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Does Atmospheric Pressure Count Toward NPSH?

Does Atmospheric Pressure Count Toward NPSH?

Does Atmospheric Pressure Count Toward NPSH?

(OP)
I am trying to pull water out of an open tank, heat it and put it back into the tank.  I'm pulling off near the bottom of the tank, going through basket strainers, then into a pump, out of the pump, through a heat exchanger, and finally back to the tank.  

My question is with the amount of NPSHa I have.  My water level in the tank is only about 2 feet.  I thought that meant that my NPSHa was only 2 feet (actually less due to drop across the strainer).

My pump vendor is saying that I have the 2 feet, plus atmospheric pressure contributing to my NPSHA, so it's actually more like 36 feet.  I disagree, especially since we are returning the water back to atmosphere.  Don't those pressures cancel each other out?    

RE: Does Atmospheric Pressure Count Toward NPSH?

BronYrAur,

get a copy of cameron hydraulic data.

npsh is the total suction head in feet of liquid (absolute at the pump centerline or impeller eye) less the absolute vapor pressure (in feet) of hte liquid being pumped.

pump vendor is correct.

good luck!
-pmover

RE: Does Atmospheric Pressure Count Toward NPSH?

In terms of definitions of terms, you are right and he is wrong.  In terms of pump performance, if the pump has a required NPSH of 10 ft, you are pretty sure to cavitate it.  You need to look at the height of water, and then subtract any significant pressure drops (certainly the strainer, you need to look at pipe sizes and Reynolds Numbers to figure if friction is significant in this case).

Every NPSH-r I've ever seen published is some number added to atmospheric pressure (I'm not going to argue about whether head and pressure are the same thing or not).  So if your pump has 10 ft of head required that is relative to local atmospheric pressure, not to zero psia.

David

RE: Does Atmospheric Pressure Count Toward NPSH?

(OP)
Thanks.  Found a copy of "Hydraulic Handbook" by Fairbanks Morse which explains it very well.  So now I just need to get data on the strainers to see pressure drop when they are loaded up.  

  

RE: Does Atmospheric Pressure Count Toward NPSH?

There is NPSHR and NPSHA.  The NPSH available is referenced to the suction flange and should bel=>than NPSHRequired
   NPSHA  based on inlet flange conditions
 = (P-Pvapor)/rho +V^2/(2g)

or NPSHA  based on upstream conditions with referece to flange =
 (Patm-Pvapor)/rho + V^2/(2g)+ (change in elevation)-losses
Sinve for stationary inlet water level, V^2/2=0

NPSHA  based on upstream conditions  at suction flange=
 (Patm-Pvapor)/rho + (change in elevation)-losses
Hopefully your NPSHA is = > than the NPSHRequired.

RE: Does Atmospheric Pressure Count Toward NPSH?

zdas04, are you sure about that?  In my 20 years in this business, every single NPSH calculation is based on absolute, and the NPSHr numbers shown on pump curves are the same.  It has to be that way; not every pump is on water, not every suction supply is vented.



RE: Does Atmospheric Pressure Count Toward NPSH?

zdas,  Here's where the difference in pressure and head comes back to haunt you.  Head is never referenced as being either an absolute or a gage head.  No need.  Head is always absolute.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Does Atmospheric Pressure Count Toward NPSH?

Head is always absolute????
Please check my posted equations.  For NPSH you will see there is a difference in pressures.  As long as both pressures P and Pvapor are in the same reference gage or absolute, it doesn't matter since one is taking a difference.

Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

TenPenny (Mechanical)Perhaps you are stating absolute because chances are when you get Pvapor, it's going to be in absolute units.
Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

More than chance.  P vapor is always absolute.  Least I've never seen one that wasn't.

Given what NIST has to say about vapor pressure and boiling points being equal at 1 atmosphere, if VP were ever referenced to 0 psig, we would have to define that as the "Standard Abnormal Boiling Pressure".  Never happened.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Does Atmospheric Pressure Count Toward NPSH?

TenPenny,
I've looked at a lot of NPSH-r references on pump curves and they always say "ft" not "ft(a)" or some such.  I've always taken that to mean that it used height of fluid column plus any applied gauge pressure.  Looking at Gould's
excellent web page (http://www.gouldspumps.com/cpf_0006.html) I can see that my assumption for lo these many years has been wrong.  Thanks for setting me straight.

