Bladder tank pressure

Discussion in 'Pumps and Tanks Well Forum & Blog' started by TheTank, Sep 22, 2007.

  1. TheTank

    TheTank New Member

    Messages:
    4
    I am curious to know why the instructions for my bladder tank says "cut-in pressure must be the same as factory set pressure of 38PSI".
    I have done a little searching and seen most online recomendations state that bladder pressure (with the tank empty) is to be 2psi less than the pump cut in pressure. Anyway, I can understand that the bladder pressure needs to be the same or less than the pump cut in pressure for the pump to to turn on, but why so close? Why can't I bump my cut-in pressure up to 45psi without fooling around with the tank?
    Thanks in advance.
    Last edited: Sep 22, 2007
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,391
    Location:
    Lubbock, Texas
    It won't hurt anything to increase you cut-in to 45 PSI. You will get a little less drawdown from your tank and it will stretch the bladder a little more. Still won't hurt anything but, I would not go more than about 10 PSI higher than the air pressure in the tank.
  3. With the pump off and the water pressure drained down, all that you have to do to increase the system cut-on pressure (besides adjusting the pressure switch), is to air up the pressure tank to the desired increased psi - per manufacturer's recommendation - with a bicycle pump, or portable air tank or compressor.
    Whatever psi you have in the pressure tank will kick the pump on at that pressure when the pressure drops to it, and the pressure in it will increase until it reaches cut-off pressure and will stop there of course.
    Mike
    Excellent information source for wells and pumps by Jess Stryker:
    www.IrrigationTutorials.com
  4. TheTank

    TheTank New Member

    Messages:
    4
    Valveman, thanks for the reply, but my curiousity is still there.
    Currently, my system cuts in at 38PSI and cuts out at 70PSI (was that way when I bought the house). As such, when the system is fully charged and the pump cuts out at 70PSI, the bladder is exerting 70PSI (by the way, the tank is rated for 100PSI) back onto the system. As stated before the bladder was precharged to 38PSI from the factory. If I change the pump cut-in pressure to 45PSI (and leave the cut out pressure the same at 70PSI), I as a layman can not understand what difference there would be whether the bladder is at 38 or 45PSI. It seems to me that it will still see the same streching (or actually less since it won't have to fully expand) and pressures regardless. I know that the pump will run more often due to the shortened differential, but I don't see the effects of the revised bladder pressure on this either.
    I know that I should do what the instrucions and professionals say to do, I would just like to benefit by knowing what the logic is.
  5. TheTank

    TheTank New Member

    Messages:
    4
    Rancher & Mike,
    Thanks for replying to this post. But I still do not see a clear picture of this.
    For example:
    At 38PSI cut-in and 70PSI cut-out, a 38PSI bladder will see a range of 32PSI with a maximum compression of 70PSI

    At 45PSI cut-in and 70PSI cut-out, a 38PSI bladder will see a range of 25PSI with the same maximum compression of 70PSI.

    I guess my question is this: If I take the system as is, charge it up till the pump stops at 70psi, open the hose bibb until the pressure drops to 45PSI and then close the hose bibb; How are the effects on the bladder any different between the two different cut-in pressures? It seems that they would be the same except that with the 45PSI cut-in, the bladder wouldn't have to expand as much. I don't see any difference in bladder "stretch" (as described by Rancher & Valveman) since the cut-out pressure hasn't changed between the two scenarios.

    I'm just a homeowner with a strange curiousity and I just don't understand where this "2 psi lower or equal" rule comes from.
  6. wondering

    wondering New Member

    Messages:
    105
    I too am just a homeowner with a well but I am going to try to explain this to you the way I understand it:

    If you run your tank with 38psi then it starts to run at 38 and stops at 70psi.
    When your pump kicks on your tank is empty and then fills the tank and kicks off at 70. The reason for the bladder stretching is because normally pumps are run with only a 20lb diff such as 40/60-50/70. With a 32 lb diff you are putting more water in the bladder and it causes it to stretch.
    Now--

    If you change your cut in to 45psi and cut out at 70psi but you do not change the air charge and leave it at 38psi you are still going to have the same set up that you have now with the cut in at 38 cut off at 70 EXCEPT
    the pump will start at 45 and off at 70 but when the pump starts your tank won't be empty there will still be water in there.

    If you change your air charge to 43psi and change your cut in to 45psi and the cut out to 70psi then when your pump starts your tank will be empty.
    Also there will be less bladder stretch because you only have a diff of 25psi compared to the 32psi with the other setting.

    If you change your cut in to 45psi and leave the air at 38psi let the pump run and kick off. Then turn your power off and drain the tank. Listen for the pressure switch to click while draining the tank.After the click notice how much water is left in the tank before it runs dry. That is the amount that will be left in there each time the pump starts if you change your settings to 45/70 with 38psi air charge. If you change your air to 43psi with the 45/70 setting then the tank will be empty each time the pump starts.

    I hope this helps without being confusing and I hope I have all this worded right. If not someone here will tell me if I am not right and help reword it better.
  7. TheTank

    TheTank New Member

    Messages:
    4
    PRD823,
    Fantastic explanation. I did not think about the tank being designed for a specific differential (it just says "pre-charged to 38PSI, Maximum 100PSI". Boy, I guess it has been subjected to some abuse with the 32PSI differential (I had thought that this was the proper setting for the system since this was the way it was when I bought the place). I wonder how long it has been that way. All this confusion could have been eliminated had the tank instructions stated the 20PSI diff info to begin with.
    Thanks again for the explanation.
  8. wondering

    wondering New Member

    Messages:
    105
    Remember that if you change your cut in/out on your pressure switch that for example if you were going to change it to a 50/70 setting that you will need to turn the large nut clockwise to increase your CUT IN pressure but at the same time it is also raising your CUT OUT pressure so you will need to turn the small nut counterclockwise to do this. You will have to cycle it several times to get it just right. Also remember to change your air charge in your tank.

    What brand and size is your tank? I have read that some brands are more tolerent to different pressures than others.
    If you leave your pump at the current setting,well or any setting I would drain the tank and check the air charge in it if you haven't done that in a while. I know you said you have the 38psi air charge but I wasn't sure if you have checked it or are just going by what it says on the tank label. It could be different than 38 if it hasn't been checked in a while.
  9. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,315
    Location:
    Cave Creek, Arizona
    tank

    The bladder in the tank is secured by the center "joint". As such it is "streched/expanded" to fill the lower section of the tank when there is no water entering it. As the water fills the tank, the membrane is acutally relieved unless the air pressure is zero, which case it tries to move to the other side of the tank and fill that, but even then, the air that was in the tank will be compressed and create a minimal air cushion so the tank will still have some limited function. If the air is pressurized above the cut in point, you will run out of water for a millisecond as the system pressure continues to drop and turns the pump on.
  10. speedbump

    speedbump Previous member

    Messages:
    4,540
    Location:
    Riverview, Fl.
    PRD823,

    Great explanation!

    For years I used to set all tanks that I installed or serviced 40/70 if the pump would allow. The reason was to make the drawdown a little bigger than it would be at 40/60. This saves wear and tear on the motor. Now I just use the Cycle Stop Valve and went back to a 20 lb differential.

    bob...
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