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What does the following user notification mean and how can it be resolved?
"There is an error in the data for this pipe. The associated data is incorrect"
One reason this could occur would be if the flow or headloss in this pipe is very low in the initial conditions. Basically the transient calculation engine is less forgiving in terms of the accuracy of the initial condition values. Even more so with small flows and very small friction head loss. Friction factor calculation with these very small numbers could result in very inaccurate values and the program may not be able to converge on a solution.
Try the following steps to resolve the issue.
Pipes that should have exactly zero flow but do not
Go to Analysis > Calculation Options. Under "Steady State/EPS Solver," double-click the active calculation option to view the properties. In the Properties, make the "Accuracy" value smaller, such as changing the value from 0.001 to 0.00001. Then, compute initial conditions and try computing the transient simulation. A smaller value for the accuracy may allow the flow in the pipe to be closer to zero and thus treated as such by HAMMER, avoiding the pipe friction factor calculation error.
Pipes with "real" low flow
In other cases, the low flow in the pipe may be real (expected), but the friction factor calculation settings may not be set correctly for this situation. First, compare the flow through the pipe to the "Tolerance" transient calculation option. If the pipe flow is greater than the flow tolerance, HAMMER will try to compute the Darcy-Weisbach Friction Factor (f) based on the total headloss across the pipe:
f = hl / [ (L/D)(V^2/2g) ]
Where f = Friction factor, L = pipe length (ft/m), D = pipe diameter (ft/m), V = velocity (ft/s, m/s) g = gravity
In cases where the flow is above the tolerance but resulting in a very small velocity, HAMMER can have difficulty solving the above equation, with certain lengths and diameters. The solution to this is to either lower the Flow Tolerance below the flow seen in the pipe in question (which causes it to directly calculate the friction factor based on the roughness factor in the pipe), or set the "Round pipe heads" calculation option to "true" and enter a number of decimal places that results in the pipe headloss rounding to zero (in which case it will use a zero for the friction factor).
What equation does HAMMER use to compute the Darcy-Weisbach friction factor (f)?
What should the "flow tolerance" be set to?