Warning!!! The following articles are NOT plans for building a model boiler. There are no drawings or engineered specifications posted on these pages for a reason. These articles only serve as a record of my experience in building a vertical boiler. Please DO NOT try to use these articles as a guide to build you own boiler. They simply aren’t written for that purpose. Thank you...
Posted on April 23, 2011
Well folks I have almost have this boiler ready for testing. The last two things to complete are to install the boiler tubes and boiler bushings.
This boiler will use a set of 31 “fire tube” boiler tubes which will be used as the primary heating surface to make steam. The boiler tubes will be held in place by swaging or flaring the ends with a swaging tool. This will mechanically hold the end plates in place and keep them from bowing out. However this will not seal the tubes very well so I will add a silver solder caulk to the boiler tube joints for a leak free assembly.
The first step is to cut all of the boiler tubes to the exact same length. This was done with my lathe and a tubing cutter (seen below). I cut a master length of type “L” copper tubing to 10-5/8” long and then used this piece to measure 30 more pieces for cutting. Note: It is better to use a tubing cutter over a band saw or circular saw in that there won’t be a rough edge to file off after the cut.....
To swage the ends of the copper tube I will be using a swaging tool made for forming “bell ends” on copper tubing. These bell ends eliminate the need for a coupling in a plumbing application but in my case I won’t need to form the entire bell end. I will only need to form the beginning cone shape to lock the tubes into the holes of the end plates. I used the swaging tool on a short piece for tube to see how far I would need to drive the tool into the tube to get the desired flare (below). Some types of full scale and model boiler designs use a method of “rolling” the boiler tubes into place. This is where a special tool is used to expand the tubes diameter inside the boiler end plate which mechanically locks and seals the tube ends without soldering or welding. I looked into this option for my boiler project but was discouraged by the rolling tool’s price :oP Instead I found my swaging tool for about $10 on eBay :0)
After all of my boiler tubes were cut I cleaned the ends with a Scotch-Brite “green pad” in preparation for soldering. I then proceeded to swage one side of each of the boiler tubes as seen below. A plywood board was used to protect the unswaged end of the tube while I pounded the swaging tool into the other end of the tube with a hammer. A piece of black tape was used to indicate when to stop driving the tool into the tubes. Each tube received a 1/4” deep flare that will mechanically hold the end plates into place....
I installed the boiler tubes into the end plates to see if the flares were even in depth. Any tube that seemed to be too shallow was swaged a little more so they all sat evenly....
I installed the rest of the boiler tubes after all of the height adjustments were made. I was now ready for soldering!!! Note: I have yet to swage the other end of the boiler tubes. I am just locking in the tubes with solder so they maintain proper alignment at this point...
I heated up a small amount of the non-acid soldering paste and brushed it on the entire face of the end plate. I made sure that the liquid was “wicked” up against every boiler tube to ensure proper coverage....
To solder in the first side of the boiler tubes I will be using a high strength non-lead silver content solder (seen below). This solder has a tensile strength of about 9500 PSI and is comprised of 96% Tin and 4% Silver with a melting point of 430° F. This temperature will be ideal to seal the boiler tubes without melting the end plate caulking I had soldered earlier. Note: Many model boilermakers will insist on using a “hard” silver solder (+50% silver content) for all copper joints in boilermaking. This is mainly for strength and longevity of the joints in question. The solder I am using is considered “soft” in that it melts at a much lower temperature and in theory can degrade over time. Since I am not using my solder as a mechanical means for keeping my boiler shell together I feel the use of soft solder is acceptable....
I heated up the boiler end plate with the Map-Pro torch and commenced soldering the tube joints. I had noticed that it takes only a little solder to do the job as too much will just drip into the boiler :0/ After the boiler was cool enough I removed any solder droplets that collected in the boiler shell with a wire hook (not shown).
Now that the tubes are somewhat locked into place I can swage the other side which will lock the end plates together. To do this I needed a platform to support the bottoms of the boiler tubes while I drive the swage tool into the tubes. For this I used the former blocks (below) that were used to form the end plates as an “anvil” to absorb the forming blows of the hammer...
