11 replies to this topic

[Drakz]

Hi all,

 

I have one question. Why does the velocity head is included in NPSHA calculation for centrifugal pump? is it because the velocity head is converted back to pressure at impeller entrance (due to velocity decrease)? Is the velocity head is converted back to pressure at all pump suction regardless of pump type?

 

Hoping you all can help me in this matter

[breizh]

Drakz ,

 

Consider reading this document , it should clear your doubt .

 

Note : velocity head is negligible .

 

Breizh

Edited by breizh, 06 August 2015 - 10:45 PM.

[samayaraj]

Hi,

 

In general, velocity head (V²/2g) is neglected in NPSH calculation. Since, the head addition because of velocity is very less. For pumps, suction velocity will be less than 1 m/sec and suction head for velocity 1 m/sec will be 1²/(2 x 9.81) will be nearly 0.05m.

Edited by samayaraj, 06 August 2015 - 04:41 AM.

[Drakz]

Thanks all for your reply,

 

velocity head is usually very small and usually we assume it negligible.

For my understanding, I want to know why we add it to our NPSHA calculation? I guess it is because the velocity head is converted to pressure. Why is that? Is it because the velocity drop to zero right before it enter the impeller thus causing the velocity head to be converted to pressure?(bernoulli principle, velocity decrease, pressure increase)

[breizh]

Hi ,

To support your work.

 

Breizh

[fallah]

 

For my understanding, I want to know why we add it to our NPSHA calculation? I guess it is because the velocity head is converted to pressure. Why is that? Is it because the velocity drop to zero right before it enter the impeller thus causing the velocity head to be converted to pressure?(bernoulli principle, velocity decrease, pressure increase)

 

Drakz,

 

Velocity head added to NPSHA calculation, because it's among all heads available at the pump's suction nozzle; no matter it's in the form of velocity head itself or converted to static pressure at, let say, pump's impeller eye or elsewhere. Then, even if it's converted to static head still is among all available heads at pump's suction nozzle and can be considered in NPSHA calculation if more accurate value of NPSHA is to be expected...

[Drakz]

Thanks breizh and fallah,

 

To further my understanding i have a question,

 

Let say i have total dynamic (flowing) pressure of 3 bar a, ("static" pressure of 2.9 bar a + velocity head of 0.1 bar a), and my liquid vapor pressure is 2.95 bar a. Will my liquid vaporize?

[fallah]

 

Let say i have total dynamic (flowing) pressure of 3 bar a, ("static" pressure of 2.9 bar a + velocity head of 0.1 bar a), and my liquid vapor pressure is 2.95 bar a. Will my liquid vaporize?

 

Draks,

 

How about the friction loss? If it's already considered in total dynamic pressure liquid will not be in its boiling point and will not vaporize...

[Drakz]

Dear Fallah,

 

Thanks for your reply. Friction loss is already considered in total dynamic pressure.

So based on your reply, even though its static pressure (2.9 bar a) is lower than its vapour pressure (2.95 bar a), the liquid will not boil because its total dynamic pressure (3 bar a) is higher than its vapour pressure (2.95 bar a)? Is it correct?

[fallah]

 

Thanks for your reply. Friction loss is already considered in total dynamic pressure.

So based on your reply, even though its static pressure (2.9 bar a) is lower than its vapour pressure (2.95 bar a), the liquid will not boil because its total dynamic pressure (3 bar a) is higher than its vapour pressure (2.95 bar a)? Is it correct?

 

Draks,

 

Theoretically is correct, but at real world if you want having velocity head of 0.1 bar, the velocity in suction line should be higher than 4 m/s...

Edited by fallah, 07 August 2015 - 05:39 AM.

[Drakz]

Thanks fallah,

 

The value i put is just a value i randomly create only to create a case study.

Edited by Drakz, 07 August 2015 - 09:07 AM.

[shan]

Yes.  Vapor will flash out if the vapor pressure is 3.0 bar and the stream static pressure is 2.9 bar at whatever velocity head is.