Minor losses in maintenance shafts
So we all know of head loss factors for stormwater pits, big square boxes.
We also are all familiar with minor losses for fittings, valves, tees, long radius bend etc.
But does anyone know of data or factors best used for maintenance shafts, running under head.
Say a 300dia shaft, with 225 pipe in and out, hgl maybe 1.5m above invert. How to best think about these?
Posted Sep 1, 2022, 10:31 AM UTC
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I'd consider it as a 'tank' or a free surface and place an exit loss (K=1) on the incoming pipe and an entrance loss (K=0.5 for flush or K=1 for a protruding entrance) on the outgoing pipe.
We could apply the same logic to a stormwater pit. But they will have a K of 1 or even 0.7 for a straight through arrangement.
I'm thinking the maintenance shaft with the very small relative size of 'tank' operates much more like a pipe than a tank, and should have a lower k
It could also be treated like a manhole instead of a free surface. There is some historical data out there showing K factors are less than 0.25 for that case and decreasing as diameter of the hole approaches the diameter of the pipe.
Search "Head Losses at Sewer Junction Manholes" by J. Marsalek for more. It was in the ASCE Hydraulic Engineering publication I believe.
In terms of the design, though, it's a question of how conservative you want to be. What velocities are you experiencing? If they're pretty low, this really won't matter. If they can get fairly high, then it's important to not be too conservative, but also not to be too light either. It's all a question of what the rest of the overall system is doing that may rely on this point.