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Rate of flow for an open pipe under pressure

Posted: Thu Jul 31, 2014 1:56 am
by Blue Max
We are trying to resolve what we thought was a fairly straight forward question, but are having problems using the calculators to find a credible answer. In our case, we need to verify the amount of irrigation water being delivered through a large pressurized golf course system to an open air turnout. The question is as follows:

What is the flow rate, in gallons per minute (GPM-US), through a 6-inch pipe connected to a large system pressurized at 90 psi?

We have no idea how long the pipelines are or how expansive the system might be, only that we have been told our point of delivery is out of a 6-inch pipe (or 6 inch valve) at 90 psi. Do we require additional information or can we figure the flow rate with just this information? Thank you for any help.

Re: Rate of flow for an open pipe under pressure

Posted: Thu Jul 31, 2014 7:54 am
by admin
You must know pipe length and position and size of branches if you have any.

Re: Rate of flow for an open pipe under pressure

Posted: Fri Aug 01, 2014 4:17 am
by Blue Max
admin wrote:You must know pipe length and position and size of branches if you have any.
Unfortunately, we do not have any of this information. All we know is that the water is delivered through a 6" pipe that is at about 90 psi at the point of open discharge before the valve is opened. The golf course system is likely complex and our line likely branches off a larger main line, but we do not have any information on the actual design of the system. Would it be helpful if we knew the length of the 6" feeder branch and the size of the mainline it connects to? Or does one have to know the design clear back to the source of the pump or other means of pressurizing the system?

We imagine that this is a common issue, so perhaps a common frustration also. How can one estimate the flow rates to their home, business, or even a faucet in their home, when they only have the basic information we have above (pipe size and pressure)? In most practical cases, the user has no knowledge of the design of the system back to the initial source, whether it is the city system or their own home system.

Are there any rules of thumb or minimum/maximum thresholds for calculating what the flow rates might be if they are fed by a larger main line or source that would appear adequate for providing all the flow the pipe can handle while maintaining a constant system pressure?

Thank you again for your interest in the practical problems of our question.

Re: Rate of flow for an open pipe under pressure

Posted: Tue Aug 05, 2014 11:27 pm
by Blue Max
Thank you for the reply, Admin. Unfortunately, we don't know the exact configuration of the system from which we receive our water, but might be able to obtain a little more information from the golf course delivering our water.

Nevertheless, there is also a question of practicality. We imagine that questions of this nature are not uncommon and that there are many practical situations where a water user does not know the exact design of the delivery system. For example, a homeowner seldom knows the design of the city system delivering their culinary or secondary irrigation water or even the design of their house system behind a given water tap or outlet. Does this mean the user has no way to estimate the flows other than by a physical test of water delivered out of the tap? Furthermore, in our case, the water delivery levels are voluminous, and under such pressure, that it might be difficult to test without considerable effort.

As such, we would like to know if there are some useful theoretical limits that might help us make a rough estimate. For example, if our 6" supply line is connected to a 12" main pipeline, that is maintained by the city at 90 psi, could that become a practical point of origin and allow us to perform a reasonable calculation from that origin versus requiring a system design clear back to the reservoir or pump, for example? Are there theoretical maximum/minimum limits for flow that would allow us to calculate a range of possible flows through a 6" pipe at 90 psi, without knowing the rest of the system design?