Depth of Flow 2

[beastmodeactivated]

Howdy,
I am an intern working with a water district this summer. I am working on an audit of the sewer system in our town and I am stumped. My first calculation is to find if any of our lines are running above maximum capacity (1/2 full for 12" diameter and less; 3/4 full for 15" diameter and larger) I was told to use manning's for this, but I can't seem to contort the equation to give me d/D. Tips?

Sincerley,
Daniel

 

[RWF7437]

You can do this in either of two ways:

You can calculate the full flow capacity of any size circular pipe at any slope. It is suggested you make a table of these capacities if you don't already have one. The you can use the "hydraulic properties curves" for circular pipes to derive the half full and 3/4 full capacities. You can find these curves in any good hydraulics text book and in many pipe handbooks.

Or, you can calculate these partially full capacities directly using Manning's equation and using the actual flow area and actual wetted perimeters for each pipe size. This is more work but can be done.

Finally, look around your office. Has somebody already done this ? I would be amazed if they hadn't.

good luck

 

[sam74]

What are your known variables other than the pipe size? A hydraulic elements diagram might be useful if you know V or Q.

 

[RWF7437]

You DON'T need to know V or Q to use the chart. You first calculate the full flow V and Q (using Manning's Eq.). Then you assume d/D and use the chart to calculate V and Q for each assumed depth ratio.

 

[beastmodeactivated]

RWF7437,
Thanks for your response, That's help me better understand the equation. And unfortunatly, no one has done this at the distric where I am working. I spent the first two weeks counting houses, calling businessess, calculating lot sizes and average sewer usage amount of my town. Most of the maps here are out dated too, there's a good amount of work to be done here and I am unfortunatly working alone.


sam74,
The info I have is an estimated GPD usage for the sewer, I split the enitre town up into different sewer sheds to isolate problem areas. I can dig up files on slope, pipe size, pipe length and pipe material and that's it if I'm lucky.

Thanks for the help

 

[sam74]

RWF7437, yes it is true he doesn't necessarily need V or Q there are a few other useless variables he could use. But from his original post you can tell he knows limits of d/D for different variations of pipe size and he would logically need to know V or Q (or in this case an educated guess) to solve his problem. Area, wetted perimeter, or hydraulic radius are his other choices but arriving at an educated guess of these values seemed unlikely.

I think we had the same idea just a different way of explaining it.

 

[RWF7437]

Sam74,

I think we can agree to disagree. But.....look at the opportunity Beastmodeactivated has. He is on the ground floor and can initiate standards and practices for his employer which will serve his district and successors in good stead. Now, if we can get him to look OUTSIDE his district at what others have done in nearby cities and towns he can save himself a lot of this work. None of these problems are really new and many have been solved by others in ways he, "working alone" may not have thought of.

 

[beastmodeactivated]

I worked on figuring out a solution today and I am still stumped, honestly, now I am more trying to decide what route to take. I can formulate a chart like RWF7437 mentioned and I like that idea because I can get it done, but I am discouraged at the use of that system because it is more reader friendly to have a simple equation that approximates D/d right into a spreadsheet. Can I get some advice on breaking that down further with manning's into excel?

thanks again

 

[RWF7437]

You can develop equations. You can even do it in Excel by deriving the regression equations that best match your data. But they won't be "simple" nor will they be "user friendly". Most people I know would rather look up a number on a table than calculate a third order polynomial.

Ask yourself, who will use this data? Are they mathematicians? Engineers ? Engineering Technicians ? Maintenance guys ?
City Council people ?

Craft you answer for the intended users.

good luck

 

[bltseattle]

beastmode,
I would set up a simple tool to perform these checks. The key steps would be:
1. From a hydraulic reference, say the Civil Engineering Reference Manual, copy tabulated values of d/D vs A/D^2, etc, to a spreadsheet. If you don't want to enter manually try scanning the table to a pdf, then selecting the text from the pdf and pasting into the worksheet.
2. Setup your worksheet to compute Q given the pipe parameters, using the lookup table from Step 1 and manning's eqn. I recommend that you spend the time to set up an interpolation function to use when retrieving tabulated values, or else step 3 below might not always converge.
3. Use Excel's goalseek to solve for the parameter of interest, in your case d/D. Specifically, if you set up an equation like "=(Computed pipe Q per mannings) - (known or target flowrate Q)", you can then solve it to value "0" by changing your value of "d/D" using Excel's Goalseek.

