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Pump Discharge Flow Is Fluctuating


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#1 pronab

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Posted 23 May 2018 - 10:32 AM

We have a flow fluctuating of an amine circulation pump discharge flow. Amine flow route is as: From Surge tank to booster pump folllowed by circulation pump discharge flow control valve to Amine absorber. Recently we found circulation discharge flow is fluctuating. There are two flow control valves(FCV) and one can be on line at a time. FCV change over from A valve to B valve however no improvement. All the Instruments parameters are checked and found normal. As per instrument maintenance transmitter and the related instrumentations are checked and found ok. Due to this fluctuations down stream level of the column also fluctuate. However both the booster and circulation pump discharge pressure are steady. Pump also chnage over from A to B and found no improvement.What could be the reason of fluctuation?

 

 

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#2 Bobby Strain

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Posted 23 May 2018 - 12:57 PM

Check the amperage on the running pumps.

 

Bobby


Edited by Bobby Strain, 24 May 2018 - 02:18 PM.


#3 thorium90

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Posted 26 May 2018 - 12:45 PM

The control valve may probably be sticky. Clean the valve packing. WD40 might be useful.

If cleaning the valve stem as above doesnt work, then maybe the compressed air being used is dirty, it might have choked the nozzle controlling the air flow inside the pneumatic controller. Open the controller cover and clean the valve controller air nozzle.

 

The other reasons I can think of are:

Flowmeter electronic amplifier faulty, giving fluctuating flows even thought the flow is actually steady.

Electromagnetic interference with nearby equipment.

Electromagnetic interference along the instrument cable due to broken shielding.

Disconnected flowmeter grounding rings, or some other grounding related issue. I assume its a magflow?



#4 breizh

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Posted 27 May 2018 - 01:16 AM

Hi ,

Do you observe fluctuations when the controller is set in manual ? if Yes you may have issue with the pumps or other equipment .

If No , you may consider to revise the tuning of the controllers .

 

Hope this is helping you.

Breizh


Edited by breizh, 27 May 2018 - 01:19 AM.


#5 pronab

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Posted 27 May 2018 - 01:54 AM

Dear Thorium,

As I mentioned earlier, Instrumentation of the Control valves have been verified and found ok. Controller posiotioner has been changed. However valve sticky not checked. Flowmeter amplifier not checked, however I will persue maintenance people to check it. Electromagnetic interference with nearby equipments can be ruled out since there are similar two other trains and they are working fine with the near by equipments.

 

Dear Breizh,

Both the flow control valves FV145 & FV146 were kept manual and found flow still fluctuate. Pump discharge pressure was not fluctuate remarkably. Tuning of controller parameters not required since on manual also flow is fluctuate.

Please note that recently in february-18, these flow controll valves FV146A & B changed to new control valves as a part of modification jobs. However fluctuation started after one and half month smooth running and only one of the three trains. Other two trains flow are steady.



#6 breizh

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Posted 27 May 2018 - 03:01 AM

Hi ,

What about the fluid ? Can it affect the reading ? let say having gas passing ! what about having the flowmeter damaged ? Can you replace the flowmeter with a new one ?

 

Thanks for sharing your findings.

 

Breizh


Edited by breizh, 27 May 2018 - 03:13 AM.


#7 thorium90

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Posted 27 May 2018 - 04:24 AM

Breizh is right, if there was gas passing through, it would impact the flowmeter reading as well. But this fluctuation would be quite substantial and it would be quite observable by the pump discharge pressure instrument. In the first post, it was mentioned the pump discharge pressure is steady, but it is now mentioned that the pump discharge does indeed fluctuate but not remarkably. The drawing does seem to imply this is the lean amine flow, might not be that much of CO2 gas remaining, and it would be odd to have such a substantial quantity of gas to affect the magflow anyway.....

 

Perhaps a short video of the observed fluctuation or maybe some trends will help. 

 

Also, does the valve have a manual override handwheel? I presume when you mentioned manual, you are referring to the PID controller manual right? If a handwheel exists, might you try to move the handwheel to the current valve position and force the valve to remain steady at the current position rather than by using the PID controller manual. If the flow becomes steady after you have forced the valve to the fixed position, (and that the valve is not moving at all) then the problem is due to the valve. Try this on both sets of valves, the FV-A/B and the valve linked to FT145, or maybe try on both at the same time, see if it stops the fluctuation.

