Boiler passes hydraulic test!!

Hydraulic test

It would be nice to say that the boiler passed the test first time but I found two stays which weeped very slightly inside the firebox. The first was in the middle of the row next to the foundation ring and was dealt with using the needle flame nozle in the torch to reheat the Comsol along with a liberal application of more flux. The second weep was on one of the stays close to the top of the firebox so was a bit more difficult to get to. I had missed putting solder on the end of the stay to seal it to the end of the nut and it was leaking along the threads. I dealt with this using one of the old fashioned copper soldering bits heated up in the gas ring along with a bit of background heat from the torch. The stay area was heated with the torch and then a blob of solder melted onto the end of the stay using the heated copper bit. On the subsequent retest all the stays were bone dry at 180psi.

To be honest, the worst part of the test was getting all the blanking plugs and the screws holding the dome cover and the regulator boss to seal! The screws are tapped all the way through the bushes so the threads have to seal as well. They will eventually have jointing compound on them when the boiler is finally assembled so this problem will not arise.

The big pressure gauge used was an Ebay purchase and is reputed to have come from a British Rail engine shed! Before using it I checked it against a smaller but brand new gauge and the two tallied very closely. I'll have to clean it up and give it a polish!

I've been in contact with Steve Eaton who is the Association boiler tester and he recommends having the boiler 'officially' hydraulically tested before fitting it to the frames so I'll probably have a trip up to Chesterfield in the next week or so to get this done. I can still make a start on the boiler fittings etc. and get them ready for assembly after the boiler is tested.


I decided to have a go at the smokebox next. I had been waiting for some suitable brass tube to arrive at our local ME supplier but gave up on this and rolled a tube up from a strip of 16swg brass sheet instead. This was fairly easy actually and worked out a lot cheaper! The strip was cut slightly too long, annealed, and then formed around a piece of thick wall copper tube I had lying about using a combination of hand pressure and judicious taps with a rubber mallet. When the diameter was about right I gradually trimmed the edges of the ring which would form the seam until the tube was exactly the right diameter when the edges were pressed together. The joint was then silver soldered after clamping the edges together. A bit more tapping with the mallet on the former removed any distortion caused by the soldering. The ends of the tube were squared off in the lathe and at the same time the tube trimmed to length. The tube was mounted in the lathe using the usual practice of turning a couple of wooden 'bungs' to fit in the ends, gripping one end in the chuck and supporting the other end with a centre.

The smokebox door ring is a casting actually intended for the Flying Scotsman and needed careful setting up in the lathe as it was only just big enough in diameter and I could not afford to take much off when machining it to fit the smokebox tube. The outside diameter of the ring was turned with a step in it so that the front of the ring overlaps the edge of the tube and gives the appearance of the smokebox tube being a lot thinner than it really is. I think this looks a lot better than just turning the ring to the same diameter as the inside of the tube. The other end of the smokebox needs a ring fitting inside the smokebox tube to reduce the inside diameter to that of the boiler barrel. This was simply rolled up again from a strip of 16swg brass and made a tight fit inside the smokebox. This 'adapter' ring and the door ring were both soldered in place using Comsol as I did not think that it would be necessary to use silver solder again. If the smokebox gets hot enough to melt Comsol then there is something drastically wrong with the design of the boiler and a lot of the heat from the fire is being wasted!

The holes for the steam and exhaust stubs were marked out on the bottom of the smokebox and also the hole for the chimney liner at the top. These were then drilled using a stepped drill from a set I got from Aldi of all places. Quite a few suppliers sell these and they are excellent for cutting holes in thin sheet metal. Much better than ordinary drills that tend to snatch. The drills have increasing diameter cutters on a single spindle and you start off drilling a pilot hole with the smallest and then carry on until you reach the diameter that you want.

Using a step drill to cut the hole for the chimney liner

The smokebox saddle was then positioned on the frames, the smokebox rested on top, and after checking that it was in the right position, the position of the saddle flanges marked on the bottom of the tube. The smokebox saddle and the tube were then removed from the frames and the tube clamped to the saddle using the previously made marks to position it. The holes for the 10BA bolts which will fasten the tube to the saddle were then spotted through from the saddle and drilled in the tube. The bolts will fit from the outside with nuts inside the smokebox. A bit fiddly perhaps but I thought the smokebox tube a bit thin to tap for the bolts.

