There is an assumption that those who are reading posts in an engineering
newsgroup have sufficient knowledge to understand enough of the thread so as
to be able to ask for clarification of a post, and to not make stupid
comments until they do.
you know, Dougie, if you have little experience or background in the subject
matter as you appear to have, and your reading skills are as poor as yours,
you should let the engineers who do this regularly, speak.
Especially engineers with decades of experience in the particular subject
and who have several well pumps of their own (two shallow installed 1981
and 1984 and one deep 1978 at the lake places, one jet 1970 at the lodge -
still running with no problems, precharged to 30 psi.)
see below for cites refuting specific neophyte comments
"Doug Miller" wrote in message
...
In article , "--"
wrote:
IMHE, the bladder is pressurized too high for efficiency, AND too
close
to shut-off. The air bladder should be lower than the low switch
setting.
I would first set the bladder charge to 25 psi.
Not if the pump is set to come at at 40. Tank precharge should be
approximately 2psi below pump cut-in pressure.
Who says? For what purpose? One can select a precharge for the parameter
one desires to maximize: for max usable volume, then precharge closer to low
set point, and for max damping of pump-ripple in the water in order to
minimize parts wear or to get maximum control stability, precharge at a
lower setting-- or one can precharge somewhere between, as a compromise.
There are four reasons that 38 psi precharge with a lower set of 40 psi is
narrowly considered and the recommendation is poor engineering. And I don't
care WHICH junior engineer made that recommendation.
First, few people have a water pressure gauge that has better hysterisis
than 2psi, let alone accounting for the system hysterisis, so calling for
38 psi precharge for a 40psi switch setting is total BS from an execution
standpoint.
Second, a precharge of 38 psi on a 40 psi start? So you can have pump
ripple cycle all your control parts? A precharge set above 60% of upper pump
setting would only be made by a pump salesman or switch salesman looking to
sell more repair parts. 38 psi precharge on a pump starting atr 40 psi is
BS from a basic design reliability standpoint.
Third, set for 38 psi so you can have how much water stored under pressure
at 40 psi? Enough to keep the pump from running except on draws above the
rule-of-thumb of a gallon?
A five gallon tank gives a gallon reserve at 25 psi precharge 40-60 psi,
and the same five gallon tank gives a gallon and a half reserve at 38 psi
precharge 40-60 psi.
Try the old P1*V1=P2*V2 equation, using the differential from 40 psi and 60
psi.
(Ppchg*Vtank^1.1)/Ppres = Vair-vol^1.1 and then use (Vair-vol 60)-(Vair-vol
40) to find water stored in the operating range
for a given tank V between 60psi and 40 psi, V usable =[(Ppc/40) -(Ppc/60)]
Vtank
Fourth, experienced engineers avoid control settings near system spring
constants unless the controls have dampers or they latch. 38 psi in an air
bag is close enough to the switch setting of 40 psi such that oscillations
in the 38 psi air volume from the moved-water momentum changes can adversely
affect a non-latching switch. And they do.
I also refer you to the Fluid Power Engineers Handbook, Parker Hannifin,
page a-3.
The pump in this thread is closer to isothermal operation, and so I quote
" pre-charge..to about 1/3 to 1/2 system pressure. "
I have gone to a higher precharge than 1/2 for specific design
applications, and I knowingly accepted the loss in the other parameters.
Next, since if the pump is not securely mounted and the switch is on the
pump, the switch will move as the pump moves and the pump motion then
moves
the switch back. I would move the switch/secure the pump and the pump
stand.
If one of these does not work, you might need a 90 degree turn in the
water
line just before the securely mounted switch, but not a 90 turn supported
by
the switch.
one of those should do it.
-------------------------
background---
1) - as incompressible water fills the accumulator tank with a bladder
charged to 20 psi, the compressible air will stay at 20 psi until the
water
pressure rises above 20 psi.
Complete nonsense. As water enters the tank, the volume of air decreases,
and
its pressure necessarily increases immediately.
What, in your world P1*V1 = P2*V2 does not apply?
20psi air pressure*Volume air before water = 20psi air pressure* Volume
after water
means volume of air is constant. Same volume means no water can displace
air.
Apparently in your world, you can compress 20 psi air with 10 psi water and
change its volume without changing its pressure. Patent it , quick.
Neat trick in your mind, but not possible in the real world.
"As water enters the tank, the volume decreases". That "water enter the
tank" cannot happen below the 20 psi of my example, as I said.
perhaps I should have written "tries to fill" rather than "fills"
If the pump isn't developing
more than 20psi, _no_ water will enter the tank whatsoever.
Gee, that is EXACTLY what I said. "The compressible air will stay at 20 psi
until the water pressure rises above 20 psi"
If the pump is
developing more than 20psi, water begins to enter the tank immediately,
and
the air pressure also begins to increase immediately.
Which is what I said.
[snip]
Basically, #1 - if you have the bladder pressure set at the lower
setting, the switch vibrates as part of a spring-mass-spring
rigid-water-mass/pump with the switch spring set at 40 psi fighting the
40
psi air spring of the bladder. Setting the bladder to 25 psi should cure
that.
More nonsense.
You really should not be posting to engineering newsgroups other than to ask
a question.
Read carefully again the comment by one who has done this for decades.
After the initial fill, as water is withdrawn from the tank,
the pump will kick on when the pressure drops to the cut-in setting of the
switch (40psi in this case), regardless of whether the precharge pressure
was
25psi or 38 as it should be. The only difference is that, at the moment
the
pump kicks on at 40psi, there will be more water in the tank if it was
precharged to 25psi than if it was precharged to 38.
In spite of the fact that I was addressing force interactions and you are
fixated on precharge and have wandered off the path, I will comment on your
error.
Contained air is a spring, with a spring constant. The higher the
precharge, the higher the spring constant.
All pump-fluid-control systems are spring -mass systems that interact.
When pumps false start, it is a sign of unintended interaction, There is
no discussion among engineers that changing a spring constant changes the
interaction, only discussion as to what amount.
Either way, the pump
comes on at 40 and shuts off at 60. The precharge pressure does *not*
affect
that.
apparently the original poster's comments refute your statement. His pump
shut off after a second at what appeared to be 40 psi. Off at 40 psi. Not
off at 60 only, as you claim.
In the real world, precharge pressure can and does affect controls.
--
Regards,
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
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