 

These new valves that you mentioned, these are new globe valves? Same type as the previous valves or a new type? Possible valve trim issue? 

 

When you mentioned modification job done previously? What was the modification? Many a time when people tell me they modified something and then a problem appears and they ask me what to do next, I just tell them to undo whatever modification they made. Maybe the modification is somehow linked to this problem.

 

If the valve is not the issue, probably the next thing is to check the pump. Might you be able to measure the pump current? Clamp on perhaps?


Edited by thorium90, 27 May 2018 - 04:37 AM.


#8 pronab

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Posted 27 May 2018 - 08:32 AM

Hi ,

What about the fluid ? Can it affect the reading ? let say having gas passing ! what about having the flowmeter damaged ? Can you replace the flowmeter with a new one ?

 

Thanks for sharing your findings.

 

Breizh

 

Yes fluid density and purity can affect the flow, however they are within the soecifications. Possibility of gas passing  is very slim. Flowmeter has been calibrated and positioner of the flowmeter has been changed by new. As per Instrument maintenance flowmeter is fine.

 

 

Breizh is right, if there was gas passing through, it would impact the flowmeter reading as well. But this fluctuation would be quite substantial and it would be quite observable by the pump discharge pressure instrument. In the first post, it was mentioned the pump discharge pressure is steady, but it is now mentioned that the pump discharge does indeed fluctuate but not remarkably. The drawing does seem to imply this is the lean amine flow, might not be that much of CO2 gas remaining, and it would be odd to have such a substantial quantity of gas to affect the magflow anyway.....

 

Perhaps a short video of the observed fluctuation or maybe some trends will help. 

 

Also, does the valve have a manual override handwheel? I presume when you mentioned manual, you are referring to the PID controller manual right? If a handwheel exists, might you try to move the handwheel to the current valve position and force the valve to remain steady at the current position rather than by using the PID controller manual. If the flow becomes steady after you have forced the valve to the fixed position, (and that the valve is not moving at all) then the problem is due to the valve. Try this on both sets of valves, the FV-A/B and the valve linked to FT145, or maybe try on both at the same time, see if it stops the fluctuation.

 

These new valves that you mentioned, these are new globe valves? Same type as the previous valves or a new type? Possible valve trim issue? 

 

When you mentioned modification job done previously? What was the modification? Many a time when people tell me they modified something and then a problem appears and they ask me what to do next, I just tell them to undo whatever modification they made. Maybe the modification is somehow linked to this problem.

 

If the valve is not the issue, probably the next thing is to check the pump. Might you be able to measure the pump current? Clamp on perhaps?

 

Yes it is a gas sweetenin process and pump discharge is delivering lean amine solvent. The H2S and CO2 in the lean amine are with in the range(200 ppm H2S and 200 ppm CO2).

When I said it is on MANUAL, it means PID controller kept on MAN and not the valve valve handwheel on manual. Both sets of valves FV146Aor B and FV145 kept on manual(PID on manual) and found still flow fluctuate.

The new valves (FV146A/B) are replaced as old valves difficult to repair and not getting spare parts. If two trains with same valves wrok fine then third train should work also fine.

The remaining part is to change the pump which will be done shortly and I will come to you.

 

Thank you very much for your time.



#9 pronab

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Posted 13 June 2018 - 08:37 AM

It was an maintenance issue and they solved it. Positioner feedback guard was too tight and clashing, a result of not smooth travelling upon change of signal.

Thank you for yoiur response and time.



#10 thorium90

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Posted 13 June 2018 - 10:23 AM

Glad to know the problem is solved. Hope we have managed to help you in some way.



#11 Bobby Strain

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Posted 13 June 2018 - 01:41 PM

 

There is a lesson to be learned from Pronab's experience. Troubleshooting should always begin with the root element. In this case the valve and attached hardware. One will find this to be the probable cause about 90% of the time. This approach should always be used except when an element in the system is new or modified. Then start with that.

 

Bobby






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