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Analog, self powered boiler controls

Posted by ChuckW 
Analog, self powered boiler controls
August 23, 2011 10:17PM
I'd like to build a variable pressure boiler that has self powered controls on both temperature and pressure, not using electricity at all.
-could you use a sealed, water filled tube, placed into the steam stream at one end and feeding a large Bourdon tube at the other to read temperature and act as an actuator with a few pounds force (to control the inlet valve of a plunger pump)?
-is there a better way to achieve such an objective?
Re: Analog, self powered boiler controls
August 23, 2011 10:35PM
Look up the flowmotor control used on the White steamers. Jim is the expert on White.
Re: Analog, self powered boiler controls
August 23, 2011 11:43PM
I've always wondered why people want so badly to make mechanical boiler controls? There's several electronic solutions that are way more accurate and much less prone to failure. The most notable plc solution would be an arduino duemilanove. Dirt cheap, mountains of sample programs, tutorials and thousands of users willing to offer support in forums.

Re: Analog, self powered boiler controls
August 24, 2011 12:01AM
Isn't the White Flowmotor a proportional valve? I'm hoping for a variable pressure boiler so the fuel rate will not be matched to the water rate, though the flowmotor does seem like a clever analog control for a constant pressure/temperature boiler.

It's true that the electrical route is very simple these days. I'm looking for a low parts count and rugged simplicity... and I'm a sys admin and after working on computers all day I'd really rather not see another onesmiling smiley
Re: Analog, self powered boiler controls
August 24, 2011 12:39AM

Ruggedness != simplicity.

I doubt you could find a mechanical solution that would be more reliable/efficient than an electronic controlled one. But I could be wrong.

I guess the biggest issue is that with a mechanical solution, the system wouldn't be able to react as well to when things go sideways.

That would be pretty badass if you rigged up a mechanical solution though, can't wait for pictures.

Re: Analog, self powered boiler controls
August 24, 2011 01:24AM
I've made four test prototypes so far and the principle seems very straight forward. Two of them were stressed beyond the yield point at 3000psi, one was so overbuild the deflection was barely discernible at 3500psi, and one seems about right; .125 deflection, several pounds of force, and it returns to a reliable resting point.
It took less than two hours to build and test them so its definitely simple. Its a little tricky getting all the air out but that is the only difficulty. It seems to respond to heat changes rapidly when 3/16 stainless brake line is used as the sensing element.

I can't find any Bourdon tube design aides online so its been cut and try. I know that the tube should be spring tempered but few other details have turned up.

As I understand the saturation curve for water, 700 degrees should produce around 3000psi in a sealed vessel, yes?

Edited 1 time(s). Last edit at 08/24/2011 01:31AM by ChuckW.
Re: Analog, self powered boiler controls
August 24, 2011 10:18AM
Dear Chuck W, I am a big fan of the John Wetz analog control system and have several examples of it here in my shop. Someone asked me why I did not use that system more on my own steam projects and that got a very long answer that included time and money constraints and a lack of mechanical ability myself. The person never asked that question again. Wetz had a feed back mechanism that was pure genius and simple. He used the heat of the steam from the boiler to elongate a rod and the cool intake water when the feed pump was engaged to shorten another rod and both rods were lengthened by the fire when it was on as they were both in the fire. So the system sensed (1) when the fire was on and thus some water would be needed (2) when the steam got very hot and thus feed water was needed to cool things down and (3) when the feed pump was pumping water so that after a short delay the pump was turned off before the monotube boiler was flooded. If the fire was still on and the steam still hot then there was a very short time delay before the feed pump came on again. As for pressure controls, that was a bourdon tube, although White and Smith used a small cylinder and piston pushing against a spring to measure steam pressure. The trick there was to put this all near the water pump so that cool water was being used instead of hot steam to measure pressure. Hot steam leaks a lot and expands things causing them to bind up. The real answer to your question is the R. J. Smith control system and the problem is that the only physical example of that is lost in San Diego so we do not have one to look at and it was too complex to duplicate just from a photo. Tom Kimmel
Re: Analog, self powered boiler controls
August 24, 2011 12:30PM
Thanks Tom,
If I understand correctly:
-the steam sensing rod lengthened on hot steam and fire
-the feed water sensing rod lengthened on fire and shortened on feed water.
-the rod ‘outputs’ could probably be combined in a bar linkage and the water pump control taken from the center of the bar (would that be summed or differential?)
Hot-Cool=off (pulse width modulated like a Doble)

