Boiler tube Analysis

[Champion09]

Can somebody volunteer information on the best practices that should be adopted for the testing of High Pressure high temprature Steam boiler ( ca 170 bar 515 C )tubes for scale accumulation & tube thickness verification.

Jimmi

 

[metengr]

The technique is called tube ID oxide thickness testing and determination of remaining tube life. There are various boiler OEM's and other 3rd party vendors that provide this service. The requirements are to grit blast clean a band on the OD surface of the tube. Ultrasonic testing is performed from the OD surface to measure the tube wall thickness and oxide thickness on the ID surface. This information is feed into a program to determine remaining service life per tube.

As far as best practices, you need to determine test locations within the boiler pendant or platens. Usually, we test every other pendant or platen and several tubes deep, from the lead tube. What is most important is after this testing is completed you should remove several tube samples for lab analysis to compare tube wall thickness, ID oxide thickness and the condition of the tube material microstructure.

 

[Guest102023]

I think you mean inspection rather than analysis. Search the EPRI website. Here are two I found in my library (sorry too large to post here):

EPRI 1007347 (2002) Proc. Int'l Conf. on Boiler Tube Failures & HRSG Tube Failures & Inspections

EPRI CS-3272 (1983) Failures and Inspections of Fossil-Fired Boiler Tubes; 1983 Conf. & Workshop

Try websites from some of the bigger inspection companies.

 

[Champion09]

thanks metenger, for the info on method of testing, ok this is the detail for life assessment exercise. In case of transient phase off spec operation in water quality should the opertor go for tube ispection & testing case is a new boiler, off spec of water chemistry 12 hrs and water quality 10% off spec?

 

[metengr]

Ok, you are referring to waterwall tube evaluation versus steam tube evaluation. In this case, the concern will be deposition of solids and the potential for under deposit corrosion.

You do not mention what limits are out of specification. The duration of 12 hours is significant and can result in deposits and subsequent corrosion. As a minimum, I would try to snake a borescope into one or more waterwall tubes for inspection. If deposits are found, you will need to remove a tube sample and evaluate the amount and extent of the deposits. Under deposit corrosion can be serious and result in hydrogen damage and or caustic corrosion.

 

[Champion09]

what is the concept of hydrogen corrosion in boiler tubes?

 

[metengr]

Underdeposit corrosion results in a local hot spot because of lower heat transfer. Increased temperature means corrosion and the formation of hydrogen atoms that diffuse into the tube wall. The hydrogen atoms combine with carbon in the steel causing methane gas that forms along grain boundaries. The voids get severe enough to result in a window breakout of tube material(aka plug corrosion). This is why deposits are to be taken seriously in waterwall components.

 

[puigi]

Champion:

What do you mean by the term "off spec"? Was there actual contamination of the feed-water with scale forming minerals, or were the residual treatment levels in the boiler-water out of the desired control range by 10%?
Do your operators test for either turbidity, total dissolved solids or dissolved iron as part of their normal testing routine. If so compare the results during the "off-spec" period to results when condition are within range.
If you suspect that scale deposits may have formed during this event; a remote visual internal inspection of the water wall tubes at the soonest availability would be a good place to start your assessment.

 

[Champion09]

puigi

The off spec condition occured due to marginal transient deviation in chloride contamination in a high pressure boilers feeding to a back pressure turbine. The low pressure steam is utlised for MSF desalination.The steam condenser leaked which resulted in slight chloride contamination up from a maximum of0.1 ppm ( allowable) to .11 ppm.All other standard test values were under control.

What is the remote visual internal ispection of boiler tubes you are referring? How you do the inspection on LIVE BOILER?

 

[puigi]

Champion:

The RVI would be during the next outage, that is why I stated soonest available opportunity. High boiler water chloride levels increase the propensity for carry-over, not scale formation; choride ions are inversely soluble. Typically at that low of a chloride level, carry-over is not a concern but without knowing the specifics of your steam generator that would be impossible for me to determine.

Do you have a in-line sodium analyzer used to monitor the steam enterning and or leaving the super-heater. A spike in this reading would be indicative of carry-over. Carry-over is more of a concern for the super-heater tubes, especially if they are made of one of the austenitic steel alloys.

 

[rmw]

If any of your heat exchangers in the feedwater train are 300 series SS, chlorides would be a concern there too.

rmw