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Steady State Npsha Calculation


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#1 IonCube

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Posted 30 October 2018 - 08:15 AM

I tried to be as explanatory in the pic as I can be. To model tank with a water level (not tank level) of 10m I used a single vertical pipe segment because in steady state tank's level has no impact on NPSHa which is to be calculated.

 

Pipe's ID is chosen such that friction gradient per unit length is kept minimum to only account for static head. Manual calcs are are shown side by side.

 

Why NPSHa calculated by UNISIM is 4m higher? Per my understanding NPSHa at the end of vertical pipe (assuming no fittings & constant dia) will be

NPSHa = VelocityHead + StaticHead - FrictionHead

 

I have taken into account velocity head due to Pump suction nozzle dia; increasing it reduces NPSHa drastically ...its understandable as the suction velocity goes down

 

Attached Files



#2 breizh

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Posted 30 October 2018 - 08:26 AM

Hi,

To support your work !

Good luck.

Breizh

Attached Files


Edited by breizh, 30 October 2018 - 08:32 AM.


#3 shan

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Posted 30 October 2018 - 10:48 AM

NPSHa = Static Head - Velocity Head - Friction Head - Vapor Pressure

 

You have errors on Velocity Head (positive instead of negative) and on Vapor Pressure (missed)



#4 fallah

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Posted 30 October 2018 - 12:17 PM


 

IonCube,

 

Why you ignore vapor pressure head? It might the fluid is at its boiling point hence atmospheric pressure head offset by vapor pressure head. Is it right?



#5 fallah

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Posted 30 October 2018 - 12:20 PM

Shan:


 

Velocity Head should be applied with positive sign, hence OP is correct in this regard...!



#6 IonCube

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Posted 31 October 2018 - 03:21 AM

Why you ignore vapor pressure head? It might the fluid is at its boiling point hence atmospheric pressure head offset by vapor pressure head. Is it right?

 

Thanks for the replies.  Fluid is water at 88 C & feed conditions are intentionally set at 0 barg; so that head effects are solely due to pipe segment & pump suction nozzle dia

 

However nonetheless what the vapour pressure will be; it will reduce my manual answer further!  How UNISIM is calculating 16.3 m NPSHa?  The problem is fairly easy & self-explanatory as I have indicated velocities in Pipe & at suction nozzle; both velocity heads will be added

 

UNISIM case attached

Attached Files



#7 fallah

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Posted 31 October 2018 - 04:51 AM

IonCube,

 

Lack of adequate info on ambient and operating conditions; anyway i think UNISIM has reached 16.29 m as NPSHa as follows:

 

101.325 kpa (as local pressure)+94.75 kpa (as static pressure)-2.5 kpa (as frictional pressure=193.575 kpa or almost 20.36 m (as P100 feed pressure)

 

Of course appears UNISIM has ignored to consider velocity pressure mostly due to its small value.

 

Then appears 20.36 m-16.29 m=4.07 m to be the corresponding head of the water vapor pressure at relevant operating temperature.



#8 breizh

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Posted 31 October 2018 - 05:14 AM

Hi ,

To me , your document is very confusing !  what means  -10 m  elevation with the drawing provided ?

 

You should stick to the definition of NPSH with  the relevant terms and perform simplification afterwards .

 

NPSH is about Head  ( meters or feet), nothing else .

 

Consider the document I submit and submitted  to support your calculation .

All you need is there.

 

Good luck .

 

 

Breizh

Attached Files


Edited by breizh, 31 October 2018 - 05:20 AM.


#9 IonCube

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Posted 31 October 2018 - 05:40 AM

I am sorry if I am appearing as you put it confusing; I am trying to be as clear as I can be!

 

Nasir,

My understanding is that UNISIM does take velocity head into consideration. You can see NPSHa decreases DRASTICALLY with increasing suction NozzleDia. My understanding is that increasing dia reduces suction velocity & this reduces velocity head which reduces NPSHa? Is my understanding is correct? 

 

Secondly refer to my latest uploaded pic. The atmospheric pressure (under steady-state) is NOT taken in consideration by either UNSIM or HYSYS. NPSHa then is entirely dependent upon feed pressure at pump inlet (thats why I surrogate a liquid level of 10m in a tank by a vertical downcoming pipe segment of 10m (thus its elevation is in -ive)

 

breizh,

Please read above paragraph regarding -ive elevation. You can see that at pipe inlet the pressure is 0barg while head has been gained at the pipe's outlet

Attached Files


Edited by IonCube, 31 October 2018 - 05:42 AM.


#10 breizh

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Posted 31 October 2018 - 07:20 AM

hi ,

For those interested I've attached a calculator .