David

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

RE: Does Atmospheric Pressure Count Toward NPSH?

Biginch
I believe water at 100C boils at 0psig at sea level or 1 atm (abs)  0psig or approximately 14.7 psia.
With regards to  head being absolute. Consider elevation, it is always relative, ie from the bottom of a tank, from tlhe center of the earth.  The thing is we are dealing with DIFFERENCES in elevation. Similary, the same is with pressures, differences in psia or psig yield difference in psi.

Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

No argument.  Head is in itself a measure of difference, as is pressure and is in fact any measurement, right?  Thus a reference point, even if it may be highly localized, is always implied, however a reference point should always be chosen as the most convenient reference point possible.  If you want to get technical, even mean sea level varies so much around the world that Mt Everest is not the mountain having the peak that is the farthest distance from the Earth's center, although it is the mountain with the highest elevation above (nearest) mean sea level.  

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Does Atmospheric Pressure Count Toward NPSH?

BigInch (Petroleum) 6 Apr 07 2:19  

No argument.  Head is in itself a measure of difference, as is pressure and is in fact any measurement, right?

Almost.  There are some absolutes.  IE, speed of light.
But now, we may be heading way off track from the original question.
Regards

 

RE: Does Atmospheric Pressure Count Toward NPSH?

As you haven't been asked the question yet, I will -- is the tank above or below the pump centreline? You post is confusing - you could have 2 ft in a tank positioned below the pump centre or 2 ft in a tank above the pump centre.

RE: Does Atmospheric Pressure Count Toward NPSH?


This subject has been discussed in numerous threads.
From the question by BronYrAur I gather it means the pump is below the water level by 2 feet.

When the water in the tank is heated its vapor pressure will go up and the available net positive suction head (=NPSHA) down, but still the atmospheric pressure would be added to it. When estimating NPSHA you better consider the highest temperature attainable on circulation.

As explained by sailoday28, the NPSHA referred to the centerline of the pump equals the pressure, in units of liquid height above, the vapor pressure:

NPSHA = (Ps-Pv)/ρg + u2/2g

Ps = Absolute suction pressure measured at the pump inlet; it includes the pressure over the liquid surface, any static head (positive or negative depending on the location of the pump and the tank) and any friction loss (as negative) in the suction piping
Pv = vapor pressure of liquid at the pumped temperature
u = velocity of liquid at the section where measurements are taken
g = acceleration of gravity
ρ = density of the liquid

u2/2g is usually small and neglected.

Although a bit out of context, sailoday28, what do you mean by "absolute" when referring to the speed of light ? Non-variant ?

RE: Does Atmospheric Pressure Count Toward NPSH?

25362 (Chemical)
My experience in filling out data sheets/specs is to relate conditions to the inlet flange of the pump-Here the v^/2g, elevation and suction pressure/rho/g are known.

With regard to absolute, I was just trying to show that for incompressible flow, the engineering Bernoulli equation is made up of differences (except for the loss term) and therefore absolute and gage, elevation,kinetic energy etc differences are just differences.
With regard to speed of light, it is a fixed invarient number.  When we talk of fluid velocity, it is a relative number with regard to a point on the earth.
Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

The pump required NPSH is based on standard atmosphere as reference. Howeer to calculate available NPSH you must use the actual absolute ambient pressure acting on the water level at the pump suction. This is important because the available NPSH would be reduced if the pump is at high elevation or if the pump is drawing from a vacuum condenser.

RE: Does Atmospheric Pressure Count Toward NPSH?

Have you ever seen a pressure indicator in a pump's suction line read below 0 psig and not hear "gravel" running thru the pump?  Atmospheric pressure counts.

DB

RE: Does Atmospheric Pressure Count Toward NPSH?

Atmospheric pressure definitely counts. In your case, your source pressure is 14.7 psia (depending on your elevation with respect to sea level).
Source Pressure: 14.7 psia
- Vapor Pressure: x.x psia
- Line Losses:     x  psi
- Misc Items:      x  psi
(suction strainer)
Subtotal:          x  psia
Subtotal:          x  feet (subtotal x 2.31/sg)
+ Static Head:     2  feet
NPSHa:             x  feet

RE: Does Atmospheric Pressure Count Toward NPSH?

NPSH deals with absolute pressure, not gauge pressure.

