How to calculate maximum flow rate through a 12" pipe 3

[thien2002]

I am trying to figure out the maximum daily output for our 12" sewage piping to our wastewater basin. In order to calculate the maximum flow rate, what data do we need to include in our formula or what formula to use?

Thank you in advance for your help and contribution!

 

[RWF7437]

I'd suggest you use the Manning equation. You need to know the slope, diameter, roughness "n". For sewage many people use an n=0.013. The maximum flow occurs at 0.94 full but most engineers just use the full flow capacity. If the pipe is under pressure the Manning equation still applies but the slope is the slope of the hydraulic grade line.

 

[thien2002]

Can you please give me an example of the equation, thank you so much!

 

[RWF7437]

The Manning Equation can be found in any hydraulics textbook and many other places. Here it is using US Customary Units

Q = (1.486/n)*A*(R^2/3)*S^0.5

Where: Q = flow in cfs
A = Area in sq ft
R = hydraulic radius in feet
( for a circular pipe R = D/4 )
S= slope of the hydraulic gradeline in feet/foot

For your 12-inch pipe at a slope of 0.003 flowing full the flow would be 1.68 cfs and the velocity would be 2.1 feet per second which is close to the minimum usually specified for sanitary sewers.

 

[thien2002]

1.486 << is that your k value. Where did you get this value from...



Thank you so much...

 

[thien2002]

I forgot to ask you something, I see this manning equation:

V=k/n (A/P)^2/3*S^1/2

P= wetted perimeter, I don't know how to obtain wetted perimeter when it defines as the cross section dimension of the fluid to the culvert.

Is the above formula identical to yours?
Q = VA

 

[RWF7437]

Yes, 1.486 is the "k" value when using "English" units. If you are using metric units then k=1.00.

For a full circular pipe the wetted perimeter is simply the pipe surface that is wet, i.e. that is in contact with the fluid. Thus it is equal to Pi times the diameter.

 

[25362]

To RWF7437: the Pipe Friction Manual of the Hydraulic Institute gives for water flowing in a ID=12" asphalt-dipped cast iron pipe with a rugosity ratio of 0.0004, a friction drop of 0.131 ft/100 ft which is about 1/3 the assumed slope of 0.003.
Kindly explain whether the difference stems from the assumed rugosity or from the quality of fluid, sewage waste water vs clean water, or from any other reason. Thanks.

 

[PEDARRIN2]

Friction and slope are different entities.

The friction is the value of the friction head usually computed per 100 feet of pipe.

The slope is the distance a pipe drops vertically over a certain length.

Inside pipe is usually sloped 1/16" to 1/2" vertically down for every 1' of horizontal run.

Outside pipe is usually sloped similarly, but is denoted by decimal values.

But to answer the first question, you can find out how much flow a 12" pipe will produce under gravity flow conditions. If you do that, then you may vastly oversize your treatment equipment. You need to know what you are putting into the system before you try to determine what you will be getting out of it. Once you know what you are putting in, then you can determine if your pipe is big enough.

 

[thien2002]

Pedarrin2: We are discharge wastewater treatment from building and residential housing units of military base. We are averaging about 30% of total flow discharged. There are months deviation but the pipe never get to pass 50%. Now it makes me wonder if 12" pipe is too big for our military discharge pipe. This base is continuing to expand with new construction activities.

RWF7437: Your explanation is very accurated and I calculated on and got a value that is exactly matched the output from our plumbing shop data. I couldn't believe there is so much help on this Forum to facilitate our jobs. Thanks to savant like you!!!

 

[RWF7437]

To 25362:

The n value of 0.013 is commonly used for wastewater because it is not clean water. Although the pipe may be smooth when new, in wastewater it can become rougher over time because of erosion, corrosion and chemical attack in a sewer line.

rwf7437

 

[PEDARRIN2]

For residential housing, you can estimate it in two ways.

1. Use 50-75 gallons per person per day. This would be a peak flow.

2. To figure a more diversified flow rate, you would have to calculate the plumbing fixtures being used and then use the expanded fixture to gpm curve that plumbing designers use. Your plumbing shop probably has a copy of it. This will give you a more realistic value for a continuous flow to design to.

 

[PEDARRIN2]

When sizing for sanitary piping (at least inside a buiding which is where I am familiar), you size the piping to flow about 1/2 full. This is done for venting reasons.

Outside, I believe you can run a little fuller, although I do not think you want to run completely full because then you may surge the pipe causing all kinds of air pressure problems upstream and downstream of the pipe.

 

[semo]

Pedarrin2 is correct that for design purposes, a gravity pipe is not intended to run full. Now, that is not to say it won't if the facility experiences growth and the effluent pipe is not replaced.

Under this condition you will use another equation such as Hazen-Williams.

Mannings equation has already been provided, and the wetted perimeter formula is not a simple equation relating to the depth of flow. You can see that by the pipe geometry using the depth of flow you will have to calculate an angle between the water edges first in order to calculate the wetted perimeter and area. Here is a link to a manning's equation calculator to make things easier. You should understand the equation first; but, the calculator can make things easier.

http://www.waterengr.com/ManningSolver.html

Now then, under non full conditions, you will experience the peak flow through the pipe when the pipe is approximately 94% full.

