7 replies to this topic

[paraschemix]

Hello all,

I am doing a pump calculation. In that, the tank has two suction nozzles. Bottom nozzle(n1) will be reserve for some other purpose. Second suction nozzle (N2) will be located above 2m from tank bottom.

N2  -------->  pump nozzle (N3)

NPSHa = 10 m

Tank details:

Diameter : 62 m

Height : 20 m

Working volume = 51500 m3 (excludes 2m height)

But my suction pressure at N3 is negative (i.e. - 0.13 barg).

And i cant increase my tank height.

What should i do?? Is it ok to have negative suction pressure in the pump.??

[thorium90]

Perhaps you can show your calculations?

 

Anyway, for your info,

http://www.engineeri...head-d_634.html

[Dacs]

There's nothing wrong with negative suction pressure as long as your NPSHa is alright.

 

What matters is that your suction pressure is above your fluid vapor pressure to avoid cavitation.

 

I have to add though, can you furnish your calculations? 10m static height and negative suction pressure don't add up for me

[fallah]

Hi,

 

In evaluating the NPSHA you should use absolute pressure, then you have 0.87 bara at the pump suction and it could be possible. It is adequate the absolute pressure at the pump suction would be higher than the fluid vapor pressure with a reasonable margin.

[Bobby Strain]

I interpret your statement to mean that your pump is located above the tank nozzle. This is not the same as a "negative" suction pressure. So maybe you can clarify your situation.

 

Bobby

[chemsac2]

Negative suction pressure can cause air ingress in fluid to be pumped. Air bubbles in liquid to be pumped can cause pump vibrations and damage (not cavitation exactly, but damage is similar).

 

Negative suction pressure of upto 0.1 barg is considered acceptable as this pressure drop is not sufficient for excessive air ingress considering small opening in a gasket joint.

 

Consult pump vendor if suction pressure of -0.13 barg is acceptable.

 

Regards,

 

Sachin

[paraschemix]

Hi,

Thankx for the immediate replies.

 

I like to add some more.

1. I have a strainer in my pump suction assuming it will offer a delta P = 0.07 bar.

2. and Line drop

 

Tank LLLL

^

|    ( 2 m)

|

Pump suction

 

 

Now at the tank nozzle pressure is 0.02 barg.

i.e.Pressure at pump suction nozzle = Pressure at tank nozzle - strainer & line drop

                                                        = 0.02 barg - (0.07 + 0.01) = -0.06 barg

[Dacs]

What's your NPSHa?