 

other resources ;

http://www.pumpfunda...e.htm#download6

 

Breizh

Attached Files


Edited by breizh, 31 October 2018 - 07:38 AM.


#11 fallah

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Posted 31 October 2018 - 08:37 AM

IonCube,


 

I think that graph you attached is about the pump nozzle dia. vs NPSHR rather than NPSHA....!  On the other hand, the velocity head coming into NPSHA calculation is based on the velocity in the pump suction line rather than in the pump suction nozzle...!

 

Hence, i think your understanding cannot be correct...!



#12 IonCube

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Posted 01 November 2018 - 04:13 AM

Breizh,

Thanks for the info but my purpose here is NOT to understand NPSHa/r but how simulator is reaching a particular answer. That is why I posted this question here
 

 

On the other hand, the velocity head coming into NPSHA calculation is based on the velocity in the pump suction line rather than in the pump suction nozzle

 

Nasir,

I have attached x3 pics which should be seen sequentially. The graph attached is indeed NPSHa as clarified here because NPSHr will require me to input some additional pump curves!

 

Only x1 variable I have changed (there is no suction line or fittings) that is nozzle suction dia (pic.2) which reduces NPSHa (pic.3)

Attached Files



#13 fallah

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Posted 01 November 2018 - 05:45 AM

 

Nasir,

I have attached x3 pics which should be seen sequentially. The graph attached is indeed NPSHa as clarified here because NPSHr will require me to input some additional pump curves!

 

Only x1 variable I have changed (there is no suction line or fittings) that is nozzle suction dia (pic.2) which reduces NPSHa (pic.3)

 

I haven't worked with UNISIM; but i'm thinking there is a misconception in this software regarding the distinguishing between NPSHa and NPSHr because based on a rule of thumb: 

 

NPSHr in centrifugal pumps is normally reduced by increasing the diameter of the impeller eye hence the pump suction nozzle.

 

Then as the NPSHa is mostly a function of the pumping system (not the pump itself) and the NPSHr is a function of the pump itself, again i think that attached graph is about the pump suction nozzle diameter vs NPSHr.



#14 shan

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Posted 01 November 2018 - 07:37 AM

I don't see any problem for Unisim NPSHa calculation.

 

Pump Inlet Pressure: 0.92 barg

Water Vapor Pressure (88 C): -0.35 barg

Water Density (80 C): 955 kg/m3

 

NPSHa = (0.92 - (0.35))/0.98/0.955 * 10 = 13.6 m (Unisim 13.7 m)

 

I believe the discrepancy is because the accuracy of converting factors applied.  



#15 fallah

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Posted 01 November 2018 - 07:51 AM

I don't see any problem for Unisim NPSHa calculation.

 

Pump Inlet Pressure: 0.92 barg

Water Vapor Pressure (88 C): -0.35 barg

Water Density (80 C): 955 kg/m3

 

NPSHa = (0.92 - (0.35))/0.98/0.955 * 10 = 13.6 m (Unisim 13.7 m)

 

I believe the discrepancy is because the accuracy of converting factors applied.  

 

Water vapor pressure is to be submitted in the form of absolute pressure and, on the other hand, the vapor pressure of the water at 88 degrees C is 0.65 bar.



#16 shan

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Posted 01 November 2018 - 08:51 AM

0.65 Bar = -0.35 Barg

NPSHA = (0.92 Barg - (-0.35 Barg)/0.98 Bar/(Kg/m3)/955 Kg/m3*10/(m/(1000 Kg/m3) = 13.6 m (Water @ 88 C)



#17 fallah

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Posted 01 November 2018 - 09:13 AM

0.65 Bar = -0.35 Barg

NPSHA = (0.92 Barg - (-0.35 Barg)/0.98 Bar/(Kg/m3)/955 Kg/m3*10/(m/(1000 Kg/m3) = 13.6 m (Water @ 88 C)

 

I didn't see negative minus of the vapor pressure; and appears the NPSHa=13.7 m in the last calculation sheet which was attached by OP is almost correct and conformed by hand calculation but NPSHa with the same conditions and another pump suction nozzle diameter was reported in the first calculation sheet attached by OP higher than 16 m. Then the main point which is followd by OP is effect of suction nozzle diameter on NPSHa which i mentioned it affects NPSHr not NPSHa.



#18 IonCube

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Posted 01 November 2018 - 11:50 AM

Nasir,

Then the main point which is followd by OP is effect of suction nozzle diameter on NPSHa which i mentioned it affects NPSHr not NPSHa

yes I think now I have clarified myself!






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