| Applies To | |
| Product(s): | Bentley HAMMER |
| Version(s): | 08.11.XX.XX, 10.XX.XX.XX |
| Area: | Modeling |
| Original Author: | Scott Kampa, Bentley Technical Support Group |
When calculating a transient simulation in HAMMER, the follow user notification is generated:
"The Darcy-Weisbach friction coefficient xxx is too large. The head loss for the given flow, diameter and pipe length is excessive. Subdivide into 2 frictionless pipes with an inline orifice between them where all losses are concentrated."
This relates to the equation for computing the friction factor. This particular issue happens mainly in pipes that have an excessive amount of headloss over a short length of pipe. This typically happens in cases where the user is modeling some kind of minor loss with the pipe element. For example several losses through the discharge assembly of a pump. HAMMER looks at the total headloss across the pipe, including both friction and minor losses, when attempting to compute the friction factor. The Darcy Weisbach equation that HAMMER uses during the transient simulation doesn’t work very well in such cases because you end up with a really large “f” (as the notification shows), which isn’t valid for that friction method.
First, check the minor losses on the pipe to confirm that they are correct and that the total headloss across the pipe is expected.
As the user notification indicates, adding the "Orifice Between Pipes" node element is the best first option. This is more appropriate in cases where the pipe represents a significant headloss, because the loss will then occur over a discrete point, rather than HAMMER trying to disperse the loss over the length of pipe and attempt to use the Darcy-Weisbach equation.
If this happens for a pipe with low flow, see this article. It could be due to the combination of velocity, length and diameter and you may need to simplify the model or adjust the Flow Tolerance or "Round pipe heads" calculation options.
Equation used to calculate the Darcy-Weisbach Friction Factor (f)