CSV , oversized pump/undersized tank, and other considerations

Discussion in 'Pumps and Tanks Well Forum & Blog' started by birkie, May 25, 2014.

  1. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    Hi all,

    This is my first post. We recently bought a fixer-upper that had been vacant for a few years (short sale), and were pleased to discover that the well pump works (submersible, size unknown), and the water is of good quality.

    It currently has an AO Smith V60 20 gallon bladder tank that fills in about 20 seconds. I figure the pump is putting out around 15 gpm. Running some of my own quick capacity tests, it looks like the well is actually capable of maybe 7gpm sustained - so it's possible to overdraw the well after a while.

    I'm wondering if just putting a CSV in is an acceptable solution, and what things need to be considered when doing so. The pump and associated piping is in unknown condition. I believe all the piping from the house to the well is HDPE. If I were just to slap a CSV into the system, is there anything I need to be worried about, or watch out for? Do I need to worry about the higher pressures encountered at the pump and piping when the valve is in 1gpm mode? Which parts of the system are under increased stress/pressure because of the CSV?
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,549
    Location:
    Lubbock, Texas
    The backpressure from a CSV is entirely dependent on the pump. If you have measured the pump putting out 15 GPM, then finding out the horsepower of the motor will help us figure out the amount of backpressure.

    None of this is really important and you could just slap a CSV on that system as long as the horsepower of the pump is 1 HP or less, and the poly pipe from the well is 160# or better.

    The only ones I worry about have that thin 100# poly pipe underground. I don’t know why anyone would use that anyway.
  3. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    Thank you very much, this was quite helpfu! I have been unable to find any records on the well, but did do some measurements related to current draw and flow characteristics.

    Here's what I found: so far
    • The pump can supply a steady 14-15 GPM at 50 psi.
    • The well runs 'dry' (lower, erratic flow as if it were pumping water and air) after about 2 minutes of running full-bore (15 gpm), but recovers very quickly. Recovery rate appears to be about 6 gpm.
    • Amp draw starts at about 8.5A with a fill well, drops to 8.4A before it 'runs dry', at which point it bounces around 6-7A.

    From this, I think I can draw the following rough conclusions. Not being an expert in wells or fluid dynamics, let me know if anything seems off base, or a big assumption.
    • The pump is probably a 1hp pump
    • The well is probably about 100' deep (this is by looking at various pump curves of 1HP, 15GPM pumps)
    • There is only about a 17 gallon reserve in the well casing above the pump (based on running dry at 130 seconds, with a 6gpm recovery rate. The relatively small decrease in pump amps between full and dry also suggests a small-ish difference in water level between extremes).

    This tells me that the well itself could be workable as-is, that a CSV could be quite helpful for taming that large pump, and that I should probably invest in pump protection (detect dry conditions). Does that sound about right?

    Doing some back of the envelope math, I find about 115 PSI at the pump when pushing out 15GPM against a 60psi tank. About 10 of that is friction loss. So I would surmise that pressures are of no concern when installing a CSV, given that it looks to be 160psi HDPE coming in from the well.
  4. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    Hm, I was just thinking - how would 'dry' condition detection interact with pump protection (e.g. cycle sensor)/ 'dry' in this case means the well is still producing 6gpm, only that the pump is trying to draw water faster than that rate when the water level is at the pump's inlet. If you disregard the air, the pump is still managing to deliver 6gpm. I observed the amps bouncing between 6.x and 7.x under this condition. The CSV can throttle the pump back to 1gpm under certain conditions. I haven't measured the pump's current draw at 1gpm, but couldn't the 'dry' condition actually draw more amps than the throttled 1gpm condition, as it's still moving more water? What would the cycle censor do in this case?
    Last edited: Jun 6, 2014
  5. valveman

    valveman Moderator Staff Member

    Messages:
    4,549
    Location:
    Lubbock, Texas
    Sorry for the delay. Been busy as a one-leg man in a butt-kicking contest. Anyway, I think you are very close. If it is pumping 15 GPM open flow, it is probably a 10 GPM pump. But your backpressure still should not be a concern.

    A CSV will make the pump into a 6 GPM pump as long as you don’t open more than 6 GPM of faucets. Then maybe you won’t ever pump the well dry.

    But even if you do pump the well dry, I don’t think it will bounce and continue to pump 6 GPM when working against some pressure. Open flow it will do that but not against pressure. So yes the amps should drop more when the CSV restricts the pump to 1 GPM. But they should drop even lower when it starts pumping air. The Cycle Sensor can be set at any amp draw. So after you see the amps when the pump is restricted to 1 GPM, set the Cycle Sensor just a bit lower and it will still shut the pump off when the well is dry.
  6. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    Thanks - your insight is quite helpful. "one-leg man in a butt-kicking contest" - I'll have to remember that one!

    The pressure gauge was reading a steady 50 psi while the pump was putting out 15 gpm. That's what made me think it was a 15 gpm pump. This was actually achieved just by attaching a 50' garden hose to the manifold and throttling it until it reached an equilibrium (i.e. the pressure didn't climb or fall). So it was 15gpm at this equilibrium point.