To personify what is happening at the face of the pump impeller...

The impeller "wants" to have the fluid delivered to it with enough pressure to force the fluid up against the impeller, such that th impeller can try "grabbing" and "pulling" the fluid on at a higher velocity.

Ever tried to pull water along with your bare hand?  If you scoop water away toward you with your cupped hand and the water "left behind" does not have enough pressure to move toward you fast enough to fill the spot where you just "grabbed" a handfull of water, then a "hole" forms. At depth in the ocean next to a ship's/submarine's propeller or in a sealed full pipe, that "hole" means a vacumm is formed and quickly gets filled with vapor from the surrounding liquid. You see the same thing in the sky, when a jet leaves a trail of what looks like "smoke" trailing off its wing-tips, where the wing-tip vortexes created low pressure in the air. Pull or push fluid (liquid or gas) out of the way too fast for the surrounding fluid pressure to fill the "hole", and a low pressure area forms.

In all of this, considering a pump impeller, the pump does not "care" what is happening at the discharge side of the pump. It is only interested at the pump impeller suction at being able to "grab" a full amount of liquid, not vacumm. The pump further could "care less" what is happening outside the pump or incoming pipe, it just "knows" / senses the pressure it "feels" inside. The pressure ramming the fluid toward the pump impeller could come from anything: elevation change (potential energy), another pump pushing, or in the case you are considering the pressure of the air pushing down on the fluid trying to force it into the pipe and eventually into the pump, or even (to make the point) someone back along the line trying to push oatmeal in to the line. The pump does not care, does not "see" what is creating the pressure in the fulid at the impeller suction. It only "wants" sufficient pressure. The pump's "world" is only what it feels at the impeller suction, not what someone could "see standing on a box looking back up the line." It is like the person in the chain of an old fashion fire-fighting bucket brigade. "Get me the next bucket....I do not want to be standing here 'twittling' my thumbs!"

The "penalty" for not supplying sufficient NPSH to a centrifugal pump impeller's suction is cavitation, those little bubbles you see forming a trail on the old Hollywood movies of WWII submarines.  Those tiny bubbles quickly collapse. Two bad things happen when the cavitation bubbles collapse: they make noise (bad for submarines, modern submarine propellers are designed to resist doing this) and the collapsing bubbles act like tiny descrictive jack-hammers, if they collapse next to anything (like the pump impeller they would be going through).

How do you know you have pump cavitation (and it is always due to NPSH problems)? It sounds like the pump is pumping marbles. Pull the impeller on a pump that has been badly cavitating for a couple of months and you will compare the shrunken size of the impeller to a silver dollar eroded down to a dime. Those little jack-hammer cavitation bubbles get destructive.


RE: Does Atmospheric Pressure Count Toward NPSH?

exCLP (Civil/Environme) 13 Apr 07 19:35  
NPSH deals with absolute pressure, not gauge pressure.

Please define NPSH available.

Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

"NPSHa" = Net Positive Suction Head ABSOLUTE........NOT! smile "a" = available

...................................(apologies in advance for my obtuse sense of humor)

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Does Atmospheric Pressure Count Toward NPSH?

BigInch (Petroleum)Please provide a credible
reference for you definition.
Regards

RE: Does Atmospheric Pressure Count Toward NPSH?

......as a standup commedian???

Sailoday28, that little yellow smiley face means that Biginch was joking. He was not seriously suggesting that the little "a" after NPSH really represents "absolute". He was just having some fun, which I think was a good idea because this thread has now been done to death.

Harvey

RE: Does Atmospheric Pressure Count Toward NPSH?

¿¿¿ IT DOESN'T ????

...and some of us, if not most, are in serious need of a good humor injection from time to time.  Have a good weekend.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Does Atmospheric Pressure Count Toward NPSH?


For an interesting discussion on NPSH see thread798-129328.

RE: Does Atmospheric Pressure Count Toward NPSH?

I will second the comments re humour - quiet often this forum become too pedantic - mostly posters are looking for a quick easy answer or guidance and some sensible answer to to their problem.

RE: Does Atmospheric Pressure Count Toward NPSH?

Agreed.  And many don't even have the time to say thanks.  But after the question has been answered at least once, I don't think it hurts to try killing a thread on a humorous note once in awhile, if served in limited quantities and done in good taste.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

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