Corrosion in High-Temp Hot Water Boiler System, specifically makeup water piping 1

[GetAtMeWolf]

Hi, I'm looking to better understand the degradation modes in a system that I'm currently analyzing.

[ul]
[li]This is a (High Temp Hot Water / ASME IV Boiler) HTHW system with multiple boilers that operate in a parallel configuration between a boiler inlet header and a boiler outlet header. The system operates around 350psig with inlet temperatures of 300F and boiler outlet temps around 350F. Water makeup is softened, ion-exchanged water. The issue lies in the fact that corrosion inhibitor and pH control is injected into the boiler outlet stream and not into the makeup water. Makeup water in injected on an as-needed basis, based upon level control of an adjacent expansion tank. The expansion tank is connected in a direct line to the outlet header with makeup water pumps injecting straight into this line. So when makeup water pumps kick on, the water flows primarily to the expansion tank and then slowly drains into the overall system. This is a 100-200Gallons/day makeup therefore it is definitely just a very low flow.[/li]
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[ul]
[li]What we're seeing is repeated cracking which develops in the makeup water piping. I know it sounds like a simple fix, but hear me out here. We've had two pieces sent for failure analysis which both confirmed "corrosion fatigue". This is kind of a chicken and the egg answer. There's both corrosion and fatigue. We don't have a definitive on if the system has cracks initiating, which are then attacked, filling with corrosion product, further propagating them. Or if there is some form of corrosion that is then causing cracking to form. What we do know is that we're only seeing this in the weld metal and the HAZ, but not in base metals.[/li]
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[ul]
[li]I'm just looking for some discussion on what I believe is happening, which is as follows...The makeup water system is generally unprotected from a corrosion standpoint. The high-temperature operation of the system will allow the steel to naturally form a passive magnetite layer, which does provide protection to the system. When makeup water pumps kick on however, we believe that the thermal shock of cold water entering the system is causing cracking and breakdown of the magnetite layer. This is causing fresh metal to be exposed which is then forming micro-pits. These micro pits are then propagated by corrosion, causing cracking to propagate.[/li]
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Does this sound like a reasonable explanation or is there something that I'm missing here. Obviously you all don't have all the information so I'll answer any questions as best as I can.

 

[mrfailure]

By definition, any fatigue you have on the ID of water piping will be corrosion fatigue because of the aqueous environment. You really want to know if it is driven because of the applied cyclic stresses or the environment. I would suggest looking at the characteristics of the cracks and pipe to help with this regard.

Are your cracks linear and (i.e. transgranular) ands sharp tipped? These may be indicators of fatigue driving the mechanism. Do you have multiple parallel cracks or just one? Multiple cracks can be an indicator of thermally-driven fatigue. On the other hand, do you see evidence of pitting or deposits on the pipe ID? Do the cracks meander as if intergranular or start at pits or show evidence of oxide or deposits in the cracks? These suggest looking at corrosion as the likely source.

Good luck!

 

[Dhurjati Sen]

The expansion tank should have an inlet steam distributor that increases the temperature of the make up water before it gets into the overall system. My experience says that in most cases it is not working or damaged which leads to a temperature difference and hence fatigue failure.

DHURJATI SEN
Kolkata, India

 

[TugboatEng]

Steam heating the feedwater also dearates the water.

 

[MJCronin]

DHURJATI SEN and Tugboat touch on the problem, IMHO ...Improper makeup water configuration and lack of deaeration

Once again, eng-tips only gets a tiny peek at the problems ..... Sir, what is your overall system configuration and what, chemicals specifically, are you using for raw water treatment ? ... How can you tell if they are working ?

How, for example, do you know how much disolved oxygen is in your system ?.. and how often do you perform blowdowns


It sounds likes you have an older boiler system that expanded over the years from, perhaps a single boiler to many ? ... and while you can get by with a single boiler scheme and your half-assed injection scheme .. your final "multi-boiler" system perhaps "grew" into the need for a proper deaerator, non-condensible ejection and regular (not-ad-hoc !) chemical additions !

Your owner should be taking carefull notice because boiler erosion/corrosion is common, real and can cost him tremendous amounts of money in both new capital equipment and excess chemical usage. .... But owners only care when things either blow-up or becone very costly

I recommend that you engage the services of a boiler consultant for a certified system review and a plan for the future ... Many many firms offer these services !!

I also recommend that you keep your friends on eng-tips of your situation and your eventual fix

Best Regards and Good luck

MJCronin
Sr. Process Engineer

 

[shvet]

https://a.co/d/a4EqBsm

https://a.co/d/88w4Kdi

https://www.epri.com/research/products/1021199

https://www.csb.gov/loy-lange-box-company-pressure-vessel-explosion-/

http://mmengineering.com/wp-content/uploads/2015/07/Conduit-Vol-15-No-2.pdf

 

[EdStainless]

Deaeration is where I start, O2 and CO2 will raise havoc with your piping system.
The thermal cycling isn't helping, get better pre-heat.
You could use direct steam injection.
The inlet temp should be much closer to boiler temp.
You are using boiler capacity to heat water.
Boilers are meant to turn hot what into steam.
Since the crack are in HAZ tell us what alloys have been used?
Any PWHT? (guessing no)
You have some work to do.

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P.E. Metallurgy, consulting work welcomed