    Exactly - that's why I thought the CSV would be an ideal application for this scenario.

    Maybe my methodology was a but odd, but I actually first came at the 6gpm figure by shutting off the hose once it ran dry, and watching the pump muscle through and try to fill the tank. It filled at a slower rate than usual... at just under 6gpm. I measured how much it pumped into the tank, and how long it took. Roughly two gallons in 20 seconds. This is also when I measured 6.x-7.x amps - as it was trying to slowly fill the tank while it was running dry.

    This is all new to me, so I might not have full grasp on the theory. When any well runs dry it usually still is being filled with water, just at a rate lower than what the pump is capable of delivering at the time, right? These are conditions under which the cycle sensor has been proven to work, so I guess there's no reason to believe that my well is any different from any other.
  7. valveman

    valveman Moderator Staff Member

    Messages:
    4,549
    Location:
    Lubbock, Texas
    I have seen a few pumps do that. However, if it is still moving 6 GPM the pump is not running DRY. So it won't overheat from a lack of flow. That surging is not really good for the spline and thrust bearing so you don't want to run it that way. Just use less than 6 GPM for long term uses. If it does lose prime and run dry it will only pull about 4 or 5 amps and the Cycle Sensor will shut it off.
  8. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    Fascinating. Does that condition have a name (where the pump is able to surge and still deliver water)? Can the cycle sensor detect that condition (i.e. look for oscillating or unsteady amp draw)?
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,549
    Location:
    Lubbock, Texas
    Surging is what we call it. You can set the Cycle Sensor to shut the pump off for any amp draw. It can even detect a slight drop in the water level and shut the pump off. But the Cycle Sensor will look at low amps for 10 seconds before it shuts the pump off so it doesn’t shut down as long as the well is still surging water.
  10. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
    OK, so I hooked up the pressure tank to the valve apparatus. Everything seems happy so far. One thing I noticed is that the amp draw only drops by about 18% when trickling through 1gpm vs running at full bore (i.e. it drops too just under 7A, vs 8.5A). Is that typical, or may I be running on a slightly worn thrust bearing?

    Here are some cool time lapse videos of it in action:

    Cycling as usual:
    http://youtu.be/G7_LM-akc6Q


    Constant pressure/cycle stop activated:
    http://youtu.be/Fk8cKpgl0hg
  11. valveman

    valveman Moderator Staff Member

    Messages:
    4,549
    Location:
    Lubbock, Texas
    Cool videos. I couldn’t get the sound to work. Which I can understand if it because it’s a “time lapse” video. It doesn’t say how long the time lapse is for, but with the CSV it would be boring because it would stay that way indefinitely as long as the hose is “on”.

    18% drop in amps is typical of some brands of pumps. It probably means the pump has a “floating stack” impeller design. There are actually two kinds of floating stack designs.

    On one design the impellers float completely free (up and down) on the pump shaft. Down thrust causes each Impeller to drag on the diffuser below it, and there is actually no load on the motor thrust bearing. This is a good thing for the motor thrust bearing, as I have seen these run hot for many hours without destroying the bearing in the motor. However, the impellers dragging causes more load and the amps don’t drop as much as the designs where the motor thrust bearing carries the load. This dragging of the impellers also causes more heat in the pump end. Therefore, in a deadhead situation (high heat) the pump end locks down before the motor is destroyed.

    Other designs allow the impellers to float up on the motor shaft. Downward pressure is held by a stop on the pump shaft, and the impellers do not touch the diffusers below. The pump shaft transfers all the down thrust to the motor thrust bearing.

    Yet another design called a “fixed stack” locks the impellers to the pump shaft. In this way the impellers as well as the pump shaft can float up until an up thrust bushing is hit. However, the spacing of where these impellers are locked onto the shaft prevents said impellers from touching the diffusers below them. The down thrust load is transferred to the pump shaft, which in turn transfers the entire load to the motor thrust bearing.

    The last two designs where the thrust bearing carries the entire load usually have a better drop in horsepower when the flow is restricted. This is because the hydroplane effect of the single Kingsbury type thrust bearing in the motor has very little friction. Multiple impellers dragging on multiple diffusers have more friction and pull higher amps.

    The actual design of the impellers can have a further impact on the amp draw. Some “fixed stack” designs will drop 50% to 60% in amperage, while other “fixed stack” designs will only drop 30%.

    Even an 18% drop in amps is enough to de-rate the motor load. This allows the motor to run cooler and requires less flow to prevent motor heating. In other words running at 1 GPM is not going to hurt anything.

    Only dropping 18% in amps is not going to make much difference in the electric bill for house water. However, when pumps are used for long terms at low flow as with irrigation systems and heat pumps, a pump that drops more in amperage can make a big difference in the electric bill.
  12. birkie

    birkie New Member

    Messages:
    8
    Location:
    NY
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Forum Title Date
Pumps and Tanks Well Forum & Blog Is my system oversized too much? Sep 14, 2007

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