Automatic Motor Restart in the Plant

[EmmanuelTop]

I have come across one interesting subject that I have never experienced before in my career, and I would like to hear your opinion on this topic, particularly from those people having a hands-on experience with such systems.

Someone came up with an idea to provide an Automatic Restart Function (ARF) of all electrically-driven motors in the plant (pumps, fin-fans, lube oil systems, hydraulic systems etc.) in cases when power supply grid voltage falls below the minimum value and causes electrical motors to trip - but only if the voltage dip is shorter than 4 seconds (reason for such timing is unknown to me) and the grid voltage is restored within that time. This software system operates completely independently from DCS and IPS (Instrumented Protective System), which is a big concern for me. Somehow, I didn't like the idea of automatic (or almost like uncontrolled) restart in the first place, and here are my queries/observations so far:

1. As completely independent from IPS/DCS, the Automatic Restart Function system has not been reviewed as such, through a dedicated expert study (at least not by Operations/Process) , and confirmed whether it will or can act against the IPS, or adversely affect its performance. This is my strongest argument against implementation of the ARF.

2. In case there is a permissive for equipment start/restart (and many pieces of equipment have permissive conditions embedded in the start-up logic), will those permissive be ignored by the Automatic Restart system? If yes, this is unacceptable from Safety and Reliability point of view. Tripping of equipment usually requires resetting/clearing/permissive action and if these will be ignored by the ARF, it is direct confrontation with equipment protection/plant safety. As a matter of fact, can the restart of any equipment from remote point be executed without clearing all permissive requirements?

3. In case of simultaneous power loss and – indirectly – the trip of hydraulically operated systems, are there any concerns with respect to interaction of fail-safe position of motor-operated or hydraulically operated valves, and the ARF function, assuming that the equipment will be restarted by the ARF and the valves remain in the position caused by voltage dip/power loss?

Do you have thoughts on this, and have you ever seen such system being functional in any of the existing plants, and what would be the prerequisites for its successful implementation?

Thank you in advance,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[JLSeagull]

For any automatic start is performed after a power interruption, consider start time-delays, strobes and warning signs near associated equipment to warn of automatic remote start.

 

[sshep]

Hey Emmanual,

I have never heard of such a system at any site I have worked at in over 25 years. Also, a 4 second dip sounds like an eternity- I am not an electrical engineer but most dips and spikes that I see investigated are quantified in milliseconds.

Having so much electrical equipment start at once seems crazy, so presumably it would be a staged process to bring stuff back on line- otherwise how many consecutive trips might be expected. Presumably you would set up a system and then justify the need and safety of adding each piece of equipment on a case by case basis.

Seems like putting the effort and capital into improving reliability of the existing electrical would be better. We have some automated dual electrical feeds and load shedding systems, but nothing that automatically restarts electrical equipment like pumps, etc.

good luck and best wishes,
sshep

 

[ash9144]

Auto restarts in this case are dangerous and will lead to much worse trouble than recovery from power outage. You will have no idea what condition your process is in and what else has failed due to the power outage or why your power even went out.

For example you could have a vapor compressor that ends up with liquid in the suction and your auto restart craters the machine or comes apart. Another common one is cooling tower fans if not equipped with reverse rotation preventer the blades could be turning backwards due to wind then auto restart rips the blades apart.

I've been in the industry 19 years and I have seen both of these failures, due to other reasons than auto restart but the auto restart could cause them as well.

 

[unclesyd]

We have what we call "automatic ride through" incorporated on the majority of our electrical motor systems. I don't know the exact mechanics of how this system works, but it does prevent shutdown of critical systems during a power blip. I know from personal experience that the location of ride through function in the switch gear is very critical especially on equipment with an "E Stop".
My particular experience was with a 1100 hp motor driving an all SS two stage screw compressor. After a newly reground set of rotors were installed in the second stage the machine was started from the field panel and after the motor came up to speed we were listening for any signs of contact of the rotor with the casing. You can hear any noise aside from a bunch of "bees" before the compressor will trip out for vibration. I detected a rubbing
and hit the E Stop and turned to walkaway and as the motor was coasting down it restarted around 550 rpm. I had to manually hold the E Stop down for several seconds to get the motor to stop, The investigation that followed revealed that the location of the ride through was located in the wrong place in the power supply at the substation.