Was the Wetz system self powered or did the rods activate switches?
Could the system produce enough displacement and power to be a reliable non-electric control?
How did he house the rods to get measurable linear displacement? Were they used in conjunction with quartz rods?
Re: Analog, self powered boiler controls
August 24, 2011 01:56PM
Isn't the Smith system a proportional control?
I'm trying for a variable pressure system following the Carters; water controls heat, fire controls pressure (with some safety controls as well).
If a water based self acting temperature actuator will work, controlling the steam temperature ought to be fairly straight forward.

The water based sensor is what I'm hoping to get some feed back on.

Any reason a water filled tube in the steam flow couldn't be used as a self acting temperature control?

Re: Analog, self powered boiler controls
August 24, 2011 02:04PM
Hi Chuck,

John's control was made with a spring steel steet metal ring that circled the boiler. There were two tubes passing through the boiler at 90 degree angles, both tubes rigidly affixed to the ring on each end. Steam passed through one tube, feed water through the other. If the steam became too hot, that tube expanded and elongated the ring in that dimension, or vice versa if too cold. Same for the feed water tube in the perpendicular plane.

Affixed to the ring was an arm that operated a microswitch that tripped the feed pump.

Actually, the description would be more accurate if I noted that the fuel tank was an open mason jar, a worn out bandsaw blade comprised the ring and other parts were more certainly found than bought. John had almost no money. I spent a lot of time at a couple of SACA meets watching a small burner regulated by that mechanism cycle on and off regularly and the control seemed reasonably tight from what I could gather from the sparse instrumentation.

Rather than opt for a quartz rod to act as a reference length, John used the cold feed water leg and the steam leg and had them reference each other. Hardly a stunningly new idea, Ofeldt did something similar with some water level regulators in launch boilers. On the other hand, I've seen stuff that looked infinitely nicer and cost much more that didn't work half as well.



Edited 1 time(s). Last edit at 08/24/2011 02:04PM by frustrated.
Re: Analog, self powered boiler controls
August 24, 2011 02:11PM
The White flowmotor system they introduced in 1907 is a fully proportional mechanical system with two overrides, pressure and temperature. The control system used three components, the water bypass pressure control; the thermostat water valve and the flowmotor itself.

1) All the water normally flowing to the coils goes through a precise tapered groove in the wall of the flowmotor. This is why when restoring a White steamer, the clearance of the flowmotor piston has to be exactly the same as the factory made it originally, or the car will never control properly. Water must not be allowed to flow around the flowmotor piston. A modern hydraulic lapped fit. Starting by having the bore absolutely round and Sunnen honed, then a new flowmotor piston was made to fit precisely after being lapped in.The lapped fit with my OO White was .0007".

2) The tapered groove must be precisely identical to the factory original and flow the right amount of water for the horsepower size of the car.
The 20 hp car flows one gallon in 1:40, the 40 hp car flows one gallon in 1:00. If the original groove has changed due to wear, or usually by the flowmotor being frozen and bulged during it's life so a new one has to be cut, it is wise to first make a plastic cast of the groove and use that for filing the new one. Then test-test-test until the flow rate matches what the factory used originally.

3) The two engine driven water pumps are oversize and feed exactly 100% excess water when the car is in high gear on a level road and going 18 mph. The excess water is bypassed back to the tank by a valve on the lid of the flowmotor. The pump check valve ball seats and the main boiler check valve ball seat must also be the correct height. Too deep from wear and the flow is reduced enough to throw the system off.

