YELOW COATING ON HX TUBES - WHAT IS IT? 1

[BRT549]

I'm having a shell & tube heat exchanger retubed, and was inspecting the unit while the shop tore it open. There was a thin yellow coating (about 2'x2'section) on the tubes in front of the condensate discharge, along with the same coating on the inside of the shell in the same area. It looks like elemental sulfur, but that seems impossible.

We generate steam as a byproduct of burning elemental sulfur, but that shouldn't be able to traverse the H2SO4 plant, the steam plant, and the steam piping and come to rest in this particular location. Could it be a boiler water treatment chemical? The tubes are hastelloy G-30, the shell is carbon steel.

 

[Dpearcey]

Grab a sample and have a lab analyze. They can tell you what it is. If the temperature is right at this location then sulphur could be condensing out. The melting point of sulphur is somewhere around 300F.

Dpearcey

 

[BRT549]

DPearcey, your post put all the pieces of the puzzle together. Here's what's happening:

The sulfur pit is the only direct contact area with the steam system. Steam and liquid sulfur are not particularly reactive, and steam is used to put out sulfur pit fires. Sulfur is being siphoned into the steam condensate system in the pit through a small leak on the inside of an elbow. It makes it through the condensate system and bypasses the boiler since condensate is injected for desuperheat. Once in the heat exchanger, it plates out at the condensate discharge since the condensate temperature drops down there to the freezing point of sulfur.

Pretty wicked scenario, huh?

 

[NickelMet]

BRT549:

Interesting scenario. But I have a side issue question for you since you seem to be dealing with phos acid. I'm pretty new to the metallurgy field in the petroleum industry, so please pardon any immediate ignorance.

The situation is I have an exchanger with a low-level concentration of phos acid (mainly benzene & cumene at >90%). The stream is on the tube side and experiences temperatures around 400-deg F (not much drop). The bundles also see this as a 2-phase region (making it even nastier). I inherited really poor life bundles at <3yrs old with 316L & 317L steels. We're upgrading, but I can't find a good reference of what to use for the metallurgy.

Any suggestions?

Thanks

 

[BRT549]

For the range of phos acid concentration 28% to 54% P2O5 (roughly 40 - 70% H3PO4), there are some standouts:
VDM Alloy 31
Haynes Hastelloy G-30
and a step down in cost and performance:
Sandvik Sanicro 28
Carpenter 20Cb-3 (Alloy 20)
Allegheny Jessup 700

These are pretty good up to about 220 F. Seamless tubing is preferred over welded, but be prepared for a price increase. 317L is our standard piping material, but in the heat exchangers we have to upgrade to higher nickel and moly alloys.

Getting up to 400 F and 85% H3PO4, Inconel 625 is excellent.

A rough guide for phos acid resistance is an alloy's Pitting Resistance Equivalent (PRE). While impurities like chloride and fluorine can skew things a bit, it does give a pretty good indication of how it will hold up.

316 PRE of 25
317 31
20Cb-3 30
Jessup 700 36
Sanicro 28 39
G-30 48
Alloy 31 54
625 51

A reference for phos acid corrosion resistance is &quot;Phosphates & Phosphoric Acid&quot;, Pierre Becker, 1989, DEKKER publishing
It has a chapter on corrosion, but it is mainly directed at wet process phos acid production for fertilizers.

 

[NickelMet]

BRT549:

Thank you for the information. It will be most helpful as I spec out these new bundles.

NM