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RyanTW

Why is my centrifugal pump shutting down?

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Dear Engineering forum,

I have a particular problem and I am hoping that the larger engineering world could help me. I will do my best to describe all of the important factors and answer any questions.

I do not necessarily expect anyone to solve it straight away (although it would be convenient) but, giving me some ideas as to what I could test for or look out for would be fantastic.

Situation:

  • I have a 22kw electric motor driving a centrifugal pump at approx. 45% load pumping at 25 - 30 m3/hour,
  • The electric motor is driven by a Variable Flow Drive (VFD) manufactured by ABB, which in tern is controlled by a PLC,
  • The inlet of the pump is connected to a 20,000ltr tank,
  • The outlet of the pump is connected to 4x pressure vessels maintaining 200kPa (2 bar) for high pressure delivery to an open outlet,
  • The speed of the pump is determined by maintaining the pressure on the outlet of the pump
  • The pump has the usual wire strainers, butterfly valves for maintenance and pressure indicators after the pump (no indicator installed before pump, will be occurring in the next couple of days)

The Problem:

  1. My motor/pump keeps shutting down during its cycle (while it should continue pumping)
  2. The VFD gives me a "Over current" fault warning as a result of the shut down.
  3. This usually occurs a couple of hours after the fault is cleared and pumping has resumed operation.

The IMPORTANT facts:

  • I have personally witnessed the motor/pump shut down momentarily (stop for 1sec before commencing operation) multiple times before the complete shut down and the "over current" fault is given, like it is tripping the power switch,
  • Every time the fault has occurred, the VFD records current supply anywhere between 7 - 21 amps, RPM at 0, Hz at 0 (the motor is rated at 27amps)
  • My industrial qualified electrician assures me that there are no irregularities in connection or communication, everything checks out electrically that he can test for.
  • I have dismantled the pump and there are no signs of excessive wear, plus during operation there is no odour, no strange noises and no excessive vibration,
  • Neither the pump nor the motor are showing any leaks, signs of rust or any other obvious sign of degradation at least on the outside.
  • The pump and motor have been running continuously (approx. 22 hours/day) without problems for 3.5 years. The pumps mechanical seal was only replaced 6 months ago and periodic maintenance is always kept up to date.
  • The pressure and flow rate on the line following the pump obviously drop every time the motor shuts down for that 1sec but there are no other changes during operation.
  • The water that is being pumped is particularly brackish (salty), but has been filtered multiple times so contains no particles or other additives.
  • I have a globe valve that I use to limit the flow rate by applying back pressure, which is of course is maintained at 200 kPa which is when the VFD comes into play.

I have no reward to offer for the person with the suggestion that leads to the cause, other than the pride of having the knowledge the engineering forum experiences for the solution and that you really helped out a fellow engineer.

Regards

RyanTW

 

 

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Ryan,

This is an interesting, real-life problem. Tell us more about this VFD drive. Is is a Pulse Width Modulated device sending variable frequency AC to the motor? If so, that may suggest some possibilities regarding your outages, but I will wait to hear from you before getting into those matters.

DrD

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It sounds like it might shut down because of a misread of the outlet preassure, and then the quick stop of the motor will generate too much current and the VFD can trip because of this.

Try to increase the Down Ramp and see if it gets any better.. my guess is that the down ramp is around 0,1 sec now..

 

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On 04/04/2016 at 9:41 AM, DrD said:

Ryan,

This is an interesting, real-life problem. Tell us more about this VFD drive. Is is a Pulse Width Modulated device sending variable frequency AC to the motor? If so, that may suggest some possibilities regarding your outages, but I will wait to hear from you before getting into those matters.

DrD

Hi DrD,

The VFD is a PWM device with specific Scalar U/f control method for the electric motor. I have not had a great deal of experience for the failure of these devices. It is supposed to be a simple system of control and therefore very reliable. What are the possibilities for these outages you are suggesting?

21 hours ago, manoranjan424 said:

Which mode is running in VFD?

