Thursday, 22 January 2015

Making Parallel Chimneys

My previous article on Sentinel 7109's chimneys left a problem to be solved.

One solution suggested was to take off the LHS tubes and grind away the casting to enable the tubes to be realigned. By elongating some of the fixing holes in the tubes, this might have achieved a result. However, I was not happy with the idea of reshaping the castings in this way as it would end up with a vague fit that could come loose when subjected to vibration and heating/cooling during service.

I enquired of one of my mentors (Justin Goold) about how concentric the blast from the blast nozzles needed to be inside the chimney tubes. It would seem that it is important for the blast to be right up the centre of the tubes. Things would work if the nozzles were out of alignment but not very well.

So the first thing to be done is to measure how well the nozzles are aligned at present and hence where any adjustment is needed as it may not be the LHS chimney tubes only. I've had to make an alignment tool to investigate this.

It's important to note that the orifice of each nozzle has a machined area around it which is perpendicular to the direction of the blast. This can be used to work with an alignment tool.
 (The nozzles themselves are a converging cone shape which will not support a rod to check alignment).
Nozzles showing machined flat area around each nozzle orifice
The tool needs to sit in the orifice and follow the line of the blast. I've made it using M10 threaded rod with a foot having a ledge to rest on the orifice surround.
Threaded rod and alignment foot
Alignment foot with ledge to fit into nozzle orifice
The foot has been made carefully and the threaded rod selected so that the latter will accurately follow the line of the blast.

Thus it will be possible to check the alignment of each chimney tube with its own blast nozzle.

The next challenge is to make the chimney tubes all parallel to each other whilst still in line with the nozzle blast. Assuming the nozzles are already well aligned(!), this involves increasing the gap between the LH & RH pairs of tubes at their tops.

I'd noticed previously that when the superheater was tightened down too much, the central part of the boiler top plate was pulled downwards and no longer flat. When I loosened the superheater fixing, it lessened the inward leaning of the chimney tubes slightly although far from enough. However, if the tubes could be made to lean inwards by pulling down the superheater, they could also be made to lean further outwards by lifting the centre of the boiler top plate. Possibly this could be done and avoid grinding the castings.

But how to do it?

What's needed is a device which can push apart the two castings far enough for the heat shields to be fitted (such as a car jack)! With the heat shields fitted, they will keep the chimney tubes in place.

So the plan is:
Check the current alignment of the nozzle for each tube.
If not satisfactory, make minor adjustment (somehow!).
If aligned OK, jack the castings apart until the tubes are parallel.
Reassemble the heat shields.
Remove the jack.

Cross fingers!

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