Two vertical Pumps with high discharge pressure

[ram_3]

Hi Gents,
We have 2 vertical pumps (1 & 2) (one in service and other one in stand-by). Pump-2 was recently overhauled, normally used to deliver 5200 m3/hr (Head:70m & current: 82 amps) after the overhauling pump-2 is developing around 1000 m3/hr (head: 85m (approx 8.5 bar) & current: 66amps). We have a RO (with diff pressure:3.5 bar). Upstream of RO is 8 bar & downstream is 4.5 bar which is fine). When the pump-1 discharge valve is opened, the flow of pump-2 is increasing (i.e discharge pressure reduces & current increases to 83 amps), as partial flow is put back into pit. With this condition, can we say the Pump-2 is working fine but there is back pressure is coming from the down stream discharge pipe line which is offering obstruction in pump flow making the discharge to go high. Pump-1 is also seems to behave like this, but the discharge pressure is around 7.7 bar estimating to be 3000 m3/hr. as per the performance curve.

Can anyone tell why Pump-2 developing more discharge pressure (less flow)compared to Pump-1.

 

[JJPellin]

That is the very nature of a centrifugal pump. It delivers higher pressure as the flow is decreased and lower pressure as the flow is increased. Plot the data on the pump curve to see if it is close to the predicted performance.

Johnny Pellin

 

[LittleInch]

I've read this three times and I'm still not really following what is going on.

Can you sketch up your system with the RO and the pump and also post the pump curves.

Your pump must be operating in a quite an inefficient region at 1000 m3/hr because for the same pump, power (Amps) is basically flow times differential head.

does this imply that flow is going backwards through pump 1 when you open the discharge valve?? If so that's not a good idea.

I don't understand how an "overhaul" reduced the flow from 5200 to 1000 m3/hr?? Did the RO go in at the same time?

Why is the RO there?

How can pump 1 flow more than pump 2 if it has the same RO?? This doesn't make sense.

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Also: If you get a response it's polite to respond to it.

 

[ram_3]

Hi Gents,
Please I had enclosed the pump performance curve & Pump piping layout indicating the RO position and also the comparison of pump discharge pressure Vs. Motor current when Pump 1 & 2 are in service (If Pump-1 is in service then Pump-2 is in stand-by and vice versa). Please can go through the performance curve and provide me the possible cause for high discharge pressure between the Pump-1 & Pump-2. Also be informed that, the normal pressure required is around 7.2 bar, whereas both pump is reading higher (Pump-1 is 7.8 bar and Pump-2 is 8.1 bar). Recently we had inspected the RO and there is no any clogging/blockage.

 

[LittleInch]

No piping layout attached, only pump curve

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Also: If you get a response it's polite to respond to it.

 

[ram_3]

Hi Little Inch,
Please can you check now.

 

[LittleInch]

Well there's a few possible issues here.

I can only assume that you're estimating flows based on discharge pressure which assumes that the inlet pressure into both pumps is basically 0m for both pumps??

Taking that assumption one potential issue is that when pump 1 is operating there is flow going either back through pump 2 or through the control valve.

Otherwise everything seems to make sense. Lower flow uses more current than in proportion to the flow due to much lower efficiency.

What exactly is the issues / problem here?
Note that at the 1000m3/hr you're operating lower than the recommended minimum.
What flow rate are you calculating through the RO at those pressures??
Why is it there?

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

 

[ram_3]

Hi,
Pump-1 is consuming Motor amps-72
Pump-2 is consuming Motor amps ----.
With same pit level (suction) and same piping condition, Pump-2 develops discharge pressure of 8.0 bar whereas Pump-1 develops discharge pressure of 7.8 bar meaning Pump-2 flow is less than Pump-1 as Pump-2 was recently overhauling.

Issue: As the flow rate of Pump-2 is less, concentration of the minerals in the suction increases thereby increasing the conductivity of the fluid.

Why the Pump-2 is not delivering the rated flow: 5400 m3/hr where is the limitation is coming?.

As I told you earlier if the MOV of Pump-1 is opening, there is increase in the Pump Motor current meaning the flow is increasing.

Can we doubt on the piping for any obstruction or Pump-2 performance itself.

