Pump Affinity Laws

[fayazdin1]

Pumps basics:
Centrifugal Fire- Pump:

Impeller dia = constant, Power = Constant
Q1 = 1000-gpm, H1 = 335-ft
Affinity Law: (H2/H1) = (Q2/Q1)^2
If Q2 = 100-gpm, H2 = 3.35-ft
Is this correct? The head should go up for lower flows for constant power and constant impeller diameter.
Why am I confused? Please help me understand

 

[skdesigner]

You don't need to use an affinity law to find pressure at a specific flow. Use the manufacturers pump curve. Flow will be on the x axis, pressure will be on the y axis.

 

[skdesigner]

http://www.gouldspumps.com/cpf_0010.html

 

[chicopee]

The reason that it does not work is because Q1 and Q2 are porportional to n1 and n2; H1 and H2 are proportional to n1^2 and n2^2. The only time Q1/Q2= (n1/n2)^2 works is when you have one curve (Q1,H1) is based on n1 and the second curve(Q2,H2) is based on n2.

 

[chicopee]

To finish off what I have stated, to apply the affinity equations that you presented, you'll have two curves; one curve will be based on RPM N1 and the second curve will be based on RPM n2 which would incorporate your 100 rpm @ 3.35 ft of head. Follow skdesigner's original reply.