PLASTIC FASTENERS TO BE USED IN DEEP SEA 1

[FastenerDude]

I have a customer looking for plastic fasteners that will hold up well in a deep sea environment but will also hold its load over a long period of time. Is there a plastic out there that would lend itself to these requirements?

 

[TVP]

The short answer is no. Plastics simply do not possess sufficient strength or resistance to creep/stress relaxation. Someone else will tell you that plastic fasteners exist, etc. However, this knowledgeable but totally anonymous person is telling you otherwise. Is there a compelling reason not to use proper metallic fasteners?

 

[FastenerDude]

Thank you for your quick response! The reason they do want to use metallic fasteners is b/c they are trying to avoid conductivity. He did tell me that if plastic fasteners were not feasible, he would resort to metallic fasteners (Inconel 625) and use an insulating medium to reduce conductivity.

 

[CoryPad]

If the insulating medium is a thick polymeric material clamped within the joint, then you have the same problem with stress relaxation.

 

[FastenerDude]

I suggested a Xylan Coating but I am not sure of how it will hold up in a deep sea environment. He didn't seem to be too keen on the xylan either.

 

[CoryPad]

Can you provide more information on the application? Is it permanently subsea (structure) or a vehicle that is periodically subsea? Does the joint need high clamping?

NORSOK M-001 Materials selection says this regarding coatings like Xylan:

For sub-sea installations the use of poly-tetra-fluoro-ethylene (PTFE) based coatings can be used provided electrical continuity is verified by measurements.

 

[FastenerDude]

The bolts are for a sensor. The sensor costs millions of dollars and cannot come in contact with conductive material or it will destroy it. The customer has proposed to use Inco 625 bolts with a Xylan Coating. The hole where the bolts will go will be coated with Delrin plastic. They will also use delrin washers on each end of the bolt and nut. Finally, there will be a ceramic spacer between the two metallic mating pieces that hold the sensor in place.

Greg
System 22, Inc.

 

[stevenal]

Have you considered shoulder washers? I would be wary about trusting a coating to do the job.

 

[OGMetEngr]

Xylan coatings alone on metallic products will not provide anything but a consistent torque to preload relationship. They are used for lubrication so fasteners will provide the correct preload regardless of additional thread lubrication or not. They are normally used for this as well as to prevent atmospheric corrosion prior to deployment.

The insulator sleeve coupled with ceramic washers (to prevent stress relaxation and loss of preload) sounds like a fine idea.

 

[OGMetEngr]

But without the plastic washers.

 

[btrueblood]

"The sensor costs millions of dollars and cannot come in contact with conductive material or it will destroy it."

Isn't seawater pretty conductive?

 

[OGMetEngr]

Not even close to the conductivity of a metallic material.

carbon steel conductivity = 6.99 x 10[sup]6[/sup] S/m

seawater conductivity - 4.8 S/m

(from wikipedia)