Pumps for high viscous fluids

[cowpen]

For context:
I'm currently upgrading a pump for Low Sulphur Waxy Residue- the existing pump is a centrifugal pump with the following parameters (rated):
[ul]
[li]Q = 70 m3/hr[/li]
[li]h= 160m[/li]
[li]viscosity at pumping temp (83 degC)= 10.2 cP[/li]
[li]density of fluid at pumping temp: 875 kg/m3[/li]
[/ul]The upgrade consists of increasing the capacity and head; process fluid remains the same:
[ul]
[li]Q2= 120 m3/hr[/li]
[li]h2= 250m[/li]
[/ul]

Question:
Is there a rule of thumb/threshold for viscosity where a screw pump would be preferred over a centrifugal pump?

 

[georgeverghese]

Though I cant tell where this viscosity cutoff is, you may also need to consider the highest viscosity which may perhaps be at startup or some other lower bound on operating temperature, not just at normal conditions.

 

[LittleInch]

The maximum viscosity I was told was 300cP for a centrifugal pump by a pump supplier, but I don't think it's written down anywhere.

We have looked at centrifugal pumps for pumping waxy Crude Oil which has cP of 60-100 without an issue, so 10 is fine. If the current one works OK, why change pump type just because flow and head has gone up?

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

 

[cowpen]

Georgeverghese, thanks for the input, will remember to consider the startup viscosity.

Littleinch, the pumps are working fine and probably wont' be changed (as you mentioned, only the head and flow have increased). Just checking if there's a rule of thumb for pump selection- thanks for your input, much appreciated.

 

[EdStainless]

Some styles of centrifugals work better with higher viscosity, and sounds like you have one. Generally a mixed flow style with larger passages as a lower operating speed will be the directions to go.
In the process world your viscosity isn't very high so you are still good.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[DubMac]

Go find "viscosity correction curves", which should be easy enough to Google. The charts will allow you to obtain some correction factors to flow and head for varying viscosities, which will allow you to properly size the pump.

One of the main issues will be the reduction in efficiency, which can be drastic, and many times require an oversized motor.

You then should balance the overall cost against using a positive displacement pump.

 

[snassd]

I have one in my hand which is Chevron Equipment Guideline.

The guideline is that for the viscosity above 65 centistokes, rotary or reciprocating pumps are recommended.

However, as discussed above, it's depended on the application, condition and equipment design.

The 65 centistokes is just for a general guideline which shall be verified/considered when you received the pump curve/data sheet from vendor.

 

[cowpen]

EdStainless, DubMac, snassd, thanks for the input. Was generally curious regarding the threshold when it is considered "too viscous" for a centrifugal pump- glad to have my questions answered.

Thanks again all.