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Re-Design Centrifugal Pump With Smaller Impeller Diameter

centrifugal pumpimpeller overdesign re-design minimum flow valve vsd inverter speed motor

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#1 zavtranguyen

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Posted 04 November 2022 - 10:27 AM

Dear all, we have a over-design pump in the plant, so the minimum flow control valve (hand control valve) can not close fully even when the pump is operating over 100% load that results in high electric power consumption. Here is some data of this pump:

1. PFD process required:

Standard API 610 10th Edition

 

- Density at P.T kg/m3: min/norm/max = 950/920/900

-Viscosty at P.T cst: <3.33

- Vap. press at P.T. MPa(a): 1.73

******Operating condition:

- Suction temperature (deg.C) Min/Norm/Rated= 75/79/90

- Suction press. MPa(g) Min/Norm/Rated= - / -  / 1.98

- Discharge press. MPa(g) Min/Norm/Rated= - / - / 15.4

- Diff.press MPa: Min/Norm/Rated= - / - / 13.42

- Head m: Min/Norm/Rated= - / - / 1487

- Capacity m3/h: Min/Norm/Rated= - / 89.3 / 103

- NPSH available m: 10 (when taking saturation of pumped solution into consideration)

- Max.Shut-off press MPa(g): 19

- FLow control: continous

- Flow control by: Range: 30-100%

- Seal type: Mech.

******CONSTRUCTIONS FEATURES

- Casing: Centerline

- Split: Radial

- Impeller: Close

 

2. Designed: Main pump with booster pump

a. Booster pump:

- Suction temperature (deg.C) Min/Norm/Rated= 75/79/90

- Suction press. MPa(g) Min/Norm/Rated= - / -  / 1.98

- Discharge press. MPa(g) Min/Norm/Rated= - / - / 2.702

- Diff.press MPa: Min/Norm/Rated= - / - / 0.722

- Head m: Min/Norm/Rated= - / - / 80

- Capacity m3/h: Min/Norm/Rated= - / 89.3 / 103

- NPSH available m: 10 (when taking saturation of pumped solution into consideration)

- Max.Shut-off press MPa(g): 3.0

- FLow control: continous

- Flow control by: Range: 30-100%, minimum flow: 35 m3/h

- Impeller dia. 

- Seal type: Mech.

b. Main pump:

- Suction temperature (deg.C) Min/Norm/Rated= 75/79/90

- Suction press. MPa(g) Min/Norm/Rated= - / -  / 2.702

- Discharge press. MPa(g) Min/Norm/Rated= - / - / 15.4

- Diff.press MPa: Min/Norm/Rated= - / - / 12.698

- Head m: Min/Norm/Rated= - / - / 1407

- Capacity m3/h: Min/Norm/Rated= - / 89.3 / 103

- NPSH available m: >80

- Max.Shut-off press MPa(g): 20.8

- FLow control: continous

- Flow control by: Range: 30-100%, minimum flow: 35 m3/h

- Impeller dia. 

- Seal type: Mech.

C. Motor Driver:

- 800 kW

-Speed 2978 rpm

-6600V/3Ph/50Hz

- Gear for main pump: Kw/ratio: 800/2.265

 

 

3. Current Status in operation:

- Flow rate: >90 m3/h

- Temperature at suction: 79 deg.C

- Suction pressure: 1.86 Mpa(g)

- Discharge pressure: 16.3 MPa(g)

- Discharge control valve: 71%

- Minimum control valve (circulation): 65%

- Amper: 77A, 6.6kV

- Consumed Power: 795 kWh per hour

 

4. Questionare:

Easy to see that the pump is overdesign (when we want to close minimum control valve, we must close more discharge control valve which cause discharge pressure increasing over acceptable value >170 barg), kindly please give me your recommendation for each following ideas:

- To use VSD/Inverter to reduce Motor speed, but this will change all performances of booster and main pump/System performance with a high CAPEX and maybe not effective when the booster pump power/head decreases and maybe can not protect the main pump avoid cavitation if any...

- To reduce impeller of the main pump

- To reduce inpeller of the booster: the booster pump power/head decreases and maybe can not protect the main pump avoid cavitation if any...

....

How do you think about above solutions or any other ideas please,

Thank you,

Nguyen

 



#2 Pilesar

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Posted 04 November 2022 - 06:44 PM

In general, if reducing the impeller diameter solves the problem, it will be the simplest solution. You give a lot of data, so apparently you have enough to analyze the situation. Would the pump circuit run fine with the main pump impeller cut down? If so, why look any further?



#3 breizh

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Posted 04 November 2022 - 09:25 PM

Hi,

Why don't you issue a specification sheet and submit it to your existing supplier and ask them what will be the best solution according to your existing set up? In any case you will have to contact them if you need to downsize the impeller.

 

A sketch with the description of the full system is required to understand the need for the booster pump.

 

Good luck

Breizh 



#4 zavtranguyen

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Posted 05 November 2022 - 10:48 PM

In general, if reducing the impeller diameter solves the problem, it will be the simplest solution. You give a lot of data, so apparently you have enough to analyze the situation. Would the pump circuit run fine with the main pump impeller cut down? If so, why look any further?

Dear Mr.Pilesar, we are doing feasibility study for re-design solutions

We haven't made a decision yet, so the pump is operating with 65% opening of circulation valve



#5 zavtranguyen

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Posted 05 November 2022 - 11:06 PM

Hi,

Why don't you issue a specification sheet and submit it to your existing supplier and ask them what will be the best solution according to your existing set up? In any case you will have to contact them if you need to downsize the impeller.

 

A sketch with the description of the full system is required to understand the need for the booster pump.

