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# Axial Flow Pump Affinity Law

## Axial Flow Pump Affinity Law

(OP)
Hi everyone,

I hope somebody can bring some light over this query.

I have a 24 inches axial flow pump which handles 1 m3/s vs 8 meters of head and I need to design a new one that can handle 0,5 m3/s vs 8 meter. As you can see I need to reduce the flow to the half keeping the head at the same level.

According to the affinity laws, reducing the size of the pump to the 50% the flow will reduce in the same proportion, so far so good. The, the original outside diameter of the pump (24 inches) becomes 12 inches. My problem comes with the head, according to the affinity laws commonly used for centrifugal pumps, the head will be reduced to its 25%, it means 2 meter. I'm not quite sure that this will work with axial flow pumps beacuse there is not necesarily a proportion between the outside diameter of the pump and its head since the head is not based in the centrifugal efect. If I reduce the diameter to the half and keep the vanes shape shouldn't I maintain the head?

### RE: Axial Flow Pump Affinity Law

No.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

### RE: Axial Flow Pump Affinity Law

The second set of affinity laws, describes the relationships when the impeller diameter is changed under constant speed conditions. The second set of affinity laws applies only to radial flow pumps and is not appropriate for your pump selection.

Rather than using the pump affinity laws, it is recommended that pump operating data should be obtained from the pump manufacturer's data (pump characteristic curves).

### RE: Axial Flow Pump Affinity Law

The only thing you can say for certain is that at 50% of the flow but the same head you will use about 50% of the power.

Axial flow pumps are different to centrifugal pumps and don't follow the same affinity rules.

At a guess you are probably looking at 50% area reduction so about 18" diameter to keep the axial velocity similar, but easier just to ask the vendor for a new data set.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Axial Flow Pump Affinity Law

The affinity laws create similar curves based on the known pump and follow the system curve (assuming no static head). However, you want to change the shape of the pump curve to match a different system curve, maintaining head but reducing flow. To maintain head but reduce flow in an axial pump you will need to consider changing the pitch angle of the blades. Think of it like a propeller or a fan with steeper angle blades.

I used to count sand. Now I don't count at all.

### RE: Axial Flow Pump Affinity Law

Contact the manufacturer with your enquiry and ask for their advice. Very possible they can either supply or manufacture an impeller to suit your duty and use the existing pump.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

### RE: Axial Flow Pump Affinity Law

I think David V is actually designing the pump:

#### Quote (David V)

I need to design a new one that can handle 0,5 m3/s vs 8 meter.

In that case, there would not be an existing manufacturer's curve. I hope he comments back to clarify. I see this is his only ever post.

I used to count sand. Now I don't count at all.

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