Assuming we're talking about 3/4" Sch40 PVC, the pressure loss tables indicate that 8-9 GPM is the max you should attempt to push water through 3/4" plumbing (regardless of the pressure). At 9 GPM in a 3/4" Sch40 PVC, the water has to travel 5.4 ft/sec. 5 ft/sec is considered the max safe water flow rate.
The reasons to upsize the pipe would be to reduce pressure loss due to pipe friction losses.
Just wondering if anyone exceeds the 5 fps rule of thumb for velocity in PVC?
In main lines? In laterals? What about the max velocity in copper tube?
The 5 fps is a guideline and not necessarily a hard and fast rule. If you read the flow charts it says that velocities above 5 fps should be used with caution.
Also the ASIC irrigation consultants started by Bob Cloud et.al. many years ago, regularly use velocities or 6 fps or more.
The reason for controlling velocity is to lessen the effects of a valve closing thereby creating water hammer. A properly solvent welded joint will withstand the effects of water hammer very efficiently at 5 fps. But they can also handle more velocity, but where is the limit?
Also the velocity in a lateral line can be higher due to the fact that the energy of flowing water is easily dissappated out through the nozzles. It may make trhe flow tubes flutter a bit, but it does not damage the piping. I have talked to agricultural drip designers that use velocities in their submains that regularly exceed 8 fps. And they have no problem as there are several hundreds of outlets for the energy to be dissapated.
Just pointing out that 5 fps velocity in PVC is not a mark that cannot be crossed. Just use a little common sense.
Also, flow in a pipe is unlimited. If you put 1 gallon in, you are going to get 1 gallon out. If you put 50 gallons in, you are going to get 50 gallons out. Pressure may be greatly reduced and velocities off the chart, but the water will flow. So 15 gpm through 1" pipe may be the norm based on industry standards and practices, but you can get 30 or 40 gpm through that 1" line. If you don't think so, look at the flow chart for a Hunter I-40 @ 90 psi with a #45 nozzle. That thing is pushing 27 gpm through a 1" inlet and out through a nozzle.
Just some thoughts while it is raining out this evening and me with some ditches open.
You're pretty much spot on. There are variables. Just think that to keep everything on the safe side the flow/friction charts err to that side based on many years of testing and use.
Another example is valves. Depending on the valve (we use Irritrol Centrurys and Hunter ICVs) it can safely handle much higher flows than the inlet/outlet size indicates. On field systems it's not uncommon for me to design/install a 2" valve with 2-1/2" or 3" in/out of the valve. Never had a problem. One thing we do here is NEVER use male adapters. We always use SCH 80 TOE nipples for the inlet and outlet of valves. Of course we have the potential of vehicular traffic including large cranes and cement trucks and have rarely had a valve snap off under vehicular load. Only time I can remember was when the area around the valve was damp.
I know in a different post I stated that I usually "undersize" zones so the zone is not GPM maxed but on very rare occasions I have exceeded the reccomendations without problems.