Modeling minor losses at a valve or a tee

Product(s): WaterGEMS, WaterCAD
Version(s): CONNECT Edition, V8i
Area: Modeling


How are minor losses applied to valves in WaterGEMS and WaterCAD?

How should I model minor losses at a "tee"? The default minor loss library contains coefficients for branch and line flow, but what about when the flow direction changes?

Minor Losses at a Valve

It is possible to apply minor losses to valves in addition to any losses that will occur due to the activity of the valve as well. However, the minor loss is only applied when the initial or calculated status of the valve status is Inactive, meaning that the valve is fully open. This will applied to valves like FCVs, PRVs, and PSVs.

For instance, if you have an FCV with a flow setting of 100 gpm but the flow through the valve is 50 gpm, the FCV will be considered to be fully open and have a calculated status of Inactive, and any minor loss entered for the FCV will be applied to the results.

For valves like PBVs, there is no Inactive status. The valve is either closed or active. If you enter in a non-zero hydraulic grade or pressure setting, that setting will be applied and the PBV will be partially closed. If you set the PBV setting to zero, you would see the minor loss applied to the results.

For GPVs, minor losses are not applicable. 

The Help documentation in WaterGEMS and WaterCAD has additional information on applied minor losses and how different valve types work. 

Minor Losses at a Tee

Typically the coefficient is applied to the pipe which has the combined flow of the other 2 pipes for a T-section. For example, if flow travels from 1 pipe to 2 pipes, then the minor loss is typically applied to the upstream pipe and
similarly if flow comes in from 2 pipes to 1 pipe then the minor loss is applied to single, downstream pipe. The same principle can be applied to pipes at a cross-intersection.

However, this is not a definite rule, because the above case might not apply if flow changes directions. Sometimes, you may need to split the value and apply it individually. Though in the end, the results should not vary much between the methods you use. Basically in practice, it generally makes little difference, which is why most engineers ignore minor losses.

For example, let's say the velocity is 1 m/s. The velocity head is 0.05 m which when multiplied by either 1.28 or 0.35 is still essentially 0 when compared to other sources of head loss in a distribution system. If you want to be conservative, you can always pick the bigger value but you will find it doesn't make a significant difference.

Unless you are working with a piping system where the velocities are huge and lengths of piping are short, the inaccuracy in these minor losses are not significant.

Source of Minor Loss Library Values

*The values can be used as provided or adjusted if desired

The source of the default minor loss library included with WaterCAD and WaterGEMS has be acquired from the table seen in our Advanced Water Distribution Modeling and Management book, page 642.

Walski, T. M. (1984). Analysis of Water Distribution Systems. Van Nostrand Reinhold, New York

See Also

Minor losses for Isolation Valves

Adding Multiple Minor Losses To a Pipe

How are headlosses determined for TCV's with the different initial status settings?