 

Good luck

Breizh 

Thank you Mr.Breizh,

Yes, we have sent the issue to the vendor to find the best solutions from them, they are doing a analysis, however i want to listen more recommendations from you, experienced experts in petro-chemical.

 

Back to above issue, why the booster pump is required, i think when the supplier received the spec.sheet, because of low NPSHa (10m) (when taking saturation of pumped solution into consideration) they must have designed a booster upstream the main pump to increase NPSHa (and head) so that the main pump can operate more safely.

 

Note:

- the balance line of the main pump is conneted from the main pump to the discharge line of the booster pump.

- The booster pump and drive motor has same speed (no gear), but the main pump speed is higher with a gear.

 

Thank you,


Edited by zavtranguyen, 05 November 2022 - 11:06 PM.


#6 breizh

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Posted 06 November 2022 - 12:33 AM

Hi,

Thanks to share with us a sketch of your system (booster and main pumps) with all engineering data to perform some hydraulic calculations.

Can you share the pumps curves?

What is this material with 17.3 bar vapor pressure?

What is the NPSH required for your pumps @ Operating conditions? 10 meters of NPSHa is plenty!

 

Note: VFD is expensive for big motor.

 

Breizh



#7 zavtranguyen

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Posted 06 November 2022 - 04:06 AM

Hi,

Thanks to share with us a sketch of your system (booster and main pumps) with all engineering data to perform some hydraulic calculations.

Can you share the pumps curves?

What is this material with 17.3 bar vapor pressure?

What is the NPSH required for your pumps @ Operating conditions? 10 meters of NPSHa is plenty!

 

Note: VFD is expensive for big motor.

 

Breizh

Dear Mr.Breizh, here is curves of the booster and main, plus NPSHa calculation (calculation before the pumps were designed, noted when P1+ Pa- Pv = 0 it means you have the boiling point, in this case NPSHa = 11 (Fluid is carbamate solution in urea synthesis process)

https://postimg.cc/gallery/kVSR38V

 

Thanks


Edited by zavtranguyen, 06 November 2022 - 04:07 AM.


#8 breizh

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Posted 06 November 2022 - 04:32 AM

Hi,

Thanks for sharing the curves, we need a sketch of your system with process data to better understand (pipe length, elevation, fittings, ...). 

Are you sure about the Vapor pressure of your product?

I've added a booklet about centrifugal pump to support your work together with a document to calculate accurately NPSH a in case dissolved gas in liquid is very significant.

Good luck

Breizh  



#9 zavtranguyen

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Posted 06 November 2022 - 08:34 AM

Hi,

Thanks for sharing the curves, we need a sketch of your system with process data to better understand (pipe length, elevation, fittings, ...). 

Are you sure about the Vapor pressure of your product?

I've added a booklet about centrifugal pump to support your work together with a document to calculate accurately NPSH a in case dissolved gas in liquid is very significant.

Good luck

Breizh  

Hi,

Here is what you need:

1. Sketch https://postimg.cc/WtrmnrkV, in which calculated press.losses were done based on isometrics (extracted from Resistance calculation)

2. Process data that was provided to Pump supplier to design the pump unit matched process requirement,

https://postimg.cc/hQtbtZ2q 

 

Thank you for your documents



#10 breizh

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Posted 06 November 2022 - 08:46 AM

Hi,

Thanks,

no process data available, empty file.

Breizh 



#11 zavtranguyen

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Posted 06 November 2022 - 09:21 AM

Hi,

Thanks,

no process data available, empty file.

Breizh 

Hi, 

sorry sir

https://postimg.cc/hQtbtZ2q

https://postimg.cc/wR1qdP1r

The vapor pressure given 17.3 bar(a) is considered at critical condition when the suction solution reaches 90 deg.C and vaporization (boiling) appears


Edited by zavtranguyen, 06 November 2022 - 12:23 PM.


#12 zavtranguyen

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Posted 06 November 2022 - 12:23 PM

 

Hi,

Thanks,

no process data available, empty file.

Breizh 

Hi, 

sorry sir

https://postimg.cc/hQtbtZ2q

https://postimg.cc/wR1qdP1r

The vapor pressure given 17.3 bar(a) is considered at critical condition when the suction solution reaches 90 deg.C and vaporization (boiling) appears.

Thanks

 



#13 Bobby Strain

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Posted 06 November 2022 - 01:32 PM

Your vendor will likely recommend a gear change rather than to trim impellers. A gear change only requires one equipment item. Impeller trim is likely multiple impellers.

 

Bobby



#14 zavtranguyen

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Posted 06 November 2022 - 07:52 PM

Your vendor will likely recommend a gear change rather than to trim impellers. A gear change only requires one equipment item. Impeller trim is likely multiple impellers.

 

Bobby

Hi Mr.Bobby, it's a good solution also,

Thank you for giving me more viewpoints



#15 breizh

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Posted 06 November 2022 - 08:56 PM

Hi,

Thanks for updating your file, your calculation is correct.

Let see what the vendor's recommendation will be (trim the impeller or reduce RPM).

 

Breizh 



#16 zavtranguyen

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Posted 07 November 2022 - 04:01 AM

Your vendor will likely recommend a gear change rather than to trim impellers. A gear change only requires one equipment item. Impeller trim is likely multiple impellers.

 

Bobby

Hi sir,

Your recommendation made me think about motor and pump torque, if we change gearbox to reduce gear ratio, how the driven inertia at motor will be changed? The driven inertia of the motor will be reduced, is it? Do you have any experience of selecting pump torque and motor torque to match the required pump power?

 

Thank you so much,

Regards,



#17 breizh

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Posted 07 November 2022 - 05:12 AM

Hi,

To add to this thread an interesting link:

https://www.pumpsand...avitation-blues

 

Breizh






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