Only on the suction side of the pump. Not where the pressure
switch is reported to have been.
And how did this "steel rod" of water become already moving,
since it has distributed inertia? Did it perhaps require
*pressure* and *time* to accelerate? The pressure spike had
nothing to do with:
Acceleration of mass requires force. In the case of a fluid,
this is usually a gradient in pressure, and the line pressure
stays pretty close to tank pressure, even at startup. Note that
the OP indicated that changing to 1.25" line did not have any
effect, where with your "model" it should have had an effect.
David A. Smith
That may help. You might also consider just getting another small
bladder tank for the pumphouse. That would be even better than the
standpipe that others have suggested.
-jim
-
It won't. Likely the "kick off" occurs simply in accelerating
the flow between the pump and the tank inlet.
If you like to play with electrons, you could drive a
time-off-delay relay with the pressure switch, and use the relay
(through a contactor) to power the pump. This would give you a
"variable orifice plate or accumulator" effect that could be
dialed in to meet the need.
David A. Smith
Nothing new to add but I thought I would wrap up all the good
suggestions in one batch. Lots of good brainstorming in this thread.
Cause: Looks like the 75 ft run is causing the pressure switch to see
overpressures at start-up. That could be due to two things:
1. Just the physical restriction getting the colume of water moving:
Most likely. Your saying that you can force the pump to stay on by
holding the contacts closed "for a few seconds" is the clue.
2. A restriction in the pipeing somewhere. The size pipe you have
should not be enough to cause it. If you have a water
filter/conditioner, it could be partly plugged. That is one common
cause of restrictions but I don't think it is in your case.
Cures:
1. Move the tank: I wouldn't. Running a trench and wire for 75 ft is
not that much work and you won't have the nuisance of having to keep
the tank from freezing or maintaining it in a hole in the ground.
2. Move the pressure switch to the tank - I would. Again adding
whatever wiring is needed for 75 ft is not that much work. It also
moves a maintenance item to an easily accessable location (the
basement).
3. Add a delay timer: Good but adds another thing to the system that
can fail.
Your plan of changing the cut-in pressure to 30 will probably fail. The
problem is not the differential between cut-in/cut-out. The problem is
that the pressure switch is seeing pressures at or above 60 psi at
start-up. The switch will still see overpressure.
Harry K
That will work if the extra pressure the switch 'sees' at start up is
between 20 and 30 pounds. Your cures didn't include adding an air filled
stand pipe in the pump house. Nomad can simply put a stand pipe filled
with air where his pressure switch is now located and put the pressure
switch at the top of the stand pipe. Having a column of air between the
water pressure and the switch should help a lot. He may need some kind
of reservoir of air in the pump house anyway (another small bladder tank
would be the most reliable) as he may also be getting a 'hammer' when
the pump shuts off.
-jim
-
Huh?? The cut-off is set at 60. The overpressure has to be hitting at
least 60 psi and lowering the cut-in pressure won't affect that. As
for the extra air cushion, yes it might work but why not fix the system
the right way instead of trying to band-aid it?
Harry K
The system pressure starts at 40 and hits 60 when the pump is activated
if it starts at 30 it may not reach a peak of 60.
Putting an air reservoir in the pump house is the right way. Moving the
sensor to the house will solve the immediate problem, but the pressure
spikes when the pump kicks on an off will stress the piping system and
pump and will lead to shorter life span.
-jim
-
Yes, an air reservoir -may- be also needed. I doubt that the pressure
pulse is going much above 60 psi. It would be interesting to know just
how high it is going. I really do not see the need for the air
reservoir at the well. In all the systems I have worked on, there has
never been one. I don't have one and my run is just about the same
distance. I can hear the pump kick on and never hear any indication of
a hammer.
Harry K
Okay, I'm a bit slow this morning, not enough coffee. Adding another
10 psi to the cushion might do the trick by giving a bit more time for
the pressure pulse to subside but again it is a band-aid approach.
Harry K
"Mutually exclusive cures". Don't try and implement all of them.
If you do 1 or 2, you don't need 3.
The pressure switch need not switch the entire pump power. It
can take 115/230 vac from a wall socket, and direct a half-amp or
so to power the contactor near the pump. So you don't need more
than one run of small sized "Romex".
"nomad", if you implement #1, you'd still likely need to move the
pressure switch to the tank. If you implement #2, you'll need
the suitable for being buried wiring, a power cord for pressure
switch power, and a dedicated purpose contactor (you can look
this up at http://www.grainger.com). If you implement #3, you'll
need a time off delay timer, a socket for it, a power cord the
timer, and the dedicated purpose contactor. The dedicated
purpose contactor will last far longer than the pressure switch
contacts, if that is any consolation. If you need more detail,
let us know.
David A. Smith
The problem, which I feel has been diagnosed correctly as surge pressure
might be solved by adding a timer into the pressure switch circuit.
At cut-on, a timer holds the contactor in for 10 sec or so to let the
surge pressure subside, then drops out and allows the pressure switch to
do it's job.
Just a thought....
--
Anthony
You can't 'idiot proof' anything....every time you try, they just make
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.
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.
