Bolting material for piping on LPG barge 4

[MechAdam]

Dear all,

We are preparing material spec for pipelines mounted on small LPG barge.
Owner wants to use carbon steel pipes and flanges (pressure depend on cargo, max 18barg). Deck of barge can be exposed to seawater piplinees, so pipelines will be painted to avoid corrosion.
Because of seawater, i would like to use stainless steel studs with nuts: ASTM A320 B8M/ ASTM A193 Gr. 8M. I know that main players (SHI, HHI) in LPG carriers design and manufacturing using this grade of stud/nuts.

Have You got any remarks or comments?

Best regarda,
MA

 

[desertfox]

Hi MechAdam

You might end up with corrosion issues if you use stainless fixings with carbon steel flanges especially with moisture around.
Look at this link:-

http://www.roymech.co.uk/Useful_Tables/Corrosion/Cor_bi_met.html

desertfox

 

[ash9144]

Stainless has different expansion properties from carbon. So if the line is ever heated (steamed out) the stainless bolts will stretch and you will have a leak.

 

[MechAdam]

Thank You for prompt answer,


Desertfox:

I heard about some issue with corrosion in that type of connection. But i'm wondering if that painting agianst corrosion on piping components can be helpful and isolate materials.
Most of temperature and pressure indicators which i have seen (onshore, offshore) are made of SS and bolted to carbon steel flanges by SS bolts/nuts. For longlife service it could br subject to control. But is it really big issue?


Ash9144:
Design temperature is 0 - 50 deg C, Max ambient temperature is 40 deg C. Pipelines will be used to carry gas (liquid, vapour). So temperature would not be higher then mentioned above.
I do understand that for high temperature service differences in thermal expansion are very important, but for ambient it should not be a problem ?.

MA

 

[desertfox]

Hi MechAdam

Well paint should insulate it however are you sure the paint will be consistant in the flange hole.

desertfox

 

[SJones]

316 will not be very resistant to the marine atmosphere and could suffer pitting, crevice corrosion and stress corrosion cracking. Consideration also needs to be given to break out ease. Have you priced up fluoropolymer coated B7s for comparison? Such coating would be Fluorokote or Takecoat-1000 for example.

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

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

 

[desertfox]

Hi MechAdam

Look at this link on stainless 316 grade:-

http://www.azom.com/details.asp?articleid=2382#_Typical_Applications

desertfox

 

[CoryPad]

I concur with Steve - consider fluorocarbon coated steel fasteners. In addition to the two he mentioned, you could also consider Xylan.

 

[MechAdam]

Thanks for info.

Have You got any idea what is the difference in price between B8M and coated B7?

MA

 

[CoryPad]

Coated B7 will be much cheaper, probably 1/4 the cost of stainless steel B8. A lot depends on quantities and configurations, and if you purchase from manufacturers versus distributors.

 

[racookpe1978]

The USN has used regular steel fittings for many years in exposed situations: fully painted over everything in site though.

Most - but not all - do not corrode if NOT exposed to wear or rubbing. Those that ARE "touched" or bumped or exposed to contact from deck gear, chains, ropes, people's hands and tools, cables or power hookups, moving hatches, loads, cargo, hoses, people walking by .... etc, etc, etc, do immediately begin corroding. Obviously anything regularly bolted and unbolted MUST NOT be preserved by painting - you'll have to accept the cost penalty of chrome or stainless.

Yacht fittings and rope gear and deck cleats are chrome and brass and stainless for a reason!