De-firing & Shutting Down of Marine Steam Boilers

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Archimedes
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De-firing & Shutting Down of Marine Steam Boilers

Postby Archimedes » Thu Nov 11, 2010 5:56 pm

hiya, yes, you guessed right, this is a sort of homeworky type question, but bear with me, I'm only asking for somewhere to start! :oops:

I just can't find anything on defiring and shutting down steam boilers!

Any pointers please?!

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The Dieselduck
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby The Dieselduck » Thu Nov 11, 2010 9:23 pm

I'm sure some steam guys here will have a more precise answer but in practicality... take as much time as possible before opening access panels, the rush of cold air will do thermal stress damage, and damage the refractory. Make sure you lock out tag out all relevant valves. And when you open up after draining, first make sure there is not pressure left in the boiler by opening indicator cock or the relief valve manually, although on a large boiler, its plumbed in to the the top of the stack so impossible to see. There will still be plenty of heat inside, possibly some hot water so take caution not to get yourself burned when opening the hatches. Before going in, ventilate boiler spaces and test atmosphere for LEL (explosion limits); hydrocarbons may have entered the water space, as such you shouldn't use non intrinsically safe devices inside.

Also, before shutting down, make sure you run on diesel instead of the usual HFO, this will make restarting a heck of allot easier.
Martin Leduc
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JK
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby JK » Fri Nov 12, 2010 1:49 am

You shutting the boiler down for an extended time or water wedging it?

Do you have access to boiler manuals? The operation manual should have the type of info you are looking for.

Archimedes
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby Archimedes » Sat Nov 13, 2010 2:09 pm

Thanks for the help DD, much appreciated.

JK, no, I'm just a humble cadet... still with dry boots!
No actual boiler manuals are available where I am.

Archimedes
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby Archimedes » Sat Nov 13, 2010 6:10 pm

How about the actual de-firing and shutdown process before tag out? Is there anything much to think about? It's got to be more than just flicking a switch!
Is there much difference in the processes between water wedging and long-term shut down beyond what sounds like the obvious?
I presume waterwedging is what you do when alongside for a while?

Is long-term shut down a bit like this: slow down the turbines to manoeuvring speed, start up a diesel alternator, shut down the turbo alternator, let the cylinder cool down, purge naughty gas, do bottom blowoff to clear out sludge at the bottom? Then do the tagout and valves/cocks, and wait for a while for it to cool down before opening up and looking inside? Would waterwedging be filling it full of water after bottom blowout instead?

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Big Pete
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby Big Pete » Sun Nov 14, 2010 5:14 am

Hi,

As previously advised put the boiler on gas oil or diesel oil firing rather than HFO and flush the fuel system through. GIVE THE BOILER A GOOD TOP AND BOTTOM BLOW DOWN. If it is a multi boiler steam plant, and you are going to continue to use steam, you will gradually throttle in the main steam stop valve, on the boiler you wish to shut down, until the burner controls shut down the flame on high pressure, then switch off the burner.
As the rate of steam generation reduces the water level will fall. (Look up "SWELL" & "EBULLITION".
When the steam pressure has fallen to a couple of p.s.i. or 0.2 bar, open up the steam/ air cock on the top of the boiler and allow the steam to vent.
NEVER ALLOW THE BOILER TO COOL WITH THE COCK SHUT, AS THE BOILER COOLS, A VACUUM WILL BE CREATED WHICH CAN START THE TUBES LEAKING.
Usually, for a short shut down you will fill the boiler completely with feed water, MAKE SURE THAT THE HOTWELL TANK IS OPERATING OVER 80c (99c IS PREFERABLE) SO THAT THE LEVEL OF DISSOLVED GASSES INCLUDING OXYGEN ARE AT A MINIMUM and there is no air trapped in the water space to cause corrossion, and dose the boiler with Oxygen scavenging chemicals (Hydrazine etc)
For a longer shut down, you will drain out all the water, open up the water spaces and put a drying agent inside to absorb any moisture and prevent corrossion, then close them airtight.
Again for a long shut down, you should wash out the fire side of the boiler and remove any carbon deposits, these often contain Sulphur compounds that will contribute to corrossion on the surfaces.
To bring the boiler back into service, check all the alarms safety devices etc. If the boiler has been shut down dry, remove the drying agents and fill the boiler to the minimum level to permit firing (swell & ebullition again).
Open the air cock, give the furnace a long purge to remove any combustible gases then manually fire the boiler according to the makers instructions. The larger the size of the boiler the slower that temperatures must be raised. Usually initial firing will be for a minute or less, on low flame, followed by a 10 minute wait before firing again.

Good luck with course work.
It is always better to ask a stupid question than to do a stupid thing.

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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby Archimedes » Sun Nov 14, 2010 7:06 am

Thanks for all that Pete.

I think now it's all starting to make sense... couldn't find much of this sort of info elsewhere.

=)

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JK
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Re: De-firing & Shutting Down of Marine Steam Boilers

Postby JK » Sun Nov 14, 2010 2:56 pm

I hesitated to post, because my steam time is all Scotch marine boiler all manual firing and it has been pointed out that it is obsolete. But on an exam, I don't really remember them specifying what type of boiler you had to talk about.

We always bottom blew the boiler down to empty it when maintenance was due. The boiler was first isolated from the steam range, and the fires pulled. The pressure was allowed to drop to 100 psi before we blew it down. The air vent was opened. Then after the boiler cooled, you could open the manhole doors.

If the boiler was on a long term shutdown, it was water wedged. To put it into service it was pumped it down to working level with the GS pump then fired. As BP said, it was with the smallest burner tip, 1 minute firing, 10 minute soak. What this does is set up of thermal circulation in the boiler to prevent localized hot sports which could lead to furnace collapse. When the pressure starts to come up, the firing is increased to 3 minutes every 10. ( When you hit 100psi, it was time to get the steam pumps running on steam instead of air, if you run out of steam, you started all over on air-happy daze :cry: )

If you had to dump the steam pressure in a hurry, the boiler was isolated and the pressure relief valves "eased" allowing the steam to vent to atmosphere. In 4 years I only saw it done twice, one to repair a leaking gasket on the main steam stop flange to the boiler top (that was fun) and another time to repair the gasket on the main feed valve.

For a drying agent we used barbecue charcoal.


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