Need explanation of SS Tees in Flare Header 4

[djregier]

Hello,

I am working on a chemical plant which has a main flare header of 14" diameter made of A333 Gr. 6 pipe. We are planning on installing a new 10" A333 Gr. 6 branch into this flare header. In the previous design all of the 10" lines that tie into the 14" header use Stainless Steel tees to connect the 10" line to the 14" line.

Can anyone explain why you would make the tee that connects the 10" to the 14" out of stainless steel and make the rest out of carbon steel. Temperature of flare gas going into the 14" branch is approximately 8 deg. F

Thank you for any help that you can give.

djregier

 

[tickle]

Availability.

 

[SJones]

The stainless would be more erosion resistant than carbon steel so maybe somebody had the idea of making them quasi target tees? Having said that, I'm lining up with tickle!

Steve Jones
Materials & Corrosion Engineer

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

 

[JLSeagull]

It is common for a branch that auto-refrigerates downstream of a blowdown valve (if calculated below -20 degree F) to run stainless for a short distance to a flare header connection. This may not fit djregier's situation.

 

[SNORGY]

Could be any number of nebulous but not necessarily bad reasons. What kind of stainless are they (austenitic or martensitic or other CRA)? Besides the tees, are the elbows carbon steel or stainless steel?

The first thought that comes to mind is that someone might have been thinking that tees and fittings have a higher predisposition towards pitting corrosion than seamless pipe, so maybe they wanted a higher PREN number in the tees. I am not sure about the increased erosion resistance of a stainless tee relative to carbon steel, especially if it's austenitic and, if so, likely softer than the pipe, but someone more knowledgeable than I in metallurgy would be in a better position to judge that.

I probably wouldn't have thought to do this, so the "availability" explanation seems as plausible as any.

Regards,

SNORGY.

 

[djregier]

Thank you very much for all of your posts. Maybe it is an availability issue. Maybe someone couldn't find a 10 x 14 tee in carbon still. As it is an emergency blowdown header, I don't think corrosion or erosion is the issue, and no the elbows are not made of stainless.

I am going to look and see what the availability of 10 x 14 tees are.

Thanks again for all of your help.

Daniel

 

[SJones]

SNORGY - sorry, I meant 'erosion-corrosion'. Just to clarify. Your premise about hardness and erosion is correct.

Steve Jones
Materials & Corrosion Engineer

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

 

[SNORGY]

SJones:

Thanks, but upon further reflection, I might not be completely right about erosion resistance being expected to increase with hardness. Perhaps it depends to some degree on the nature of the material and the geometry of attack of the erosion.

We did some work recently on frac sand separation for starting up gas wells. A client was experiencing extremely fast erosion rates in well production piping and asked if there was some theory that could be applied towards predicting the corresponding expected reduction in service life. I did some analysis and concluded that, in general, the solids loading profile (in terms of solid particle size distribution) followed a sigmoid (S-curve) function. For a typical particle size analysis, as you increased the solids fraction in the gas stream for a given particle size - especially after correcting for stream void fraction - it became clear that the kinetic energy carried by the particles began to exceed that carried by the "gas" by a multiplying factor of about 4 to 5 at 10% solids by volume. We had some data in the form of a research paper on the corresponding accelerated wear rate for a particular elbow made of a particular alloy, and at least in "order of magnitude" terms my findings were in the ball park.

I took it a bit further and attempted to rationalize things in terms of material fracture toughness and the angle of particle impact. I concluded that there are two modes of abrasion resistance: resistance to scratching and resistance to impact. Accordingly, I concluded that for a 90-degree elbow, probably the latter effect would dominate. I have not been able to put numbers to it yet, but if I hypothesize that abrasion resistance generally increases with toughness (loosely correlated to the area under the stress-strain curve to rupture at UTS), a material might have "toughness" by having either more ductility at a given yield / UTS, or higher hardness corresponding to higher yield with shorter elongation to UTS. If that is true, then resistance to impact abrasion might be better afforded by higher ductility (probably corresponding to "softer") material, whereas resistance to scratching / gouging abrasion might be better afforded by a less ductile (probably corresponding to "harder") material. In very poor and imprecise engineering terms, highly ductile material might be expected to sustain millions of little plastic "dings" and deformations without having material fragments start to break free from the surface, although it might "wear" more rapidly at more acute angles of particle attack. Conversely, harder materials might be expected to behave in the reverse manner to that.

If one believes that, as I do to a certain extent, then erosion resistance of a tee with inflow from the branch might indeed be higher for a softer material than it is for a harder material. Regardless, the resistance to erosion in terms of service life comes down to some function of the rate of kinetic energy transferred as the solids (and gas) are decelerated against the material, and can be expressed in units of power. That makes sense, since the more powerful the "sandblaster", the faster you can "blast stuff".

So, I might not be completely right at all.

I think, too, that the analogies I drwa reflect the "cowboy engineering" side of me.

Regards,

SNORGY.

 

[SNORGY]

drwa

Yep...some cowboy I'd make if I can't even get *that* word right.

Regards,

SNORGY.

 

[tickle]

Snorgy

Below is a reference for an interesting paper developing further the erosion velocity in API RP 14E.

NACE Paper 58, Corrosion 98 – Erosion in multiphase production of oil and gas by Ken Jordan.

Daniel - the availability is most likely not the same as the availability at the time of fabrication.

Tickle

 

[SJones]

Well, there's hard and there's brittle. One will govern abrasive wear behaviour and the other will govern impact wear behaviour. Of course, we would have to throw work hardening into the equation since particle impact would peen the surface (if it wasn't wearing it away!).

Let's stick with 'availability' eh?

Steve Jones
Materials & Corrosion Engineer

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

 

[djregier]

I can't think of a better explanation than availability in this instance. Thanks again for everyones help.

Daniel