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.

 

[1503-44]

Upside your pipe diameters now. I can't believe you have so much head and you wind up with less than 2.

Those are not calculations. They are values of variables.
I'm tired of asking, so bye bye from me.

 

[Artisi]

It's nearly bye bye for me as well, I suggest you reread the questions asked numerous times, think about what has been asked and give some feed back.

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]

Basically you have about 18.5m driving head (ATM press + positive head difference).

Subtract your vapour pressure, a bit less than 18m.

Then subtract your friction losses

Result is your 1.8m NPSH.

That's too low for most pumps.

Now you might want to show us how you calculate your friction losses as these seem quite high and was the original question....

So you have three options
Make your inlet pipe bigger,
Reduce flow rate
Lower the pump.

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

 

[HTURKAK]

If the friction loss is 16.0 meters ( for dia 1000 mm, q=1152 l/sec, C=140 for steel pipe ) the length of inlet piping should be in the range of 10000 m ( 10 km )

Pls check your head loss calc . and if you want to get better responds, provide full picture.. the problem is not clear with the info. that you provided ..

 

[MJCronin]

Well, after twenty or thirty more questions we may come to a resolution for this simple problem ....

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

"Hydromechanics" may be within Civil Engineering scope ..... But it does not seem to include simple pump NPSH calculations or even prudent pump piping design

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

In a couple of weeks, your client will be adding a raw water strainer to the inlet piping. Straining raw water at its source makes much maintenance sense but will significantly affect your pump NPSH calculations.

Depending on the strainer design the maximum pressure drop could be from 1.5 to about 3 psid ....Are you certain that there will have no raw water strainer ?

https://scontent-ort2-2.xx.fbcdn.ne...=91fb6d36926a27fe23c40211c1fe2ce5&oe=615BCC1F

MJCronin
Sr. Process Engineer

 

[POLYENGMOR]

Thank you @LittleInch for the complete answer.

@HTURKAK: The thing is the head loss has been calculated by the client but I will look into it for sure, thanks.

 

[LittleInch]

How long is the inlet pipe??

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

 

[1503-44]

He does not seem to know.

 

[POLYENGMOR]

The length of the inlet pipe is 600m, the nominal diameter is 1200mm.
Actually, I found out that the head loss I used in my calculation (16m) is the maximal head loss between the canal and the entry of the water plant (with Roughness Coefficient k = 1mm), not the entry of the pumping station.

To summarize, we have :
Canal / 600m of pipe Ø 1200 / Pumping station / 2700m of pipe Ø 1200 / water plant

I redid the calculation of the head loss between the canal and the pumping station, I got Hf = 2.9m and then I recalculated the NPSHa again and I got NPSHa = 15.1m

 

[Artisi]

Amazing, had you responded to my first reply, and similar requests from others, could probably have saved all the back and forth nonsense. Chalk it up to experience.

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]

Your pipe size keeps changing?

One post is 1000mm, the next it's 1200??

It's quite strange to have your inlet pipe and discharge pipe the same size. Adding a pump to overcome 14m of head and not look at reducing the pipe size and increasing the size of the pumps seems a bit strange.

Short distance of pipe you could easily run at 3 to 4 m/sec in the discharge pipe.

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

 

[POLYENGMOR]

Actually, the (new) pumping station is supplied by two sources, a canal, and a dam. The water level in the dam is high so we don't have a problem with the NPSHa calculation. Also, these two sources supply a nearby existing pumping station.

The size of the inlet pipe changes a few meters before the (new) pumping station.

I omitted these informations for simplification purposes.

 

[Artisi]

Is the canal level is higher than the pump inlet or just the dam 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]

The lowest level of the water in the canal: 102m
Level of the pipes in the pumping station: 93.7m

 

[MJCronin]

Well, only another 15 or twenty questions more before POLYENGMOR decides upon what his problem really is ...

It is so much fun dealing with Civil Engineers !!!

Hey, on a closely related matter ..... Has anyone calculated the Horsepower required by these monster pumps ? ... This will, of course, kill the project

Only two more weeks until the client walks in and tells POLYENGMOR that he wants strainers in his system !!!

MJCronin
Sr. Process Engineer

 

[Artisi]

As data is changeable as you go along, please confirm the canal level in ABOVE the pump 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.)

 

[catplank]

@MJCronin - Just clarifying something here, in the thread you've cross posted. It has designed and constructed by a mechanical engineer - so whilst we should stay in our areas of competency for the most part. In my case it was the mistake of a mechanical engineer. I was simply picking up the pieces to determine what the issue was...

It seems no one is immune to making a mistake or two..

 

[pierreick]

Hi ,
You may consider to review the document attached supplied by KSB,in particular the part related to NPSH.
Review your calculation for the head loss on the suction pipe, something is wrong there based on data supplied ( length 600m,diameter 1200 mm, rugosity 1 mm , flow rate 384 LPS . head loss is about 6 mbar, not taking account minor losses.
NPSHA is about 17.8 m
Note : a simple sketch should have saved hundred of words !

Are you sure about the pipe diameter because the head loss is so small ? The velocity is less than 0.35 m/s ? Something is very weird in term of design .

Good luck
Pierre