Velocity Head 2

[tmartin125]

I need some direction on velocity head for calculation of centrifugal pump TDH. Is the velocity to be used that measured at the pump discharge nozzle or at the pipe connected to the nozzle (usually larger than the pump nozzle)? Isn't the velocity head at the pump nozzle included by the manufacturer in the pump curve? For example I have a pump with a discharge velocity of 27 fps and its a big difference if this velocity is used for the velocity head calculation instead of the 10 fps which is the velocity in the discharge pipe. Thanks in advance for your comments.

 

[tmartin125]

Also, if the velocity head at the pump discharge nozzle is required for calculation of pump TDH, why isn't velocity head considered at the pump suction nozzle?

 

[katmar]

I have always taken the TDH to include the velocity head and the static head. It does not make much difference whether you use the pump discharge nozzle size or the connecting pipe size. There would of course be some small loss in the reducer between the pump discharge and the pipe, but as a first approximation the sum of the velocity head and the static head would be the same in the discharge nozzle and the piping (thanks to Bernoulli).

Strictly speaking, the velocity head at the pump suction should be taken into account. But the suction nozzle is almost always bigger than the discharge nozzle and because the velocity is proportional to the square of the pipe diameter, and the velocity head is proportional to the square of the velocity, the velocity head at the suction is usually very small compared with that at the discharge. Ignoring the velocity head at the suction makes the sizing slightly conservative and we usually like to hide a bit of extra safety factor where we can.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[e43u8]

Katmar,

You mentioned:"...the velocity is proportional to the square of the pipe diameter, and the velocity head is proportional to the square of the velocity, the velocity head at the suction is usually very small compared with that at the discharge..."

I am sure in the first part of above mentioned statement you meant:"...the velocity is [highlight #EF2929]inversely[/highlight] proportionalto the square of the pipe diameter,.."

 

[katmar]

That is why humans will always be better than computers - humans know what you mean, even when you say it wrongly!

Thanks for the correction e43u8, that is exactly what I should have said.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[BigInch]

Velocity head can be added to suction head when calculating net positive suction head available, but since there is rarely enough suction head anyway and margins of error of NPSHR for products that typically are not cold water, it is far better to keep that ace up your sleeve.

you must get smarter than the software you're using.

 

[tmartin125]

In my situation the pump is a 3196 Goulds with discharge nozzle size of 1.5" compared with 3" for the discharge piping. Difference in velocity is significant, 36 fps compared with 10 fps. The velocity head is either 20.4 ft or 1.5 ft depending on which velocity is used.

 

[katmar]

At that velocity you will lose about 10 or 11 ft of head in going through a standard reducer (used as an expander). Depending on the speed of your pump (ie TDH developed) this could be significant and you should base your calculations on the rated TDH being in the discharge nozzle. It might be worth considering a gently tapering conical reducer (expander) if the loss affects your process - and if you can physically fit it into the pipe layout.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[racookpe1978]

At startup, the NPSA cannot include suction velocity head anyway, since the inlet liquid is not moving yet.

 

[tmartin125]

Thank you all for your comments, they are most helpful.

 

[miya2014]

Its often advisable to consider the difference in velocity head, suction to discharge

 

[BigInch]

You usually can. Fortunately at startup the NPSHR is normally far less than NPSHR at design capacity.

you must get smarter than the software you're using.

 

[tmartin125]

Miya2014, why is it advisable to consider the difference in velocity head? Or what is gleaned by comparing the two?

 

[miya2014]

well you mentioned that you wanted to calculate the TDH
my understanding is that TDH = algebraic sum of discharge heads - algebraic sum of suction heads

you add the "static heads' and subtract "friction" and "suction lifts heads" based on your specific application.

 

[miya2014]

you may find this intresting

http://www.pumped101.com/velocity head.pdf