Advice on motor protection system 3

[Gyo]

Hi. I am a new starter with a company which manufactures water-filled motors. There was an incident on a 6kV motor 400kW 2960rpm motor driving a centrifugal pump. It was summarised that the impeller damaged to the extent of fully worn out. As a result, the rotor shaft became unstable and caused damage to the bearings.

There was only thermal trip and starting current protection system for that motor installed by the client. As a result of the lack of protection system, the client decided to

My opinion is that the damage to the bearing can be prevented by adding a simple overload current protection. Since worn out impeller causes higher motor current above the allowable limit, the overload trip can be activated to switch off the motor to prevent another set of failure. Thermal protection does not effectively protect the motor as there is a lag before the thermal limit is reached, therefore allowing the motor to run abnormally for an extended period of time until it tripped on thermal overloading.

What is your experience on this?

 

[wroggent]

To my knowledge a thermal trip and overload relay offer the same protection. If the pump vibrates badly before it fails, you might try using a vibration switch with a contact placed in series in the motor control circuit - at a certain level of vibration the motor will shut down. Or, you could use the contact to drive a signal that would indicate an imminent failure.

 

[controlsdude]

Dont you have head/discharge pressure high low limits on your pump that would of detected the impeller problem before it became a motor problem?

 

[CREED01]

Trending of the process, motor current, pump vibration etc. would have detected a problem before the failure.
Without it RTF is the default mode.

 

[electricpete]

As a result of the lack of protection system, the client decided to

You forgot to finish your sentence.

I agree with wroggent, thermal trip and overload trip are roughly interchangeable - itended to detect mechanical overloads. Often the overload trip setpoint is based on some margin above nameplate, if it is known that the machine oeprates lower then it may be possible to lower the overload trip setpoint.

In addition to other fluid system monitoring mentioned, for large machines it may be helpful to have continuous vibration monitoring to develop alarms or trips.

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(2B)+(2B)' ?

 

[electricpete]

And also bearing temperature may be appropriate... for example it's common on oil lubricated sleeve bearings.

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(2B)+(2B)' ?

 

[jraef]

Also, an impeller im-plies this was a centrifugal pump. A "worn out" impeller would NOT cause the motor to over load, it would pump LESS water, so the load on the motor is LESS, not more. You need some other way of detecting this situation as the others have mentioned.

"Will work for (the memory of) salami"

 

[rhatcher]

My experience would lead me to believe that the bearings failed first and that the impellor was then damaged due to contct with the pump casing wear ring. I'm not sure how overcurrent protection could detect this before it was too late. The suggestion of using vibration detection devices or bearing temperatue detectors is a good one but, I do not think that I have ever seen this on a submersible pump.

 

[LionelHutz]

I've never seen a worn out impeller pump more water either.

Overload protection and thermal protection are the same thing. Both work on a curve. A small overload has a long trip time and a large overload has a short trip time. I believe you're thinking an over current relay which has a definite time trip. You'd have to bypass it during the motor start or it will trip due to the high starting currents.

I agree with the others. Try pressure or flow or bearing temperature or vibration protection.

There are many people who think an overload or over current can protect against a failing bearing believing the motor current will rise significantly as the bearing starts to go bad. Well, I personally believe the bearing will be to the point of causing significant damage to the motor before monitoring current will tell you it's failed.

 

[waross]

Enough years for the impeller to wear out is enough years for the bearings to wear out.
Add me to the list of people who are skeptical about using current to detect failing bearings. Even when trouble shooting motors with failing bearings a current reading won't tell you much.
Consider that pumps seldom run at the full rated power of the motor.
Your 30 HP pump may only be producing 25 HP, 20 HP or less. Overload protection is sized for full output. Even if your bearing failure causes extra loading, it will be destroyed long before the overload protection trip point is reached.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Severe cavitation can destroy an impeller. Blocked flow? That would again mean that the load would be DECREASED and neither thermal or current based overload would have picked up the problem until there was further catastrophic failure.

"Will work for (the memory of) salami"

 

[Gyo]

Thanks all for sharing your thoughts. Rhatcher, I thought of the same that bearings should have failed than impeller. I believe there was more political than technical outcome to this, but I was tasked to keep "within the scope" as described above, and determine adequate protection system for this type of motor as our client wanted a protection system solution to this.

If the impeller wear ring becomes fully worn, I would imagine that the efficiency will drop, and pump will absorbed more power. This led me to believe that current should increase, and and if current is for example 10% higher than full load current than it should immediately trip - I guess I was mistaken. Like Lionel pointed out that it will cause ".. significant damage to the motor before monitoring current will tell you it's failed".

If vibration was monitored for the motor, does it trip when it detects high amplitude velocity?

 

[Gyo]

More current means more heat, but there would be a lag in temperature rise with the onset of higher current. My belief is that monitoring current is more effective than temperature.

Also, if the pump is operating outside of this intended duty, it would also tell that it is not within its normal operating mode.

 

[waross]

Read my lips;
When bad bearings cause enough increase in the current to be noticeable it is too late.
A trend that I am seeing is trending a combination vibration monitoring at each bearing and temperature monitoring AT EACH bearing. The transducers are computer monitored and the software alarms on sudden changes of temperature or vibration. These were skid mounted pumps in petroleum service. I am not sure how effective bearing temperature monitoring will be on a submersible motor.
One advantage of vibration monitoring is that it will also respond to impeller damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I agree with Bill. I clearly remember a medium-sized LV motor - about 45kW or so - with white-hot sparks being thrown from the end bell as it tore itself to bits. The overload relay didn't do anything at all to protect the motor and it was shut down remotely because no one was stupid enough to go near it as it shook itself apart. The pump it was driving was wrecked as well. At low percentage overloads an OLR takes 'forever' to respond - far longer than it takes for the small thermal mass of a bearing to reach incandescent temperatures.

 

[KllrWolf]

In a submersible motor, temperature monitoring is next to useless. The internal water masks hot spots and is good at slowing the response of RTDs and such to a point that when they detect something, the motor is scrap.

The best way to protect a submersible motor is by setting the current overloads based on the actual operating point and not the nameplate. If the name plate says FLA of 60A and the motor is running at 55A, base your overloads on the 55A. Also monitor the voltage, current, and pump flow for trends. A slowly increasing current and/or decreasing flow will indicate things are starting to get out of spec, and a time for repair can be scheduled before then entire thing gets destroyed.

The problem with the hydrodynamic bearings used in water filled motors is that if they lose their fluid wedge while in operation, it does not take long for them to be destroyed. The fluid wedge can be overcome by vibrations and such, as well as by the internal water heating up too high.

 

[ScottyUK]

Thanks Kllrwolf, water-filled machines are obviously quite different in their behaviour as well as in their construction compared to air-cooled machines.