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Replacement of Sea Water Carbon Steel elbows with plastic

2003

Hi,

We have a chronic leakage problem in carbon steel elbows; the fluid passing is sea water. We have an idea to replace the material of elbows with other but we are in doubt whether to use one of the following: GRP (Glass Reinforced Plastic) or RTRP (Reinforced Thermosetting Resin Plastic). Could you please explain why the carbon steel elbows have pits (holes)? Is it from erosion due to high velocity (velocity 3.7 m/s) or because the corrosion due to sea water quality the pH for sea water is neutral. AND which is better for replacement, GRP or RTRP?

Thanks & regards,

Sami ZOBEIR
- Yanbu Industrial City, Kingdom S.A.

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2003

Hi Sami,

Firstly (but none of my business!) I ask myself why in the heck are you using carbon steel for saline conditions?

Then you mention GRP and RTRP ... and I haven't a clue what the latter means. To me, they are both the SAME !

You don't give diameters ... so I'm assuming that you are using flanges ...but then from other inquiries from your neck of the woods I've noticed horrendously high operating pressures ... and that you didn't give, either.

l. No, your velocity is not excessive.
2. Presuming 'welded' joints, have you ever considered NOT using standard elbows (which have a lousy aspect ratio) and instead get long radius elbows made, say at a ratio of l:4 or l:6. 3. With a standard elbow you'd probably easily double the velocity on the outside of the bend !

PLASTICS ... i.e., my background

Fibreglass is good and strong ... and FRP elbows are not standard but 'made up' to suit the chemistry (i.e., the resin selection) and the pressure. They are ALWAYS glass reinforced hence GRP and RTRP are synonymous terms !

The other option is to use PVC elbows and fibreglass them. PVC being far more abrasion resistant than FRP. Better still, form the PVC into long radius and then fibreglass. This procedure is commonly known over here as DUAL LAMINATE construction ... and the world's first successful ones were made in Canada (Alcore Fabricating in l962). If you have any further questions, let me know.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).