The chimney liner was turned from a length of 7/8" dia. brass bar. The dimensions for this and the blast nozzle were calculated using some ideas given in a past Model Engineer article by Harold Barton (Vol. 163, issues 3853 and 3855). Harold did some experiments on the draughting of several locos and improved their performance quite considerably by redesigning the exhaust arrangements. It may be necessary to do some experiments with Helen's draughting on the steam tests but at least I have somewhere to start from. There's a lot of variables to take into account such as the grade of coal used, how hard the loco is working, etc. I shall probably make a selection of blast nozzles with different size holes and try each one to see which gives the best all round performance. The nozzles simply screw onto the exhaust stub in the smokebox so it will only be a moments job to change one. Strictly speaking the dimensions of the choke in the chimney liner should also be altered to suit the different nozzle size but it's a big job to change the chimney liner as well as the nozzle!

The inside bore of the liner was bored to the correct taper of two degrees and the bottom end flared out and blended in with the taper. Harold suggests that it is not necessary to have a wide flare on the bottom of the liner, in fact this may be detrimental to the extraction of the smoke from the tubes by obstructing the flow of the gasses. The finished liner was then silver soldered to a curved flange to enable it to be bolted to the top of the smokebox. Some designers suggest silver soldering the liner to the actual smokebox but that makes it very difficult to change it if the dimensions prove to be wrong!

Boring the taper on the inside of the liner

Top and bottom views of the chimney liner

A couple of pictures of the loco so far:


The next item tackled was the superheaters. These are radiant type as opposed to flue type i.e. they extend to the back of the firebox and collect a lot more heat, hopefully not too much though! They consist of two lengths of 3/16" diameter stainless steel tubing per superheater flue joined at the firebox end by a stainless steel return bend. These return bends were made from short lengths of 9/16" diameter stainless bar drilled on the end to take the two tubes. A cross hole was then drilled to connect the two drillings for the tubes together and tapped to take a thin stainless plug to seal it off. The tubes were then silver soldered into the blocks and the plug silver soldered over. I used Silverflo 24 for this job as its melting point is about 750° C which should be high enough I think. Normal practice is to either braze or weld these joints but I can't see the ends getting that hot? I found that you had to be very quick when doing these joints as the flux soon loses its effectiveness at these high temperatures. I took too long heating up the first joint and the solder would not run. I had to clean it all up and start again. The second one went perfectly. After soldering the return bends were sawn and filed to a rectangular section to fit through the superheater flues.

The wet header that feeds the steam from the regulator to the elements and the dry header that feeds the superheated steam to the cylinders were turning and drilling jobs from bronze bar. The wet header connects to the regulator boss via a short length of 1/4" copper pipe. A similar piece connects the dry header to the connection on the steam manifold that feeds the cylinders. I thought it much easier to do it this way than try to bend the superheater elements and join them directly to the regulator boss. All the joints at this end of the superheaters was done with Easyflo which was much easier than soldering the return bends.

The tubes are bent up slightly where they enter the firebox so that the return bends are tucked up in the top corners of the firebox over the firehole.

The only item left for the smokebox end of the boiler is the blower jet. This was made from a slice of brass bar drilled to fit over the blast nozzle. This was drilled in the side to take a length of 1/8" copper pipe and a vertical hole drilled with a No. 60 drill to connect with the copper pipe and form the actual blower jet. The other end of the pipe was fitted with a nipple and nut to fit on the end of the blower stay. The pipe was made such a length that it could be curved out of the way of all the tubes to make access for tube cleaning easy.

Smokebox fittings assembled without the smokebox

It was time to think about a regulator. I spent quite a bit of time trying to decide on a suitable type and eventually went for the simple screw down type as there's very little to go wrong with this and few parts to break or fall to pieces! It can also be made very sturdy. The only problem with the screw type is that they can jam solid if left shut when the boiler cools down due to differential expansion (contraction?) of the different metals used. They will usually free up again next time the boiler is steamed though. I decided to try and avoid this by making the actual valve seat from PTFE which will deform slightly rather than jam. PTFE will probably seal better and require only light pressure to shut off properly.


Construction of the regulator follows normal practice. The main body was made from some hexagonal bronze as I didn't have any round bar the right size and I didn't want to have to turn down a much larger piece of bar. The corners of the hexagonal were skimmed off to enable the body to fit through the backhead bush. The seating for the actual valve was turned from a length of Peek rod and is screwed into the end of the body. The seating is also threaded to take the 1/4" dia. steam pipe to the front of the boiler. The body was turned down at the other end to fit into a length of 1/2" dia. brass tube which extends to the backhead and has the boss and gland for the operating rod. The valve itself was turned from stainless bar and threaded 3/8" BSW. The bore of the body is threaded to suit and this coarse thread gives a fairly rapid opening of the valve. The 'working' end of the valve has a 120° point on the end which also helps to give a rapid opening. The other end is drilled to take the 5/32" dia. stainless operating rod which is secured with a stainless pin. The cab end of the rod is turned down and threaded 6BA to take the regulator handle. The boss that bolts to the backhead has an 'O' ring seal for the operating rod. The thread on the valve was a fairly slack fit and so I fitted a long stainless spring to the operating rod to remove any backwards and forwards play that may affect the operation. Steam is collected from the shallow dome on the boiler via a stub of 1/4" copper pipe screwed into the top of the body.

