Power plant explosion in CT 3

[ScottyUK]

Hi Keith,

My intent wasn't to stifle intelligent discussion, especially as a few folks here are in the general area of this plant and will likely pick up some information, but PLEASE don't put much faith in anything you read or hear in the media.

I mentioned an accident where I used to work. That accident killed three of our friends, colleagues and critically injured a fourth. In the hours after the accident the media interviewed some brainless moron who stated that it was a gas cyclinder explosion, presumably because that was all he could think of which might go bang, and straight after that the stupid reporter authoritavely blathered some complete crap about the explosion having taken place inside the turbine hall. The plant didn't have a turbine hall, and it was a big transformer which blew up. Anyone who didn't know the plant would have listened to this report and taken it at face value. I threw something - the remote control, I think - at the TV in rage and walked out.

Don't trust the opinion of anyone in the media to be anything other than convincing lies. Look at the photographs, listen to the people qualified to comment, and judge for yourselves.

/rant


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If we learn from our mistakes I'm getting a great education!

 

[lz5pl]

ScottyUK, sometimes not only reporters, but even qualified people can talk nonsense. I remember a case from first years of my career as commissioning engineer. It was in middle of 80's, when I worked as commissioning engineer in specialized commissioning company. We made rehabilitation of 110 kV switchyard of power plant. As part of the work we energized power plant auxiliary services transformer - 110/6,3kV, 16 MVA. Transformer was not new, so we did only maintenance tests on it, as well as change of relay protections. Transformer was put in operation around 11 p.m. and we went home. In the morning at 6,30 we met with a colleague in a company bus on the way back to the plant and he told me that in the night transformer exploded with no one protection operated ! I was really surprised, as I personally did setting and testing of all relays and everything was OK. We came to the plant, ran in control room and asked operators from daily shift what happened with the transformer. They said "Nothing, they energized it during the night shift and it is on-load now". I came to the control board, saw ammeter showing about 50% of full load and still didn't know whom to believe.
What have been the actual situation? After we left the plant around 1 p.m. operator from night shift looked on remote temperature indicator and found that it shows 105 degree Celsius. Overtemperature protection have to trip, so he went down to the transformer and found local indicator showing about 60 degree (it was in January, very cold like it is now!). So he decided to call National Dispatching Center and report them that discrepancy. They say that probably it is an instrumental mistake, but to be on safe side ordered him to check hourly what is the situation. After that dispatcher in NDC phoned home (3 a.m. !) the CEO of our holding company and told him that transformer we commissioned is overheating and nothing operates. CEO immediately phoned General Manager of our company and told him that transformer exploded and protections didn't trip. Our GM phoned my colleague and ... you know the rest. I have to add that after 2-3 checks during the night operators understood that it is really instrumental mistake and forgot to say to colleagues from daily shift about the case. Of course something have been written in the logbook, but who wants to read about the solved problems of previous shift!
Most funny point in that stupid chain was that it was not our fault. We were external contractors and did our job perfectly. Remote temperature indication was obligation of Control Instruments Team of the power plant. These guys simply replaced the indicator without going in troubles to calibrate it together with the sensor!

Well, after we calm down we found the story quite funny, but these 30 minutes we traveled in the morning to the plant were very, very long!

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

Tangent: I suppose that the temperature transducers were Ni100 and that the new instrument was for Pt100. Had exactly that several years ago. The VBCC (again) checked the 6 kV grid and found lots of problems - unbalanced, phase shifts, around 20 percent harmonics.

I found that they had measured phase-gnd and also that the transducers were Ni100 and not Pt100. The interesting thing is that the VBCC saw no problem with 20 % voltage distortion. Neither with severe phase shift and voltage unbalance. BTW: VBCC reported directly to the production manager - not to electrical department.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Yes Scotty. Good point there. (media)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rmw]

My post agreeing with Scotty was in reference to the media reports. While the true cause is still conjecture, we who 'speak the language' can engage in a discussion of such conjecture without actually thinking it is more than what it is. To Scotty's point, who knows what "expert" at the plant gave the meida the buzz words that they did use in that report. The blackened surfaces on that one HRSG and the blown out building panels probably say a louder message to us than the words in a news account by suzie que reporterette.

rmw

 

[PHovnanian]

Well, I'm heading over to Slashdot to discuss this. Where we formulate our conclusions without even having to read TFA. ;-)

 

[ishvaaag]

To be scientific ... like my brother says, again, you are talking of technicalities!

And unfortunately, many, many times he says so, it is well brought.

 

[rcw retired EE]

First- My condolences to the family and friends of the deceased and injured.

A former co-worker who is an experienced startup engineer was working on commissioning this plant. (Our company was not involved.) Here's some information from a personal conversation he had with a friend in my office. (It's second hand information; may be inaccurate.)

Gas line blows were in progress, scheduled for Sunday morning when minimal personnel were on site. Lines were vented outside to a safe area. The first blow using pipeline gas was done without problems, the line secured and the coupon target removed for inspection. (That’s the tattletale metal target at end of line that collects evidence of any dirt or debris in the pipe.) The construction workers were told to standby and not start the next blow until the startup engineer returned. He inspected the coupon in the office and went into the control room. Then the explosion occurred. He regained consciousness lying outside the building uninjured but covered with debris.

