Fire Tube Boiler Tube Leaks

To give any comment on this we require lot of parameters.

Please share your contact details sp that we can talk and get parameters

I am the facility manager for a US govt project in Panama. In our boiler room we have 3 CB 250 4 pass fire tube boilers. We have a maintenance contract with the local CB rep, and we do water treatment. Makeup water is softened to zero ppm hardness. The deaeration system needs some work (the auto pressure regulator doesn't work and the steam pressure and temperature is maintained by opening a manual block valve). Given all this we shouldn't have suffered double failure on two different machines, unless something else is afoot. Maybe my question should have been: what could cause tube failure at the flaired ends (first pass) in a fire tube dry back boiler?

hughes838@yahoo.com

(the auto pressure regulator doesn't work and the steam pressure and temperature is maintained by opening a manual block valve).

Two things jump out at me, pressure and thermal shock. If you have instrumentation on your boiler and trend the data to a chart recorder or SCADA, I would look for temperature and pressure spikes, both positive and negative. Human operators are notorious for doing things the wrong way and breaking things, then lie about what they did.

I suspect that your operators are letting too much air/gases to build up in the boiler then they vent too fast causing the boiler to expand and contract destructively. Boilers do best when ran in a steady state with slow changes.

If you do not have pressure and temperature for each boiler being automaticly recorded I would stronly suggest that you include it as part of your repair of the boiler.

i think you have to check the refractory brick in the reer end

may be it causes thermal expansion makes the leak

Corrosion occurs due to carbonic acid attack and oxygen pitting. Carbonic acid attack occurs due to CO₂, which is the breakdown product of carbonate alkalinity in the boiler, condensing with water to form H₂CO₃. This results in the "grooving" of condensate piping, which usually shows up first in leaks at threaded sections. Oxygen pitting occurs as steam condenses and the vacuum created pulls air into the system. Because of the localized nature of oxygen pitting it can cause relatively quick failure in boiler tubes and condensate piping system.

The most common method of addressing carbonic acid attack is through the use of neutralizing amines. These chemicals, such as morpholine and cyclohexylamine neutralize the carbon acid, and increase the pH of the condensate

This is also known as Ferric Oxide Attack caused this. This is a mixture of Oxygen + Water (CO2) and obvious due to presence of oxygen because:

1. Feed water temperature to be 212F where the oxygen content is 0.

At your lower feed water temperature of 160F the oxygen content was 3.75 cc/litre.

2 . deaerator not at par.

This damage has been due to your own negligence as not maintaining a higher feed water temperature at 212F as recommended by all Boiler manufacturers and neglecting the De-aerator.

Check also the ph of your condensate return for not being acidic.This must be maintained at 8.3 or higher.

Reading your para I am able to figure out that your pipelines both steam & condensate have aged and the return is corrosive.

Regretfully, the damage inside the Boiler is more than that you've already discovered only with the tubes leaking. Other tubes may have also been pitted and sitting to be explored with passage of boiler operation.

I would like to thank all the professionals who answered my question with such a wealth of knowledge on the subject matter! I am an electrical engineer saddled with the responsibility for O & M at this facility, so my knowledge of mechanical engineering is rudimentary at best. Maybe I'll keep my job! Just one more detail that remains unanswered, in my understanding of boiler technology. What is the optimum temperature for the feed water to avoid possible cavitation of the feed water pumps (DA tank bottom is 10 feet off the floor)?

hughes838

What is the optimum temperature for the feed water to avoid possible cavitation of the feed water pumps. Stay below 80C. But for every drop of 6C feed water temperature there is a 1% increase in the fuel consumption and drop in Boiler efficiency.It is better off buying preferably a Grundfos/equivalent Feed Pump that can withstand above 110C feed water temperature.

(DA tank bottom is 10 feet off the floor)?- This is the cause for your pump cavitation.

Recommended height for 100C feed water temperature - 4.5 metres minimum tank bottom

In my experience, we heated feed water to reduce thermal shock & to release dissolved gases such as oxygen & CO2- we mainly found that 90'C to 95'C was enough- despite low height of tank- the reason for cavitation was found in every case to be nrv leaking back(using centrifugal pumps)- the steam was forming vacuum in pump inlet- refurbish of nrv fixed the problem in every case.

Deaerator is very essential to remove dissolved oxygen from the feed water.Dissolved oxygen may be one of the cause for you failures. Please heat the feed water upto 100 Deg.C (min.) and then fed to the boiler.

Thanks again to all! The DA tank is working properly now (95 - 100 C) and the water chemistry is indicating that we may be OK so far. The boiler tubes have been repaired also. The check valves (nrv's) in the boiler feed lines are holding (thanks Neil!) well, so far. Thanks again to all who provided such great assistance!

hughes838