At that point, any higher pressure further compresses the air and
squeezes the bladder, which fills the part of the tank vacated (by the
compressing of part of the bladder) with water. Thus, when the water is at
60 psi, the air in the bladder is 60 psi and the tank has more water and
small bladder. At 50 water psi, 50 air psi and less water in the tank and a
larger bladder - and at 40, 40 and even less water -or in your case, no
water, just air.
That is, as the water is drained for use, the air pressure provides the
force to expand the bladder and empty the tank, from 60 psi down to the pump
lower setting
2) the switch uses a spring to sense pressure. In your case, one spring is
set at 40 psi.
3) the bladder is a spring. In your case, the air spring is set at 40 psi
initially
4) the water has mass, and it has momentum which will be felt as a force on
anything in its way on along run. Like water dynamics being misread as
static pressure.
5) a switch is a part with mass - if you move it as it is closing a circuit,
that mass may move and open the circuit again. Rule of thumb is to not
mount sensitive switches to parts they control.
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.
As to #2 - If the cycling is because you have long run straight at the
switch, you might be able to stop the cycling by adding a turn in the line
to stop the running-watermass from hitting the switch spring. Of course,
putting that 90 on the switch won't help, because the water-hammer force
will go right into the switch again.
As to # 3 - If the cycling motor start is caused by switch moving its
contact to closed which then moves, and puts the switch contact back to open
as motor twists - move it off the motor
( or, if you don't have to drain it and thus have freezing to worry about
that will shrink the pump more than the mount, secure the motor more
rigidly.)
(note that your switch and gauges even when used carefully are likely no
closer than 5% accuracy, so 40 can be 38-42, and your 38 can be 36-40, an
overlap)
Not if the pump is set to come at at 40. Tank precharge should be
approximately 2psi below pump cut-in pressure.
Complete nonsense. As water enters the tank, the volume of air decreases, and
its pressure necessarily increases immediately. If the pump isn't developing
more than 20psi, _no_ water will enter the tank whatsoever. If the pump is
developing more than 20psi, water begins to enter the tank immediately, and
the air pressure also begins to increase immediately.
[snip]
More nonsense. 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. Either way, the pump
comes on at 40 and shuts off at 60. The precharge pressure does *not* affect
that.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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
wrote:
close
setting.
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.
> >
moves
stand.
water
by
water
and
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
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
and
Which is what I said.
40
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,
was
the
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
affect
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.
That's a typical recommendation by manufacturers of well pumps and pressure
tanks.
[snip]
Obviously your experience with the reliability of residential well systems is
purely theoretical.
If you think that 40 and 60 are the right numbers to use, you don't know as
much as you think you do. Hint: pressure guages read -zero- at an actual
pressure of 14.7 psi.
[snippola]
I think I pretty clearly said that it does.
Read what I wrote.
Ya know, I'm not a mind reader. If you don't say what you mean, then you
should expect to be misunderstood.
fine...
No, that's not what you said. But there's not much point in arguing about it.
So whose pump is false-starting? The OP's problem is that it *stops* when it
should not.
Obviously not, if you've read the entire thread. The pump was shutting off
_at_60_ due to pressure surge, not at 40 as it _appeared_ to be.
The OP's problem was not the precharge pressure; as noted by others in this
thread, his problem is an improperly installed system. As you would know if
you had any _real_world_ experience with residential well systems.
--
Regards,
Doug Miller (alphageek at milmac dot com)
Harry, I have no problem challenging engineers on the breadth and scope of
their recommendations. My questions have on several occasions caused
industry "rules-of-thumb" to be flat-out changed.
As a chief engineer and principal engineer for almost 30 years, I am not
cowed by an engineer because an engineer was hired by a firm or because he
works there. To increase the firm's engineering team's outlook and
experience and knowledge is often part of my job. Ironically, if the
engineer always does their job right, they never have any problems nor any
non-book feedback to learn from.
Almost all work I do for a firm is done at the firm's request, done when
a firm's engineering teams are unable to solve their company's technical
problems. And often as not, they missed fundamentals and relied on their own
misinterpreted experience.
Engineers are human. Like all humans, they give their best guess based on
their knowledge and experience, and like all humans, their knowledge and
experience may be misapplied or it may be inadequate or through poor
management they may have misinterpreted their own experience. The better
engineers ask, so as to interact and expand their understanding.
Others.........
wrote:
too
pressure
pump
looking to
is
systems is
pressure
the
psi,
psi
and 60
as
bladder
the
decreases,
Volume
displace
and
the
20 psi
You are obviously well qualified in your field. Your field very
obviously is not in practical applications, at least not from what I
have read. Well systems have been in use since before recorded
history. Insisting that changing an industry standard that has stood
the test of time is not 'practical application'. The '2psi below
cut-in' is the standard and for very good reasons. One is that it
minimizes pump cycle times, i.e., max water delivered between cycles.
That at least is what I was taught. Since it works in practice I see
no need to get a masters in theoretical physics or calculus.
There is fun in trying various things to see if something can be made
to work better but there comes a point in development (well systems for
example) where further tinkering is a dead end.
The guy has a problem with a pressure wave spike going over 60psi at
start-up. He band-aided it. It is not now a problem and would never
have been a problem if the switch had been at the tank to begin with.
That is the simple cut and dried version. All your theoreticals isn't
going to change it.
Harry K
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