He thinks the crew may have started the second blow without checking the valve line up and speculates that valves leading to pipes in the building may have been opened.

There’s probably some memory loss from his concussion and I’m sure he is still very distraught at the deaths that occurred.

Another reminder to pay attention to our designs and our instructions to others on how to implement them.

 

[itsmoked]

Is there normally anything out at the ends of the pipes that route the blow outside? Like blowers to help disperse the gas or are they just pipes/hoses that end?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[blackseaman]

what is purge cycle ? i cant find anything...

 

[Skogsgurra]

That is when you get rid of air (or oxygen) in a system before you put explosive prone gas into it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GTstartup]

For Keith,

No, normally the gas just vents to atmosphere.

Even during normal operation of a gas turbine, when the unit trips, or shuts down, the length of pipe between the main gas shut off valve and combustor is just vented to atmosphere with a motor operated vent valve. The vent pipe is normally quite high but the exact height depends on various factors

 

[rcw retired EE]

The main purpose of gas blows is cleaning the gas piping after initial assembly is complete. It is common to use the pipeline gas as a cleaning medium although compressed air might be used until the final purge.

Temporary piping is added to the ends of the pipes to route the discharge outside and up to a safe elevation. Hoses have too much pressure drop and may not handle the mechanical forces. The discharge piping and temporary supports should be designed to allow the maximum flow with minimal pressure drop and no pipe movement as the gas slams into bends. High velocity is needed to properly clean the pipe and any restriction slows the flow. It's like an impulse test on a transformer. You want a fast, steep wave front.

A silencer/muffler is placed on the outlet to reduce the exhaust noise, mounting that can be a problem.

The process is a little more complicated than it sounds. Complications also mean there are more places to go wrong.

I may have some of the details/reasoning wrong. I’ve usually left the site when gas blows are in progress.

 

[itsmoked]

Thanks GT and rc for the info. Most interesting all this!

I ignorantly always thought that you just plumbed everything up and turned on the 'main' gas valve. The burner controls go thru their purge cycle venting the combustion chamber and stack, then open the controlled gas valves for a start attempt. Maybe the first few start attempts fail because of air in the lines not supporting combustion. Eventually the the gas shows up and ignition occurs and you're off to the races.

It seems like the purges here are for 'pipe cleaning'.

I can also envision that in a large pipe, say 6" or bigger, that during the start attempts as you run out blocks of air and some gas that eventually you might have an explosive mixture in the plumbing near the burner. Could the burner flame backfire into the gas line? Is this a different reason to purge?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[GTstartup]

Pipe cleaning by blowing gas is a one time only (normally) event during first commissioning.

After the unit is commissioned and in operation there is a different type of purge each time the unit is started.

When the GT is not operating (standstill) the length of pipe between the trip valve (last valve before combustor) and the main shutoff valve is vented by a block and bleed system.

During startup there is a purge about 800-1500 RPM depending on the GT type. This is achieved by using the SFC to turn the generator as a motor. After a predermined time (I believe 5 HRSG air changes required by NFPA - about 5 minutes) the sequence continues and the vent valve is closed and the main valve opened. Gas is sent to the trip valve immediately prior to the combustor and when that valve opens the ignition system is turned on. This is normally a propane system with spark plug. Once this ignition system has started there is a perdetermined allowed time for main flame (about 10 secs). If the flame detectors do not detect flame within this time, the unit trips automatically and the whole cycle must be reinitiated.

During shut down the gas shut off valve is closed, the trip valve is closed and the vent valve vents the air in the pipe between the shut off valve and the trip valve to atmosphere

For those GTs in simple cycle (no HRSG) there is no purge required as long as the stack is immediately after the GT exhaust (no long duct section).

 

[GTstartup]

To address your question about combustible mixtures between combustor and main valve, this is normally taken care of by check valves and by the normal venting that occurs when a GT coasts down and the block and bleed system.

I did have a strange experience once though when some water injection check valves were installed backwards and gas filled up the top 20% of a 500,000 gallon water tank. I was told that one of the reasons there wasn't an explosion was that the top of the tank was 100% gas.

 

[itsmoked]

Youch!


Thanks GT.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rcw retired EE]

An experienced startup engineer corrected me: gas purge is only used on outdoor systems where a gas leak is not as dangerous.

Our procedure on any indoor piping is to clean with high pressure air (1000 psi), reassemble piping, leak test under pressure, purge with nitrogen, then displace nitrogen with gas just prior to first firing of the equipment.

 

[rmw]

GTstartup,

So, how did you get the gas out of the tank. I think some of the hairiest moments I have ever spent were after a boiler flame-out when the fuel didn't trip immediately filling the entire boiler, back passes and all with gas. Not a problem at that point. Pure gas isn't explosive.

BUT, you ultimately have to purge it and when the purge starts and the air begins to displace and mix with the gas, when (if) it goes stoichiometric, kaboom. Not always, but potentially always.

Those were moments when certain body parts puckered the worst.

rmw

 

[rmw]

And, that last post wasn't intended to take the thread off topic, but to amplify on the problem of gas/air mixtures especially in the presence of any potential ignition source, even if it is nothing more than static electricity.

rmw