Node Elevations (m)

Hydraulic grade from model prediction

Hydraulic grade calculated by me

44.91

 

3.6

7.87

44.81

44.59

3.6

8.21

44.81

44.93

4.2

2.14

44.81

44.98

4.3

0.98

44.81

44.84

3.9

5.3

44.81

45.08

3.4

9.98

44.81

44.66
Parents
  • +1 Offline

    Hi Christen,

    WaterGEMS first calculates the hydraulic grade based on energy balance across the network (see more on this in our Advanced Water book). The pressure result field is then calculated based on the difference between the hydraulic grade and the physical elevation. 

    You may have a rounding issue with your math. Try looking at the pressure in bars with at least three decimal places. For example with a reservoir at 44.91 m and 0.1 m of headloss between the reservoir and a node at an elevation of 7.86 m, the pressure head would be 44.81 - 7.87 = 36.94 m. With the conversion factor of 10.197 m per 1 bar (at a specific gravity of 1.0 - the default in the calculation options is 0.998), the pressure would be 36.94 / 10.197 = 3.623 bars. Conversely if the pressure is 3.623 bars as the elevation is 7.87 m, the hydraulic grade at a specific gravity of 1.0 is (3.623 * 10.197) + 7.87 = 44.81 m

    See: What is the difference between pressure head and pressure?


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Christen Crique 

Reply
  • +1 Offline

    Hi Christen,

    WaterGEMS first calculates the hydraulic grade based on energy balance across the network (see more on this in our Advanced Water book). The pressure result field is then calculated based on the difference between the hydraulic grade and the physical elevation. 

    You may have a rounding issue with your math. Try looking at the pressure in bars with at least three decimal places. For example with a reservoir at 44.91 m and 0.1 m of headloss between the reservoir and a node at an elevation of 7.86 m, the pressure head would be 44.81 - 7.87 = 36.94 m. With the conversion factor of 10.197 m per 1 bar (at a specific gravity of 1.0 - the default in the calculation options is 0.998), the pressure would be 36.94 / 10.197 = 3.623 bars. Conversely if the pressure is 3.623 bars as the elevation is 7.87 m, the hydraulic grade at a specific gravity of 1.0 is (3.623 * 10.197) + 7.87 = 44.81 m

    See: What is the difference between pressure head and pressure?


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Christen Crique 

Children
No Data
Communities
Support and Services
Training and Learning
Social Media