NPSHA calculation (low value) 4

[POLYENGMOR]

Hello,

I am currently trying to calculate the NPSHA for 4 parallel horizontal pumps but I find a low value (2meters). Is this value reasonable?

I don't know if I'm doing the calculation right. I'm thinking that, since we are dividing the flow per 4 then maybe I should divide the friction head per 4 as well in my calculation of NPSHA, this would give me a higher value, is this right?

Thank you in advance.

 

[Artisi]

How about posting something that is understandable, like pump detail, system layout, product, temp, pipe sizes, static head, system head, flowrate.
Currently it's like asking how long is my piece of string.

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.)

 

[LittleInch]

It could easily be if you're pumping say gasoline and your common line is small.

Without some sort of drawing or schematic we can't see what you can see.

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

 

[POLYENGMOR]

@LittleInch: we are pumping raw water from a canal upstream to a drinking water treatment plant.

Below a little schematic :

 

[LittleInch]

Well that's a start, but you now need to add in elevation data from the canal water surface to the pump inlet, pipe size, flow rate, atmospheric pressure at your location, pipe lengths for the main pipe to the junction and also perhaps your calculation and then we might be able to help better.



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

 

[1503-44]

That schematic doesn't cut it. For NPSH you need
Pipe sizes, lengths, elevations, fittings, valves, flowrates. temperature and NPSHR. Lowest water level.

 

[Artisi]

And sometimes atm. pressure.

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.)

 

[MJCronin]

How about this latest internet whiz-bang ?

https://inventory.powerzone.com/res...u=null:pidu=null:rhu=null:rfvu=FT/Sec:rnu=PSI

That magical place you marked as "IN".... Now.... is the inlet pipe at the canal twenty. thirty. forty feet above or twenty, thirty, forty feet below the centerlines of the pumps ? Since this is the first time you have done this, you must realize that it will make a huge difference.

MOST IMPORTANTLY, WHAT IS THE FLUID VELOCITY IN ALL OF THE PIPING SEGMENTS !!!

Is there significant pressure drop in the pump inlet piping from something like a Strainer ???....Will your MBA boss add a strainer to the raw water system at a later date ???

A poorly designed system can mean years of future problems ....

https://www.eng-tips.com/viewthread.cfm?qid=486590

Why is it that so many Civil engineers feel that they can design mechanical systems ?

I would not drive down a road or cross a bridge designed by a Mechanical engineer

MJCronin
Sr. Process Engineer

 

[1503-44]

That must be why they call it "fluid mechanics".

 

[POLYENGMOR]

Hello fellow engineers,

Excuse me for the lack of information in my first post.

My goal is to choose the right pump for the pumping station.

Below is a more detailed schematic :

Below is the information I used to calculate the NPSHa :
The lowest level of the water in the canal: 102m
Level of the pipes in the pumping station: 93.7m
Friction Head: 16m
Pvpa (30°C) : 4242 Pa
Patm (worst case) : 98500 Pa

 

[1503-44]

Still missing a lot of information. See requests above.

All 4 pumps run at the same time.
Flowrates, min and max.
Pipe lengths
Etc.

 

[Artisi]

3 x 1 standby, not that it matters currently with complete lack of data and what appears to be 8.4 m suction side water level.


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.)

 

[POLYENGMOR]

That's it, 3 x 1 standby.
Flowrate 384 LPS per pump (total 1151 LPS), I didn't need this value in the calculation of NPSHa.
Regarding the other data (Pipe sizes, lengths, elevations, fittings, valves...), are these useful to calculate the friction head or the NPSHa because the friction head is already calculated and it's 16m.

 

[1503-44]

"I didn't need this value in the calculation of NPSHa."

Then you made a major mistake and probably have no idea of what you are doing, or maybe your world has no friction. Which is it?

 

[Artisi]

Or your goodself, LittleInch, MJCronin, and others don't know what we are doing.

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.)

 

[POLYENGMOR]

Well, I am just trying to find out what I did wrong in my calculation and learn from it.

 

[POLYENGMOR]

@1503-44: The friction head between the canal and the entry of the pumping station is already calculated, it is 16meters.

 

[HTURKAK]

- Hydromechanics is the scope of civil engineering.. Almost 50% of civil engineering scope is related with water works..

- If you are not a MILITARY ENGINEER, literally you are also a CIVIL ENGINEER !!

 

[Artisi]

"Well, I am just trying to find out what I did wrong in my calculation and learn from it."
lets see the cals so comments can be made - at the moment its guess work to everyone except yourself.

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.)

 

[POLYENGMOR]

Below the full calculation :
Level of the pump (m) 93,7
Lowest level in the canal (m) 102
P atm (Pa) 98500
P vpa (Pa) 4242
Hf (Friction head loss) (m) 16
M*g (995*9,81) 9760,95
Hst (m) 8,3
Hf (m) 16
Hvpa (m) 0,43
Hatm (m) 10,09
NPSH (m) 1,96