How to calculate flow in a pipe from water level masurements

[member 1218030]

Hello,

I need to calculate flow in a pipe from water level masurements taking in the upstream and downstream manhole. It is important that especially single loss of energy in both inlet and outlet of the manhole is taking into account. Some of the manholes have sewer overflows and thereby an edge, which should make single loss zero, but not all of them.

I have placed about 50 measurement point in my sewer system, so the specific information will vary. The points are either placed: (1) in a manhole that recives combined wastewater, and let it out to a combined pipe. (2) in a manhole, that recives combined wastewater and if the water level reaches the overflow edge, lets it out into the rainwater pipe. (3) in a manhole, that somtimes recives combined wastewater but otherwise just handle rainwater.

A simplyfied sketch:

I do not know which assumption is best, as I want to use my flowrates to validate my hydraulic model. There are (almost) always water in my pipes, so open channel makes sense, but when it rains, there is a chance that the pipe will be full flowing. That of cource depends on the rainfall and the diameter of the pipe. So I do not know which assumption makes the most sense open channel or full flowing or if I need to combine the two. I have attached the "best" try I got in calculating a flow - but I do not think it is very realistic. It can also not handle if the downstream water level becomes higher than the opstream water level, but that could happen in real life, so I also need to take that into account somehow.

I first tried with the asumption that the pipe had full pipe flow with:

But that assumption does not seem accurate. Moreover the equation did not take into account the energiloss.

Then I tried using the following equation:

This is not really correct either eventhough it is closer. But I do not know how to make a better estimation of the flow between two manholee whith the waterlevel measurements.

 

[Snickster]

I would think you would need to have some type of weir in the manholes so that for a given height above the weir you could calculate the flow. How else would you do it?

If the pipe is flowing full then you can get the difference in water level between two manholes to give the difference in head available. If you know the diameter and length of your conduit between the manholes and friction factor you could predict what your flowrate should be using pipe full flowing equation such as the darcy equation.

If you pipe is not flowing full you can do the same estimate of flow using equations for non-full flowing conduits knowing the elevation difference between water level such as the Manning equation.

 

[pierreick]

Hi,
To support your work:

https://www.cedengineering.com/userfiles/Partially Full Pipe Flow Calculations.pdf

More accurate is the use of a weir

https://www.youtube.com/watch?app=desktop&v=gxJWAUqGX9w

Pierre

 

[1503-44]

The formula is for a full flowing pipe.

For partially full pipes, see Manning formula and use the hydraulic radius in place of the full flowing pipe area.

https://en.m.wikipedia.org/wiki/Manning_formula

For whatever formula you use, if downstream water level is higher than upstream, reverse your flow direction. Upstream becomes downstream. Problem solved.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Heaviside1925]

OP,
I'm no gatekeeper but double posting can waste people's time, please try to avoid it next time. If you feel you need to move a post to a more appropriate forum, please remove or provide a link in your original post before doing so.

 

[bimr]

Measuring manhole water depth will not work.

People working on sewer I/I here use the Flo-Dar by Marsh McBirney which is an easy and accurate flow measurement:

Flo-Dar

 

[member 1218030]

Thanks so much for all your answers. I am trying to edit the manning formula to my situation by incoorporating the weir using your suggestions.

Moreover, it was suggested to me, that I post my question in this forum insted of the one I chose to being with, and I do not know how to delete the original post, or what I should link where, so hope you will forgive my oversight.

 

[1503-44]

Click on "Delete" under your first post in the thread you want to delete. You should see a confirm Delete option pop-up, if you do it within the alloted time.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

Measuring the height of water over the weir in a manhole is not going to work. The width of the manhole is not probably not long enough. You will also accumulate waste solid materials on the upstream side and on the weir.

This flow measurement is usually accomplished with the Flo-Dar. See the link above.