Calculating flow variables from a water pump

[cocopops]

The setup I have is a water pump, with a pipe out leading to a nozzle of some sort. I know the mass flow rate of the water through the pump, and wish to calculate the exit velocity of the water from the nozzle.

Which other variables do I need to know?

Also, what dictates the limit to the exit velocity, as from conservation of mass, it would appear that simply making a tiny nozzle would give a high velocity. I'm fairly sure I'm missing something really simple and fundamental, but it's been a while since I've done this sort of thing.

Cheers.

 

[gmax137]

It depends upon the nature of your pump. Most pumps discharge pressure varies with the flowrate (or vice versa, if you prefer). So, a smaller hole in the nozzle raises the discharge pressure, reducing the flowrate, etc...

 

[cocopops]

I see. Well the pump that is proposed is a centrifugal impeller pump, and the only real data I can find on it is the mass flow rate (250 m³/hr).

Lets say I know the diameter of the pump outlet, and the diameter of the nozzle (and assuming density is constant), do I need any other data to calculate the velocity of the water exiting the nozzle? I know pressure must come into it somewhere, I'm just not sure how...

 

[gmax137]

Well yeah, if the flow rate is 250 m3/hr then the velocity in the nozzle is just 250/A where A is the flow area in m^2. That's called 'continuity.'

But if you want to know what the flowrate really is, you need to know how the pump flow varies with pressure (and how the nozzle pressure drop varies with flow rate).

Also, if the nozzle area is too small, the flow will 'choke' -- this has to do with the speed of sound in the fluid.

 

[cocopops]

Thanks for your help so far! I've got this chart for the pump in question, but I just want to make sure I understand what it is displaying.

Does this mean for a flow rate of 300m³/hr, then the head can be between 70~450m? I don't understand why it isn't a fixed value?

On that same note, is head just another way of saying pressure? i.e. Can I convert 450m into bar/pascal/psi, etc?