Ignition problems on a MW-701DA

[ScottyUK]

Looking for a few ideas:

We have a fleet of eight Mitsubishi MW-701DA turbines. The 701 is essentially a 50Hz variant of the Westinghouse 501.

One of our units suffered a baseload trip while running on distillate in the past week. We are having tremendous problems igniting the machine on natural gas where normally we would expect a first time start. The turbine uses dual igniters in adjacent combuster cans at the approximate 5 o'clock position. Igniters are known to be working. Fuel valves are known to be working. The engine is reaching the normal cranking speed of 650rpm.

We are able to detect gas flow into the engine by measuring the DP developed between the fuel manifold and the combuster shell. The DP is about 14kPa which is the typical lightoff value for this engine design. We can't verify fuel flow to individual cans, so a blocked or damaged nozzle would not be detected by this method. We've tried a slightly lean and slightly rich gas mix relative to normal conditions with no success.

I'm inclined toward thinking we have suffered combuster damage as a result of the trip, but am being fought by others who would like to avoid an internal inspection. I think we have checked everything which is likely to affect ignition - is there anything else I have forgotten to check? Can anyone familiar with the internal design of these machines - or the 501 - offer any other ideas, or either lend support to or discount my theory of internal damage to the combuster?

Thanks.




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I don't suffer from insanity. I enjoy it...

 

[rmw]

In that the igniter cans are at are located low in the 5 o'clock position, is there any possibility that while running on oil there was a back-flow or leakage from some/any nozzles into the gas or igniter gas piping so that there is a liquid leg in the gas piping for the igniter (lower) cans?

Can you break the gas piping to the igniter cans and remove/inspect and/or remove the nozzles from these cans to make sure that they are not plugged/defective?

If there was such a liquid leg, normal ignition gas pressure probably wouldn't be sufficient to push it out into the combustion chamber to clear the blockage.

You did determine that the igniters are working and not oil fouled?

By the way, L'bro is cold and dreary. Otherwise, everything is going well.

rmw

 

[ScottyUK]

Hi rmw,

We've seen the liguid-in-the-manifold problem previously, although it was a result of a bad purge steam isolation valve. You're right, it is tricky to dislodge once it is in there. We have found the simplest way is to use fuel gas, at a pressure somewhat higher than normal ignition, to eject it into the combuster. This is followed by a very long HRSG purge before any ignition attempt is made. The ignition system needs to be isolated while this is going on! This has been tried a couple of times without any improvement on this engine.

The igniters were pulled out and inspected. One was dirty and was replaced with a new item which was verified functional before mounting.

Our machines are triple fuel capable: nozzle removal is a nightmare because the distillate pipework is fully welded. I agree that it is probably the best way forward, but the amount of work involved is causing the management to try to find any other possible cause before major surgery takes place.

How is the Jarvis Hotel BTW? L'bro is dreary in the summer too, just not so dark. Be grateful - the darkness is sparing you from the true horror! Hope you get a chance to see some of the nicer bits of England too before you go.


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I don't suffer from insanity. I enjoy it...

 

[rmw]

Scotty,

Have you gotten the machine lit yet?

The liquid in the manifold, short of a dirty or non functional ignition device, or plugged crossover tubes (my experience was more with GE machines where the igniters were in certain cans, and the flame detectors were in cans on the other side away from the igniters so as to prove that all the cans in between had flame if the flame detectors saw fire.)

The Jarvis is interesting. It isn't as bad as "grim" a word you used earlier. I think there must have been some renovation recently. Parts of it are quite nice. I had forgotten how small European hotel rooms are. I feel like I am living in a match box. But, it is functional and comfortable.

The lobby has wireless internet, so what else would one want.

I watched someone vs Ipswich yesterday, and it proved that the sun does shine somewhere in the UK. They had a beautiful day. Too bad Liverpool didn't have a beautiful day. It wasn't the same Liverpool I watched beat AC Milan last year. Chelsey seemed to have come to play.

Hope you get your igniter problem fixed.

rmw

 

[ScottyUK]

Yes! We have the engine lit! We found an undocumented contactor in the igniter power circuit which had a handful of loose terminals, in addition to the problems we picked up late last week with water in the HV terminal box and a damaged igniter cable. We're unconvinced that the bad contactor was the sole problem because that contactor was definitely allowing power to the igniters, albeit perhaps intermittently. I personally witnessed the igniter test because I have such a trusting nature, and they were operational at that time.

I believe it still took a couple of attempts to get the machine to light, so I'm still suspicious that things are not entirely as they should be with this engine. However, the engine temperatures are fairly normal and we haven't got any peculiarly cool or hot cans so the combusters themselves must be more-or-less ok. I half wonder if the cross-flame tubes are damaged. As long as the damned thing doesn't trip between now and the April outage we should be ok!

The arrangement for flame detection on the '701 used to be exactly how you describe, but we've got an officially-sanctioned mod to use a combination of bladepath temperature rise and shaft acceleration to detect ignition instead of relying on the UV scanners. The scanners were suffering a high mortality rate, and the opportunity arose to get rid of them. The new system is proving very reliable.

Glad to know the Jarvis has improved in case I have to stay there: during my last visit the improvements I had in mind would have involved a taxi to the Hilton at the airport for me, and a bulldozer for the Jarvis.

Good to know you are enjoying the football. Chelsea have such deep pockets that it is hard for anyone to compete. They can simply outbid any other team if they want a specific player. Manchester United still have an excellent coaching structure, but they can't compete head to head with Chelsea in the financial sense. That may be the only nice comment I ever publicly record about Man U, so take note! Liverpool of today are a ghost of the team I remember while I was growing up in the seventies and early eighties, but still doing better than my team Newcastle.


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I don't suffer from insanity. I enjoy it...