I placed the boiler over the former blocks on the concrete floor to start swaging the boiler tubes. I worked my way from the outside to the inside in sort of a spiral pattern only swaging the tubes till they barely contact the end plate. I had noticed that there was some “spring” in the former blocks which was causing me to hit the end plate ever so slightly. This spring was dishing in the end plate :0/ I solved this problem by adding a 1/4” metal disk to the face of the former blocks as an additional support so the wood could not deflect each blow of the hammer (not shown).
After all of the tubes were swaged in place I was ready to solder them in. Once again I fluxed the end plate with the liquified soldering paste and then started to solder the outer tubes. I had immediately noticed that once the solder cooled down to a solid that the joints would crack. In fact they seemed to “pull” into the end plate which lead me to believe that the tubes were expanding while soldering and contracting before the solder could harden. To solve this problem I packed wet paper towel into each tube at the other end of the boiler (not shown). This was done to keep the torches hot gasses from traveling down the tube’s length and expanding them during soldering. This fix worked quite well and no cracks formed after this point. I finished soldering all of the tubes and then revisited the other side of the boiler to fix any cracks that might have formed during the swaging process. I flipped over the boiler and installed the wet paper towel on the freshly soldered side so I could “smooth” over the solder joints with some heat (not shown). Soldering paste was brushed on the end plate to help the solder “flow” and then I applied heat to smooth out the solder fillets...... Note: Soldering paste or Flux is used to clean and help prevent oxidation of the base metal being soldered. However it also helps the solder to flow into cracks and become somewhat “wet” producing nice glassy fillets. Without flux the solder will not flow correctly (it reduces the surface tension of the solder)....
Addendum 2/1/2012:
After many hours of running this boiler I had noticed that pinhole leaks were forming at the bottom of the boiler tubes. Apparently the thermal cycle of heating up and cooling down causes the solder to flex which causes these pinhole leaks. I decided to swage the tubes again with a swage tool of my own design that will put a lot more mechanical pressure on the joint where the solder is. I made the swage tool out of cold rolled steel with a taper starting out at .530” and widening to .590” @ 1.4” long. I cut some flat spots on the end so an open end wrench can spin the swage tool to remove it once it has been driven into the tube. I used some high pressure moly grease to lube the tool and then drove it in each tube with several sharp blows leaving about 1/4” of the taper exposed. Of course I supported the other end of the tubes with a wooden block so the end plates did not get deformed (not shown). This solved the pinhole leak problem without having to resolder the joints.
The next step was to solder in the boiler fittings with the 4% silver solder. This job was fairly straightforward and required very little effort. Note: I would suggest that if you plan to operate a boiler over 60 PSI or 307° F you should use hard solder or brazing rod to secure your boiler fittings to the shell. Soft solder can weaken at elevated temperatures and fail without warning. I have since reduced my original working pressure of 80 PSI to 60 PSI (75 PSI max pressure) as to not stress the soft solder on the boiler fittings...
The last thing I needed to add to the boiler was a set of 10-24 brass nuts as threaded inserts that will be used to mount the boiler shell over the burner box.
I drilled a set of four holes into each end of the boiler shell at 90° apart. I then soldered on brass nuts (seen below) to secure the boiler and the smoke stack/firebox later on.
And now the moment of truth !!! Will my new boiler hold up to pressure or will it leak like a sieve ??? I proceeded to plug up all of the boiler fittings with brass plugs to do a hydrostatic pressure test. This type of test is used to ensure that a pressure vessel can withstand a minimum of two times the working pressure that the vessel is rated for. During the process of testing, the vessel is filled completely with liquid as to not allow any air to remain inside the vessel. This is to insure that no pressure energy can be stored up inside the vessel during the test thus preventing an explosion if the vessel fails. Since water cannot be compressed it is a safe medium for this kind of test and is typically used. I filled up my new boiler with water and proceeded to heat up the boiler tubes with my torch. Since I have filled the boiler completely with water it won’t take much heat to generate a rapid rise in pressure with only a little heat applied. I carefully watched the pressure gauge as I heated up the boiler tubes and sure enough the pressure started to climb. According to the Harris book it is unwise to hydrotest a boiler at much more than twice the working pressure. This is because the boiler may be damaged without any visible evidence. In fact a boiler can be stressed beyond it’s elastic limits which can cause a failure at even low pressures later on. I am going to take the author’s advice and not over stress my boiler during this test. I continued to heat the boiler till the pressure gauge reached 210 PSI. I let the boiler sit at this pressure for about 1/2 hour looking for any leaks along the boiler tubes. Luckily there were no leaks and the boiler survived the first phase of testing !!!