The "n" value is also based upon the pipe roughness (ie friction loss). PVC and steel pipe have an N value of 0.009 to 0.011, Conc pipe = 0.011 to 0.013, etc. As the pipe ages and increases in roughness, the value will increase to the high end.

Getting past your first question and into some of the latter responses. You need to look more at the flow entering the treatment facility based upon population, industrial flows, inflow/infiltration, etc as stated above. You can perform some flow measuring for your existing facilities and base future expansion upon your results. If you size the facility for the volume the pipe is capable of you will probably drastically oversize your treatment facility and not get the treatment result you are looking for.

If the high flows are intermittent, you might consider some flow equalization storage or bypassing with primary treatment (if allowed in your state).

 

[thien2002]

semo:You can perform some flow measuring for your existing facilities and base future expansion upon your results. If you size the facility for the volume the pipe is capable of you will probably drastically oversize your treatment facility and not get the treatment result you are looking for.

If the high flows are intermittent, you might consider some flow equalization storage or bypassing with primary treatment (if allowed in your state).
____________________________________________________________
Can you please explain the above more precisely? You have a good point but that is way beyond my comprehension. How do you design flow equalization storage or bypassing with primary treatment? I am very interested to explore into this subject.

PEDARRIN2: Where did you get your data source coming from? Would you please share since I am still new to industrial wastewater treatment.

*** Are there any good web site or books to understand more about industrial wastewater treatment system? Specially on military bases. Thank you so much!

 

[RWF7437]

thien2002,

You really should get hold of a good sanitary engineering text and some references. The ASCE Manuals are a good place to start. You can find them at

http://www.asce.org

Designing collection system and treatment plant for a military base is a fairly large and complex task and requires some experience and judgement. Seek out a Civil Engineer in your area and ask him/her for some help. Expect to pay for it because it will be a good education.

 

[semo]

Item 1 - flow monitoring

If you don't already have flow measuring for the treatment plant that is continuously monitoring the flow, you can temporarily install some to see what the average daily, peak daily, peak instantaneous, wet-weather flows, etc. are at that plant. This data can be very useful when designing upgrades or replacement facilities. ie. If you know your average dry weather flow, subtracting it from the wet-weather flows will give you your inflow/infiltration which must be taken into account in the plant design or reduced through collection system improvements.

Knowing your average daily flows along with some water sales data you can see what the average residences or other building types are producing in your location. This information can be used when designing for future expansion. The flows from various facilities (say residences) are not the same in all locations throughout the world. Having site specific data can help keep your design more cost effective.

Item 2 - Flow equalization/bypassing

If the plant is sized adequately for the average daily flow; but, you have peak instantaneous flows that exceed the plant design flows and wash sludge into the stream or upset the treatment processes, you can design holding facilities where the excess flows (peak minus design) is diverted from the head of the plant. Once the incoming flows are below average, this stored waste can be returned at a lower rate (when combined with the incoming flows do not exceed the plant design flow) into the plant influent for normal treatment.

Some states also allow this excess flow to be bypassed, partially treated (with primary treatment) if it is inflow/infiltration, and discharged with different limits or combined with the plant effluent to meet the plant effluent limits. That is something you will have to discuss with your local DNR/DEQ.

Now then, I'm saying this so you can understand the concepts; but, the actual plant design can have a lot of pitfalls if you don't understand the treatment processes and how all of this can effect it. I would go one step further than RWF7437 and say that you really should have an experienced engineer design or help design any improvements.

 

[PEDARRIN2]

My source is "Facility Piping Systems Handbook" by Michael Frankel. It is published by McGraw-Hill and also can be obtained through American Society of Plumbing Engineers.

It is a good "overview" type resource that goes just deep enough into each area to allow you to get good preliminary results but not so deep that the book is too large to use.

 

[thien2002]

Dear Semo, RWF7437,PEDARRIN2:

I would like to introduce myself to you, I am currently a Mechanical Engineer working on HVAC system on military base. Due to the ISO 9001 compliance, every engineer will have other responsibities outside of their assignments. That is why I have other technical duties that are beyond my knowledge such as the industrial wastewater treatment as one of the area. Being a contractors on an Air Force base with very tight budgets I have to find way to survive on the jobs. Fortunately I am meeting dedicated people like you, otherwise I am packing to the next job by now.

 

[PEDARRIN2]

Another good resource that I have used is

"Standard Handbook of Environmental Engineering" by Rober Corbitt. It is also a McGraw Hill Book.

I, like you, have had to modify myself to meet the changing engineering job market. I have a Chemical Engineering Degree, but I am doing inside piping/plumbing design for institutional and educational facilities. I do a little bit of "outside the building" design, but not much.

My motto in this is that if you know how to do a lot of things a little bit, it is easier to train/reeducate yourself than if you know how to do only one thing.