Loss of NPSHa when Pumps in parallel

[davincigee]

Hi guys,

I have a bit of a problem with a system I've been analyzing these past few days. Please see attached images. The system consists of a tank feeding to 3 centrifugal pumps (pumps are at an elevation below -5m of the tank foundation). These 3 pumps rated at 576cu.m/hr discharge 12 loading arms at a flowrate of 2400l/min. My problem is whenever I simulate the system model on AFT fathom or PIPENET, there's alway seems to be less NPSH available to the pumps. However when I run just one pump feeding just a third of the loading arms(4 arms) there isn't any issue with NPSHa. Infact I obtain ample ie 11m. Can anyone please tell me what is going on? I've always thought that NPSHa is not affected by pumps in parallel configuration.

Tank level = 0.6m (this is in sizing for a pump in a worst case scenario)
Suction pipeline length =580m
Discharge pipeline length = 440m.

 

[danschwind]

You have more flow, therefore more velocity, therefore more pressure drop, therefore less NPSHa in your pumps.

Daniel
Rio de Janeiro - Brazil

 

[LittleInch]

Daniel is totally correct and succinct.

A 580m long inlet line of what size exactly? whatever size it is pressure drop is approximately proportional to flow rate^2 for the same fluid and pipe size and length.
So lets assume your frictional losses are say 1.5m with one pump, they will be 2.25m with 2 and 3.4m with 3.

oh and how does 3 pumps at 576m3/hr (I know that's rated) equal 2,400 m3/hr to the loading racks? The further to the right of the curve, the worse the NPSHR gets usually.

It only doesn't affect it if you had three equal inlet lines from the tank feeding each pump....

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

 

[davincigee]

Thanks Dan and LittleInch.

And LittleInch, regards your question, yes they will certainly run further to the right of the curve. But its 2400 litres/minute for each arm if you do the conversion and computation.

 

[LittleInch]

Sorry, you changed units and I don't notice

So a total of 1728 m3/hr is a bit better alright.

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