Dear all,

We have 2 Nuovo Pignone MS5001 gas turbines installed in our site for the past 10 years. Normally, those turbines are running on gas fuel. Occasionally (once every 3 months or so) they will be switched to liquid fuel firing due to the shortfall in natural gas supply.

Since installation, we had a problem with liquid fuel firing because the tubing connected to the liquid fuel check valves before it goes into the combustion liner always leak. In the past, teflon tape was used to seal the connections. However, recently we understood that teflon tape is not suitable for high temperature applications such as this. We changed teflon tape to loctite. It ends up that the leaking still persists. 6 out of 10 tubing assemblies leaked and the worse leak is 1 drop every 5 secs which is pretty bad.

We have tried to take out all of them to tighten and to perform pressure tests before. However, this problem still cannot be resolved. I would like to know whether anyone else has experienced similar problems before and how should we work to resolve this issue. Thank you very much.

Best Regards,
Dawei

 

Dawei,

This is not really a controls-related problem.

You haven't really told us if the leaking is occurring at the compression fittings (Swagelok or Parker or similar) or where the body of the compression fitting is threaded into the check valve or fuel nozzle, usually with a NPT pipe-thread.

Teflon tape isn't probably the best sealant for high temperature applications, but it's not going to dissolve the stainless steel at high temperature, either! If you've had good luck with Teflon tape over the years, and you don't use too much of it when assembling the components, it's probably not going to be a problem. There are high-temperature thread sealants which are available which can be used for the pipe thread portions of the compression fittings.

But, it should not be necessary to use any type of thread sealant on the compression fitting nuts if they are properly assembled and tightened.

The most common problem with leaking fuel tubing compression fittings is improper tightening techniques. Properly tightening the compression fittings requires two (2) wrenches, one to turn the "nut" of the fitting and one to hold the "body" of the fitting stationary. For some reason, this is an <b>EXTREMELY</b> difficult concept for many people to understand. One <b>can't</b> just use a single wrench on the nut and expect the fitting to be properly tightened.

There are actually hex flats on the body of the compression fitting just below the nut, just for using a second wrench (sometimes called "backing wrench") to hold the body of the fitting stationary while tightening the nut of the fitting.

I can't tell you the number of times I've seen <b>alleged</b> pipe-fitters use two wrenches to disassemble tubing and fittings, but absolutely refuse to use two wrenches to assemble the same tubing and fittings. That's just insane logic. And it's insanity trying to convince them otherwise, because it just falls on deaf ears.

Most of the tubing fittings are stainless steel, and stainless-on-stainless can very easily gall threads. Also, it's very easy to cross-thread the nuts when trying to install them if the fitting and tubing aren't properly aligned or spaced.

I don't know what damage using Teflon tape can cause other than making the fittings difficult to disassemble, or, if too much Teflon tape is used then it could melt and create a void that could cause a leaky fitting. You might have to use a wire brush to clean the threads after disassembly (internal and external) but I don't think Teflon tape will cause any failures of stainless steel fittings, regardless of temperature. It shouldn't be necessary to use any thread sealant on any compression fitting nut (the one that compresses the ferrule onto the tubing; but it won't hurt anything, either, as long as it's a very light amount of sealant.)

Many times, because tubing fittings are usually compression fittings (meaning that a ferrule has to be compressed onto the tubing) if the ferrule isn't properly aligned on the tubing it will not properly compress and form a seal.

Lastly, there is a limit to the number of times a compression fitting can be reused. The typical rule of thumb is that when re-assembling compression fittings that the nut be hand-tightened as much as possible (assuming properly alignment of tubing and fitting, that means the nut will be nearly fully tightened) and then the two-wrench method be used to tighten the nut to the fitting by turning approximately half of a flat (1/12th of a turn) or so. Doing this repeatedly over years will eventually cause the nut and ferrule to be incapable of being tightened any further.

Oh, and there's one more thing that's absolutely <b>IMPOSSIBLE</b> to get people to understand about the little knurled "rings" used in many compression fittings on GE-design heavy duty gas turbines. Those rings (they are actually spacers) are only there for the <b>INITIAL TIGHTENING</b> of the fitting, and they must be removed and <b>THROWN AWAY</b> on the first disassembly!

If the ring is reinstalled on subsequent installations, then the ferrule cannot be compressed with the 1/12th turn on re-tightening!!! But, everyone insists that the rings must <b>ALWAYS</b> be re-used. (Unless the spacer rings are lost, and then it's okay not to re-use them! I don't get the logic!!!)

Those knurled spacer rings are only there to ensure that the factory personnel tighten the fitting to ensure proper compression of the ferrule onto the tubing during the initial assembly of the fitting. Nothing more and nothing less! And only on the first tightening during the initial assembly; after that, they are useless and actually cause more problems! Just try to buy those knurled, spacer rings from Swagelok or Parker; most of the local suppliers won't even know what you're asking for! They're typically only sold to factories for initial assembly.

So, make sure the knurled spacer rings are not being re-used<b>!!!!!!</b>

Make sure that the tubing and fittings are properly aligned during re-assembly<b>!!!!!!</b>

Use two wrenches during re-assembly<b>!!!!!!</b>

Properly tighten the fittings during re-assembly.

Examine every fitting as it ages to ensure it's not being used past it's usable compression.

Don't over-use thread sealant on any threaded fitting, compression or pipe-thread.

 

Dear CSA,

Thanks a lot for your prompt reply. You are such a helpful and knowledgeable person. I will definitely keep you posted about the result after our next GT shut down in about 2 weeks' time!

Best Regards,
Dawei

 

Hi Dawei,

No hard-sell here, but as a control measure, many sites are using 'SAFETY SHIELDS' as a control measure to prevent hazardous spray-outs from leaking pipe joints.

These shields won't solve your problem, as they do not contain the leak - but they WILL prevent consequential loss, as a result of fire/explosion etc.

Our shields are multi-layered teflon or st/st, made to exactly fit specific pipe joints.

During general web surfing, I stumbled upon this thread so sorry if this reply appears inappropriate. More inf www.allison.co.uk

Hope this helps

 

Hi,

I have some question regarding the compression fitting. Normally some of the fitting have front ferrule, back ferrule and B- nut.. If the back ferrule is missing during installation:

- what will happened, is it the fitting will be leaked?

- is it the pipe can be pulled out if there's only left front ferrule and B-nut which can cause major leak?

QuF..