Calculating Flow @ a given pressure 1

[smgo]

I'm trying to find a standard flow rate in gpm for PVC pipe on a city line at a give psi. I can calculate gpm from velocity, and can find pressure drop, but am unable to find a table or equation for this calculation

 

[CountOlaf]

I don't understand. You state that you can "calculate gpm from velocity..." There you go, right?

 

[smgo]

I have a specified pipe diameter, and a specified supply pressure. I would like to know how to calculate either volume in ft/sec or flow rate in gpm. I know how to get from volume in ft/sec to flow rate in gpm, but not how to get from psi to max flow rate in gpm.

 

[cvg]

it depends upon length of pipe from your "source" eg: tank, well or booster pump. At a given flow rate, you will have a pressure drop in the pipe(s) leading from the source to your point of connection. At higher flow rates, the friction loss will increase and the pressure drop will increase. Unless you have a very simple system with just one pipe, this involves a pipe network analysis which is normally done using a numeric computer model (way to complicated for hand calcs)

try this web calculator

http://www.lmnoeng.com/Pipes/PipeNetwork.htm

 

[CountOlaf]

You can't get flow rate (or velocity) from just a single pressure reading. If it were that easy, we wouldn't need flow meters, we'd just need pressure gauges. An orifice meter, for example, measures the pressure drop across a restriction with a known geometry and empirically determined correction coefficient. Similar stuff for a venturi meters, etc.

If you knew, for example, the pressure at some other point downstream (and any elevation changes) and the equivalent length of pipe between the points (equivalent including other minor losses such as changes in direction, etc.), you could figure out the velocity by way of the Darcy formula which relates pressure drop or head loss (due to friction) to velocity.

If you're simply looking for maximum "acceptable" flow rates through PVC pipe, etc. of different diameters, then you need to look at charts which are derived from the Darcy formula (or do it all manually, or use a computer program--someone on the previous thread had a link) where you can attempt to limit the pressure drop to something reasonable (say 4'/100') or the velocity to something reasonable (say 8 fps for 8" and smaller piping) and then see what the gpm is. You can obviously pump more or less water though the same size pipe, it just changes the pressure drop through the system. The losses may or may not be acceptable depending on the pressure you need at the point you're trying to to get the water to.

I hope that made sense. Someone else may be able to explain better.

 

[vzeos]

Try this website:

http://www.martindalecenter.com/Calculators4.html

and go to the Fluid Dynamics & Mechanics section.

 

[Montemayor]

smgo:

To understand and accept that what you stated is wrong, refer to either Crane's Tech Paper #410 or Cameron's Hydraulic Data book. There, you will find the basic relationship of fluid flow in a pipe. The Darcy-Weisbach equation is what defines the flow of fluid in a pipe and it requires that you define the allowable pressure drop (the driving force, or head) in order to obtain the related flowrate of liquid in the pipe. Cameron gives you tables of expected water flowrate in a variety of pipes with a variety of pressure drops. You can create your own similar table for PVC pipe.

You need a Driving Force to obtain a flowrate, not a simple pressure value. Fluid flow is analogous to the flow of electricity: you don't have coulombs flowing because of voltage; you obtain coulomb flow due to the existance of a voltage drop.

And by the way, there is no "standard" gpm. Volume for incompressible fluids (such as water) is simply volume. You don't get into "standard" conditions until you mess with compressible fluids (gases or vapors).

 

[vzeos]

smgo:

I assume you're trying to set-up standard capacity tables for various sizes of plastic pipe at various inlet pressures given a fixed terminal pressure

Go to the link below and order - Performance Pipe (Plexcalc) CD-ROM. – It's free.

http://www.hdpe.com/Freebies.shtml

The CD has a complete polyethylene industrial and municipal piping catalog. Also includes an engineering manual on properties, design and installation. Along with this you get Plexcalc to assist in your flow calculations, design and construction.