 

[ram_3]

Sorry,
Pump-2 is consuming Motor amps: 66

 

[Artisi]

Are figures for pump 2 true as first posted, was 5200 now 1000 m3h - - - is pump 2 now running in the right direction. Don't believe anyone - check yourself.

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.)

 

[JJPellin]

Based on the provided diagram, I would suggest that pump 2 is producing more flow than pump 1, not less. I base this on three observations. Pump 2 produces more pressure working in the same system. The pressure drop across the restriction orifice is greater when pump 2 is in operation. There is no flow meter in this system. Pump 2 likely produces more pressure (and thus, more flow) because it was recently overhauled. It has tighter internal clearances and is generally in better condition. This is also why pump 2 is drawing lower motor amps. You seem to be basing your concerns on the belief that all pumps always run exactly on the published curve. This is not a valid assumption.

Johnny Pellin

 

[LittleInch]

Please get your numbers straight - your diagram say pump 2, 66 A, 8.6 bar outlet, but your post above now says 8.0 bar.

on that pump curve it makes a difference between 1000 and 3000 m3/hr.

Without a reliable flow measurement I don't think you're going to get anywhere.

I agree with the check rotation issue.

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

 

[quadtracker]

Check size of both impellers to make sure they are up to specs and check the cut waters in the casing to be the same for both pumps. Some times factory sends new impellers that have to be trimmed to meet specs. The repair company is obligated to trim impeller to correct size and balance it. Also the cut waters are eroded due to corrosion of fluids being pumped therefore they need to be built back up. These checks may eliminate pump performance differences.

 

[ram_3]

Dear Gents,

Based on the response, I have made a comparision table based on the Pump motor power consumption.

As per the rated condition:

Pump flow: 5200 m3/hr & Head: 70 m

Pump-1 estimated flow: 3428 m3/hr & Head: 76 m

Pump-2 estimated flow: 3109 m3/hr & Head: 79 m.

Pump-2 was recently overhauled with same Pump Impeller (as condition of impeller found in good condition).

Issue:

1. Why both pumps are not delivering near to the rated flow, especially Pump-2 which was overhauled one month back.

2. Issue like Impeller undercut & reverse rotation can be ruled-out as there is no impeller change & Motor DOR check was done before pump start-up.

3. Any restriction of Discharge piping, can create flow reduction of 2000m3/hr.

Please provide you opinion/suggestion.

 

[LittleInch]

So

1. Why both pumps are not delivering near to the rated flow, especially Pump-2 which was overhauled one month back.
Because the downstream system curve won't let it. Both pumps appear to be on curve. The issue is that to achieve 5200 m3/hr your RO needs to be a bit bigger so that the system resistance to flow at 5200 is 70 m head, not 76 to 79 you're getting at the moment.

Please get it into your head that the duty point is just one point on a curve based on some estimate or calculation of the system requirement at that flowrate. Centrifugal pumps are essentially fixed pressure devices with flow varying depending on the system resistance. Positive displacement pumps are essentially fixed flow devices with pressure varying depending on the system resistance. Just because your duty point is 5200 means nothing if your system resistance requires more pressure to flow at 5200 compared to what the pump puts out.

An analogy is that your car has a maximum speed of 70MPH, based on a flat road with no head wind. You won't be able do 70 MPH if you're going up a hill and / or have a head wind...

Your pump with duty point of 5200 as per the curve you supplied is happy anywhere between 3640 to 6240 m3/hr.


2. Issue like Impeller undercut & reverse rotation can be ruled-out as there is no impeller change & Motor DOR check was done before pump start-up.
Good

3. Any restriction of Discharge piping, can create flow reduction of 2000m3/hr.
You have deliberately introduced a Restriction orifice which appears to be slightly too small. However be careful as your pump curve is really quite flat so literally 5m (0.5 bar) difference in friction losses can make 2000 m3/hr difference.

If you really want 5200 you should really replace the rather brutal single fixed RO with some sort of variable flow control device like a control valve or perhaps a set of parallel RO branches which you can open and close as many as you need to maintain the flow you need??

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

 

[ram_3]

Thank you all for your help.