4) The pressure control is by a diaphragm operated valve that bypasses all the water back to the tank when pressure reaches the set point, 600 psi. It cheats the flowmotor of water and it then thinks the engine has stopped and the flowmotor piston goes back to the starting point powered by a 22-24 pound spring. The flowmotor piston moves a tapered fuel needle that controls the flow of fuel to the burner so this movement gradually shuts down the flow of fuel to the burner. It also opens a bypass water valve that starts bypassing the water back to the tank when the piston has moved down exactly one inch.
This actually causes the White system to operate in two modes. Below the point where the bypass valve starts to open, all the water is pumped into the coils. The fuel needle is biased a bit to feed more fuel than the system actually needs and causes the temperature to slowly rise. The thermostat sees this and starts feeding more water to the coils in a very unique manner. Not enough to completely close the fuel flow down; but enough to reduce the heat level where the steam temperature starts to go down slowly. See #5 below.

5) Temperature is controlled by the differential expansion rates of a copper rod in a cast iron tube that operates a tapered needle water valve. This valve starts opening around 425°F and is fully opened at 750°F and then feeds the extra water into the flowmotor behind the piston. The flowmotor then thinks the car is going slowly, because the flowmotor piston starts to go back up and starts shutting down the fuel flow to the burner. This extra water is not flowing through the flowmotor groove; but cheating the flowmotor and feeding the coils extra water around the backside of the flowmotor piston. This is not a really big movement; but rather small, just enough to start closing the fuel needle down.
An observant driver will see the steam pressure gauge stay rather steady just below the cutoff point. The dual fuel pressure gauge, tank and vaporizer pressure, will remain at 50-55 psi most of the time. When the steam temperature starts to rise, the vaporizer pressure will go down to perhaps 30-35 psi as the system has fed more water to the coils and reduced the fuel flow rate, then slowly climb back up as the extra water has caused the fuel valve to open more as the thermostat starts to reduce this extra water flow rate because the temperature is going down.

6) The White pyrometer on the toeboard is generally a liar. One permanently installs a 0-1,000°F Weston dial thermometer in the test well of the thermostat, bores a neat hole in the aluminum floorboard and uses that to see what is going on. Then, loosens the needle of the White pyrometer and goes for a test drive. When the Weston indicates a steady 750°F, taps the pyrometer needle down on its stem. Then when that gauge shows the needle at the red mark at 12:00 O'Clock, uses that gauge and not the Weston. White did not put numbers on the pyrometer, probably didn't want to scare the driver. The early Whites didn't even have a pyrometer at all.

It cannot be emphasized enough that modifying one's White steamer is an invitation, no, guarantee, to ruining a fine and dependable steam car.
The Edmonson Manual and the Owners Manual give all the correct clearances for the check balls and the fuel needle offset, believe what they say. They also say that there should not be any water leaks and they really mean not one drop, or it throws the system off.
The White is not really fussy, just very precisely made by people who really knew what they were doing and it has to be restored back to that same level of precision.

The 1909 O and M Owners Manual has beautiful engravings of the three main control units, and it can be purchased from the Club Stores.
Once restored properly, the first go around had too many mistakes out of ignorance, my White ran perfectly for 14 years with only yearly cleaning of the pilot and main vaporizers, keeping the water tank cleaned out, and blowing down both sides of the coil stack hard after every trip. I now bitterly regret selling the car and the Stutz.
The little White was used as a regular car on every nice weekend and never caused one bit of trouble. After restoring and driving all three major makes, I would take the 20 hp White over the Stanley and the Doble any day. It sure is not the fastest one; but a whole lot more reliable.
Why add the complications and lack of reliability of some "modern space age electronics" computer control system when something like the White worked so well for a hundred and one years??? We have the accurate drawings and patterns and core boxes for the White parts and interestingly enough there is a way to add a mechanical normalizer to the White system if you just cannot live without one. Although, it controls so well that you don't really need one, +- 5°F is good enough for any steam engine.