Please elaborate about pump /motor specification sheet

 

Hi manoranjan424,

I am not certain about the mode, but I will check the next time I am onsite.

The motor specifications are

  • Voltage: 415V
  • Frequency: 50Hz
  • Power: 22kW
  • Rated RPM: 2950 RPM (this obviously changes with conditions)
  • Rated Amps: 37.4amps
  • Cos f: 0.87
  • Delta connected

IMG_20160406_114401.jpg

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15 hours ago, Van der Loeb said:

It sounds like it might shut down because of a misread of the outlet preassure, and then the quick stop of the motor will generate too much current and the VFD can trip because of this.

Try to increase the Down Ramp and see if it gets any better.. my guess is that the down ramp is around 0,1 sec now..

 

Hi Van Der Loeb,

I can understand your thoughts on your first point to do with quick stopping the motor and then re-start causing on overcurrent situation. It is something I have considered, just not sure why the VFD does not record this overcurrent when it faults. There is nothing to indicate that the PLC is misreading the outlet pressure, what are your thoughts behind this?

In regards to Down Ramp, I am not sure what you are referring to? Is this decelerating ramp? If it is then the factory default is currently set at 0.1sec. What would you suggest changing it to? And please, for my knowledge can you elaborate on why you think this may make a difference? Is it to give the motor a slight time delay to adjust?

Thank you for your contribution.

14 hours ago, George Munoz said:

Possible cause of shutdown is due to overspeed.Please try to check the suction line if their is sufficient supply.

George Munoz,

The overspeed possibility can definitely be related to the overcurrent state. All this could be because of inlet issues to the pump. I am currently in the process of installing a pressure indicator for the inlet of the pump to hopefully have more information during operation.

While the pump was inspected for possible inefficient operation section of the inlet and outlet pipework as well as the all butterfly valves were checked for blockages and everything was ok. The outlet of the tank has a grate that still needs to be checked, but monitoring the inlet of the pump DURING operation will probably be more helpful I think.

Thanks George

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Noise on a badly grounded cable can cause this misread, or a bad connection.

I was thinking of the DCC time yes. Excuse my bad English.

I will suggest you try to 0,2 or 0,3 because that will not force the motor to work against the centrifugal force.

If you get rid of the over current thing by changing the DCC, then it should be a lot easier to work out what the main problem is

 

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DrD and Van Der Loeb,

I have spent much time researching the controller (I did not specify or install) to see what was available to me to change and help compensate for the motors age.

Some of things that I have changed and found could help improve its performance (continuous operation) are the following points below:

  1. Current limit setting reduced to 40A from 77A. The Over Current Protection is fixed at 84A, so this gives the unit more time to slow the pump down if the current limit setting is exceeded before it reaches the over current protection (at which point it will fault).
  2. Pump ramp up/down time changed from 0.1sec to 0.5sec to prevent over draw.

In my mind these are only temporary fixes, the fault occurs at random times during CONSTANT operation, not during start up or wind down. So I suspect that as DrD has suggested that the very high peak voltages have possibly broken down the insulation in the connector and the motor windings. Which can result in a short which would trip the motor. Because as we all know, up time is the most important factor in the operation, but I only have the budget to wait for my April monthly O&M before my electrician can check a number of things.

Thank you everyone, I will continue to provide feedback as I find out more information about the system conditions

Ryan TW

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I am very curious what you find out. I have no experience with ABB, I am using a lot of Danfoss inverters, and the only time I get over current is if there comes air on the inlet of the pump. Then it get over current on 45 hz even if it could run on 50 hz with no problem. It seems like if it is running with too little work load, then it gets over current. 

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On 4/4/2016 at 5:04 PM, RyanTW said:

Dear Engineering forum,

I have a particular problem and I am hoping that the larger engineering world could help me. I will do my best to describe all of the important factors and answer any questions.

I do not necessarily expect anyone to solve it straight away (although it would be convenient) but, giving me some ideas as to what I could test for or look out for would be fantastic.

