flanges leaking during sail-away? 2

[mort78]

We're about to sail away with a converted FPSO, and as part of the pre-start-up checks we need to check a sample of flanges to determine if vessel motion has caused leaks.

Are there flanges that are particularly prone to leaks in terms of type, pressure rating, material, location (length up/down-stream, proximity to bends), fittings, etc?

Thanks,
M

 

[BigInch]

The ones with the smaller diameters, lower pressure ratings (and not uncoincidently, the fewest number of bolts) are usually the most problematic.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

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[TGS4]

NPS 8", Class 150 are the worst flanges for leakage. Check those first.

 

[mort78]

Thanks, hopefully I'll be able to say that you were both wrong come April

 

[SJones]

Would it not be a case of 'if they were all assembled and tightened correctly in the first place then there shouldn't be any preference for leaks'?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

No, I think because load on a 150# 8" flange from bending could easily be the same as that on a 900# 8" flange from bending. Stress is more or less directly proportional to pipe wall x-sect area for tension and compression which would probably show some direct relationship, but from bending load it is mostly a function of pipe diameter to the 4th power, so you shouldn't expect a direct relationship to pressure class for bending resistance. I also think transport produces higher average bending stresses and stress reversals in bending than you'd get from tension and compression.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[jmw]

On the off-chance use heavy fuel oil (most offshore vessels seem to use gas oil because its free... that is the oil companies usually supply the fuel to the support vessels) the fuel lines may leak during shut down.

This is possibly because from running on HFO at 140-150degC, shut down involves switching to gas oil at ambient. Not everything cools at the same rate.... so not related to vessel movement.

JMW

http://www.ViscoAnalyser.com

 

[weab]

Check ALL of your FRP piping joints if you have any. That stuff leaks like a sieve.

 

[TGS4]

BigInch - the 8" Class 150 flange as the worst bolting available of any flange. It should have 4 more bolts, but it doesn't. The Class 900 8" flange has more (and bigger) bolts, and is better able to handle external bending moments - even bending moments proportional to the effective cross-sectional area of the attached pipe. I've done the math - the Class 150 8" pipe is the worst for leakage.

 

[BigInch]

Yes I totally agree. I might have been writing a bit cryptically, but that's what I meant to say. The lesser # class can have about the same bending moment stress as the higher classes flanges, due to the relatively similar Second Moment of Inertias of the attached piping, but as you say, they have less bolting points, which can really increase the bolt tension when the bending moment is in the right (or better said, wrong) plane. The flange thickness is also thinner, so more distortion of the flange would be expected for the same bending moment. Both could add up to a lot of distortion and separation.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[unclesyd]

You forgot the add the 3" Class 150 flanges. It has the same characteristics as the 8" Class 150.

Our internal piping specifications essentially prohibits the use of 3" and 8" class 150 flanges.

 

[TGS4]

unclesyd - that's right. The 3" Class 150 flange is only a 4-bolt flange. I think that most internal piping specs prohibit the use of 4-bolt flanges. And if they don't, they should.