jamesgangnc wrote in
:

On Jul 25, 7:31*pm, Reno wrote:
jamesgangnc wrote in news:e2726069-c968-4f1a-
:





Thanks for the pointers on the last one. *Much appreciated. *I had
no
idea there were suction limits. *I have not talked to the corp yet
bu
t
I have heard via others that they do issue permits for personal
irrigation usage. *And from the response to my last post I'm
thinking the only practical solution for me is a shallow well jet
pump. *The pump can remain on my property and I can easily supply
electrical power. *Running power to the lake is out of the question
because o fthe flood issues. *The corp is very restrictive about
that. *I jus
t
have to run water lines down to the jet foot in the lake. *That's
the part I'm investigating now.

I'd like something cheap and easily taken up or moved if needed. *I
see that normally some pretty large pipe is used for a home jet
pump. And it's not cheap. *But I also only need to accomplish 30
psi at about 2 gpm. *I'm wondering if I can get away with 1 inch
poly for th
e
main pipe and 1/2 inch poly for the jet supply? *Any thoughts? *Aga
in
the lake is about 250' away with a 25 to 30' rise to the house.
Thanks to all :-)

Are you sure of the 2 gpm rate? Seems a bit low to me but if all you
need is 2 gpm then you could use 3/4" for the main pipe and 1/2 inch
for the jet supply. Smaller pipe is easier to prime so don't
over-size it. Get a good foot valve to make priming as easy as
possible and hopefully one priming will last a long time. Priming and
keeping prime is the main disadvantage of long pipe runs from the
source.

For your 2 gpm *you need a pump that will supply 2 gpm at 102 feet of
head - 3 ft friction losses, 69 ft for the 30 psi pressure and 30 ft
for the static lift. The equation for horsepower is *hp = (gpm x head
in feet)/ (3960 x efficiency) where efficiency for these pumps can be
as low as 60% (0.60). That is only 0.09 hp for your 2 gpm need. You
can use the smallest pump you can find that will make 102 ft of head
at 2 gpm and that may be hard to find as small pumps usually don't
make that much head.

I did the calculation for 5 gpm and get 22 feet of friction loss in a
3/4 inch pipe. That means a hp of (5gpm x 121 ft)/(3960 x 0.6) =
0.255 hp.
So
a 1/4 hp pump would supply a lot more than you need and that size is
easy to find but again the head rating is important.

There is no problem with an over-size pump, in this case. The pump
only draws as much power as it needs for the amount of water being
pumped so even a 1/2 hp pump wouldn't waste much electricity. Running
a pump lower than it's best range makes it operate in a less
efficient range but that might be 50% instead of 60%. It wouldn't
harm the pump physically because 2 gpm is enough to lube and cool.
So, if you can only find the pressure requirement in a 1/2 hp pump
and the price is right that is OK. Check the shut-off head (head at
zero flow) to make sure that your pipe can handle that pressure. In
the 1/2 hp range there are many pumps that make a lot of pressure so
be careful of that.

Also, use a pressure tank and probably a 30/50 switch - starts pump
when pressure falls below 30 psi and stops pump when pressure reaches
50 psi. You divide psi by 0.433 to get feet of head. The pump's
shut-off head must be higher than the higher number of the switch or
it won't make enough pressure to cause the switch to turn off. These
switches are usually adjustable with a screwdriver so you can likely
get a setting that will work with your pump - just turn all the taps
off so flow is zero and if pump turns off at 50 psi, great. If it
keeps running adjust the shut-off screw until pump stops and then
turn screw a bit more.- Hide
quoted text -

- Show quoted text -

Thankls everyone for all the info. It is a great help. Reno, you
think a tank is really needed? I was originally thinking I'd just
couple the output to the drip system and let the automatic controller
turn the pump on when it opens a valve. I have seen some combo tank/
pumps that were not unreasonable but still a good deal more than just
a pump. I realize I would need to group enough drippers so I was able
to run a good deal of water through them.

I'm leaning towards the two pipe solution because we have had the lake
drop during severe droughts. My suction requirement could go up
another 8 to 10 feet if that happens. And that would be a period when
I needed the system the most to irrigate.

I'm going to call the corp today and see what the deal is. Sounds
like the engineering is not a show stopper.


If you can make the pipe system leakproof and keep it that way for years
then a tank is not strictly necesary. Small leakage only does harm when
the pump is not supposed to be running. The leak lets pressures drop
which starts the pump but since the major water use is closed the
pressure immediately rises and stops the pump. It can cycle every minute
or worse. More than 6 to 8 cycles per hour kills pumps. You could try
going without a tank and monitor the pump for cycling when it is supposed
to be off. If it cycles, find and fix the leak or if that proves
difficult install a tank. Or skip the automatic controller and manually
operate the pump valve.