Good luck!

 

[bltseattle]

beast,
Elaborating on that last thought, last week when I needed to do something very similar to what you are doing, I setup a macro that went down each row of my table, using the "solve difference to zero" approach I outlined above. This is what it looks like (very simple). If you know a little VBA you could use a macro like this that runs through all of the pipes sequentially. It is a *lot faster* than picking Goalseek from the menu repeatedly/manually if you lots of pipes!

Sub FIND_DEPTHS()
'
' Macro 6/25/2007 by BLTSEATTLE
'
'
Range("AG5").Select
Range("AG5").GoalSeek Goal:=0, ChangingCell:=Range("z5")
Range("AG6").Select
Range("AG6").GoalSeek Goal:=0, ChangingCell:=Range("z6")
Range("AG7").Select
Range("AG7").GoalSeek Goal:=0, ChangingCell:=Range("z7")
Range("AG8").Select
Range("AG8").GoalSeek Goal:=0, ChangingCell:=Range("z8")
' etcetera, you get the idea
End Sub

 

[GregLamberson]

I may be missing something here, and apologize if I am over-simplifying, but getting back to RWF7437's original post, can you not calculate 1/2 full for 12" & under and 3/4 full in 15" & over for each line size you have in the system, that will give you a base flow rate for each diameter.

Then you have a "pass or fail" flow rate for each size. All you need are the pipe diamter (which you have), the depth of the liquid (which you have been given), the slope (which you stated you can find) and the coeffecient of friction (I would use Perry).

Once you have that table, then you can calculate actuals and compare each quickly to see if you are above or below the criteria.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[RWF7437]

To GregLamberson,

In sanitary sewer design one also needs to be aware of minimum velocities and slopes ( also called, self cleaning velocity ) which is most often taken to be 2 feet/second.

Also, because sewer flows are not clear pure water, the roughness is usually conservatively estimated to be n=0.013 (Manning's) ,regardless of the pipe material.

And finally, flows are very unpredictable so large "factors of ignorance" are usually employed.

 

[GregLamberson]

Obviously "factor of ignorance" applies, I should stick to oil & gas. Perry does suggest .013

As a note, since this is an existing system and not one under design, I would assume the slope of the installed lines take into account the minimum velocity requirements, i.e. it is what it is and you use the existing slopes for the calculations.

It is 2 phase flow of a little different variety. Again, apologize for "cross responding" but thought it an interesting post/dilema and thought I'd give a perspective on it.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[beastmodeactivated]

GregLamberson,
Thanks for your input, I really do appreciate every and anything that is posted. You are correct in the fact that I can simply make a chart and cross examine and just tell the gov't, this pipe is bad, this pipe is good. That is truly all that is required of me to do for my report. Unfortunately through polotics and what not. my boss strongly desires for me to recreate an equation of approximation that he can use in excel so he has it for furture use.

Again thanks for all the comments guys, I really appreciate the input.

Daniel

 

[BigInch]

You might find this website helpfull.

(I don't stick to oil and gas, cause hydraulics is hydraulics)

http://www.ncpi.org/EngineeringManual/eng_manual02.htm

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Nice present for your boss...

http://www.ncpi.org/hydcalc.htm

http://virtualpipeline.spaces.msn.com

 

[beastmodeactivated]

Howdy all,
Thanks again for all the help, I found a quattro pro sheet that was written partially for what I was looking for, I used that to rewrite it into excel and it works great. I will be posting the .qpw and .xls files on here sometime, but for the next week or so I will be busting away on the audit. Thanks again.
Daniel

 

[queque]

BigInch,
I have no actual hands on experience on sewer design. Your input was great. Thx.

 

[cr1973]

If this is an existing system, I wouldn't trust your calculated flowrates based off # of people being served. Odds are the actual flowrate will be much higher due to groundwater - especially after rain events!

A little field work may be in order - watch for manholes that surcharge beyond the top of the outgoing pipe, etc.