User Notification: "There is an error in the data for this pipe. The associated data is incorrect"

Applies To 
Product(s): HAMMER
Version(s): 10.00.xx.xx, 08.11.xx.xx
Area:  Calculations
Original Author: Scott Kampa, Bentley Technical Support Group

Problem Description

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"

Reason

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.

Steps to Resolve

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 increase the Flow Tolerance to be above 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 - but this could be problematic if set too high), or set the "Round pipe heads" calculation option to "true" and set the number of decimal places to a number that results in the pipe headloss rounding to zero (in which case it will use a zero for the friction factor). You can making the "accuracy" steady state calculation option smaller (for example 0.0001 instead of 0.001) as a more accurate initial conditions may avoid this situation.

If you experience such low flow pipes, you should consider simplifying the model. Eliminate tees by placing demands at the junction that connects the main line to the tee, a process called skeletonization. This will eliminate the low demands and some of the short pipes. See more here: Preparing a model for analysis in Bentley HAMMER

If you can make adjustments to your model similar to those described above, you will eliminate the user notifications you are getting, plus assure yourself that the proper friction factor is calculated for your model. Looking at the Transient Analysis Output Log will be helpful in eliminating the latter, since this Darcy-Weisbach friction factor is not reported in the properties or flextables. Open the Transient Analysis Output Log from the Report menu. The report will open the Wordpad text editor. You can use CTRL+F to "find", then search for "pipe information" to jump to the section that lists the friction factor per pipe.

See Also

What equation does HAMMER use to compute the Darcy-Weisbach friction factor (f)?

What should the "flow tolerance" be set to?

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