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Coupling failure with no motor trip 6

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EEENGRX

EEENGRX

Electrical
Sep 11, 2021
89
A 75 Hp centrifugal pump in operation suddenly lost its coupling plus the coupling guard. The bolts were dislodged and the motor kept spinning but did not trip. All motor control set points appeared normal. Can anyone explain why there was no trip when this occurred as the motor had to be turned off via a local STOP button. Thanks
 
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Loss of coupling bolts means motor runs on no-load aka open shaft = no cause to trip. A pressure/flow sensor on the pump side is the answer for such events.

Muthu
 
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Thanks for the response. Do you mean a flow sensor with a set point/ run status to trip whenever such occurs?
 
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That's quite a serious incident and quite rare. Think you're better off seeing why the coupling failed than working out some strange trip function.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Dear Mr EEENGRX (Electrical)(OP)30 Jan 23 05:18
"... A 75 Hp centrifugal pump in operation suddenly lost its coupling plus the coupling guard. The bolts were dislodged and the motor kept spinning but did not trip. All motor control set points appeared normal. Can anyone explain why there was no trip when this occurred ;;;;"
1. The basic electrical protection are current base on (a) short-circuit and (b) over-load.
In your case the motor is running free (no load), the current is < than the rated current. There is NO over-load or short-circuit. Both of these protection do NOT come into operation.
2. There are other protections based on mechanical detection you may have to consider.
3. Attention: Your case is very serious !. More frequent inspection is required.
Che Kuan Yau (Singapore)

 
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Thank you very much guys. I’m grateful for all the insights
 
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Why did the coupling fail? Was there sudden jamming of pump impeller that caused this??
Does the motor protection include protection against Locked rotor (Stall during Run) protection??
 
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Any idea why the coupling failed? Pump couplings don't normally just "break".

As there's no particular issue with the motor running no load, there is not normally a trip for this rare malfunction. That's what local stop buttons are there for.



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

It's shear pin coupling to limit the peak torque to a safe value protecting the motor and the pump against sudden torque demands for whatever reason.

Severe misalignment.

Poor quality bolts.

Weak coupling.

You could mount a non-contact speed sensor on the pump coupling to monitor speed and use it to trip the motor.



Muthu
 
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... bolts work loose, due to misalignment or vibration, or maybe they were never torqued properly. Or maybe the bearing in the pump is starting to go bad.

... and probably nobody has looked at it for years and there's no monitoring of bearing temperature or vibration, both of which are possible, although still no substitute for someone walking around looking and listening once in a while.
 
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As many said we don't expect a motor to trip on loss of load. Unlike say a steam turbine that needs to be tripped to prevent overspeed on loss of load, an induction motor doesn't need any protection from loss of load.

in terms of reason for the coupling failure, that is the more unusual / interesting question. if you can describe the coupling, provide photos of the failure and any other detials, that would be interesting to see.

=====================================
(2B)+(2B)' ?
 
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You can now very easily get a Solid State Overload Relay for your motor starter that can be programmed to detect a load loss. What you do is to measure the no-load current, then set the trip threshold for the Load Loss trip at a value slightly higher than that. So let's say your 75HP motor has a FLA of 104A, which is where you program the Overload Trip. Then with the motor uncoupled, lets say that the motor current is 40A and coupled, but with the lowest permissible flow, it is 50A. You set the Load Loss trip to 45A, so if there is no flow or the coupling breaks and the current drops to 40A, the relay trips.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Underload protection! And performed solid state rather than using the ol' electromechanical motor current hold-in coil, no less. Nice!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
 
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Yes that’s what I thought too. Under voltage protection should be able to detect load loss and initiate a trip. I would have to check the trip settings to confirm
 
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All of this talk about sensing a loss of load to trip the motor but at the point the load is lost the damage is already done, the coupling is broken, the parts have been ejected. There isn't a whole lot of benefit to having an additional trip point. OP said that an operator was able to safely stop the motor from the local control after the failure.
 
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@OP - What Jeff said was undercurrent tip, not undervoltage trip which would do no good in your coupling failure.

tugboat - An automatic trip is far safer than a manual local trip given this disaster. All kinds of process monitors and controllers are used to trip a motor automatically for any abnormality.

Muthu
 
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Apologies I meant to say UC. For solid state protection devices. I was able to pull out the logic settings and found UC set to 0 which automatically disables the function for trip wrt UC. Thanks
 
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Dear Mr EEENGRX (Electrical)(OP)31 Jan 23 01:02
"....Yes that’s what I thought too. Under voltage protection should be able to detect load loss and initiate a trip. I would have to check the trip settings to confirm...."
1. I wish to draw your attention that what we are talking about is to install an additional under-current protection relay, NOT under-voltage protection. Over-current protection relay/thermal over-load, which are always fitted. The under-current protection relay. is usually NOT provided. Caution: do not be confused !
2. Besides the electrical detection, another option is by mechanical speed switch/relay. This comes in various forms. There are simple forms that are not too expensive and with easy installation. The working principle is if the starter is on the load shall come up to speed within a certain time delay. If the speed failed to come up speed or drops in speed while the stater is on after some time delay, the relay issues a shut-down command. Note: The load will drop in speed to zero rpm, with the coupling halves decoupled.
Che Kuan Yau (Singapore)
 
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Load loss trip also protects against other situations in which the desired work is not being performed. In the case of a centrifugal load such as a pump, it could also be used to detect a flow blockage or worn out impeller. Sure, this can be done with a flow monitor, but adding one after the fact is often more difficult than just replacing an OL relay with one that is capable of this. In the OPs case, it appears this was already there but not being used!


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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We used to monitor actual power used by our pumps, and we set high and low trip points based on power (as well as current).
With a centrifugal power tells you a lot more about what the pump is actually doing.
Coupling failure can be catastrophic to motor bearings, pump bearings and seals, and anything else in the area.
You need some expert forensic help with this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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