I am modeling a pipe breakage event for a Fire Fighting Water Network using a "Discharge to Atmosphere" element in Bentley Hammer.
Can anyone suggest what values should be used for Initial Flow (Typical) and Initial Pressure Drop (Typical) for a 300 mm dia pipe.
Is it correct that Hammer will compute an "emitter coeff" based on these inputs and then use that emitter coeff and pressure available at the D2A element to calculate flow through D2A?
I have already used the flow and pressure values at the breakage point as computed in the steady state as the typical initial flow and pressure drop values, but I get the same values as results from Transient calculation.
Can anyone suggest a correct method to model a pipe breakage using D2A as pressure dependent demand?
Thanks in advance.
The Flow (Typical) and Pressure Drop (Typical) will be dependent on the model. In the case of a main break, you would set the discharge element type to "Valve", with the "Time to Fully Open or Close" set to zero. This is because it is conservative (for a design scenario) to model the rupture occuring quickly and producing a large opening.
Essentially the initial conditions describe the normal pipe and appropriately conservative flow conditions just before the break, then the transient simulation instantly opens the 'valve', to initiate transition to a ruptured condition. To represent the opening's size, it is recommended that the user set the "Pressure drop (typical)" to the steady-state pressure (observed prior to the break), and only vary the "flow (typical)" according orifice equation (Q = C A (2 g P)^0.5).
For more information, I would recommend reviewing the Discharge to Atmosphere technote, found at the following link: communities.bentley.com/.../modeling-reference-_2d00_-discharge-to-atmosphere.aspx
Bentley Technical Support
I have already done that, but my question is how do we calculate 'C' in the given equation for a sprinkler or a breakage opening.
For example, it is mentioned in the Tech note for D2A element that you need to enter any known pair for typical pressure and flow and Hammer calculates a C value accordingly and uses this value to calculate the flow depending on pressure variations.
So my question is "How can we assume a set of typical flow and pressure drop for a pipe break while assuming the breakage opening to be equal to the diameter of the pipe?"
Your help will be highly appreciated.
Deriving these values is largely up to your engineering judgment. However, using a value of 0.7 for C is a good approximation. You can then enter the area and the pressure drop to find a flow value.