Marine Boiler Blow-down 1

[Rjeffery]

I remember performing UT thickness testing on surface, soot, and bottom blow piping. what does this blow down process actually do and why would the thickness measurements be required every 6 to 18 months?

Thanks, in advance.

 

[vesselfab]

Marine Boilers have their unique sets of problems.

one is corrosion from external sources....salt water/vapor

another is lack of feed water treatment

another is lack of regular maintanence.

 

[RossABQ]

Dissolved oxygen in the feedwater is the primary culprit, that's why any large boiler system has a deaerator. In the bottom blow down there will also be a collection of mud that prevents the protective chemicals from reaching the pipe walls.

 

[gr2vessels]

Jeff,
The steam system is operating based on extremely clear, clean water, which normally should have the oxygen level tightly controlled, in order to control the corrosion of your piping system, boiler piping exposed to high temperature, etc.. Since the system is a closed circuit, you cannot expect much of the steam / water losses in the circuit. However, to prevent major corrosion problems, chemicals are added to the water (and particularly in the make-up water to replace those losses), to scavenge for the smallest of amount of oxygen still present in the water, also to coat internally, perhaps some equipment sensitive to even small amount of oxygen. In time, all those additives and corrosion particles in the water tend to gather in specific places, like a blowdown vessel or pipe, where from time to time, those by-products can be "blown-down" from the system, for cleaning purposes. Automatic blow-down valves or even manual valves do the trick.
Some time ago, the corrosion problem due to the dissolved oxygen in the boiler feed water lead to major plant break down, most of the time untimely and completely out of control. The equipment meant to release the oxygen and other unwanted gases from the water was called deaerator. It helped somehow to remove the gases from the water down to few ppm. It was still not sufficient and this de-aerating process was so expensive, that a new corrosion process is now used. Most likely on ships also, the controlled injection of oxygen, which is generating a particular form of stabile and somehow protective layer of "rust", helps to extend the life of the carbon steel piping and equipment in this steam system. Also, it helps in predicting quite precisely the expected life of that piping and equipment, hence the maintenance has turned into a predictive replacement operation of the damaged components, thus bringing the corrosion issue under operator's control. It aslo is comming to a price, with higher than normal corrosion rates and more additives for corrosion control. Hence larger quantities of blow-down accumulated in the system and requirement for more frequent thickness check of those places where experienced operators know the metal loss is higher than other places. You'll see, when time for replacement closes, your check will be even more frequent, in the hope that, just maybe, the corrosion stopped and miracle, the pipe is still holding..(it doesn't and the section will be replaced in the next port regardless).
Hope this helps, perhaps to get you sleep...
cheers,
gr2vessels

 

[mshimko]

Here is what I think you're looking for:

1. A boiler is a pressure vessel - duh
2. Blowdown from upper region of a boiler (surface blowdown) is performed periodically while the boiler is on-line, that is, at operating pressure.
3. Blowdown lines are "normally" empty, hence unpressurized.
4. During the blowdown process, the blowdown lines see sifnificant gradients in temperature and pressure.
5. Add in the benefits of the boiler water chemistry that DO NOT help much in the blow down lines, and you essentially have a pressure vessel-piping failure waiting to happen every time you blow down the boiler.

OK, that's a bit of an extreme description, but hopefully you get the idea & understand WHY it's important to identify possibly thinned piping BEFORE it fails.

 

[MJCronin]

To all..

Erosion and corrosion are, of course important issues for this type of piping system, but nobody has yet metioned the affects of flashing.

Flashing of water to steam occurs at the location where the pressure drops and erosive wastage of piping/valve materials can occur.

Special valves are sold with special configurations and "hardened" internal materials to mitigate these affects.

http://www.cesare-bonetti.it/Products/Valves/blowdown.htm

Additionally, "blowdown tanks" are commonly sold with internal 316SS wear plates at the expected point of erosion.

It is also common to specify XXH piping for these systems to ensure long-life.

My opinion only...

-MJC