Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Calculating Water Flow Rates

Calculating Water Flow Rates

Calculating Water Flow Rates

Hello and sorry for what I think is a dumb question, but I was unable to find what I am looking for elsewhere, then I thought you guys could easily help me here.

I am trying to calculate what the flow rate from a water service should be. I know that the simple formula is Q=AV (flow rate = area times velocity), however I do not know velocity. What I do know is that the service is 1 inch polyethylene, and the pressure of the main tested from a very close hydrant to the service is 66psi, and the service length is 20 feet. What is the formula to calculate the flow rate from the end of that service? I feel like I am missing something simple here, but I am unsure what...

Thanks for any help you guys can provide, :)

RE: Calculating Water Flow Rates

Reasonable pipe velocities depend on the application. There is no correct velocity for all applications. Here is a general guideline. Would expect the typical velocity range to be within 4 ft/sec minimum to 10 ft/sec maximum.

Reasonable Velocities for the Flow of Water through Pipe:

Boiler Feed.............8 to 15 ft/sec
Pump Suction ............4 to 7 ft/sec
General Service.........4 to 10 ft/sec
City.......................to 7 ft/sec
Transmission Pipelines...3 to 5 ft/sec

Go to a basic hydraulics book. Try Cranes Technical Paper 410 as a reference for the above velocities.



RE: Calculating Water Flow Rates

Iron Dogg

bimr is right in what he says if you are looking for general guidance.What i would like to know is why is this important in this case? If you really want to know and you dont have a meter simply use the line to fill a bucket of known volume and record how long it takes. That will give you Q and you already know A so then you can calculate V.

"Any water can be made potable if you filter it through enough money"

RE: Calculating Water Flow Rates

You have not really supplied enough information - given what you have I suspect what may be most critical is what is your exit or discharge condition (e.g. at a throttled tap or say at an extreme a cleavered end of the poly service line - i.e. full bored discharge?) While discharge formulae or nomographs, e.g. out of Crane, could then give you approximations, I believe another bit of missing info that could be some signficance in any case is what is the actual dimesnions e.g. "DR" of the poly you are dealing with i.e. a very large DR (i.e. thin pipe) would likely flow more than a very small DR (thicker pipe), the latter with therefore smaller flow area?

RE: Calculating Water Flow Rates

Hi guys,

Sorry for taking a bit to get back to this. Thanks for the links and info provided. I will check them out.

I am not sure off hand of the dr of the poly, but I was given this info in the attachment if it helps.

RConner, I guess full bore discharge is what I am looking for. Although I realize the tubing would not be straight (somehwat snaked in ditch) and there could be other minor headloss associated; full bore potential. Just curious, would there be much flow throttle through a fully opened curb stop?

Ashtree, the barrel fill method was done however months later now, there are discrepancies between the guys who remember what the results were and the documentation is lost.

RE: Calculating Water Flow Rates

get a new bucket. do it again. Why do you need this? A supply issue?

Richard A. Cornelius, P.E.

RE: Calculating Water Flow Rates

While there still may not be enough info supplied for a fully rigorous analysis, as you have described you basically have 20 feet of 1-inch plastic pipe, connected to a corporation stop at the main, and discharging "full-bore"/open to air? at the other end of same. I suspect further if you have a looped, well supplied 6" main, it's possible you may not reduce the main head/pressure a whole lot, by even rather high discharge from such a small, and short pipe. Bernoulli theorem might be applied with iteration to this situation, perhaps assuming 60 psi/138 ft of head is available at the main, and that amount of static head is basically exhausted to only flow momentum as the flow discharges to zero static head at the end of that 1" piping. There will be component head losses composed of entrance losses through whatever attachment of the corp to the main, the minor loss of a fully-opened corp/valve, flow loss through 20 feet of the small pipe (that I believe will be many feet of head at the extreme velocity), and then finally exit losses of the basically square, open-end.
Once you have iterated all that, I have noticed that the site at
http://www.engineeringtoolbox.com/water-discharge-... contains a chart that reports if you instead had 100 feet of 1" hose it apparently would discharge about 50 gpm at the far end with 60 psi at the supply. I've also seen various sites and calculators that say on the other hand the flow through just a leak hole or aperture in a pipe with area due to 1" diameter in a 60 psi pipe discharging to zero pressure would flow in the much greater range of say 160-200 gpm. Therefore, If you do your calculations properly, you may well have a result somewhere in-between these numbers (with your much lesser length of piping).

While my earlier question related mostly to what you had at the end of your 1" pipe (was there some sort of premise plumbing at the end of the 20 feet, or just an open jet etc as you have answered), yes, due to extreme velocity of the full-bore discharge I believe even minor losses could be exaggerated. (and if I understood this situation correctly, I'm gonna make a wild guess there may be say a hundred gpm or so discharge, so you might need a pretty good-sized "bucket" for a meaningful flow test!)

RE: Calculating Water Flow Rates

One simple way and within ball park, is to let the water flow horizontally from a spigot or nozzle, then measure the height and the horizontal distance from the source of flow to the impact on the ground. The horizontal measurement should be made at the center of impact as best as you can determine; then apply the trajectory equations that we should all be familiar from our high school physics to calculate velocity and then knowing orifice size you can calculate flow rate.

RE: Calculating Water Flow Rates

easier way (and probably more accurate) is to fill a bucket and use a stop watch to time it

RE: Calculating Water Flow Rates

--- or borrow a domestic flow meter from the local water authority ...

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating Water Flow Rates

You could also model the system in EPAnet.
Although if you have a water meter, it is non linear at higher flow rates.
Also in this case, minor losses will be significant.



Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close