The second phase of testing was to do a low pressure air test. This test is to rule out any pinhole leaks that water may have been to “thick” to leak out of. A pressure regulator was used to slowly introduce about 10 PSI of air to the boiler for this test. Note: Be really careful when testing with any compressed gas. A careless mistake could cost you your life !!!
After charging the boiler with 10 PSI of air I submerged it in a sink to see if any bubbles could be seen. Luckily the boiler held it’s pressure for an hour without any signs of leaks :0)
The next step was to plumb the boiler with a few fittings that will make a live steam test possible (the third phase of testing). I decided to use 1/8” NPT schedule 80 close nipples (seen below at top) in lieu of schedule 40 (seen below at bottom) to plumb the steam pipes on the boiler. The thicker schedule 80 nipples will endure mechanical stress better than the schedule 40 ones. These nipples will also endure the most chemical and thermal stresses on the boiler so the extra thickness will help out greatly !!!
Before installing the steam fittings I applied some Permatex high temperature thread sealant (Cat #59214). This stuff is really great and I use it on almost all of my pipe fitting projects. It is available at most auto parts stores....
I installed several fittings including a couple of cast red brass pipe “tees” and three cast red brass pipe elbows for various attachments. You will also notice the feed water check valve on the middle left side of the boiler. This 1/16 NPT (5/16” X 27) check valve is made by PM Research (Cat #CVA5). The valve will prevent any steam pressure from back flowing toward the feed water pump (more on that later)....
At this point I installed the 1/8 NPT globe valve (PM Research Cat #GV1/8) which will be used as the main steam shutoff valve (seen below)...
The next step was to install the water gauge kit (PM Research Cat #WG-1). Earlier I had installed the 1/4 X 40 boiler bushings so that the gauge is at about 6.75” at center from the bottom of the inside of the boiler. This is where the water level will be maintained during operation (about 66% full)...
I installed the gauge “tee” fittings into the boiler with 1/4 X 40 close nipples. I then installed the 3/16” glass tube into the tees followed by the pipe plugs. Silicone tubing was used to seal the glass tube under the compression nuts...
The next and MOST IMPORTANT fittings of the build are the pressure safety valves. These valves will insure that the boiler cannot contain any more pressure than the boiler is designed for. This special type of pop-off style safety valve features a spring loaded stainless steel ball on a brass seat. It has an adjustable pressure setting from 5 to 300 PSI and has a 0.210” clear bore orifice diameter (Kingston Model #125SS). According to the Harris book (table 7) I will need at least two safety valves for a boiler over 300 sq in of heating surface. Table 7 also states that at a working pressure of 100 PSI I will need two valves with a orifice diameter of at least 0.199” each. Luckily these valves will work perfectly for me....
Using my shop air and a pressure gauge I was able to set one of the pressure valves to pop-off at around 125 PSI which is 125% of the normal working pressure. This is the pressure value suggested by the Harris book for a live steam test. I installed the safety valve on one of the pipe tees as well as a pressure gauge. I made a gauge siphon (seen below) which is a “U” shaped pipe that traps condensed steam (as water) in the bottom of the “U” as to not allow live steam to contact the Bourdon tube inside the pressure gauge. Hot steam could damage the Bourdon tube (which gauges the steam pressure) and give a false reading.....
And now for the third and final test, the live steam test. I filled the boiler with distilled water to the middle of the water gauge and placed it on the side burner of my barbecue outside. I also set up the burner valve with a remote shut off string so I can shut down the heat without getting close to the boiler (not shown). I then lit the burner and closed the main steam valve to proceed with the live steam test.
I watched the boiler from a safe distance while the pressure increased to around 100 PSI. At this point I was anxiously awaiting the pop-off valve’s buzzing noise when it activates. At about 127 PSI the pop-off valve blasted off which quickly reduced the steam pressure to about 120 PSI. The valve cycled a few more times before I shut the gas down to the burner. I approached the boiler with a pair of leather gloves to bleed off the remaining pressure with the main steam valve. I was amazed at the energy that was contained in the steam as it created a white cloud in my backyard !!!
Well I am pretty happy with the progress so far. I now have a working boiler vessel but I still need a way to fire the boiler. Join me next week when I build a firebox and burner for the newly completed boiler.