Component parts of the regulator

Regulator assembled

The water gauge was tackled next and once again follows normal practice. The only problem was that it is on the same side of the cab as the reverser and the blowdown valve has to be as short as possible to clear the reverser handle. If I'd thought about this before I would have put the gauge on the other side of the boiler but it's too late now! I could put the reverser on the right hand side of the cab I suppose but being right handed it's easier to operate where it is.

The body of the gauge was built up from bits of 1/4" bronze bar silver soldered together with the various nuts turned from hex brass bar. I decided to make the stubs of the top and bottom fittings that screw into the boiler bushes as plain threads without any bosses and they will be fitted with loose brass nuts. This will make it easier to line up the top and bottom fittings after which the nuts can be nipped up to lock the fittings in place. Fittings with bosses on can be a real pain and usually need much fiddling with copper washers of various thicknesses to get them to tighten up in the right position!

Water gauge top and bottom fittings

The blowdown valve was turned from 5/32" stainless bar and reduced at the hand wheel end to 1/8". The bore of the gland nut is also 1/8" and so the valve cannot be completely unscrewed from the body (a requirement of the new boiler regs). The very end is reduced again and threaded 8BA to take the hand wheel. The stem is sealed with a 1/8" bore'O' ring reduced in outside diameter to fit into the gland nut. This is easily done by putting the 'O' ring on a suitable mandrel (Drill!) in the lathe and sanding down the outside with carborundum paper.

Blowdown valve assembly

While I was set up for valve spindle making I also made the spindle etc for the blower valve which is virtually identical. When fitting the valve spindles I put a bit of PBC compound (copperslip) on the threads which makes them nice and smooth to operate.

Trial assembly of the water gauge

The sealing washers for the gauge glass are made from rings cut off some 1/4" o.d. silicon tubing that I happened to have. A length of the tube was pushed onto a piece of 5/32" bar held in the lathe and 1/16" thick rings were 'parted off' using a sharp craft knife. While I was at it I cut off a load of spares as well. A couple of things to note when fitting gauge glasses is that the glass must be a nice loose fit in the top and bottom fittings (which must line up accurately) and the nuts that hold the glass must only be finger tight (no spanners!). Any stress on the glass caused by overtightening the nuts or out of line fittings can cause the glass to shatter under steam. Not nice! I remember an article by LBSC in which he described a 'safety' water gauge fitting that had a ball in the top and bottom fittings that sealed the gauge if the glass broke. I'll have to try and find it again as it seems a good idea. There's probably something to be said for fitting a protector around the gauge glass. These are really intended to protect the glass from being hit with something but would probably deflect the escaping steam and water away from the driver in the event of a breakage.

I had an email from Steve Eaton (the boiler tester) yesterday to say that he would be away for a week or so so the 'official' boiler test will have to wait for a bit. What I may do is carry on and perform a steam test anyway if I get everything finished before Steve gets back. That will at least prove all the fittings etc.


Work continues on the boiler fittings.

The next item is the steam turret which was fabricated from two pieces of brass bar silver soldered together. Rather than screwing into the top of the boiler, the turret is attached by a hollow bolt through the rear, banjo type fitting of the turret. I copied this idea from Andrew Dick's 'Josie' which is a 2½" version of the original LBSC O gauge model. Headroom in Helen's cab is very limited so this method of fitting allows a very shallow turret to be used. The turret itself will carry three steam valves, nipples for connecting the blower and the pressure gauge, and the whistle valve. Two of the steam valves will supply two injectors and the third will be a spare in case I think of something else!

Steam turret awaiting the valves

The steam valves will be very similar to the blower valve but with slightly larger outlet nipples to take 5/32" diameter pipe rather than 1/8". I had originally intended to make all the pipes 1/8" but after reading a very comprehensive series in Model Engineer on injectors by D A G Brown I decided to increase the diameter to 5/32" to reduce the velocity of the steam in the pipes to the figure recommended in the articles. I shall be fitting two 8oz per minute injectors I think and 1/8" outside diameter pipe would give a steam velocity higher than recommended. Using the calculations given in the articles I arrived at a steam consumption for Helen of 1.5oz per minute at 9mph and 35% cut off so actually just one 8oz injector should be more than adequate. The articles do describe a 4oz per minute injector which I may have a go at building but apparently it is not as efficient as the larger sizes.

I've made the bodies for the valves but I'm waiting for some 7/32" taps and dies to make the outlet nipples so I can't finish them just yet. The valves will screw straight into the manifold and be soldered in place with Comsol as there is no room really for bosses or locknuts.


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