Determine the restrictor pipe size for a run of pipe 4

[oengineer]

Hello I have been asked to help provide some Civil Engineering Design work, but my main area of experience is Structural. I have some information regarding this assignment but I this is very new to me so I am unclear. Any comments/suggestions are appreciated. I have been asked to determine the restrictor pipe size for a run of pipe.

 

[LittleInch]

What sort / size of pipe?
Flow rate?
Lenghts?
Fluid ( water, sewage, gas, air, petrol?~)

Give us a bit of clue here as to what and why you're doing

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oengineer]

What sort / size of pipe?
48" dia HDPE
Flow rate?
Q for 100 yr storm is 6.51 cfs
Q for 10 yr storm is 4.65 cfs
Q for 3 yr storm is 3.50 cfs
Lenghts?
98 ft
Fluid ( water, sewage, gas, air, petrol?~
storm water

 

[oengineer]

This project is for a proposed building. We are adding detention ponds. The land is too small to have an adequate size detention pond so I am also using the 48" HDPE pipe as storage as well.

 

[LittleInch]

That's a bit better.

What are you trying to do though? Size a short section of smaller pipe? To what flow are you trying to restrict?

Head loss available?

Diagram / section will add a lot to our understanding.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oengineer]

I am trying to put a restrictor pipe inside a 48" HDPE (see red solid line). I have attached the sample calc98

http://files.engineering.com/getfil...9-4941-be33-f0d5a6e5394e&file=Sample_Calc.pdf

and the plans

http://files.engineering.com/getfil...46e-4e67-8566-79edf4dd0e17&file=Plan_View.pdf

I am unsure about the 10 yr max warer surface elevation for the site, so i used the same value for the overall max water surface elev (WSEL)

 

[psmart]

98' of 48" pipe has a volume of approximately 1200 CF, which is unlikely to provide significant detention and peak reduction for the flows you mentioned. As a guess, you're going to need a lot more storage to mitigate those peaks.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

an orifice plate is commonly used for this application. the orifice equation used for storm drainage purposes is documented in the following:

http://sscafca.org/development/documents/DPM/DPM_4_2010/DPM_Section3_HydraulicDesign.pdf

orifice coefficient is probably more like 0.6 not 0.8

 

[oengineer]

@psmart - The total run of pipe is 448 ft. I was planing on just have the restrictor pipe in only 98 ft.

 

[Pinwards]

I encourage you to sub this portion of the design out to an engineer experienced with stormwater design. It is much easier to spend a few hundred bucks for a simple design (for them), than it is to explain to a judge a few years down the road why you stamped a design for something outside your area of expertise, based off formulas others provided you with on a public forum, that led to flooding of the building or adjacent properties.


ASCE Fundamental Canons:
1. Engineers shall hold paramount the safety, health, and welfare of the public and shall strive to comply with the principles of sustainable development in the performance of their professional duties.
2. Engineers shall perform services only in areas of their competence.

NSPE Code of Ethics for Engineers:
I. Fundamental Canons
Engineers, in the fulfillment of their professional duties, shall:
1. Hold paramount the safety, health, and welfare of the public.
2. Perform services only in areas of their competence.

#

 

[oengineer]

I am not actually stamping the drawings. I am working on calculations. I have something calculated but I am trying to double check it before I give it the final approval.

 

[cvg]

i don't believe your restrictor will handle 6.5 cfs and there is inadequate documentation to check just about everything. so i would suggest better documentation before you give this the "final approval"

 

[oengineer]

I have a question regarding the formula below:

Q = CA (2gh)^(1/2)
D = Q ½ / (2.25h1/4)
Where:
Q = outflow discharge (cfs)
C = coefficient of discharge
= 0.8 for short segment of pipe
= 0.6 for opening in plates, standpipes, or
concrete walls
A = orifice area (square feet)
g = gravitational factor (32.2)
h = head, water surface differential (feet)
D = orifice diameter (feet)

I found that this is the formula use from the city design manual to determine the required restrictor diameter. I would like to verify that the orifice area is my initial assumed restrictor diameter. Please let me know if this is correct.

 

[psmart]

The issue here is not just the sizing of the restrictor plate. When you limit the flow (with an orifice or other device) you are detaining part of the hydrograph, and this requires a certain amount of storage. My guess (as indicated before) is that you don't have enough volume in the pipe to provide the required storage, which will result in overflow and potential flooding at the pipe inlet(s). You will need to perform a complete hydrograph routing in order to verify the system behavior, and this is best undertaken by an engineer with the appropriate H&H experience.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[oengineer]

I am using 350 ft of 48" pipe and 2 detention ponds for storage volume.

 

[beej67]

One of these days, I'm going to go over to the Structural forum and post a thread titled, "Hey what kind of beam do I need?"

Body of the thread:

"I've been asked to do structural design for a bridge, but my background is hydrology and hydraulics. Can anyone tell me how big a beam I need? I think it has to hold some big trucks."



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Serious answer:

What you need, to answer the question, is the following:

100 year rainfall data
10 year rainfall data
3 year (?) rainfall data (good luck with that by the way, report back if you're able to find it)
watershed area
watershed land cover (may include soil types, depending on method selected)
watershed time of concentration
unit hydrograph (SCS? Something else?)
regional hyetograph shape, depending on method (Type I? Type II?)
stage-storage relationship for your detention pipes
stage-discharge relationships for your selected outflow control structure

Then you apply the storage-indication method for reservoir routing, which of course you learned in your senior level civil hydrology class. (right?)

tl;dr:

I'm going to go out on a limb, here, and say you're in way over your head, you need to go back to school to be able to answer this question, and your boss needs to be slapped for thinking he can hand this to someone without the proper training. He needs to be slapped with a lawsuit if he stamps it.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[coloeng]

beej67,

One of the best answers I've seen on this forum yet for this type of question.

 

[beej67]

Addendum:

Don't forget to check tailwater, which is based on downstream hydraulics of the receiving system. For that you'll need a different set of data, and probably a different procedure.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com