What is the pressure/temperature of the boiler?

Is it residential, commercial or industrial?

You mentioned condensate - is this line an atmospheric drain after a trap, fed back into feed water pump, ............ ?

15-psig boiler in an commercial/institutional application. The line is between the feedwater pump and the boiler. Suction side of pump comes off condensate receiver tank (with domestic water make-up). There is no deaerator. Oxygen scavenger is fed to condensate tank.

I guess the appropriate question back to you is "What code/standard do you need to adhere/design to?"; because, yes, there is a standard that allows it, but is not recommended (ASHRAE) - but is it the correct one to follow?

Thanks for the information. My primary goal was to determine if there is a code that allows it. As you indicate, there is a least one standard that provides for it. So, when the mechanical contractor tells me (the corrosion consultant called in to determine why the pipe failed) that it was designed for threaded Schedule 40, I can take him at this word. I can see why it's not recommended and I question the judgment of the designer (or, perhaps, the person who might have value-engineered the design to save a few dollars). Not that Schedule 80 wouldn't have failed eventually, but not in such a short time.

I came across this problem too with welded Ms Schedule 40 condensate return sysyem and presented this which will highlight further.

Proper Condensate Treatment and Monitoring

Proper treatment of your plants steam condensate is essential for two reasons:

1. To protect the integrity of your valuable equipment.
2. To minimize the amount of condensate corrosion byproducts that are returned to the boiler feed water.

Corrosion occurs in steam condensate systems due to carbonic acid attack and oxygen pitting. Carbonic acid attack occurs due to CO₂, which is the breakdown product of carbonate alkalinity in the boiler, condensing with water to form H₂CO₃. This results in the "grooving" of condensate piping, which usually shows up first in leaks at threaded sections. Oxygen pitting occurs as steam condenses and the vacuum created pulls air into the system. Because of the localized nature of oxygen pitting it can cause relatively quick failure in a condensate system.

The most common method of addressing carbonic acid attack is through the use of neutralizing amines. These chemicals, such as morpholine and cyclohexylamine neutralize the carbon acid, and increase the pH of the condensate. Corrosion of mixed metallurgy condensate systems is minimized when the pH is maintained between 8.8 and 9.2. Due to high alkalinity in boiler feedwater elevating the pH to this level may not be economical. In this case the pH should be maintained at 8.3 or higher, or a filming amine applied. A filming amine, such as octyldecylamine, provides a non-wettable protective barrier against both carbonic acid and oxygen. When utilizing a filming amine, the pH is usually maintained between 6.5 and 7.5, so a neutralizing amine may still be required. Additionally, filming amines can be difficult to apply and generate fouling of the system, particularly in systems where they have not been applied previously.

In order to minimize oxygen pitting one can utilize a filming amine or a volatile oxygen scavenger such as DEHA (diethylhydroxyamine.) In the author's opinion utilizing DEHA provides better results as it scavenges oxygen and passivates the condensate system, making it less susceptible to corrosion.

Depending on the treatment method chosen, condensate monitoring can vary. In all cases the following tests should be performed.

1. Soluble and insoluble iron levels.

pH levels at various points in your steam condensate system. It is extremely important that pH measurements be made on cooled ("unflashed") samples. If the sample is taken hot, carbon dioxide will flash, which results in artificially high pH measurements.

1. Condensate corrosion coupons.

If a filming amine is utilized, the residual should be measured. The same is true if DEHA is used as an oxygen scavenger. In the latter case, a residual of 100 to 150 ppb is usually targeted. Note that this may take time (as much as 3 to 6 months) since much of the DEHA will be consumed passivating the system.

These are some general guidelines.

Ducon, Thanks. I recall seeing this post when I was searching to see if a similar question had already been posted. It's a helpful article.

Depending on the temperatures and pressures, schedule 40 pipe should last 100 years if the water is treated properly.

In theory, yes, but threaded Schedule 40 NPS 2 pipe isn't very forgiving of lapses in proper water treatment.

Sch 40 is adequate for the intended pressure. What type of thread sealant was used? If it is thread type, it becomes wet and aids corrossion. Change to plastic tape type sealants. Many good types are available. You can also change to welded joint.

I think welded or flanged joints would have been preferable, but it was up to the design engineer. Can't comment on the sealant other than it was olive green but might have started out blue (no tape was used). I should have been more specific in my original post: the pipe failed at the root of an exposed thread (not within the threaded joint itself).