Boiler Component Life

Not railway - marine this time - but no doubt there is much in common

I have just got in from a PSPS Council meeting at which surprise was being expressed at tube end deterioration at the interface with the tube plates eight years after installation.

Waverley of course uses recirculated water that is "scrubbed" to remove impurities rather than one pass water as in loco boilers

Is there a view on this as my own was that eight years is reasonable - The WSN Board are of the view that the boiler should last twenty years. I would hope the barrel would last a damn sight longer, but that components may well need replacement within that period.

Comments please

 

Re: Boiler Compnent Life

I'm well out of my specialist area here - I do have a great interest in old warships, but have never been practically involved - but marine steam installations generally have little in common with locomotive ones. Boilers are invariably water tube (which I assume is the case here); and, as you say, the exhaust from the engines passes through a condenser for reuse. This effectively brings it to a high state of purity.

Marine boilers are usually designed to allow relatively easy tube removal and it wasn't uncommon for tubes to be changed on a ship at sea. The whole ethos of marine boiler construction was different, in fact, so different rules apply.

There are a number of issues here though: 'pure' water will still give corrosion, especially if air bubbles are trapped within the feed. So too will steam bubbles formed on the tubes' inner surfaces, although this doesn't account for the occurance at the tube ends. But what materials are being used? Could it be as simple as galvanitic action between two dissimilar materials?

 

Re: Boiler Compnent Life

It would help more if you could give detail of the type of boiler you are referring to. Is it a simple marine type with a circular firebox and is it coal fired? I'm guessing that you are getting 'necking' of the tube end just insde the water space at the combustion chamber end and I'll guess even further that it is the top rows of tubes you are having problems with?
This is a similar phenomenon to grooving caused by the flexing of the tubes as the boiler is heated up and cooled down. The end of the tube is effectively rigidly fixed into the tubeplate. As the boiler heats up, things expand and the tube flexes.
The protective layer of passive corrosion on the surface is therefore constantly being disturbed leading to active corrosion conditions and the tube is oxidised away. Even with water treatment, you'll still get this happening. I'm not a metallurgist and I'm sure one would be able to explain what's going on in far more detail. you don't get this problem on boilers that are constantly in steam and at working pressure.
I have known boiler tubes last for over 30 years on gas fired industrial boilers in hospitals where there is no cyclic loading and they work for long periods at a time. On the steam locos I'm responsible for, I usually reckon on starting to have problems with tubes as described above at about 8-9 years.

 

Thanks for the feedback

Here's a blog from the Society's Scottish Branch which describes the re-tubing carried out

It appears that the Cochran boilers installed are conventional fire tube boilers with flue gas multipass

http://pspsscotland.blogspot.com/2009/04/new-tubes-arrive-for-waverleys-boilers.html