I recently installed 1 inch pex for my main line of my sprinkler system. I have a well and my pressure tank is at 65 psi. I did the old 5 gallon bucket test and it filled in like 10 seconds. THat is 30 gallons per minute. Does this sound right?..Safe?
Not sure what your question is. If you filled a five gallon bucket in 10 second, that is 30 gallons per minute, so your arithmetic is correct. You did not describe exactly how you did the test, was it an open 1 inch hose, through a hose bibb, etc., so when you actually hook up your sprinkler system you will get different losses, so your overall flow rate may be different.
Every chart and spec sheet I find for 1 in Pex says the flow rate maximum is 18 gpm. The pex pipe is about 20 feet and right now it is open at the end. My 30 gpm measurement seems high. I know it will drop once I put the proper fittings on at the end of the pipe. I need and accurate flow rate to design my heads and zones. Should I use 30 gpm?
The term "maximum flow rate" is misleading. The actual flow rate in a plumbing system depends on the pressure at the inlet, and the losses through the system. The greater the pressure, the greater the flow for a given system, so there is effectively no "maximum" flow rate.
In your case, you can calculate the approximate flow rate through a one inch PEX pipe as a sanity check. Flow through a pipe is generally governed either by the size of the opening on the end, or by friction losses through the pipe. In any case, in domestic water supplies, the actual velocity rarely exceeds 15 feet per second through the pipe, as friction losses build up quickly when flow exceeds that velocity.
The ID of 1" PEX tubing is 0.863 inches, so the inside area is 0.585 square inches = .00406 square feet. At 15 feet per second, the flow rate would be about 0.06 cubic feet per second, or about 27 gallons per minute, which is pretty close to what you measured. The ACTUAL FLOW RATE you will get in your system depends on the losses in the pipes and the sprinklers, which requires a fair amount of computation. There is no simple way around needing to do the calculations, but if you do not want to do any calculations, you can assume a maximum velocity of about 10 feet per second, and you will not be too far off.
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