Erossion in air to steam heat exchanger

[kulkarnidinesh]

We have this heat exchanger installed in our plant. There are two modules. In module-2 saturated steam is entering @200 Deg C and heating the air from 40 to 80 Deg C. Condensate from module-2 is given to module-1 which preheats the air entering in module-2 from ambient temp (30 Deg C) by 10 Deg C . Recently 1 tube in module-2 which is on module-1 side had punctured. When opened we found errosion in bottom part of tube. There is FT type steam trap in between steam & condensate module. Steam is entering from top & condensate coming out from bottom. this condensate is taken to inlet of module-1 which is at top & outlet from bottom. These radiators are having frequent failures and already replaced two for same reason. Please guide me what can be the reason.

[Astronuc]

More information is needed, particularly geometries, flow rates, and materials (particularly tube material).

Please describe the tube failure. I would expect it failed steam/water side.

It is possible that erosion-corrosion is the problem. Is the steam-water side closed or open? If closed, one might wish to employ a corrosion inhibitor, which is usually dependent on the material being protected.

[Q_Goest]

Hi kulk,
I'd agree you need to provide much more information. Pictures might help too.

I'd also suggest trying Engineering Tips forum here:
http://www.eng-tips.com/threadminder.cfm?pid=391

[Gokul43201]

Since the corrosion is at the bottom of the tube, at the downstream end of module 2, I'd suspect it's from hot condensate, rather than from the steam itself.

Info needed:

1. tube material,
2. nature of water source; any testing results of impurity levels in water used to make steam,
3. age of heat exchanger,
4. description of the corrosion (or pictures, if possible).

[kulkarnidinesh]

1) Geometry of Radiator :-
Radiator is fitted in air duct of 2m x 2m cross section. It has one tubesheet with header at top and one at bottom (top-supply & bottom-return). Tubes are finned and tube length is 2m x 2.03 thk

2) Tube Material is BS 3059
3) Flow rate - 150 m3/hr
4) Failure description
Tube has thinned in a bottom 1 feet portion. I don’t have photographs but you can refer to sketch attached herewith which will give you some idea. It clearly seems to be erosion but what could be the reasons for it to happen. Specially at bottom only. Erosion is also not allover tube inside surafce. It is a single line where it is hapening.

[ray b]

looks to me like the cooler air is causing water to form from the steam
at the first tube in the airstream there for the coolest air temps there
and as the water runs down the cooler side of the tube
it is wearing away the tube
so I supect erosion not corrosion esp as only a thin line is causing the tube going thin in one short spot

[Astronuc]

It may be erosion/corrosion, but likely erosion if it is on the ID. Any water drop (condensate) will accelerate with gravity in the flowing steam, although I would imagine some erosion of the elbows at bottom of the heat exhanger.

[kulkarnidinesh]

I have done the condensate test and following are the findings,
1) pH :- 5.59
2) TDS :- 6.0 mg/L
3) Chlorides (cl-) :- Nil
4) Electrical conductivity :- 4.77 umhos / cm
5) Turbidity :- 0.8 NTU
6) Iron (Fe) :- 0.0358 mg/L
7) Reactive Silica (SiO2) :- Nil
8) Total alkalinity (as mg CaCO3 / L) :- 4.378 mg/L
9) Phosphates (PO4---) :- 0.0408 mg/L
10) Sulphates (SO4--) :- 0.27 mg/L

Meanwhile there is one more tube which is punctured. I will get back to you once I see the nature and location of the failure.

Thanks.

[kulkarnidinesh]

Yesterday only we visited the plant and found that 3 more tubes are leaking. These leakage is just nearby existing tubes. Iam attaching sketch of the tube layout for your reference. These new leakages are all at a height of ~1 feet from top. Problem found is again erosion of tubes. This time I have taken photographs of the same which I will give you shortly.

This heatexchanger was installed in Aug 2006.
Failure in first tube was noticed on 8th July 2007
Failure in second tube was noticed on 4th Aug 2007
Third Failure was noticed on 26th Aug 2007

Presently we are cutting the eroded tubes and blocking the tube sheets on either side.

Meanwhile can anybody tell me what is wrong with this radiator ? why it is frequently failing.