Standard Thickness of impeller blades?

[John7171]

Dear,
We have encountered a problem with an impeller in an vessel.

What is the standard blade thickness of a pitched blade impeller?
Impeller: pitched blade 45deg, 6-blades
Impeller diameter: 600 mm
Vessel diam: 1800 mm
Power input: 17 kW
Max. Vol. 4.7 m3
Speed = 265 rpm max
We have 6 mm, but blades simply sheared off (so might be not sufficient strength)
There was no abnormal unblance/wobbling of the shaft.
MoC: 316L
Shaft length: 2500 mm, Shaft thickness: 110 mm

We don’t understand why blades sheared off. First idea is blade thickness to small.

Kind regards,
John

 

[LittleInch]

That sounds like a mixer.

At 17 KW there will be a huge bending moment and torsional effect from such a basic mixer. Has this been designed or just put together.

A drawing or picture would help.

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

 

[John7171]

Hi,
sorry. 17kW is installed power. Power input is 12 kW max.

max. vol. is 4.7 m3, which would mean 12 kW/4.7 m3, which would lead to 2.5 kW/m3 (which is ok for reactions).

It's really a standard agitator set-up.

So, could you indicate what a 'standard' thickness of pitched blade impeller is?

I found 0.01 x Diameter of impeller = 6 mm, but not sure if correct.

Kind regards,
John

 

[LittleInch]

Whats the detail of the fixing of the blades to the hub. If there is no reinforcement then there is a huge stress concentration.

Look at the blades here and see the bigger root attachment

https://www.afxholdings.com/impellers/

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

 

[EdStainless]

The roots of the blades is usually thickened, braced, or gusseted.
Reinforcing the root connection is the issue.
The thickness of the blades is related to either bladed bending or vibration.
In some cases blade vibration or flutter will lead to fatigue failures.

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P.E. Metallurgy

 

[MJCronin]

"John"

How about a little more history here .... Was the mixer "repurposed" from another tank/vessel ?

Did the process change radically and you are now mixing a different, more dense product ?

Breaking of mixer blades is VERY unusual and I suspect that there is a history here...

Why do you not have any design drawings to share ? ... Were they all discarded by the new maintenance department ?

There is also the significant issue of mixer balance ....

Cantilever mixers are notorious for becoming out of balance due to repaired or corroded impellers.

Better to replace the entire impeller assembly with a replacement unit from the original vendor

Please tell us of your final solution to this problem

Was the mixer causing vibration problems for a long time ?

MJCronin
Sr. Process Engineer

 

[John7171]

Hello,

no. It's orginal design of a company.
Power input is 2.5 kW/m3 @max 4.7 m3. It's a standard set-up.
Density max. 1200 kg/m3.
Material of Construction: 316L

If you would need to design it: Pitched-blade 45°, 6-blades, power number 1.6:

What blade thickness would you chose?

Shaft is thick and not wobbling.

Kind regards,
John