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 Post subject: Developing Low Energy case for a Refinery
PostPosted: Tue May 26, 2015 10:16 am 

Joined: Sun Feb 01, 2015 12:55 am
Posts: 17
I am working on AF Studies for a refinery. The way they built and operate the refinery there is no practical low energy case that PI can determine a way to develop. The main Substation contains two 24MVA transformers operated in bus pairs. Obviously the worst case energy wise, is to parallel all four transformers. This however generally has little or no effect on the incident energy in the 480V systems, since the 480 is operated from transformer pairs with an open tie. The pair are generally 1000 kVA to 2500 kVA transformers.

I have tried several methods to reduce the incident energy to a minimum, hoping to get worst case studies from lower fault clearing. Since the major controlling factor for the IE is the size of the transformers at each substation, and the motor load on the substation. I have come to the conclusion that the "worst case" for these substations is a situation that is a real, but rare possibility. If the refinery loses all power and has to come back on line from a complete black start I suspect the IE at many of the buses would be low enough to create a problem, since there would be zero motor contribution at that time.

The question I am asking is: "Should the MCC's and buses be labeled such rare conditions,?" If you start labeling for this type of condition you may need labels for many rare conditions.

To suggest an answer to my own question, I feel I should be looking for the worst case during normal plant operation - not the worst conceivable case. The worst conceivable case is most likely a during a Black Start of the utility serving the plant. How far should we go?


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Tue May 26, 2015 11:39 am 
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Joined: Mon Apr 14, 2008 7:10 am
Posts: 142
My understanding is that you should consider the rare condition for arc flash hazard labeling. When it happens, it will cause damage or get people injured. If you don't want staff wear heavy PPE all the time, you may place two labels and indicate them at different running mode. Just my 2cents.


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Tue May 26, 2015 5:51 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 550
Location: Wisconsin
NFPA70E requires you to perform a risk analysis as well as determining the incident energy hazard.

We did a study for a data center. The owner decided that their policy would be no live work or even testing while on the backup generators. Labels were applied based on the worst 'non-generator' incident energy. the generator incident energy values were published in the report so they could be used for the live work permitting process.


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Mon Jun 01, 2015 8:24 am 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
Well I would say label for the worst case, no matter how "rare", and leave the other scenarios for the report tables. I would also say the use of "rare" here is incorrect. WHiule the conditions that get you to this worst case are "rare", this is ALWAYS the worst case. As a result there is no real need to label for other rare conditions.

I am not a fan of multiple AF labels on gear in any case.


"If the refinery loses all power and has to come back on line from a complete black start I suspect the IE at many of the buses would be low enough to create a problem, since there would be zero motor contribution at that time." I assume you meant "fault current" not IE here.


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Tue Jun 02, 2015 9:56 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
"Rare" is of course one of the issues.. If there is some way to limit this consideration then that's the way to do it or at least trigger things to where the arc flash study and not the label would have to be consulted. As an example I don't label for having both mains and the tie closed partly because it's interlocked to not allow this in the first place, but it makes the incident energy values ridiculous to look at this scenario. It also gets much higher if you look at a single ended scenario with the tie closed which is a more realistic possibility but again rare that this actually gets used except when doing maintenance.

Personally I have found it practically almost impossible to get below even 40 cal/cm^2 if you have a 480 V transformer over about 1500 kVA in the area directly off the transformer secondary. You can't possibly drive the primary side protection low enough in most cases. Best shot at it if you are going to attempt it is to go to a high speed current limiting fuse such as a Class L fuse. Install in an enclosure and coordinate it so that it would only trip on instantaneous fault within the protected line and equipment anyways. Don't bother with the disconnect handle...disconnect upstream to inspect/replace fuses (if ever). That is the best combination I've found so far that is reasonably priced. Breakers CAN do i in some instances but chances are the best you'll be able to get to is around 50-60 ms opening time based on 3 cycles for vacuum interrupters and DC relayi controls.

At medium voltage things are a lot easier and generally not really a major problem unless you have some inductive loads with some huge inductive backfeeding going on such as with large synchronous motors.


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Tue Jun 02, 2015 9:35 pm 

Joined: Sun Feb 01, 2015 12:55 am
Posts: 17
Thanks to everyone for your comments. i plan to take them to the client with your permissions. If you would rather I did not please post a reply.

To PaulEngr : My main concern at this plant is that the loss of the motor contribution will reduce the fault current sufficiently to produced delayed trips on 480V breakers, which could result in significantly higher Incident Energy. Of course I won't know for cetain until I run a study under those conditions.

To JKlessig: Your comments bring out one of my concerns. When a plant is trying to get restarted people are generally on their best safety behavior, but that is also a possible condition that could have equipment failures. Recplacign 480V buckets. inn MCC cubicles that are labeled in the 6 or 8 cal/cm2 could produce a terrible situation for someone that considered themselves as safe workers. If my study proves positive I am probably going to suggest a published work rule with a general moratorium on replacement of bad equipment under live conditions after a plant shutdown. I also don't like dual labels, because the one least used begins to get ignored entirely.

To Noah and JDB: Thanks for your comments as well.


To all: keep an eye on the subject. I will post the final plan and corporate comments, if permitted to do so.

It also occurs to me that installing the Cutler -Hammer switches where possible would insure an instantaneous trip and be be the best solutions for all situations.

Thanks,
Airjockey42


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 Post subject: Re: Developing Low Energy case for a Refinery
PostPosted: Wed Jun 03, 2015 8:01 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
airjockey42 wrote:
To PaulEngr : My main concern at this plant is that the loss of the motor contribution will reduce the fault current sufficiently to produced delayed trips on 480V breakers, which could result in significantly higher Incident Energy. Of course I won't know for cetain until I run a study under those conditions.


What I found mostly in multiple plants and sites is that typically plants did a short circuit study (because it's easy), usually ignoring conductors, and thus predict relatively high short circuit current. This is sufficient for the purposes of sizing the busses and interrupting devices. But then they take the SAME very conservative number and use it for short circuit protective device settings. Or even do an arc flash study with this value. BIG mistake. Frequently nothing will trip, motor contribution or not. I'm not finding very often that lack of motor contribution increases trip time significantly. Usually its the opposite problem...heavy inductive loads push assumed X/R ratios to the point where devices have to be derated or to where arc flash increases significantly even with fast tripping due to the assymetrical fault current contribution.


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