(Continuation of a series on the construction of a manually pumped, propane powered espresso machine:)
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Flame On!

In a previous post, I discussed my plans on converting a La San Marco FCS-85 from 220V to propane, and my concerns about doing it safely. Playa Espresso Cart 2- Thermoregulation I was fortunate to find that a propane conversion kit for the Astoria two group was still available. It had a key component: a propane valve that was pressure controlled, so that at high pressure (say 1.5 bar) the gas is cut back to barely on, and at low pressure (say below 1 bar) the flame is on high. It also had a thermocouple pilot saftey valve so that the gas would shut off if the burner went out.

My design called for placing two more safeties in the system: A pressurestat, so if the boiler pressure exceeds the preset value the gas would be turned off; and a thermostat mounted on the boiler, so if the boiler ran dry and the temperature started to climb, the gas would be shut off. The easiest way to implement this is by placing the two additional switches in series with the pilot safety valve, by interrupting the thermocouple current loop if the boiler pressure or temperature goes too high.

The only problem was that nothing seemed to fit to this assembly that I purchased! The thermocouple was nothing I could identify; the propane in and out was nothing that seemed to fit- the only thing that I could connect to was the output of the control valve with the venturi for the burner,and a BPP fitting for pressure monitoring.

One thing at a time

Just getting propane in to the assembly turned out to be very difficult. I wanted to test that the pressure regulator worked off of a low pressure, rather than high pressure regulator. I needed to get propane in to the control valve, a variable pressure in as well, and see what kind of flame came out.

I disassembled the bits, and separated the pilot safety valve from the pressure driven gas flow valve, in order to test the pressure regulator. Once the assembly was taken apart, I got a break: I could connect a 3/8" flare (commonly used in propane hoses) directly to the pressure controlled flow valve. With no pressure on the bicycle pump, I could hear gas flow, and could ignite a flame:



Hooray! Flames! BTUS! Flame Effects!

At high pressure, I had a little flame, and at low pressure, it was a real flame thrower. Without any adjustment it appears that below 1 bar, it is "all on", and around 1.3 bar it is all but off.



A big sigh of relief- the purchase secured the two hardest to fabricate components: the burner bar and the pressure regulator, somewhat close to properly adjusted, and the knowledge that the system ran off low pressure propane with the supplied venturi.

Safety Woes

However, it was not so easy to hook up the thermocouple and pilot safety valve that was supplied as well. Nothing seemed to fit. On a web search I found a reference to the valve that suggested the fittings were M8 but after tearing the system down, they are more like a fine pitch 12 mm. Nobody in town (Grainger, Parker, NAPA, BAP, Appliance Repair Store, Propane, Praxair...) had ever seen anything like it. Back to Ace Hardware for two more 3/8" flare fittings, with the hopes that I could disassemble the valve, have enough wall thickness to cut a NPT 1/8 internal thread, and convert the mystery fitting into flare fittings as used in the propane industry. (Hank Hill, thanks for the advice). I started with an 21/64th and then went to 11/32:



Dialing in the Grind

The problem with cutting a tapered thread in to a shallow depth is that you need to use progressive taps, since the full length of the tap won't fit in the hole that is allowed. "Bottoming Taps" are used to finish the thread to the desired depth: http://www.jjjtrain.com/vms/cutting_too ... ap_06.html

From what I could figure, the commerically available bottoming taps are for parallel threads. I needed a progressive tap; where the first 3/16 of the tap becomes progressively larger in diameter. If you do this, be especially careful not to cross-thread when starting each successive tap:

I made my own by starting with a regular NPT 1/8-27, starting the thread, then progressively using the ginder to shorten the tap:




with the result being that I could get the 1/8 NPT tap "bottomed"



and fit two 3/8" flares with an elbow to work around the mystery fitting:



I also cut the thermocouple so I can splice in the two safeties:



I plan to use a 12" 3/8" Flare Stainless Steel corrugated gas line to connect the pilot control valve to the regulator valve, and a quick connect on the input to the system.

The next steps should be fun- fabricating the stainless steel sheet metal to shroud the burner, mounting the burner and valve, and measuring the heat production.

BTW: does anyone know if it would be better to use the yellow PTFE tape on the fittings, or purple Threadloc? The original part was glued up with something; this is all very low pressure so it seems that PTFE might be better.