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Thise Grundfos pumps...poor head stats - Page 3

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Posted by nicksanspam on July 12, 2006, 6:24 am

That's to prevent cavitation.

You should be able to pump 7.5' above the water level.

I've been through this with Taco. I asked:

and the ap engineer replied:

Dear Nick, Attached is the NPSH required for the different 00 pumps. The
NPSH required is in ft. of water absolute. You can see from the chart
that the 008 needs 13ft up to 11 GPM. If you were operating in an open
system with a tank of water supplying the suction of the pump and 1 ft.
of water above the center of the pump and at sea level you would have
33.9ft (atmospheric pressure) + 1ft = 34.9ft at the suction end of the pump.

From this number we take away the vapor pressure of the 170F water
which is 14.2Ft. = 20.7Ft. Friction loss in the suction piping
also takes away from the absolute pressure available. IF the friction
loss is 2ft, the Available NPSH would be 18.7Ft. The pump only needs
13Ft. so there would be no cavitation. If you would be lifting water out
of a tank at sea level you would have 33.9Ft - vapor pressure -friction
-amount of lift = NPSH Available. If this is still above NPSHR no cavitation.

If we have a pressurized system you start at 0 psig = 33.9Ft at sea level.
For every 1psi you add 2.31ft to 33.9 ft. If you have a loop with an
elevation of 23ft. You would need a pressure at the bottom of the system
10psi to get the water to the top of the system. 33.9ft + 23.1ft. = 57ft.
You can see that this pump would not cavitate in most systems...


Posted by Solar Flare on July 12, 2006, 11:28 pm
With all this inlet pressure in mind it sounds impossible to use these
pumps in an open loop system despite their rating and using stainless
steel for the housing etc..

Posted by Gary on July 13, 2006, 12:00 am
 Solar Flare wrote:

The way I read the note above, the 008 pump they are discussing WOULD be OK for
an open system as long as the pump is mounted below the level of the water in
the open tank. Is this not correct?




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Posted by Solar Flare on July 13, 2006, 2:10 am
 Look for "minimum inlet presure" on the pump specs. I found Grundfos
hides that figure wuite well until I opened the pump itself. All the
Grundfos UP and UPS inlet presures of 3 feet or more for higher
temperature. This means mounting your 10,000 pound tank a few feet off
the floor or never letting the tank empty itself at all.

BTW: if these pumps run dry the seals will be ruined.

As far as Nicks figures. I can only see nonsense from it. The message
is very unclear to me even it looks as though much effort went into
the figures. He appears to be stated the blatantly obvious about
absolute pressures on earth and is not related to the question at hand
in any way I can determine.

Sorry Nick, no chart and no definitions. WTF is NPSH? WTF deos
atmosphereic pressure have to do with relative pressures across a
pump? Are you demonstraing the tech and Taco was an idiot? Where does
the email stop and your comments begin? I'm lost. LOL

Posted by daestrom on July 15, 2006, 4:17 pm

NPSH is an engineering acronym for 'Net Positive Suction Head'.  It is the
net pressure (above atmospheric) that is needed at the suction.  It is
called 'positive' because it is above the vapor pressure.  It is called
'net' because it is calculated by subtracting the vapor pressure from the
suction pressure to find the 'net' head.

When the flow through a pump is higher, the amount of pressure drop from the
inlet to the interior of the pump is larger, so some pumps include an NPSH
curve that shows that a higher suction pressure is required for higher flow

The reply that Nick posted shows how to figure the suction pressure needed
when the water is hotter.  Since the vapor pressure of water is often
*below* atmospheric, it is sometimes easier to keep everything in absolute
pressure units instead of trying to figure out +- signs.

So they worked out the whole thing for Nick's question about 170F water.
With the pump one foot below the surface of the water and 'normal'
atmospheric pressure of 33.9 ft, the absolute pressure at the suction is
34.9 ft.  But the boiling point for 170F water is about 6 psia or about 14.2
ft of water column.  So the 'net' suction head is 34.9 - 14.2 = 20.7 ft.  So
as long as the pressure drop from the tank to the pump suction is not more
than 7.7 ft, then the NPSH at the pump suction would still be at least 13
ft, the stated minimum required for that pump.

Obviously, with *any* pump, pumping water that is near boiling (such as you
might have in a solar heating setup) is hard to do without cavitating.  The
most obvious solution is as you surmised, place the pump a few feet below
the storage tank.  Easier said than done though, I know.


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