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Variable Speed Constant Pressure Well Pump

Discussion in 'Irrigation' started by 2 man crew, Aug 9, 2003.

  1. 2 man crew

    2 man crew LawnSite Senior Member
    Messages: 403

    I'm in the early stages of designing my own irrigation on my 2.5 acre lot.

    1st question of possibly many:
    My well only has a 3/4 hp pump. I know I will need to upgrade the pump. What is your opinion of the Variable Speed Constant Pressure Well Pumps? The one that has been recommended to me is a 1 1/2 hp 21gal per min. @ 50psi (or close to) pump. With new 1 1/2" PVC pipe and labor for $3000.00. The well is 191 feet deep.

    Also is there any other option with the same result?
  2. SWD

    SWD LawnSite Senior Member
    Messages: 988

    Stay away from the variable speed systems.
    Easiest, most reliable, least costly way of regulating system pressure is with a cycle stop valve.
    Go to www.cyclestopvalves.com
  3. 2 man crew

    2 man crew LawnSite Senior Member
    Messages: 403

    Thanks for the info.
    What about the new variable speed pumps should i stay away from?

    No matter what I will need a new pump. Should I go with the standard constant pressure pump and a new pressure tank for a few hundred more?

    This newer style pump is not the same as the old variable speed pump from what I've been told. I do like the idea of the pump turning on slower and building up speed to keep up with water use. Also I can use my existing pressure tank.

    Is 20gal per min @50psi going to be ideal for 2.5 acres or do I need more?
  4. greenworldh20

    greenworldh20 LawnSite Senior Member
    Messages: 659

    you want to water 2.5 acres...you better have a bigger pump than that!

    you will need more than 20 gpm @ 50 psi to water all that in less than 24 hours!!!!

    also, that is alot of water...can the well handle it? have you ever done a 'draw down' test? spend the $$$ on the 'draw down' test before you EVEN think about the irrigation system.

    draw down test is when a well company sucks all the water out of your well. then they will measure how quickly it will recover...then they will suck the well dry again and see how quickly it will recover...then using their caculations, they will tell you what your well can handle (gallons per minute for how long).

    definitely put some more thought into the system and well.

    just my .02.

  5. 2 man crew

    2 man crew LawnSite Senior Member
    Messages: 403

    When I talked to the well installer he said the well can handle at least 20gpm. How many gpm do you think I need? The area to be irrigated is probably more like 1.75 acres if you take the house, shed, driveways, and a small area that is growing wild.
  6. greenworldh20

    greenworldh20 LawnSite Senior Member
    Messages: 659


    you are talking about watering each turf area for about 30 minutes. if you have 15 zones (just a guess) @ 20 gpm you will be using 9000 gallons of water per cycle.

    and if it gets hot, watch out!!! you will have to increase run times...

    your best bet would be to put in a HUGE pump with a booster pump to give you great pressure. then, you could space heads further apart. place bigger nozzles in heads to water the large area in a short amount of time.

    like i said, just my .02

    of course, do the draw down test first...it might save you ALOT of $$$
  7. SWD

    SWD LawnSite Senior Member
    Messages: 988

    You are going to need more than 20 gpm if you intend to water effectively. I wouldn't necessarily raise the pressure too far, but based on your design and friction loss characteristics, the farthest section should have 50 psi at the nozzles. Work back from there to determine what you need at the well head.
  8. SWD

    SWD LawnSite Senior Member
    Messages: 988

    To answer your first question, regardless of how 'soft' a pump starts, several things happen, all at once, and all contribute to a shorter pump life and performance problems in the field.
    First thing is the increased amperage and voltage needed to start the impeller is highly damaging to the motor.
    Second, even with a pressure tank, cycling will occur in the system, so the variable speed motor will cycle with the variations in pressure/volume - yet this cycling is what damages the electric motor on the pump. I have seen it time and time again on commercial pump stations used on golf courses. Not to mention what occurs in a pipe under differing turbulent flow characteristics.
    Third, you are introducing a unknown variable into the irrigation design. Can you actually design around a variable rate system or is the pump manufacturer telling you the variable speed works?
    With the cycle stop, you eliminate all of these problems, with one valve, and you can use a larger pressure tank rather than one suggested in the web pages.
    No, I do not recieve money for recommending these valves. I do, however, know they work as advertised.
  9. MikeK

    MikeK LawnSite Member
    Messages: 145

    I reallyb like the VFD pumps, here is why.
    With a system on a well, you want to design the system to use ALL of the available flow at a specific pressure. This keeps the pump from cycling on and off. With large yards and different pressures in each zone, this is tough to do.
    What kills a pump is not the run time but the on and off. A VFD pump will turn on when the system turns on and throttles itself to maintain the desired flow. I have used them in several installs and wish that everyone would put one in. In theory, it should live longer than a conventional pump in the same application.

    If I remember correctly, there are 27,152 gallons in an acre inch of water. Lawns in our area need about 1 inch of water a week, minus whatever mother nature gives.

    27,152 * 1.75 acres = 47,516 gallons a week/20 GPM = 2375 Minutes/60= 39 Hours a week/7 = 5.6 Hours a day, 7 days a week is what the system would have to run if you do not get any rain that week. It's not ideal, but you could run the system from 4:00 AM to 10:00 AM every day and get by

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