Steam Trap Failure 2

[mjpetrag]

I have a 3" 50# condensate return line that has about a 10 ft vertical drop and a 2" reduction, a 90 over to a 2" Y-strainer and a steam trap all along a 3' section of horizontal pipe in the middle, and a 90 back up.

The problem here is that the steam trap keeps eroding away, along with the pipe on the horizontal run (vertical runs are always OK). The pipe is schedule 80 carbon steel and was paper thin the entire ID after about 1 year of service. It is an ongoing problem every year. Most recently the pipe upstream of the trap failed at the threads and the Y-strainer at the 10:00 position. The steam trap has been ultrasonic tested and is working properly.

We brought the vendors in and they seem to think it is chemical attack from aggressive condensate. I have a hard time believing that (see chemical composition below) so I wanted to listen to your ideas. It seems like an erosion issue even though the fluid velocity is low (Assuming 100% condensate). Possibly something to do with 2 phase flow?


Process conditions are 50# condensate
10-17 kpph flow (2-4 ft/s)
<25 ppb O2
pH 9.0
conductivity 9.6 mmhos
Iron 0.09 ppm
Total Hardness 0.0

Here's a link to the trap

http://literature.puertoricosupplier.com/003/CU2496.pdf

-Mike

 

[steamdog]

No standard answer without a lot of troublshooting. Your own answer is probably mine - 2-phase flow. You are getting flashing that is using up space in your piping, causing the water phase to flow much faster than you think. Use a bigger size pipe next time and see what happens

 

[Compositepro]

From your description it sounds like the erosion is occurring down stream of the strainer which is before the trap. Perhaps the the strainer is actually catching something and getting restricted.

 

[ione]

I tend to concur with steamdog.

Is the problem located downstream the trap? If so you could have a flashing induced erosion issue. As you have argued when dealing with a 2-phase flow, your line, even if correctly sized for 1-phase pure condensate flow, could become inadequate to handle vapour presence. As liquid flashes it undergoes a fast increase in volume, which leads to an increased fluid velocity. At this point accelerated liquid phase acts as an eroding medium.

If the line is made of CS evaluate to upgrade it to SS (if affordable).

 

[mjpetrag]

The problem is located upstream of the trap, and downstream of the strainer (from what I'm told historically). Possibly flashing across the y-strainer? The entire inside of the trap was also eroded out.

-Mike

 

[chief]

Put a collector pot in front of the trap with inlet at bottom and discharge at top. This will assist in separating the 2-phases and reduce the condensate velocity. A simple baffle layer arrangement in the pot will assist in separation.

Offshore Engineering&Design

 

[ione]

Is the strainer installed properly?

From the Spirax-Sarco site

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/strainers.asp

“Y-type strainers in horizontal steam or gas lines should be installed so that the pocket is in the horizontal plane (Figure 12.4.2(a)). This stops water collecting in the pocket, helping to prevent water droplets being carried over, which can cause erosion and affect heat transfer processes”.

 

[mjpetrag]

Ione, I think you might have found the problem. The y-strainer is installed with the pocket vertically down. The question I have is since this is a condensate return line before a trap, there's 2-phase flow.

Would you think this would negatively affect the piping from this statement:

"On liquid systems however, the pocket should point vertically downwards (Figure 12.4.2(b)), this ensures that the removed debris is not drawn back into the upstream pipework during low flow conditions."

-Mike

 

[stanweld]

Since this has been occurring regularly over time, why have you not considered changing metallurgy in the affected location.

 

[mjpetrag]

We are missing something else if we can't use carbon steel on a condensate return line. It was always just a like for like replacement and wasn't flagged as a reliability issue until recently. Metallurgy change is a band-aid on the underlying problem. Most likely the next time this fails we'll replace with stainless, but I wanted to understand the specific failure mechanism as a justification for material upgrade.

-Mike

 

[ione]

Mike,
Strainers are fitted upstream items such as valves or steam traps to remove debris and prevent clogging and consequent malfunctions. When you deal with pure liquid lines (i.e. water line) you have 1-phase flow and so you are not only allowed to install the pocket vertically downwards, but this is a recommended configuration as debris will remain trapped there with changed flow conditions. When you deal with steam line it is more than likely that you have to deal with 2-phase flow. In this case putting the pocket vertically downwards is the basis for water collection and consequent droplets carry over with erosion related issue on items placed downstream the strainer.
Likewise configuration which foresees pocket installed in the horizontal plane also reduces risks of waterhammer.

 

[ione]

I agree that a material upgrade without at least an idea of what mechanism has caused the failure is not he most appropriate approach, anyway it’s not that unusual to have SS condensate line because of corrosion issues related to carbonic acid. Schedule 80 represents the min wall thickness option for CS pipe, when SS is not economical affordable.

 

[mjpetrag]

Good stuff ione. I'll try to get this strainer rotated and post how it goes.

Thank you all for the replies.

-Mike

 

[stanweld]

If your corrosion mechanism is 2-phase FAC, changing metallurgy from carbon steel to 1 1/4 Cr- 1/2 Mo alloy will solve your problem. I agree with ione that you need to define the cause of 2-phase flow and rectify, if possible, the flow conditions.

 

[Frank1344]

Folks,

This might be off the topic but,

I just want to make sure if everybody is aware of the following useful website by Spirax-Sarco.

http://www.snapfour.com/

 

[mjpetrag]

Thanks Frank, very useful resource here.

-Mike