Repairing a stainless butterfly valve

I am preparing to build a stainless steel beer-brewing tank. In preparation, I've been buying various stainless valves and tubes from eBay. The bottom of the brew-tank needs a large valve -- preferably a butterfly valve. I found one on eBay for less than $20 shipped. The seller said it was a "great valve". As it turned out, the only thing great about it was the price.
This thing has had a very hard life. Luckily, there is nothing missing, and all of the pieces are in fair shape except for the rubber seal.


Not only does the seal look bad, but it smells bad too. It has a strange sweet smell like antifreeze. I don't want my beer touching this thing even if it didn't leak. I decided to make a replacement since there is no manufacturer listed on the valve and buying a replacement seemed like a long-shot. First, I sandblasted the metal parts.


It's mostly OK, but there is some pitting. It may not cause any leaks.

I have some Dow MDX4-4210 "biomedical" elastomer left over from a work project. This stuff is basically liquid silicone that, when mixed with a catalyst, cures into a solid piece. The cure time is almost a day at room temperature, but only a couple hours at 130*F.
I machined a two-part mold out of 1" thick aluminum.

After mixing the silicone and catalyst, there are thousands of trapped air bubbles in the mixture. This will create a part that is more like a sponge than a solid piece of rubber. The product information sheet suggested de-gassing the mixed silicone in a vacuum chamber for 30 minutes, which is what I did. It works great, and the cure time is so long at room temperature, there is no danger of running out of "pot life".

I used a custom-made punch to cut the shaft holes after the piece was cured and removed from the mold. The replacement seal is a lot softer than the original. I don't know if/how that would affect the valve's operation. I'll probably test this valve before I weld it onto the brew tank.

Rust in the aquarium / Passivating stainless steel

OK, so I owe the titanium fanboys an apology. In my original post about aquarium chillers, I asserted that a stainless steel chiller coil would not rust in a saltwater aquarium. The coil in my tank rusted after about 6 months. Take a look:

I decided to sandblast the part so that I could see how extensive the damage was.

Yeow! The metal must have been exceptionally thin, and the sandblasting blew right through it!

I used 316 SS filler rod, and TIG welded the hole shut. The blue tube is an argon hose to protect the inside of the stainless tube from oxidation while welding.


Now it's all patched up. The heavy corrosion was localized around the area where the stainless coil entered the plastic filter box (see top picture) . My guess is that vibration from the pump caused the rough surface of the plastic to abrade the passivated layer on the stainless, thus causing a localized spot where the metal was unprotected. I've heard that passivated the stainless in an acid bath can create a stronger, more uniform passive layer. There are two common acids used to passivate stainless: nitric and citric. Nitric acid is nasty stuff, and it's possible to damage the stainless parts if the procedure is done incorrectly. Citric acid seems to be just as effective as nitric, and it's non-toxic. Here is the best reference on citric acid passivation:

http://www.astropak.com/downloads/technical_papers/boeing_passivation.pdf

I mixed a %15-by-weight solution of citric acid in water. I got 2lbs of citric acid on eBay for under $10 shipped. I submerged the stainless coil in the solution for 2 hours at room temperature. I could see the surface changed a bit -- the shade of gray was a little different. Hopefully this means it built up a nice strong oxide layer.

I've put the coil back into the aquarium, and added some silicone pads to the filter box so that the plastic would not scrape away at the stainless coil. I'll keep you updated to see how effective it is.

Cutting a corner radius on the lathe

I use a round-over router bit in my lathe toolpost to leave an attractive, uniform edge on the part. I mostly work with plastic, so the tooling angles are not as critical as for metal.



Here are two 1" long 1" dia sections of white Delrin. The upper piece has its right edge rounded over by the 1/8" radius router bit.

Peltier power supply and integrated PID controller

I am preparing to build a beer-brewing tank that will be temperature controlled. The goal is to keep the vessel at a very steady 72*F. Depending on the time of year, this might require heating and/or cooling 24 hours per day, or heating during the night and cooling during the day. Since the only heat load on the vessel is the heat leakage through its insulation, the total amount of heat that needs to be pumped is fairly low. This might be a good application for a Peltier heat pump since the heat load is low and the heat pump direction might need to be changed often.

As I mentioned in my aquarium chiller post, the Peltier module must not be controlled by PWM (pulse width modulation) or on/off thermostatic control. During the 'off' part of the cycle, heat will flow backward through the Peltier device and decrease efficiency almost to zero. Ideally, the PWM output should be smoothed with an L-C (inductor-capacitor) circuit to provide clean DC power to the device.

I happened to disassemble a thermoelectric refrigerator (and replaced its guts with a refrigerant-based system in a previous post) and had a 115VAC Peltier power supply and Peltier module. The power supply uses a TL494 IC to control a 115V->12V high-frequency transformer. The output of the transformer is smoothed with an L-C circuit. The TL494 is controlled by the output of a thermistor and digital thermostat, thus sending more power into the transformer when the fridge's temperature exceeds the thermostat set-point, and reducing power when the temperature falls below the set-point. I couldn't figure out the analog side of the circuit which had a few op-amps. Instead, I discovered that I could throttle the power supply by putting voltage on the TL494's DTC (dead-time control) pin, which wasn't used in the original circuit. Cool. So now I have an efficient power supply that I can throttle from 0V to 12.5V (full power) to the Peltier.

