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Thread: Need help calculating Friction loss for IRRIGATION

  1. #16
    Moderator valveman's Avatar
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    I would use a gate valve on the discharge of the pump. Start the pump with the gate valve closed or almost closed. Then slowly open the valve to fill the pipe line. If you just turn on the pump with no valve, or no water in the pipe line, the pump will not have any head against it. This will make the pump produce too much water, too much velocity for the pipe, and possibly even overload the motor. Once the pipe is full and flow is moving, you can open the gate valve all the way. The pipe is what puts the head on the pump. If you turn on the pump into an empty pipe line, there is not enough head, so the pump will produce too much flow and use too much horse power.

    A soft start on the motor WILL NOT be slow enough to get the pipe line full before the pump goes to full speed. A VFD cannot speed the motor up slowly enough to work. Even when the pipe is full of water, you need to start the pump against a closed valve, and slowly open the valve to prevent surge.

  2. #17
    Sprinkler Guy Wet_Boots's Avatar
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    Hey Valveman, ever hear of using a ram pump for a project like this? (who, if anyone makes one this size, I wouldn't know)

  3. #18
    DIY Junior Member DGM_Jakarta's Avatar
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    Talking Cavitation

    Mr Valveman, i already held a meeting about this, they still calculating again, then again i still have some question, please do not be bored with my question. . How about the cavitation in the pipe and valve, is there any risk. by the way i love this forum, we can change ideas and solve the problem. This is kind of forum that we really need.
    back to the question, how do you get the information, is there any calculation to show me and how to calculate?

    Anyway your suggestion that i dont need bigger pump with bigger power to achieve that pipe length, from your information the pump power will not exceeded 200 Hp. we think of changing the pvc pipe 20" to pvc pipe 16". Because the total sum of $$$ we still trying to calculate how much $$$ we have to spend.

  4. #19
    Moderator valveman's Avatar
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    I would not try to push more than 4,000 GPM through a 16" pipe. If the velocity is higher than 7 FPS, the pipe will come apart. I am using the friction loss chart in an old Hunter Irrigation book that I have had for many years. You can also find friction loss charts on the Internet, check with your pipe manufacturer.

    Cavitation is only a concern on the suction side of the pump. As long as your NPSHA is higher than your NPSHR, cavitation is not a problem.

    Starting and stopping the pump without causing water hammer or surge is the main problem. I would only start and stop the pump against a closed or almost closed valve.

  5. #20
    Plumber Cass's Avatar
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    Should something this large have something similar to an air admittance valve at or near the pump for when the pump is stopped to compensate for possible negative pressure...I no nothing about systems this large...just a thought...

  6. #21
    DIY Junior Member DGM_Jakarta's Avatar
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    Great thinking, well cass this is my first time project too, i never had this big one, the one before only takes 4700 feet long but with steel pipe. thats why i got into lots of forum to looking for ideas or some reasonable thought, so far only MR. Valveman had a responsible answer. ok now back to valveman, broo where do you find the friction loss, do you got any address on the web? because i also had one but the biggest one only for 12" pipe. I found it at Goulds pump website. How do you calculate the FPM? this is i got to know, cause this is important matter to design a pipe system.

  7. #22
    Moderator valveman's Avatar
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    If the pump is started and stopped against a closed valve, everything will work fine. The valve will need to be slowly opened after the pump starts, and slowly closed before the pump is shut off. Problems can occur if the pump is running and the power goes off. For safety I would use a combination pressure relief/surge anticipator valve that is teed off after the pump.

  8. #23
    DIY Junior Member DGM_Jakarta's Avatar
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    So i put the pressure relief valve after the pump. How about if im using two pumps? one for running and the other only for backup but they have a header pipe, can i place the pressure relief valve after the header pipe. Because this is a long way from the city, middle of the country wheres no technician. So we need one pump for a backup just in case.

  9. #24
    Moderator valveman's Avatar
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    Yes you can place the pressure relief after the header connecting the two pumps. It needs to be a combination valve with surge anticipation feature as well. I am looking for a friction loss chart on the web, hard to find for 16" and 20" pipe. If I can't find it, I will scan a copy from my book.

  10. #25
    DIY Junior Member DGM_Jakarta's Avatar
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    Thanks Valveman, it is hard to find on the net, i already tried many times, but the biggest pipe i found is 12". Im really appreciated that broo. Ah one more question, according to your opinion for the pump which is the best type, a centrifugal end suction or submersible waste water pump or turbine pump. I know every type of pump has different installation on the field, im just looking at the most easy pump for maintenance. For the power im using a diesel generator set with 250KVA, for one pump only. the river only had 33 feet from the bottom. and the suction length horizontal pvc pipe 16" are 25 feet from offshore. What do you think broo???

  11. #26
    Moderator valveman's Avatar
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    I think 16" pipe is not big enough for 4700 GPM, and is certainly not big enough for a suction pipe. Small suction pipe causes cavitation. I prefer turbine pumps as they will not require priming. It is hard to use a suction pipe with a turbine, unless you use a turbine in a can.
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  12. #27
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    Ok, thats a big surprise you are really a big support that book of your is really great books. Well then if there is so much trouble in the 16" pipe maybe we change it back to 20" then, where there is less pressure and less power, as for turbine pump is a great choice too, but the problem with the pump are if the pump broke down then we have to bring it up for service, the maintenance not very simple. But i agree turbine pump has advantaged and disadvantaged, i was thinking about submersible pump waste water that pressure can up to 70 PSI, but im not sure about the flow, the one that i use before are HOMA Pump from germany, and PARAGON Pump from the State but this pump doesnt have pressure 43,5 PSI, but can pump water 7900 USGPM.

    May i lowered the pressure but added more flow rate to balance the output?

  13. #28
    Moderator valveman's Avatar
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    With 20" pipe you only need 68' of head at 4800 GPM. You might be able to find a submersible sewage type pump that will work. If it has to be installed 26' from shore, and kept off the bottom, installation with 26' of 20" flexible hose could be a problem. You would have to prime a centrifugal and use a foot valve but, the pump and motor would be on shore, and only the suction pipe and foot valve would be out in the lake.

  14. #29
    DIY Junior Member DGM_Jakarta's Avatar
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    Well valveman im thinking using the centrifugal pump, i think it is the most efficient pump for maintenance, because we dont have to change any installation pipe when the pump are broke down and can fix it on the spot. Maybe sometimes have to cleaned the foot valve. but if im using the submersible pump it takes some time to bring it offshore. anyway thanks again broo your opinion and ideas are very important to my project. we still havent decided the brand pump, because we are looking for some open clog pump with flow and pressure as we demand.

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