ASME I Preheater Fitting

[loilfan]

Hello Eng-Tips,
I am having issue with an alteration to a Sec I Preheater currently. We need to bypass the last few rows because dew point corrosion from the flue gas has corroded them through wall. The preheater tubes are finned NPS 1, horizontal, and serpentine back and forth from the inlet header to the outlet header.

The contractor has indicated that welding ASME B16 socket or butt welded fittings would be impossible due to the accessibility. We are looking at using Swagelok fittings rated for the pressure and temperature, but they are only valid for B31.1/B31.3 applications.

The fittings will be installed behind a baffle plate and not in direct path of the flue gas, so from an engineering point of view, they are satisfactory. However, we are unable to get them approved by the Regulator since they are not manufactured for this purpose and do not meet Code.

Does anyone know of any other fitting options that could work?

loilfan

 

[metengr]

What exactly are you proposing? To abandon the existing air preheater coils by severing the tubes and welding caps to the header stub tubes? Why not valve out the entire air preheater coils like most utilities/power plants have done over the years and replace the tubes or re-design it?

More information is needed for proper help.

 

[loilfan]

Hi metengr

The pre heater is for the boiler feed water, not the air. This is a waste heat HRSG and is the last set of tubes in the flue gas path. Replacing the entire section would not be feasible since we would need to disassemble that portion of the boiler And would cost sgsignificantly more than the lost productivity of a few rows of tubes.

 

[EdStainless]

You can't cut and cap and just force all of the flow through the remaining ones?
I can see that the flow velocity or PD could be too high.
How about cut and cap, and then weld in a bypass line to handle the excess flow?

Without the preheat will you still be able to reach rated outlet temp?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[loilfan]

EdStainless, bypassing the last few rows of the preheater works to our advantage because it stops the dew point corrosion at the end of the boiler. We won't need to change the flow.

Just to clarify the arrangement, the rows that we are bypassing are the first couple of coils that leave from the inlet header. We are not bypassing a row on one side of the boiler or the other, we are bypassing the rows that run perpendicular to the flue gas flow. Making a bypass to compensate for the flow is not necessary.

We also aren't going to bother capping or removing those bypassed rows.