Pressure in Pump Systems

[Ethan Barrieau]

OK, So I have been having an on and off debate with my family about this. I saw a pump at Home Depot that said it has a 30 ft head. Then it went on to show that the flow rate varied as the output of the pump changed in height. Now I thought that this was wrong. I just finished a course in fluid mechanics and in the course they made it pretty clear that the flow rate, Q, is constant throughout the system and that the dynamic pressure will decrease as the hydrostatic pressure increases due to Bernoulli's law.

My family members, two engineers and one plumber, have said that as you increase the exit height of the water, the flow and the pressure will decrease. I have looked back through my textbook, my notes and the internet and found nothing to support either of our claims. The only thing that I could think of is that by increasing the hydrostatic pressure at the exit of the pump you would need more power to create the same flow and therefore in practical applications where pumps have limited power capabilities the flow would in fact decrease.

So can anyone shed some light on the situation for me?

 

[Doug Huffman]

See the Affinity laws.

 

[mfb]

The only thing that I could think of is that by increasing the hydrostatic pressure at the exit of the pump you would need more power to create the same flow and therefore in practical applications where pumps have limited power capabilities the flow would in fact decrease.

Right.

"The flow rate is constant within the system" allows to compare different points in the same system, but not two different systems.

 

[russ_watters]

Naa, this isn't an affinity law situation. Affinity laws are for when you change something about the pump's operation, but here it is the system that is changing. That's a conservation of energy situation. If you look at a fan or pump curve, they run at fairly consistent power across a wide range of the curve (with some varying efficiency). So if pressure goes up due to an added resistance, flow must go down.

Or, you can think about it more directly: close your faucet valve halfway. Does flow go up or down?

Unfortunately, the rel world operation of pumps isn't really covered in college.

 

[Baluncore]

It is the type of pump that determines the pressure versus flow characteristics.

A positive displacement pump, (such as a piston or gear pump), pushes a fixed volume of fluid per cycle. The pressure is decided by the output restriction, it can reach destructive pressures if the outlet is obstructed.

A centrifugal pump generates a pressure that falls as flow increases. If the outlet is obstructed so that there can be no flow, the pressure will reach a limited maximum determined by diameter and RPM. That is probably the type of pump you see listed with a 30 foot head.