On site we have a triple pressure HRSG, HP/IP/LP with re-heater, 125/28/3 bar, max flow 300 tn/hr in HP, flue gas vertical flow, with exhaust gas from a 9FA GT, multi shaft arrangement. After almost 54000 fired hours we have measured reduced thickness in HP ECO tube bends, not in all of 4 modules but in the 3rd and 4th one, mainly in the middle, nominal thickness was 4.2 and measured was found 3.2-3.3mm. We proceeded with tube bends replacement but as you understand this is a high cost activity taking at least 3 weeks. My question is if with your experience, you found similar tube thickness reduction issues, what could be the causes and the corrective actions? The tube material is SA210 AL (carbon steel), operating pressure 155-175 bar, temperature inlet at 145oC and outlet at 310-320 oC.

Please advise.

 

From your description of the location of the erosion, the most likely cause of the loss of material would be sulfur in the fuel producing SOx in the exhaust gas. You did not say what the fuel is, but if it is liquid it almost certainly has some sulfur in it and if it is gas it may have some sulfur in it. Is there temperature control on the economizer? If so, set it so the gas temperature at the cold end of the economizer stays well above the condensation temperature of sulfuric acid. I don't remember what that temperature is, but I am sure you can find it on line. I do know that on the HRSG controls that I used to design, we had a temperature control valve that bypassed some or all of the water flow through the economizer depending on the sulfur content of the fuel. When on liquid fuel, it was usually set to bypass the economizer completely. This obviously reduces the thermal efficiency of the system, but usually that is a lower cost impact than replacing tubes in the economizer.

 

Thank you for the reply, the fuel is natural gas. there is no temperature control in HP Economizer. we never operate in liquid fuel, apart from early commissioning days, 23 hours in total of 55000 fired hours.

 

Hi Dear,

I had same experience in three HRC boilers connected with Alstom Gas Turbines, and it was very costly. We confirmed the cause for loss of thickness in economizer tubes is from water quality.

After that we decided to add in our laboratory result the Iron
(Fe)content on daily basis and put more controls on chemicals added to the boiler water and monitor the results which should be below certain limit depends on your tube specification.

My advice: Boiler chemical MUST be monitored closely. another point I like to address the effect of bad boiler water can not be seen immediately and its mount up after certain period.

Good Luck
AM

 

solidus1975,
It is possible that at some time during your 55000 fired hours there was some sulfur in your gas supply. I don't know how well the gas composition is controlled or how often it is analyzed. I know that for gas turbines we were supplying for some Spanish customers we were providing on-line gas analyzers and adjusting the HP Economizer temperature control based on sulfur content. I don't remember the exact algorithm we used - I retired over 7 years ago.

I would recommend having an analysis done on the economizer tubes to see if the source of the erosion can be determined, or at least which side of the tubes it occurred on, in light of the reply by ahdash who experienced water side erosion.

If the erosion is on the gas side (the tubes would have a reduced outside diameter rather than an enlarged inside diameter), then I would strongly suspect the source is sulfur in the fuel.