NPSHA- calc 2

[rajm008]

Rotodynamic pump , operating in closed system with following conditions: liquid level above pump centreline is 15 ft; suction vessel pressure at top of liq. level is 10 psi; suction friction loss is 2 ft; liq vapor pressure is 5.4 psia;atmospheric pressure is 14.7 psia; liq specific gravity is 0.8; suction velocity head negligible; suction nozzle size is 8 inch ID.

 

[LittleInch]

Very interesting but what's the question?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rajm008]

what is the NPSHA?

 

[LittleInch]

Convert all your pressures to ft of liquid.

Then it is (feet of liquid on top of the liquid ( atmos pressure plus 10psi) + static height) - (friction losses + vapour pressure)

Nozzle size is irrelevant.

My guess is >50ft.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Link to How to Calculate NPSHA

 

[rajm008]

the correct answer is 26.3 ft(please find the attached ). but when calculate i am getting 69 ft.

atmos head in ft= 14.7x2.31/0.8= 42.44
head at top of liq in ft= 10x2.31/0.8= 28.87
head loss due to friction in feet = 2
varpour pressure in feet = 5.4x2.31/.8= 15.59
static head in feet= 15
npsha= 42.44+ 28.87-2-15.59= 69

 

[1503-44]

"IT IS A CLOSED SYSTEM."
IT MEANS VESSEL IS NOT CONNECTED TO ATMOSPHERE.
It means that 10 psi is psi ABSOLUTE.
The tank is a closed tank that is not connected to atmosphere in any way, so you do not add atmospheric pressure to head. You can only do that when connected to atmosphere.
You listed, but then did not include the 15 ft of liquid in your calculation line, but then you included it in the calculation of the total.

10 psia = 28.9 ft
-2 ft
-5.4 psia = -15.6 ft
+15 ft
-----------
26.3 ft

SO .... DO NOT INCLUDE ATMOSPHERIC PRESSURE WHEN SYSTEM IS CLOSED.

 

[LittleInch]

Also this looks like a student post. There is a special forum for students as we're not unpaid tutors.

Your key error is not recognising the difference between 10 psi and 10 psia.

To be fair it is only normally process engineers who think in psia, just about everyone else and real life works in psi(g) or barg, but normally people miss off the "g". But never miss off the "a" if it there -can make a massive difference as shown here.

But next time ask your tutor please.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

No, the error was not recognising that a closed system is not connected to atmosphere. Everything else was correct. Adding 14.7 psia was the error.

Simple rule. Fluid analysis of any kind, especially NPSHA, is always easier when using absolute pressures.

 

[georgeverghese]

If 1503-44 hadnt picked up this slip, think the rest of us would have taken longer than it took the Egyptians to dislodge that container ship EverGiven on the Suez Canal to dig up this simple error.

 

[1503-44]

I have to admit that I did not see it on the first pass. I finally noticed the "10 psi" did not have the a, or g flag, so I wondered why and looked for more clues as to which it might be, then the "closed system" key words came into focus.

I think it wasn't easy to notice, because most of the time we have open to atmosphere tanks and are accustomed to seeing +14.7 in NPSHA calculations.

 

[Artisi]

Interesting, I didn't respond as it appeared to be in hand, but must admit the very first thing I noticed was closed system and only came back well into the discussion after it had been flagged.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)