Next, I need a thermostat to control the power supply and also reverse the polarity of the Peltier device (to switch from heating to cooling). I thought about using an Arduino to do the whole thing: sense the temperature, provide a UI with LCD and buttons, process the PID loop, and send output to the power supply. My biggest worry was the UI. There are actually a lot of parameters in a PID loop, and I didn't feel like writing code to make all of them user-selectable. I also had another Eurotherm 2132 PID controller (same as the aquarium project) which I really like. I decided to use the Eurotherm and build a circuit to convert its output into a control signal for the power supply.

The Eurotherm has two outputs, a relay and a driver output for an external solid-state relay. I removed its internal relay and directed the relay's coil wire connections to the Eurotherm's rear terminals. I then used an optoisolator to interface the Eurotherm's floating outputs to the rest of my signal-converter circuit. The Eurotherm can be configured so that its output 'cycle' for each output is 1 second. Thus, it essentially outputs a PWM signal at 1Hz for cooling, and another 1Hz PWM signal for heating. I wrote a little code for an Atmel AVR ATMega8 that reads the two control signals from the Eurotherm and outputs a voltage that controls the Peltier power supply. It does this by generating high-frequency PWM and smoothing it with a simple R-C low-pass filter. The current draw is very low. The AVR also controls an NPN transistor that drives a DPDT relay. The relay will reverse the polarity of the Peltier device.


All of the above-mentioned circuitry is crammed into an acrylic box that I made for this project (left). The Peltier module has a small heatsink and a large heatsink with two fans (right).





I modified the power supply (by removing a fuse, and adding a fan), and tested it with two Peltier devices hooked in parallel. It seemed fine at 12.5V and 7A.

The next step is to mount the Peltier devices onto the brew tank and test it out.

The tank will hold 5 gallons. For thermodynamic analysis, I will assume it's water. So, that's about 20 liters, and water has a specific heat of 4.2J/g*C, so the tank will require 4.2(20)(1000)=84KJ to change 1*C. If the Peltiers are consuming 12.5(7)=88W, and are 50% efficient, they will be pumping about 44W (in cooling mode), which is 44J/s.

In order to change the tank's temperature by 1*C, the system will need 84000/44 = 1900 seconds, or 30 minutes. This is good and bad. The good news is that the tank will remain very stable, as it is not in danger of being quickly influenced by the Peltier. The bad news is that the tank must start out fairly close to the target temperature, or else it will take a long time to be regulated by the system.

Stainless TIG welding update

OK, I have learned a few more things about welding stainless steels with TIG. I am far from an authoritative source on the subject, but I hope to share what I do know:

* Keep the weld bead width as thin as possible. This is really key to everything else, and making a thin weld bead will force the welder to do everything else correctly (ie keep the current to a minimum, keep the electrode close to the surface, etc).

* Minimize the heat input to the work by any means necessary: Lower the amperage, bring the electrode closer to the surface, use copper chill blocks, stitch-weld 1" sections and allow the work to cool between welds.

*Keep the tungsten electrode sharpened to a needle-point. The length of the taper should cover about 2.5 times the electrode diameter, and I normally leave the tip fully sharp for low-amperage welding. I've never had a problem with the tip breaking, and it helps direct the arc.

* The part fit-up must be superb. I always try to tell myself "sure, I can fill that gap", but no. With stainless, filling the gap means dripping lots of filler rod, which will be very hot and leave the weld in very bad condition. The excessive heat will "burn" the chromium out of the alloy.

* The electrode should be as close to the surface as possible. This is not an exaggeration. I mean really get the electrode as close as you can without touching the puddle. A small, hot arc is much more effective than a large cool arc.

* Use thin filler rod. If you're welding 1/16" thick stainless, I'd say the weld bead should be 1/8" wide or less. This will be a problem to weld with 1/16" rod, because it is difficult to dispense a small enough amount of rod to keep the bead thin enough. Additionally, since the electrode is being held very close to the surface, there isn't much room to stick the rod into the puddle. Using .045" and .035" rod is a HUGE help.


* Don't let the arc melt the rod! When adding rod, move the torch backward a tiny bit so that you can dip the rod into the puddle, then pull the rod out of the puddle, and advance both of your hands forward in the weld direction. I can tell that I've messed up when I see the rod form a ball on the end, and the arc changes color. If the weld is important, I would cut the end off the rod and start again.

* When the weld is all done, check the colors to see how corrosion-resistant the weld will likely be:

Gray, pinkish-gray, black = Not good. The stainless has been chemically changed at the surface
Dark blue, purple, turquoise = Borderline OK. Acceptable for some situations.
Gold, silver = Perfect.



* The most common fill rod for welding stainless is 308. As near as I can tell, 308 can be used on anything, but it would be unwise to use it on higher grades of stainless like 316. I tried both 308 and 316 fill rods on 304 base metal. They behave pretty similarly, but 308 seems to melt a little more easily.

* Most rods and base metals can be obtained with an "L" designation, like 304L or 308L. These are low-carbon alloys and are preferable over the non-L alloys because welding can cause carbon to precipitate in the metal. The upshot is that welding 304 will have a more deleterious effect on its corrosion resistance than welding 304L. How important is this effect? I don't know. All of the information that I found on the net was purely anecdotal and poorly documented.



This was attempted with 1/16" filler. Very inconsistent bead width.


This was done with .045" filler. It's not perfect, but much better. Both welds were stitch-welded in 1" sections.