Suction lift & vapor pressure... math check, please 1

[HopDr]

I'm working on a pump for a suction lift application & want to insure that the pressure at the pump inlet remains high enough to prevent vaporization of the liquid. The liquid is 50% sulfuric acid at ~25 deg C. Vapor pressure at this temp is 8mmHg. The static suction pressure will be 58.2 kPa or 436.8mmHg (14 ft head/lift). Appears that vaporization won't be an issue IF I've done the math correctly. Can someone please check me on this?

 

[BigInch]

Post your math please (xls, etc.), otherwise we're not checking. It would be like we're doing your homework for you.

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[25362]

For NPSHA and lifts there is a plethora of posts. See, for example,

thread407-259273
and
thread407-260505.

 

[HopDr]

I guess I should boil this down more succinctly. I know the values are correct. Where I'm uncertain is in the interpretation of the static head pressure value, 436.8 mmHg, which is equivalent to the 14 ft of lift in my application. Is the 436.8 mmHg absolute, or must it be subtracted from 760 mmHg?
This kind of work is outside the norm for me... my job is manipulating molecules to make better brewing products. However, I've found little interest from any vendors in actually providing a solution for this lift application. So, I'm learning to do it myself. It's both the benefit & detriment of working for a small company where everyone wears many hats!

 

[Latexman]

NPSH_Available = pressure on surface of liquid in suction vessel (760 mm Hg) - vapor pressure - maximum suction lift - friction loss in suction pipe.

Yes, it must be subtracted from 760 mm Hg.

Good luck,
Latexman

 

[btrueblood]

...and then compared to the NPSH_required value provided by the pump manufacturer. You want NPSHa > NPSHr, some would say by some multiple of from 1.5 to 3x.

See

http://en.wikipedia.org/wiki/NPSH

or

http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html

 

[Latexman]

My company has some complicated NPSH margin and ratio guidelines, but they always can be met if you use max. and min. values in the NPSH equation and NPSH_R - NPSH_A > 5 ft. of fluid.

Good luck,
Latexman