Situation:

  • I have a 22kw electric motor driving a centrifugal pump at approx. 45% load pumping at 25 - 30 m3/hour,
  • The electric motor is driven by a Variable Flow Drive (VFD) manufactured by ABB, which in tern is controlled by a PLC,
  • The inlet of the pump is connected to a 20,000ltr tank,
  • The outlet of the pump is connected to 4x pressure vessels maintaining 200kPa (2 bar) for high pressure delivery to an open outlet,
  • The speed of the pump is determined by maintaining the pressure on the outlet of the pump
  • The pump has the usual wire strainers, butterfly valves for maintenance and pressure indicators after the pump (no indicator installed before pump, will be occurring in the next couple of days)

The Problem:

  1. My motor/pump keeps shutting down during its cycle (while it should continue pumping)
  2. The VFD gives me a "Over current" fault warning as a result of the shut down.
  3. This usually occurs a couple of hours after the fault is cleared and pumping has resumed operation.

The IMPORTANT facts:

  • I have personally witnessed the motor/pump shut down momentarily (stop for 1sec before commencing operation) multiple times before the complete shut down and the "over current" fault is given, like it is tripping the power switch,
  • Every time the fault has occurred, the VFD records current supply anywhere between 7 - 21 amps, RPM at 0, Hz at 0 (the motor is rated at 27amps)
  • My industrial qualified electrician assures me that there are no irregularities in connection or communication, everything checks out electrically that he can test for.
  • I have dismantled the pump and there are no signs of excessive wear, plus during operation there is no odour, no strange noises and no excessive vibration,
  • Neither the pump nor the motor are showing any leaks, signs of rust or any other obvious sign of degradation at least on the outside.
  • The pump and motor have been running continuously (approx. 22 hours/day) without problems for 3.5 years. The pumps mechanical seal was only replaced 6 months ago and periodic maintenance is always kept up to date.
  • The pressure and flow rate on the line following the pump obviously drop every time the motor shuts down for that 1sec but there are no other changes during operation.
  • The water that is being pumped is particularly brackish (salty), but has been filtered multiple times so contains no particles or other additives.
  • I have a globe valve that I use to limit the flow rate by applying back pressure, which is of course is maintained at 200 kPa which is when the VFD comes into play.

I have no reward to offer for the person with the suggestion that leads to the cause, other than the pride of having the knowledge the engineering forum experiences for the solution and that you really helped out a fellow engineer.

Regards

RyanTW

 

 

Dear

Please check the suction line of the pump for any minor leakages.

Their may be leakages in suction line inside the water tank

You can also check the foot valve 

Regards 

Deepak Rohilla

 

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This is a fascinating, genuine issue. Inform us all the more regarding this VFD drive. Is a Pulse Width Modulated gadget sending variable recurrence AC to the engine? Assuming this is the case, that may propose a few conceivable outcomes in regards to your blackouts, yet I will hold up to get notification from you before getting into those matters.

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Ryan have in mind that the centrifugal pump will only consume more power (if nothing is wrong with it) by:

1) changing the fluid specs: specific gravity, viscosity

2)increasing de flow: if you increase it by 10% the power will increase also 10%. Consider the pump curve to check your actual flow using the reading of the pressure gauge at the discharche, dont forget to take out the inlet pressure. Closing the outlet valve will increase the pressure, decrease the flow but you will use less power.

3)increasing the RPM: if you double the RPMs the power will increase 8 times (yes 8!!!)

4) changing system conditions: decreasing suction head (pressure) the pump have to "work" more to acomplish the same results.

So if the pump is correctly working and you are not in one of the cases above, it will probably be an electrical problem. If you have a data sheet or something like that share it so I can check it. Which pump is it?

Hope you can fix it!

Regards

Pablo

Flowserve Pumps Service Engineer

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Mr. Ryan, 

I am facing similar issue at my location. In my case pump is in submerged in water and mounted vertically. We are planning for impeller trimming as pump is getting tripped on high amps. Can you suggest me possible solutions. 

 

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