Edited 2 time(s). Last edit at 08/24/2011 03:21PM by Jim Crank.
Re: Analog, self powered boiler controls
August 24, 2011 04:27PM
Hi Jim

I havn't seen a White flow motor except pictures and drawings. It's been a while. But what I am wondering is one couldn't be made that with adjustment so you could set the flow. It seam a poor design if one has to keep making adjust ments by filing the slot.

Doesn't the piston move in and out and the slot being tapored allows more or less flow. Isn't there some adjustment to the depth of the piston?

Is anyone else unable to past from their editos to the forum?
Re: Analog, self powered boiler controls
August 24, 2011 05:00PM
If I recall correctly, Jim did an excellent article on the White Flow Motor recently, (within the past year or so) in the bulliton (sp?). My address just changed, will have to update that pretty soon here.

Re: Analog, self powered boiler controls
August 24, 2011 05:36PM
I am assuming you want to use a once through monotube. Perhaps look at the Educator controls on [www.firedragon.com] and vary the pressure setting by pivoting the attachment point for the pressure transducer spring loaded cylinder. The design is set up to save the steam generator from overheating past a certain temperature, and to provide steam at a slightly lower temperature, so the superheat would be quite high at low temperatures. This is not necessarily a bad thing...

Karl Petersen
Re: Analog, self powered boiler controls
August 24, 2011 06:38PM
I would love to see a Wetz controller in action and doing what it claimed to do. I have experience with a very nice version of it made by Mike Cross and it did absolutely nothing in the way of controlling a monotube, so we were doing something wrong. A table top version with no boiler attached proves little in my opinion. Maybe someone will bring an actual Wetz controlled boiler to the Berrien Springs meet? Maybe even TK? Here's hoping.

Re: Analog, self powered boiler controls
August 24, 2011 06:45PM
Yes, 'once through' is the objective since it seems to be the most simple, provided the controls work of course.
On the Educator system, is it thermal expansion of a part of the boiler that provides the temperature control?

Its good to hear the details of the White system, though its a bit more complex than I imagined.
If I understand it;
- you have pressure controlling feed water directly (on/off or variable?) which controls the fire rate indirectly and proportionally through the piston and the fuel valve it is connected to. This ensures water flow whenever the fire is on...
-A differential expansion thermostat controls a bypass valve that proportionately defeats the flowmotor piston, indirectly reducing the firing rate while the water rate stays the same.
-An intentional bias within the flowmotor metering ratios ensures a continual rise of temperature instead of over watering.

The flowmotor piston and metering channel are derived experimentally based on spring force, flow rate, and burner characteristics.
Is that it?
Re: Analog, self powered boiler controls
August 24, 2011 08:55PM
Once the flowmotor groove is set for the particular car, there is no need whatever to keep fiddling with it. The whole idea is the further down the piston moves, the more water goes to the coils, because the two pumps are capable of overfeeding the steam generator under all conditions. The groove is both varying in width part way down and also in depth. That is why it is smart to make a cast of what the groove really is for a particular car, then when you rebuild the flowmotor, duplicate it.
The factory determined this when they made the car and it stayed true until the end of production. The figures came from Roland Giroux and he got them from Jack Heiser, who was the head mechanic at the L.A. White agency. He was factor trained.

That groove is a constant, you don't want to fiddle with it at all. What is variable and this you set for the individual car, is the fuel flow bias. I think the Edmonson Manual says you let the piston go clear to the top lid, then turn the needle in until it just starts moving the piston, i think it says you then screw it in five more turns. My White was happier being set at seven turns in, letting a bit more fuel flow. Whatever, the idea is that this bias causes a slow overfire condition and a slow rise in the steam temperature.

There is really a two stage operation. Before the flowmotor bypass opens, it feeds what the coils need and you bias the fuel feed to slightly overfire the system. Then the thermostat comes into action and holds the temperature under control by adding extra water behind the piston and NOT moving the piston. The second stage is when the car is running at cruising speed OVER 18 mph. Then the bypass opens proportionally to keep the pumps from overpowering the system and at the same time the piston is seeing so much water coming in from the thermostat that the piston is moved up reducing the fuel flow. This is one very subtle system in operation and most people have no idea how it really works.
Get the owners manual and read it. The explanation there is as good as anything, or the 1912 edition of the Holmans "Motor Vehicles and Motors" book.

Re: Analog, self powered boiler controls
August 24, 2011 09:56PM
Jim, I was not thinking about fiddling with the setting. But to get it set right. I was more thinking of Chuck making a simular system. But with varing pressure I am not so sure a flow motor would work. One would really have to understand exactly how it works to design a new one for a different boiler size, pressure, temperature...

We were talking about a new design weren't we. Not restoring a antique to origional condition. In that light it would be nice to have you input on all the conditions it handles and how. Is the tapered grouve stright or does it have a curve.

Duplicating is one thing. designing a new one is another. Do you think it could be modified to handle variable pressure?


Edited 1 time(s). Last edit at 08/24/2011 10:02PM by steamerandy.
Re: Analog, self powered boiler controls
August 24, 2011 11:49PM
I'm hoping that ya'll will comment on the idea of using a water filled tube connected to a large bourdon tube as a self powered thermostat.
The bourdon tube concept is appealing because;
-the entire system is fully sealed without piston seals, diaphragms, bellows, or any other wearing or leaking part.
-Bourdon tubes are known for their linear and repeatable operation (thus their use in gauges)
-the power and displacement the system would be capable of should vary with the length of tubing inside the steam flow.
-the sensing and the actuator elements would be connected with a length of sturdy tubing isolating them from motion, vibration, and thermal expansion.

Unless someone shoots me down I think I'll rig an experiment to test temperature, power, and displacement using 3/16 stainless tubing for the sensing element and the large experimental bourdon tube I've got.

The White Flowmotor system is a very elegant analog system. It has a frightening number of seals, valves, and other moving parts for this novice...
Re: Analog, self powered boiler controls
August 25, 2011 02:59AM
OK, I womped together a test rig to measure deflection, return consistency, strength, etc. I have a boiler providing steam, a thermometer, and 20 inches of 3/16 tubing inside the steam line. The home brew Bourdon tube is made from 1/4 in iron pipe and the tube is filled with water. I took a short video of the deflection under direct heat which you can see here:

bourdon tube experiment

I'll post some data when I have the time to finish the tests, but as you can see, the tip travel is more than a quarter of an inch and should be quite strong. It really should be a spring tempered tube...

Link should work now...

Edited 2 time(s). Last edit at 08/25/2011 12:40PM by ChuckW.
Re: Analog, self powered boiler controls
August 25, 2011 04:47AM
I'll be interested to see how the tests turn out. When I tried something like this, the forces generated weren't real great, much less than I suspected. My thought was that the coiled tube had enough possible deflection in three dimensions so that it just took a path of lesser resistance rather than generating the work I desired. Maybe my test rig wasn't correct, or maybe such devices need to be designed carefully to provide just one degree of freedom.


Re: Analog, self powered boiler controls
August 25, 2011 10:29AM

I get 404'd with the link.

Re: Analog, self powered boiler controls
August 25, 2011 12:31PM

Varying pressure?? Why would you not want the steam generator running at a constant pressure and then the throttle and the cutoff control what the engine gets? Where is your reserve now? Seems to me that the thermal hysteresis would be a big nasty problem no matter how small the steam generator is. Sounds like introducing a whole lot of problems that just as easily can be avoided and are totally unnecessary. I see absolutely no reason to dance down this path. The Lamont solves all the problems in one step.

The flowmotor groove is straight from end to end; but it angles down starting at the top of the flowmotor where the lid is with a point then opening up as the slot goes down when the piston is all the way down. Thus more and more water as the piston is pushed down by feedwater pressure.
I suspect; but cannot support this, is that the factory made them with a shaper and the casting bolted to a jig that was at the angle necessary to make the groove as long as they designed it to be. It is not from end to end in the bore; but goes about two thirds down. You need to look at the Owners Manual that the Club Stores sells or go look at an original flowmotor that is in good shape and not modified by some experimenter who didn't know the system or what he was doing.

I suspect that the factory spent all of 1905-6 developing the new flowmotor system over the previous one they used and did an enormous amount of testing and evaluating before putting it into production. The White Company was no knee jerk outfit. According to good information, everyone with engineering responsibility had an engineering degree.
The late Whites are an extremely well balanced system and when restored to the original factory specs, are as reliable a steam car as one would want.

There is no need to design a new one, as the original will do very nicely and we have the patterns, core boxes and drawings for all the White control units and parts.
However, keep in mind, the White flowmotor was designed to work with their superb and very powerful vaporizing burner and changing that to an atomizing type would probably be a major project. Although, if the fuel needle was inverted to work as a bypass valve, that might be a good way to adapt the system to a modern car project, at least a starting point with perhaps a butterfly valve in the air blower duct as another way to simulate the original White system.

What is so damned frustrating is that all the engineering information and files seem to have been trashed at the factory. We have all searched and asked world wide for surviving design and service information about these cars; but except for the Edmondson Manual, Owners Manual and parts book, nothing seems to have survived.
Henry Merkel, Walter White's grandson, has personally gone through the surviving family archives; but it seems from what he discovered that what happened is when Thomas White decreed there would be no more steamers and from now on only gas cars and trucks, there was one huge split in the family that lasted for decades.
The Stanley and Doble data has all survived; but not the White.

Look, several people have asked about a dependable all mechanical system. White made some 10,400 cars from 1900 to 1912, with the flowmotor system being introduced in 1907. Several thousand cars at least; but I have not counted up the cars from 1907 to 1912 on the build list to see just how many were made.
All I can tell you is that when it is done correctly, it works just fine under all road conditions. It uses, of course, a forced circulation monotube. Today I would definitely use the Lamont for many reasons. Then one only needs a level control and perhaps design the buried superheater to provide more superheat then you really want and control that with a normalizer. Infinitely easier than what one needs to do with a pure monotube IF it is being forced really hard or uses a draft booster on the burner.


Edited 1 time(s). Last edit at 08/27/2011 03:02PM by Jim Crank.
Re: Analog, self powered boiler controls
August 25, 2011 03:38PM
I think these systems are fine and dandy, but.

The last time I met with Tom Kimmel he made a remark about the 4cycle steam engine im developing, he remarked "the way you have things arranged it simplifies boiler/monotube controls".

As I was learning about steam engines in my teens and watching the old steam tractor's in action, the operators/engineers would remark "the boiler is actually part of the engine".

Re: Analog, self powered boiler controls
August 25, 2011 06:10PM
Dear folks, In regards to the Wetz control system, it is so simple that I had to sit and look at it for a year to begin to understand it and then a little while longer to notice the subtleties. Ken Helmick has not had the luxury of spending the whole year and so he got one small part of it incorrect. The hot steam pipe is only fastened at one end and the other end slides through a hole in the spring ring of flat metal with very little friction. As for the question on quartz rods, there are none. You want to use a quartz rod when you can only hang onto the metal tube at one end of it. The other end is capped and the quartz rod is spring loaded to rest against the capped end, hence it, the quarts rod, moves back and forth at the other end near where it is fastened to something. Wetz put in a mechanical advantage and actuated micro switches gleaned from an old Hammond organ and these ran the solenoid that lifted the check valve for the water pump and the solenoid that ran the propane line. The solenoid for the pump check valve was made with a 16 penny nail and a few feet of copper picked up along the road. As for the other question about how the two rods were interacted; it is possible to put them parallel and have some complicated linkage floating around out there in space to connect them; with second and third class levers being used. I worked it out once and quickly drew it up before it floated away from the consciousness. The whole trick with a monotube is to not flood the boiler. Terry Williams did a study of this once and found that there was a time factor, 11 seconds in the Smith type monotube, and I did not understand hardly a thing about where the time factor came in. Tom Kimmel
Re: Analog, self powered boiler controls
August 25, 2011 06:51PM
I remember the article by Tom Kimmel in the Bulletin about something called "GreenBoard" is this the same type of monotube control system (John Wetz).


Re: Analog, self powered boiler controls
August 25, 2011 09:30PM
The "Green Board" was Smith's first automatic system and an example of an attempt at a good control system, now simply historical.

The Educator controls corrected the problem of it changing temperature with changes in demand. This is the patented control. Smith's later controls using on-off systems, temperature sensors, pressure switches were merely electrical implementations of the Educator system, and the Educator system is the simplest. Use it.

Overheat control is from the hottest tube in the system, the last pass of the steamline straight through the fire, and it will always be there to save the tubing from overheating. The steam temperature control is a length of steamline just after the steam exits the fire, and it controls both fire and water. Pressure also controls both fire and water. I cannot conceive of a system in which you would want the water to only be controlled by the fire or the pressure or for the fuel to be only controlled by either the fire or the pressure. That's why both the fire and the water are controlled in concert by the temperature and pressure. And it works.

Karl Petersen
Re: Analog, self powered boiler controls
August 25, 2011 09:31PM
Hi Jim. Just to get this stright Chuck was interested in a variable pressure. If I remember right Jay Carter used a variable pressure also. I thinke Chuck may be trying to do the same.

It is auful that we don't have any factory info on the White flow motor control. It seams everyone that has had a White says the same thang about them that you have.


Edited 1 time(s). Last edit at 08/25/2011 09:34PM by steamerandy.
Re: Analog, self powered boiler controls
August 25, 2011 09:53PM
A Bourdon tube is a great device to repeatably indicate pressure in a gauge. It is unfortunately going to be too weak to use as an actuator in a control system which has to move valves. It would be ok with electrical contacts to drive electronics or relays where there is no change in force when the contact is made or broken. If there is any reactive force, then the contact values will not be accurate. The Meletron switches used extremely light microswitches and very heavy Bourdon tubes and were successful, although they had wide deadbands and were affected by vibration.

A spring loaded cylinder with floating seals is likely to give enough power to run a small valve directly. That valve can be used as a pilot to a larger actuator or, if it is balanced and light to operate, could be used to control fuel or water directly.

If you have committed yourself to using a Bourdon tube, it is possible to have it move a vane which cuts a stream of air flowing at right angles to the vane motion. The passing air can be balanced or unbalanced and can be the pilot for a larger actuator by back pressure or by varying the flow. This works nicely for a proportional control over a small range of pressure variation. It is not necessarily obvious how to set it up and get repeatable and accurate results, but it is a possibility if you don't mind complication. I have used it many times.

Karl Petersen
Re: Analog, self powered boiler controls
August 26, 2011 01:28AM
I'm enjoying learning about all these analog systems. Thanks for taking your time...

I continued the 'quick and dirty' tests of the water/steam powered bourdon tube this evening...
For a benchmark, the deflection at 600 degrees is very close to 1/2 inch with a force of about 30 pounds. It was almost a hundred degrees to start with so that would be one thou per degree.
This is spring scales and non contract thermometer metrology... not exactly precise. The tube is pretty strong though.
I got carried away with the direct heat and the tip deflected about an inch, which was beyond yield for whatever semi steel alloy is used in black pipe. It requires over fifty pounds of force at the tip to move it by hand.
I'm encouraged enough by these results to order some 17-4 stainless and make a few heat treated tubes.

Variable pressure boilers-
I'm using a bash valve expander so following the Carter system seemed the thing to do. I don't know how throttles vs variable pressure boilers compare thermodynamically but Jerry Peoples and Jay Carter wrote articles promoting them for some situations so that is what I've pursued.
Using water to control temperature independent of pressure also seemed like a simplification worth investigating. Am I failing to understand the Carter system? Isn't that what they did?

The only thing I'm really committed to is finding a simple, rugged, inexpensive, non electric, once through boiler control system.
So far the oversize bourdon tube looks like it has promise to provide good linear power and displacement from a self powered control, while eliminating mechanical linkage between the boiler and engine/pump.

I will continue to investigate the various thermal expansion mechanisms discussed.
Thanks again,
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