Your tank may need to be 20 gallons or more in size, for the 2 gpm flow
rate. Too small a tank will also cause cycling. Tank size must be matched
to pump capacity, pump performance characteristics and system flow rate.
If you don't know how to do this then you may be better off without a
tank at all than to have a small tank causing the short cycling that you
are trying to avoid. You can minimize cycling by using as large a spread
as possible for the control switch. Switches come in settings of 30/50 or
40/60, etc which describe the start and stop pressures in psi. These
numbers are designed for potable water applications where the pressure in
a house system should not get too low or too high. These switches are
usually adjustable so you could set it to something like 15/70. Many
factors can influence these settings. If you are pumping uphill then the
lower (start) pressure must be higher than the elevation difference
converted to psi plus a bit. The higher (stop) pressure must be lower
than the pump shut-off head or the pump will never reach this value and
thus never shut off.

If your lake level drops significantly then that can affect the pump
performance considerably. Since your 2 gpm load is much less than the
likely pump selection there should be little impact. If the pump capacity
is close to the applied load then you could have problems. Pump selection
should be made by computing a system head curve and plotting this on the
manufacturer's pump performance curve graph. Where these curves cross is
where the pump will run - when some one says a pump makes 5 gpm at 120
feet head it means the curves cross at this point. Less flow will have
more pressure and vice-versa. Some pumps have a steep performance curve
which means that a large change in pressure has a small effect on flow.
Some pumps have a flat curve which means that a small change in pressure
will have a large effect on flow. Your case has large changes in lake
level and these levels are included in the total pressure that a pump can
make - if the lift increases then the pressure head is reduced. So a
steep curve would be best as it would tolerate changes in lake level
better. You would lose pressure at the drip line so get a pump for the
worst case of lowest lake level. Add the lift, the losses and the
required pressure at the drip line to get head at the needed flow.

The tank prevents frequent cycling of the pump. Cycling kills pumps for
two reasons - 1) pumps draw about twice the amps for starting as they do
while just running. This large draw heats up the motor windings which
cool down while running. If the motor starts too often, more than about 6
or 8 times an hour, then the heat builds up and kills the motor. 2) The
motor starter uses points just like in the old car distributors. The
points carry a lot more amps than in cars which were high voltage but low
amps. The high amp tends to wear out the points more quickly so it is
best to minimize the number of starts. If you are lucky, frequent cycling
will just trip the high temp protection for the pump motor - cheap motors
might not have this. So you can monitor cycling rates by frequency of
motor re-sets or wear rate of the points. A good system never needs new
points and maybe a motor re-set once a year or two. Generally the pump
and motor wear out before they give these problems.

The two pipe solution is definitely required for high lift cases such as
yours. Suction lift is the same concept as a person sucking a drink up a
straw. It is not the negative pressure sucking that moves fluid up the
straw; it is atmospheric pressure pushing. Atmospheric pressure is 14.7
ps which computes to 33 feet of head. So if a pump could create perfect
suctiion then the absolute maximum lift would be 33 feet vertical. Pumps
are imperfect so their max lift is in the range of 28 feet for great
pumps to about 20 feet for many pumps. Lift can be as low as 5 feet for
some types of pumps such as trash pumps which have large clearances
inside the casings to allow for large objects to pass through the pump. A
cheap irrigation pump should be in the range of 18 to 22 feet. This will
decrease with time because internal clearances will increase as the pump
wears.

Say the pump can do 20 feet of lift. From this you must subtract
hydraulic losses such as; friction loss, entrance loss, foot valve loss,
and possibly other losses. Thus the 20 feet potential leaves only about
15 feet for actual vertical lifting. So if your case calls for more than
about 15 feet of lift you must use either the two pipe system, move the
pump lower and closer to the water source or use a submersible pump. I
have had a lot of problems with pumps losing prime or being difficult to
prime so I favour submersibles or locations where the pump suction lines
can be as short as possible. As you describe the situation a two pipe
system is the logical choice.

Another issue - you wish to pump for a drip system. Drips are sensitive
to clogging by algae, dirt, etc. If your source is clean water then at a
minimum you must use a good fine screen. Screens can plug and the way to
minimize screen clogging is to use a large screen so flow velocity
through the screen openings is lower. A large screen takes longer to clog
so mainenance has a better chance to keep up. I often put a second, very
large screen around an intake that is either disposable or at least
easier to clean without pulling the intake. Locate the intake so screen
condition is easy to see and easy to clean. If you have fine dirt or
algae issues then you may need a filter in your system. The filter must
be large enough and located downstream of the pump. The pump must be
sized for the extra pressure loss of the filter. I like sand filters with
backwash valves for some applications. Filter losses can be anything but
5 to 10 psi can be a good guess. Undersized filters can lose 20 psi or
more. Put a pressure guage on both sides of the filter and backwash when
it reaches the filter manufacturer's recomendation. Severe algae
conditions would preclude a drip irrigation option as the frequent filter
cleaning would be too much to keep up with. No filter and algae in the
water would result in a plugged drip system that would essentially
destroy it. Note that algae growth is worse for dry hot weather, just
when irrigation need is highest.