
Applies To 



Product(s): 
HAMMER 


Version(s): 
08.11.xx.xx, 10.xx.xx.xx 


Area: 
Modeling 


Original Author: 
Jesse Dringoli, Bentley Technical Support Group 

Problem
When using the "Orifice Between Pipes" elements, is it possible to enter the diameter instead of the "typical flow" and "typical pressure drop"? How would I determine these values based on a known orifice diameter?
Solution
Currently, you cannot enter the diameter directly. Instead, you will need to backcalculate the "Flow (Typical)" and "Pressure drop (Typical)" using the orifice equation. HAMMER internally uses these two values to compute a discharge coefficient, which it uses to vary the head loss with flow.
Basically, you would select a flow that you might typically see through the orifice (for example the flow through the pipe in a steady state run without the orifice added), assume an orifice coefficient, then rearrange the orifice equation to solve for the head. You would then enter that head as the "Pressure drop (Typical)" and the selected flow as the "Flow (Typical)".
P = (Q/CA)^2 / 2g
Q  Discharge (cfs, cms)
C  Orifice coefficient (typically 0.6)
A  The crosssectional area of the opening (ft, m)
g  gravitational acceleration
P  Pressure head (ft, m)
In recent releases of HAMMER (08.11.02.31 and greater), you can make this process easier by using a formulabased user data extension. You can create two new fields  one where you can enter the diameter, and the other that computes the typical pressure drop for you, based on the entered typical flow value. The formula used in the user data extension would simply be a rearranged version of the orifice equation, solving for pressure. See example model below (note that you must be signed in to download this)
Note that the above can also be used to estimate the "typical flow" and "typical pressure drop" for the Rupture Disk element (for the orifice size when it ruptures and discharges to the atmosphere).
See Also
How is an Orifice Between Pipes element used in WaterGEMS?