Max HP from 5 HP motor
- Thread starter junkhound
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Sometimes people want to get away with a motor service factor which NEMA discourages of late.
isn't 'HD" always a combination of dynamic/friction and static/gravity?
both use pressure x flow rate
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
Yes, but the system in question involves a pressure increase of 4000 pounds-force/inch² and a negligible elevation change.
4000 lb/in² is comparable to pumping up an 8000-foot increase in elevation.
makes no difference to the pump though
power = work/time
work = force x displacement
in this case
force = lbs of water moved
disp = head (either friction or elevation change)
time is derived from the rate lbs/time
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Say you want to pump a fluid with a specific gravity of 2.0 up 10 feet. Would you say that the static "head" is 20 feet, or 10 feet? (Edit: not rhetorical, I don't know the standard usage.)
If the answer is unambiguously 20 feet, then "head" is just a unit of pressure tied to a vertical column of water at sea level. The direct analogue to the "mm of Hg" unit of pressure.
If the answer is ambiguous, then it would be best to clarify that the unit of pressure is "head of water."
Cheers, Wayne
If the answer is unambiguously 20 feet, then "head" is just a unit of pressure tied to a vertical column of water at sea level. The direct analogue to the "mm of Hg" unit of pressure.
If the answer is ambiguous, then it would be best to clarify that the unit of pressure is "head of water."
Cheers, Wayne
Easier with SI................makes no difference to the pump though
power = work/time
work = force x displacement
in this case
force = lbs of water moved
disp = head (either friction or elevation change)
time is derived from the rate lbs/time
10 ft
the equation is then HP = (10 ft x Q gal/min x (2 x 8.34 lb/gal)) / (60 sec/min x 550 ft-lb/sec / HP x eff)
units cancel to HP
Yes, but it's true....................you just had to go there!!!
:lol:
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
No, 20 ft. It's the equivalent water column.
if we ignore friction and the pump is lifting vertically 10', I would call that 10' HD
I consider density a separate variable (as is gpm)
drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
Head is a distance, not a pressure, but lazy, sloppy and inconsistent usage is commonplace.
If you're designing pumping/piping systems with liquid densities that are significantly different from water, using shortcuts (including "head") is likely to introduce errors.
One of the nice things about standards is that there are so many different ones to select from.
Less error-prone, too.Easier with SI ...
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Consider all you like, but that's not the common practice in the chemical process industries and fire protection, in both of which I have personal experience.
I've sized a boat load of pumps and all US curves use head (total) and gpm and a correction factor for sg
Why? Because in most pumping applications gravity head >>> friction head
Agree to disagree
https://www.kleen-ritecorp.com/imag... Sheets/Gould Pumps/Goulds-Pumps - CINTRO.pdf
feet and meters
http://pfcequip.com/wp-content/uploads/2015/12/goulds-36-3742-R2.pdf
kwired
Electron manager
- Location
- NE Nebraska
When certain clients already have something, they are set on using it and can't always figure out why it is undersized for their needs:happyyes:
Grain elevators we commonly put a amp meter near the operator so they can monitor loading, and adjust inlet gate accordingly if needed.
New installation one time I had finished setting up the meter and we were unloading a truck, ower was standing right there and said "we need to be able to unload the truck faster then that". Wanted to slap him and tell him he should have mentioned to the equipment sales people when he selected equipment what speed he wanted to unload (which he probably did), but I kindly told him if he goes any higher he is going over the motor's capacity, it will overheat the motor, but motor overload will protect it by shutting it down, also that that the motor was likely sized to the equipment rated capacity.
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
I guess he wants to use feet (or meters) in his NPSH calculations, too. <g>
Head can be converted to pressure and vice versa.
obviously
since suction lift is usually in feet
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your product - loss in the piping, valves and fittings
- Static head = 5 feet
- Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute
- Gage pressure = 0
- Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg = 0.27 x 2.31/1 = 0.62 feet
- Looking at the friction charts:
- 100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet of head loss in the piping
- The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
- Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.
I'm not the only one
http://www.warrenpumps.com/resources/npsh.pdf
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