NPSHa for the pump

[gosooners]

How do I calculate NPSHa for the pump where suction of the pump is discharge of other pump? Pump 'A' is pumping liquid at 90F with discharge pressure 220 Psig. Pump 'B' has suction pressure of 220 psig, which is discharge of Pump 'A'. Pump 'A' has suction from the storage tank, and storage tank pressure is 190 psig @80F.
What value should I consider for Vapor pressure? What will NPSHa?

thanks,
gosooners

 

[JRLAKE]

Are you calculating the NPSHa for Pump A or Pump B?

I don't understand, is the inlet temperature of Pump A 80F or 90F? Or are you saying that there is a 10F temperature rise across Pump A?

Use the vapor pressure of fluid at 80F for calculating NPSHa of Pump A. Use the vapor pressure at 90F for calculating NPSHa of Pump B.

The total NPSHa calculation also depends on elevation changes, pipe system friction losses and fluid velocity. We don't know any of those with the information you have included.

For Pump B NPSHa, start out with P1=220 PSIG. Subtract friction losses, vapor pressure at 90F, velocity head, add or subtract elevation losses (whatever is the case).

Obviously, use the appropriate procedure when converting from head to PSI if necessary. Also, and again obviously, consider PSIA v PSIG.

 

[gosooners]

JRLAKE

I am trying to calculate NPSHa for Pump B. Inlet temperature of Pump A is 80 F. And discharge of 'A' is appx. 90 F. both pumps 'A' and 'B' are closest and there is negligible elevation change between these two pumps. How do I get vapor pressure at suction of pump B at 90F. From simulation True vapor pressure is at 100 F is 240 psia, which is higher than suction pressure of pump which is 220 psig. This is the confusing part.


thanks,
gosooners

 

[BigInch]

That simply means its not enough NPSH.

Pump A must be run at a higher speed-head, or its suction pressure upped sufficiently, or changed out completely so that the discharge pressure is higher than the vapor pressure at pump A's discharge temperature + the NPSH Required of the second pump B.

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[insult2injury]

Time to review fundamental thermodynamics. The saturation pressures at identified temperatures are as follows:

80 F --> 0.507 psia
90 F --> 0.699 psia
100 F --> 0.950 psia



I2I

 

[insult2injury]

Assuming water....I guess that wasn't specified.

I2I

 

[BigInch]

Ya its a pretty sure thing its not water (100 F is 240 psia). Probably LPG, propane, butane mixture ... maybe. I'm not gonna' look it up to see what it could be though.

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[JRLAKE]

So is it like BigInch says? Is it a mixture? My guess is that there is a chart somewhere with this information. If it is a mixture, and if I recall correctly that vapor pressure is an intensive property, good luck with that calculation and maybe think of creating a new post on the thermo forum with that one.

 

[gosooners]

liquid is NGL and Pump A is pumping from Storage tank.

 

[BigInch]

Close. Probably a lot of propanes & butanes in there.

For anybody needing LPG mixture tables, how to pump LPGs, etc. check Corken Pumps. Their website has a lot of info on this,

http://www.corken.com/publications/training.html

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[insult2injury]

I thought I had read water and started typing. Then reread the question....oops.

I2I

 

[BigInch]

At least you listed the right values for water. That's good.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[JRLAKE]

That's a good point BigInch. If he had listed the wrong values it would have added insult to injury to insult2injury.

 

[vpl]

JRLAKE -- ouch

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[JRLAKE]

Sorry, I couldn't resist. The irony was too much for me to handle.

 

[BigInch]

That's why I had to comment too. No chance to resist.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Artisi]

" --it would have added insult to injury to insult2injury."

Brilliant - best one for a long while.

 

[djack77494]

gosooners,
You need your simulator to tell you what the mixture's vapor pressure is at 90F. Telling us what it is at 100F is of little value. (I do note however, that the vp at 100F is 240psia = 225.3psig. Since you have available 220psig at the discharge of Pump A, and assuming little delta in getting to the suction of Pump B, you are probably OK with NPSHa. But, do the math.)

 

[BigInch]

Its PDC given he's discharging at roughly just right above the approximate bubble point now and he's got to control suction pressure exactly and temperatures at either suction or discharge can't vary by more than 10 F. Not much operation leeway there. On a hot day when the product in the tank is a bit warmer than usual before going into the pump ... ???

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain