Pressure Controls

PMJ

Consider this analogy:

A steam heating system maintaining temperature is like a hot air balloon maintaining altitude. If both systems have a modulating flame then it is possible to change the flame to suit the conditions and maintain the temperature or altitiude quite close to the desired setpoint. Unfortunately, most of us have a one speed flame.

A hot air balloon with a one speed flame will rise when its flame is on and will fall when it is off. The longer the burn times the higher above the desired altitude it will rise and so necessarily the longer the off times will be while it falls back down. If you choose to have more shorter burns closer together you can operate the balloon always much closer to the desired altitude.

Our steam systems are the same. Our boilers cannot be the right size on an average day. A boiler that will heat the place on design day is at least 2 times too big on an average day. With a one speed flame there is simply no getting around this fact. So our only choice now becomes how long to run the flame. Just as with the hot air balloon, longer burns means higher rises above the setpoint and longer waits for the temperature to fall back down. In my view any burn allowed to run to a pressure stop in these systems is a burn that is already way too long for the conditions.

Any 75-100 year old steam system was designed with extra radiation and it was intended to be operated with that radiation never full. They were operated with single digit ounces of pressure at the coal fired header and at zero or even mildly below atmospheric in the radiators. Pressurizing these systems to the point where traps close at all or vaporstats trip is proof that way more steam is in the system than is required to match the heat loss and you are now surely rising considerably above the temperature setpoint just like the hot air balloon gaining altitude.

I have found any pressure at all to be the enemy. Uncomfortable and wasteful. It causes much bigger swings in the temperature of the radiators and therefore the rooms too. It really doesn't take that many more cycles to totally eliminate it. In 2 pipe when you add natural vacuum to that it gets amazingly better yet. I am convinced that the operation of these systems in a way so unlike the original intent has only hastened the demise of residential steam by making it far less comfortable than it could be. After what I have experienced with my 2 pipe I would never consider going back to a pressure based control.

Eastman

Have you made any recent changes to your control system?

the_donut

I think varivac with modulating firing is the way to go. Match vacuum to steam temp needed and vary firing to reduce output.

PMJ

> @Eastman said:
> Have you made any recent changes to your control system?

Actually yes. I've been interested lately in coming up with something simpler so no PLC required. I am looking for a simple control where the cycles self adjust to the conditions.

I've been running the last 2 months or so using only a preheat sensor, the ultra low pressure switch that tells me whether I have vacuum or not, and a timer. I moved the preheat switch from a point on the main in the basement to the feed pipe on the most remote radiator I have in the garage so I know when steam is actually at that point.

So now on a call for heat I run until both the preheat switch is satisfied and there is no vacuum plus 2 minutes. If the system has been off a long time any remaining vacuum will disappear before the switch is satisfied. During repeating cycles the switch is always satisfied while most of the vacuum is still present because the steam flies out there so fast in the full vacuum. New burns are initiated when the preheat switch opens back up - I'm guessing 150-175F (it is an adjustable molding die temperature control switch and I never calibrated it to anything). Of course this control is just in series with the tsat cycling the burner during the call for heat.

The interesting thing (at least to me) is that after the first cycle that really requires a preheat, the cycles settled in right at 20 minutes +/- or 3 per hour. That really seems to be a sweet spot. When it gets colder, the vacuum gets deeper as more steam is condensing which requires a longer burn to kill it. The garage radiator cools faster which drops the wait time a little and the cycles get a little closer together with the burn a slightly higher percentage of the cycle. The bottom line is the system adjusted the cycles to a higher burn minutes per hour all on its own - with components that might cost $150.

Not to bore anyone but I also noticed that at the moment the burn just kills the vacuum (5-6 minutes average into the burn) the fill level of each and every radiator is exactly what it was when the burner shut off on the previous burn. This is good to know as it makes it really easy to maintain the partial fill level I talk about a lot. My plus 2 minutes accounts for any leakage and you always need to be gaining a little and definitely not losing ground.

Anyway, I was quite pleased with this and haven't changed anything for a while. One note, it won't recover a cold house fast though. For some reason my wife turned the thermostat off one night and the house was 60F when I got up. I was leaving for work and told her she had to wait it out as I couldn't change the program right then. It took 3-4 hours to warm up. Anyway, I would have to add something to handle that condition.

Jamie Hall

You know, the funny thing bout all that... is that from the standpoint of someone in the space, that's not a bad description of how the system in the main place I care for works. With a standard thermostat and a standard vapourstat and nothing. nothing fancy.

BobC

@pmj

"For some reason my wife turned the thermostat off one night and the house was 60F when I got up. I was leaving for work and told her she had to wait it out as I couldn't change the program right then."

It may not have been your fault but beware. My sister used to starch her husbands underwear for something like that and he worked construction! Do not piss them off.

Bob

PMJ

Jamie Hall said:

You know, the funny thing bout all that... is that from the standpoint of someone in the space, that's not a bad description of how the system in the main place I care for works. With a standard thermostat and a standard vapourstat and nothing. nothing fancy.

Jamie,

The system I started with was a mercury switch tstat with anticipator and a mercury switch vaporstat and vented mains both supply and return. Standing in this space my current results are dramatically better than the best I could achieve with that. The boiler still ran longer than I wanted it to in a single stretch. Much quieter now, much more even heat, and more efficient. It may be mostly the vacuum. But the old Tstat had no schedule, and I needed the PLC to be able gather some data to study what is going on.

I don't wish to offend anyone . I only wish to encourage those interested to try some of this. There is still a lot of room for improvement.

Jamie Hall

PMJ said:

Jamie Hall said:

You know, the funny thing bout all that... is that from the standpoint of someone in the space, that's not a bad description of how the system in the main place I care for works. With a standard thermostat and a standard vapourstat and nothing. nothing fancy.

Jamie,

The system I started with was a mercury switch tstat with anticipator and a mercury switch vaporstat and vented mains both supply and return. Standing in this space my current results are dramatically better than the best I could achieve with that. The boiler still ran longer than I wanted it to in a single stretch. Much quieter now, much more even heat, and more efficient. It may be mostly the vacuum. But the old Tstat had no schedule, and I needed the PLC to be able gather some data to study what is going on.

I don't wish to offend anyone . I only wish to encourage those interested to try some of this. There is still a lot of room for improvement.

Oh heavens yes -- lots of room for experiment and improvement. I just want to note that it is possible -- with a very well matched system -- to get very good results with very simple controls.

ChrisJ

@PMJ With all due respect you're wasting your time. No one is going to change the way they think or the way they do things.

A 33% oversized or even 50% oversized boiler will be mated to already oversized radiation along with slow radiator vents and a thermostat set to 1 cph. THAT'S THE WAY WE'VE ALWAYS DONE IT.

Jamie Hall

Relax. Start off with the objective: adequate heat with minimal temperature swings. Then think of what you have. Is this a new installation? Is it a new boiler for an old installation? Is it to tweak and existing system? If it's a new installation, match the system -- radiation and boiler -- to the heat loss. If it's a new boiler for an old installation, match the boiler to the radiation. In all three cases, make sure the piping and venting are correct for the type of boiler and system (there are many variations here).

Then, and only then, begin to think about the best way to control it -- and there are many good options. It is good to look at as many as one wants to. Particularly with poorly matched existing systems, it may be possible to really improve things with more complex control strategies. With a very well matched system, perhaps not. But in any event, the bottom line is does it do what it is supposed to do? That second sentence up there? If it does, well and good. If it doesn't, figure out why and fix it.

Eastman

Jamie Hall said:

Then, and only then, begin to think about the best way to control it -- and there are many good options.

All the options seem mediocre to me. Control systems for air and hydro have progressed quite a bit and residential systems are able to take advantage of that. If anything, steam has gone backwards.

Jamie Hall

Eastman said:

Jamie Hall said:

Then, and only then, begin to think about the best way to control it -- and there are many good options.

All the options seem mediocre to me. Control systems for air and hydro have progressed quite a bit and residential systems are able to take advantage of that. If anything, steam has gone backwards.

Not sure I really agree -- although in general most steam system controls are pretty stone age when compared to the computer driven wizardy for variable speed pumps and the associated controls for hot water systems and some of the rather sophisticated systems for air handlers and heat pumps. This does not mean, however, that there are not some very sophisticated controls possible for steam, many of which have been discussed on the Wall (sometimes rather vigorously!). For instance, if one is talking about Paul type one pipe steam system, or some two pipe systems, and one introduces a variable vacuum, there is nothing you can do with outdoor reset/variable speed pumping in hot water that you can't do in steam. There is a lack of ready-built hardware (condensing steam boilers are a bit thin on the ground, for instance, as are analogue (as distinct from pulse) modulating burners for steam boilers) but that doesn't mean it can't be done.

The real question is... should it be done? It is one thing for some of our more daring experimenters to do it -- and I applaud their efforts, though they may not think I do sometimes! -- but as a normal part of hardware available to the general public? Perhaps not. One of the virtues -- and not the least -- of a "normal" steam heating system is its extraordinary simplicity and reliability. This has been obscured a little in recent years by some doubtful quality control on some control components, and by shorter lifespans for boilers (largely a trade-off for the last few percent of AFUE -- a doubtful measure), but it still remains as a characteristic.

It is this kind of trade-off -- between bullet proof reliability on the one hand vs. high tech bling on the other -- which a truly conscientious engineer or salesman should consider on every job, and they also need to consider the preferences and needs of the client. There is no one size fits all -- indeed, every job will be different.

There is also a little matter of cost and craftsmanship: it's a lot cheaper and easier for relatively unskilled labour to come up with a more or less acceptable result in hot water and particularly in air than it is in steam. To come up with a really good top end system? Not that much difference -- but it's going to cost, either in installation or maintenance, and that is a factor.

SeymourCates

@PMJ

You are entirely correct with all of your analysis. Don't permit those that must live in the past to derail your precise conclusions. I do need to inform you however, that Tekmar makes an off the shelf unit that does precisely your intent (sans the vacuum of course). It's called the Tekmar 279 and you can program it for whatever cycle length you desire on the design day. The control will reduce the boiler run time as a percentage of the cycle time based upon outdoor reset for any other day. It works extremely well and will typically hold the building within 1.5C without a thermostat for the entire heating season. For those that require better accuracy, Tekmar offers indoor sensors that will shift the curve based upon the input from those sensors thereby accommodating high wind conditions.

It's a well designed system and puts the cavemen to shame. Of course they will never understand it and will certainly never attempt to utilize it.

Jamie Hall

SeymourCates said:

@PMJ


It's a well designed system and puts the cavemen to shame. Of course they will never understand it and will certainly never attempt to utilize it.

You're welcome to visit my cave, sometime, @SeymourCates . Just give me a PM and come on by. Only I must admit that I don't really appreciate being classed as a caveman -- though I'm not surprised that some of the more enlightened might think me one.

Thanks.

Gsmith

Count me in as a caveman too. I"m constantly amazed at how simple, reliable, mostly quiet and evenly and comfortably heating my one pipe steam system is, despite the boiler being about 50% oversized compared to the installed radiation (boiler EDR on nameplate versus actual radiators installed). Since i have re-insulated the steam mains in the basement, and added good main vents, and rebalanced the radiator venting, I have only once this year seen the pressure gage (0-3 psi) needle move more than slightly off the zero peg. HIghest pressure developed was about 4 oz on a very, unusually below design day. And, I should also add my system is controlled by a millivolt thermostat and gas valve, which over the 35 years I have owned it has on quite a few number of occasions proved its worth when I had heat and none of my neighbors did during extended electrical outages. Simplicity is a great virtue.

I know one pipe steam systems, particularly one as old as mine will probably not ever be as popular and more high tech newer systems with internet controls from a phone, and that it takes a little time each week to stop by the boiler and check things are running smoothly. Heck, most people don't know how to check the oil level in their cars and never do anymore. Guess that's progress.

I have been mulling over the past few months, the many posts on vacuum systems and the potential for fuel savings and have been trying to figure out a rational way to calculate the potential savings if I were to go to a vacuum type system on my own--if I could just find vacuum radiator valves and a low seating pressure check for my Big Mouth vents. As I see it, with very little pressure developed in the system, always less than 4 oz, the only real savings would be the boiler run time to push the air out of the system (total air in system divided by the boiler steaming rate in cfm), but even then, the steam produced in that time interval has the heat of vaporization which it gives up to the heated space and so I don't right now even think the boiler run time to evacuate the air is entirely wasted. I'd like to calculate the potential savings first, even approximately, before investing time and $ in the vacuum conversion; most particularly because I want to weigh the added complexity and increased reliance on the electrical power being available, against the simplicity, ruggedness and reliability I currently have.

zoom

I'll throw in a another homeowner's perspective.

There are all kinds of trade offs between accuracy, precision, reliability, safety and cost (feel free to add to the list). Different aspects are important to different individuals in different situations. Hence a wide spectrum of opinions on this board and across the industry as a whole.

For me as a steam novice and homeowner, reliability and cost are the most important. I want reasonable comfort for a low heating cost, and most of all I want a system that heats the house reliably. Safety is a must have. I could not care less about bells and whistles and I see them as a potential liability and extra cost, unless they strongly contribute to cost savings and reliability or safety.

However, I recognize that others might be deeply interested in developing their custom control systems and that these systems could highly effective. And I am always interested in what these individuals have to say because I might learn something that matches with my preferences that I could implement.

There are some basic things to get right on a steam system that every professional and every homeowner needs to know. Venting, steam pressure, insulation, boiler sizing, near boiler piping, and an understanding of how an individual system was designed to work seem to be those key elements that must be right from my experiences and readings.

Everything that comes after the essentials provides marginal optimization of the system as a whole. Marginal does not mean insignificant, but the significance depends upon what's important to the individual and will always be a matter of personal preference.

Gsmith

yes, well said.

SeymourCates

@Jamie Hall

While I appreciate the offer, I have no need to visit your cave. I fully understand that you operate a system that has the net output nearly perfectly matched to the radiation, and the combination is nearly perfectly matched to the heatloss of the structure. You operate on the pressure limit very, very infrequently and only near the design day.

Such nirvana does not require any control other than the thermostat. Your pressure controls hardly operate at all. Your system would not benefit by any of the electronic controls devised by @PMJ or other commercially available units.

What is troubling is the fact that you patently refuse to believe or understand that 98% of the steam systems out there do not have the luxury of the trifecta. They are usually oversized to the heatloss and the boiler is typically oversized to the radiation. Your generalized solution of "turn the pressure down" is wildly in error for such systems and causes the H/O extreme fuel usage and system degradation.

@PMJ has correctly identified the proper solution for these systems (as has Tekmar and others) and it is far superior to controlling the boiler with pressure. The sad part of the facts are the people who incessantly undermine the work of @PMJ and others because they do not fully understand modulation using cycle time and length and believe their system (which really doesn't modulate on anything but a thermostat) can be applied everywhere.

Jamie Hall

My dear @SeymourCates . You need to read more carefully what I write; it would help you. Your third paragraph is just plain wrong -- while the second sentence is true, both the first and third sentences are completely false. You are not helping your argument -- or @PMJ 's -- and you both have valid points, and very good solutions to some difficulties, but frankly rhetoric such as "caveman" or flat out false statements do not help others appreciate that.

Fred

I have learned there are several personality traits of the posters on this site:
- Those who love to "Tinker" to perfection (for their system) and believe that should become the standard for all systems.
- Those who have found good alternative controls and offer them up for discussion and debate/validation.
- Those who just want their system to do what it is suppose to do with a high degree of reliability and repeatability and who will consider PROVEN and generally available alternatives, when a replacement is needed .
- Those who are completely "hands off" and want to know that when they do need a service professional, that professional can come with standard replacement parts, diagnose the problem in a conventional way, fix it and leave. Anyone who has to try and figure out customized systems/ components and what they do only adds to the labor charges and more often than not will be screwed up when that professional is done.
- Those who are just cheap or very poor and just want to get through the winter.
- and those who just like to talk
Decide which you are and enjoy being that way without insulting everybody else on this site. It serves no purpose. I for one have no interest in controlling my one pipe, 35 year old boiler/116 year old system from the grocery store, just because I can, or trying to convert it to a vacuum system that might save me a very few dollars a year or keep my home temps any more even than they already are or that allows me to boast about all the changes I've incorporated into my system and it still works! I guess that makes me a "Caveman" also. I'm good with that. Oh, I guess There is one more Personality trait I should add to the above. That being "Cavemen".

BobC

@Fred At my age I just want the system to work without costing me an arm and a leg for fuel or service. I also like to keep things simple if I can, I'll reserve the complexity for my comcast internet service which is always finding ways to complicate my life.

Bob

ChrisJ

Those that feel a PLC added to a steam system makes it less reliable do not understand what a PLC is. Simple doesn't necessarily mean more reliable. For example what's more reliable, a Ford model T or a 747?

the_donut

Gary Smith said:

I have been mulling over the past few months, the many posts on vacuum systems and the potential for fuel savings and have been trying to figure out a rational way to calculate the potential savings if I were to go to a vacuum type system on my own--if I could just find vacuum radiator valves and a low seating pressure check for my Big Mouth vents. As I see it, with very little pressure developed in the system, always less than 4 oz, the only real savings would be the boiler run time to push the air out of the system (total air in system divided by the boiler steaming rate in cfm), but even then, the steam produced in that time interval has the heat of vaporization which it gives up to the heated space and so I don't right now even think the boiler run time to evacuate the air is entirely wasted. I'd like to calculate the potential savings first, even approximately, before investing time and $ in the vacuum conversion; most particularly because I want to weigh the added complexity and increased reliance on the electrical power being available, against the simplicity, ruggedness and reliability I currently have.

If it ain’t broke, don’t fix it. If you want the Mercedes of one pipe steam, then go for it, but you’ll never justify doing it out of economy. It’s like a tankless electric water heater, you can’t justify the small savings in electricity, you are after the endless hot water.

Fred

I guess I missed at least one category, "Those who think they know it all and certainly more than anyone else"
@ChrisJ , if they were both designed to serve the same purpose, I'll take the Model T, for obvious reasons.

ChrisJ

> @Fred said:
> I guess I missed at least one category, "Those who think they know it all and certainly more than anyone else"
> @ChrisJ , if they were both designed to serve the same purpose, I'll take the Model T, for obvious reasons.

Since you chose to be mean you also forgot those that ate asphalt as a child...


Now be nice and stop attacking others.

Fred

@ChrisJ , I guess that's why some say "I'm not right in the head".

Jamie Hall

An interesting and thought-provoking comparison, @ChrisJ . But first for the PLC -- if the PLC is added to the basic system in such a way that all possible failure modes do not degrade the basic system (running when it's cold, off when it's hot) then it won't have made things worse, at least. If it were programmed and connected in such a way that it would compensate for various other system failures on its own -- while maintaining that basic system functionality when it itself failed -- then it would improve system reliability. This can be done, though I would wonder if it is done (it certainly isn't for most computer applications I have encountered).

As to the Model T vs. the 747. Well... apples and oranges. It would be exceptionally difficult to really state, accurately. which was more reliable. Why? Because one would have to define what was meant. At least when I was flying, it was rare to depart without something on the bird which wasn't working. Nothing, mind you, which was actually required for flight (there is what is called a "Minimum Equipment List" which defines what has to be working to fly), but something -- or somethings. And I had an army of dedicated mechanics available to squash the bugs which might have kept us from launching. The remaining squawks were kept for the next 25 or 50 or 100 hour inspection. MEL squawks usually could be resolved in the turnaround, but not always ("Flight 992 to White Pigeon will have a slight delay. We appreciate your patience"). I have never driven a Model T -- but I learned to drive on a Model A, and it, too, often had squawks. Once in a while it actually quit on the road. But there was nothing on that vehicle that I couldn't fix with a couple of wrenches, a screw driver, a pair of pliers and a dollar bill -- and a good ear.

But it is an interesting and thought provoking comparison! You've given me something to contemplate philosophically for quite some time here, thank you!

EBEBRATT-Ed

@Fred, Well said.

ChrisJ

Jamie Hall said:

An interesting and thought-provoking comparison, @ChrisJ . But first for the PLC -- if the PLC is added to the basic system in such a way that all possible failure modes do not degrade the basic system (running when it's cold, off when it's hot) then it won't have made things worse, at least. If it were programmed and connected in such a way that it would compensate for various other system failures on its own -- while maintaining that basic system functionality when it itself failed -- then it would improve system reliability. This can be done, though I would wonder if it is done (it certainly isn't for most computer applications I have encountered).

As to the Model T vs. the 747. Well... apples and oranges. It would be exceptionally difficult to really state, accurately. which was more reliable. Why? Because one would have to define what was meant. At least when I was flying, it was rare to depart without something on the bird which wasn't working. Nothing, mind you, which was actually required for flight (there is what is called a "Minimum Equipment List" which defines what has to be working to fly), but something -- or somethings. And I had an army of dedicated mechanics available to squash the bugs which might have kept us from launching. The remaining squawks were kept for the next 25 or 50 or 100 hour inspection. MEL squawks usually could be resolved in the turnaround, but not always ("Flight 992 to White Pigeon will have a slight delay. We appreciate your patience"). I have never driven a Model T -- but I learned to drive on a Model A, and it, too, often had squawks. Once in a while it actually quit on the road. But there was nothing on that vehicle that I couldn't fix with a couple of wrenches, a screw driver, a pair of pliers and a dollar bill -- and a good ear.

But it is an interesting and thought provoking comparison! You've given me something to contemplate philosophically for quite some time here, thank you!

Jamie, the A is another good example.
Sure, it's extremely simple and easy to fix, but would you consider it even remotely as reliable is a modern car with solid state ignition and fuel injection? The "A" is dead in 80,000 miles while a modern, overly complicated car is barely broken in most of the time.

Simple tends to be more reliable and complicated tends to be less reliable, but it's not the rule. Complicated can often be more reliable than simple, if done right.

PLC's in general are unbelievably reliable.
Meanwhile, it seems like simple vaporstats are incredibly unreliable especially lately.

I'll take PLC over one any day.

Jamie Hall

And I'll take the A. When my wife's modern car, with all it's lovely things, is running it's wonderful. When it's not, it's a tow truck and anywhere from 2 days to a week in the dealership (who's the only one with the privately owned tools and manuals to work on it). The A? Well... when it breaks down, and it does, I climb off the driver's seat, mutter a bit, fix it, climb back on and I'm on my way.

My tractors are, perhaps, better comparison, though. I will NOT by a new tractor with all the bells and whistles. I'd love to have it, but when it breaks down in the middle of haying -- and it will -- I can't fix it and would have to wait a week or two for the dealer. That's lost income; timed right, it's a lost year. When my older tractors break down -- and they do -- I get the tool kit out of the truck and fix them. That's NOT lost income. End of story.

the_donut

I’d rather have reliable and simple for home. I like complicated at work. That way I have the best of both worlds, but the difficult one I don’t have live with on a daily base.

BobC

My take on this is a complex system usually has more parts, more parts tends to lesson reliability. I have found PLC's to be reliable, the signals going to and fro are not so reliable. keep in mind the machines I worked on were 60 to 150ft long and my experience is the canbuss is a bag of worms at best. Your signal lines are pretty short so that should not be a bid issue in your application.

The work that is being done on steamer control is really amazing but I note most have a switch that returns the system to simple thermostat / pressure switch operation. That switch is there for a reason, if your lucky you won't have to use it but it's nice to know it's there.

Bob

Jamie Hall

Many moons ago, @BobC , I had the privilege of working on a real cutting edge project (in a rather peripheral role, but still part of the team!). We pioneered what was then known as "Zero Defects" engineering and construction -- what we really meant was that every part or assembly had to be verified to have at least a 99.999 percent probability of functioning properly for the duration of the mission. Even at that level, we almost lost one...

What's always amazing about that sort of thing is how the reliability goes down with the number of parts. Suppose you have 100 possible failure points (connections, contacts, transistors, plugs, what have you) each of which has a probability of performing correctly over the life of the equipment of 99%. The overall assembly has only a 3 in 8 probability of functioning correctly for the life of the project...

It's amazing that anything works at all!

BobC

@Jamie Hall I agree more parts leads to less potential reliability. My days of flying on commercial aviation for work are over and with the advent of fly by wire I'm kinda glad they are.

While I was still in harnes the PO was installing robots to sort the tubs of mail into containers so they could be loaded onto trucks or planes. These were installed in cages (24x40ft), the cage had 4 rows of containers and the robot would place each tub into the correct container if all went well. These containers had a folding shelf in them so the container would be filled half way up and the robot would lower the shelf and then fill up the rest of the container. The software read the barcodes on the tubs that were delivered by conveyor and keep track of how many tubs each container had. When a container was full the software would stop the machine so the operator could swap the full container for an empty one. There were 2 computers and 3 PLC's that controlled that controlled the machines operations.

The robot moved at a good speed so it was important nobody was in the cage while it was in operation. The designers implemented the safeties in the software, there were no hard safties in the system. I always thought that was a bad idea and one day while helping another tech work on the machine we found out why it was a bad idea. Jim was in the cage while I was standing by to restart the machine - two people on opposite sides of the machine were needed to initiate a run. Jim as in the cage checking some eyes with the rolling door open (the doors were all on the estop loop) and the robot started up on it's own and started to tear don the other end of the cage. I reached down and killed the main power.

They went over that machine in detail and never found out why it did that. That machine was fly by wire, I prefer the old mechanical estop system where a dual set of switches take power off the start relay. The mechanical system can be a pita if dust gets into the switched but I'd rather waste some time looking for which switch (some machines had 40 estops) than worry about a bit of code getting confused and allowing a machine to start up.

After that day nobody was allowed in the cage without someone standing by the knife switch.

Bob