View Full Version : Pipe flow formula

10-17-2004, 04:51 AM
I need help!
What is the formula for figuring out gallons per hour of water flowing through a 1/2" and 3/4" pipe.
An example of what I need is:

1/2" pipe at 20PSI... How many gallons per hour flow through pipe, or what is flow rate?

1/2" pipe at 35PSI.....Same question.

3/4" Pipe at 20PSI and 3/4" pipe at 35PSI..

Any help would me most appreciated.
Take care,

10-17-2004, 05:56 AM
The anwer bill be different for new steel pipe, old steel pipe, PEX, PVC, CPVC, type K copper, type L copper, or type M copper. The are charts that give the answer, but first you have to use the correct one.

10-17-2004, 06:05 AM

See table 7-11 in the above link, for copper pipe. An engineer could give the formula for this, but it's a LuLu! There are many variables. So, as a practical matter this data is usually compiled into tables such as these.

I don't know what project you might be working on, but it will be obvious from the tables that there is no single number for "20PSI " pipe". The length of the pipe affects the GPM in a big way due to pressure drop in the pipe as a function of distance. This link has some useful tables directly showing the loss (PSI) as a function of length:

10-17-2004, 06:18 AM

Here is the deal.

This is in a new restaurant and the connection goes to a Hatco C36 Booster Heater and then into a Hobart C44A Conveyor dishmachine.
They were suppost to have 3/4" copper into the booster heater and then 3/4" to the dishmachine. They put in 1/2" copper.
Here is my problem, the copper is in the wall and I have no way of knowing how long the run is, I can check with the builder.
My problem is with 1/2" pipe my rinse pressure varries from 35PSI to 20PSI. This is a big problem for me because I add a rinse/drying additive in the line and at 35PSI the glasses look GREAT, at 20PSI they have streaks all over them. This is because at 20PSI my rinse agent is much more concentrated, thus causing the streaks and runs. I need to explain with simple math as a back up the reason for the problem and then see if they want to fix it. It is a brand new high end restaurant in Atlanta and they will not tolerate the sub-par results. So......... I need to have my ducks in a row when I meet with them and explain the problem and offer a solution.
Thanks again for your help and take care,


10-17-2004, 07:41 AM
Hello Danno,

I am not a plumber, I am a Mechanical Engineering Technologist with design experience in fluid mechanics.

I believe you need a practical solution to your problem and without knowing the number and type of fittings, the pipe length and other variables it is not possible to calculate a flowrate.

You can rent or buy a flow meter that will measure the flow at the outlet to the dishwasher. There are available flowmeters made in many ranges. The problem is what range are you in? Further, the test needs to be done at the low pressure figure 20psig. You'll have to be able verify 20psig before doing the test to make it valid.

Here's a simplier solution. If you can find a container such as a beaker or bucket that is measured in the units you wish use. say gallons, and are able to redirect the flow at the dishwasher into it. you can measure the flow rate.

Simply, direct the flow into the bucket for 1 minute. or 1/2 min or some other convient period. Then, measure the water you've collect in that period. for your connection, use the same id as that of the dishwasher line and as short as practically possible. 1/2"pipe, I believe you said? Attaching a long garden hose won't do!

The math is simple. If you have say, 2 gal in a 1 min test then the flow rate is 2 gals per min.

Or 3/4 of a gal in a 1/2 min the you have 1.5 gal per min.

The best part of this test is that it is accurate. It will give you the exact flowrate at the outlet to that dishwasher at that time. Again, you need to verify that the pressure at the time of the test is 20 psig.

You can then write a report simply stating how you did the test at what pressure and the results.

Also, call Hobart and ask for a product or design engineer. Explain your problem. Ask him what minimum supply pressure is required for that specific unit in order for it to rinse streak free. He will most likely be more than happy to help and may be able to give you a minimum flow rate requirement to compare against your test. or maybe another solution. After all, Hobart dishwashers are his business. Engineers always go to the manufacturers when dealing with an equipment problem. You should too.

I believe the proper solution is to change the line and I assume you are looking for ammunition to support this. Hope this has helped.

10-17-2004, 08:46 AM
The post from Daveydo gets a little involved, but he did get to the core of the issue: You need to get from your equipment manufacturer the specs on minimum required water supply pressure and GPM. The reason your pressure seems to vary from 20 to 35 is probably that your supply line is inadequate, and when some other demand ( sink, toilet, etc) is placed on the system at the same time, it cannot keep up. The only real solution in this case is to repipe or provide an alternate supply. These things may not be as difficult as you think.

Bob's HandyGuy
10-17-2004, 02:40 PM
Why not experiment with a dilution of the rinse agent that will work at both pressures? The restaurant will save money and you will not have to re-plumb. Sounds win-win to me.

10-17-2004, 07:36 PM
Since it appears that increasing the pipe size will be difficult, will your unit operate at 20 psi, if the additive volume is modulated? If so, install a pressure reducing valve at the DW set at 20 psi, so it maintains a constant pressure.