There is a fundamental principle of safety engineering that is being overlooked here.
If the manufacturer is not responsible for the explosion leading to great bodily harm, that leaves 2 other possible 'causes' (that I can think if....) 'Operator Error' or 'sabotage'.
If a possible consequence of failing to do proper maintenance is to create and detonate a bomb, clearly there is still an issue of safety engineering which is squarely with the design and manufacture.
Evaluating failure risk is about both the (however remote) possibility of multiple safety elements failing at the same time and (more importantly) the consequence of such a failure.
If such a failure produces a loud BANG, a cloud of steam, a flooded floor, and a $2000 machine rebuild, that is acceptable risk.
If potential loss of life and limb is a result of the failure, that is an unacceptable risk, however remote. Safe design is about redundancy. A seal or low pressure plate blowing, overpressure cap, intentionally weak seam splits; all these are the last ditch 'fail-safe' design elements that can be used to prevent this.
There is a theory in engineering about why we don't have more and better safety engineers; It is a basic risk/reward problem
Risk. Loss of life and limb to innocent people. Reward. A handsome plastic wall plaque.
Posted Wed Sep 15, 2010, 2:10pm Subject: Re: Safety valve maintenance for home machines
c4h5n2o Said:
I don't believe that safety valves fail very often. But pressurestats do fail often enough and I know lots of people with machines that have had the safety valve open. Since the safety valve really is your last line of defence, you shouldn't neglect it to the point where it might seize.
Testing it sounds like a good idea. I've never had much trust in pressurestats based on past experience with an hx machine. I'd like to think electronics are more reliable, although never infallible. Most domestic machines seem to have a thermal fuse or a secondary thermostat as a second line of defence - the small commercial Techno I have has a 145c secondary thermostat I think on the steam boiler. It sounds as if the big machines don't. Is this so ? (I can see with a really big boiler it might be tricky to implement).
A question for the experts. The steam boiler on mine has a 3 bar safety valve. I can (and will) give the release ring on that a tug tomorrow ! As far as the brew boiler is concerned, I'm assuming that the overpressure valve in the hydraulic system (which I think is pre-set to 11 bar) replaces this as this part of the hydraulics is presumably open to brew boiler pressure. The boiler has to be able to handle ~ 10 bar at an operational level, so presumably these sort of boilers must have quite an additional margin built into them.
Any expert reassurance - or maintenance advice - would be gratefully received.
c4h5n2o Senior Member Joined: 19 Jul 2005 Posts: 234 Location: Vancouver Expertise: I live coffee
Espresso: LM Grinder: Bunn/Vario Drip: Swiss gold Roaster: No time
Posted Wed Sep 15, 2010, 2:19pm Subject: Re: Safety valve maintenance for home machines
__________ Said:
The boiler has to be able to handle ~ 10 bar at an operational level, so presumably these sort of boilers must have quite an additional margin built into them.
Just to clarify, espresso machine boilers typically operate below 2bar and commonly closer to 1bar. It's only the group head or HX/group head that is pressurized to 9 bar. From a materials standpoint, the boiler isn't likely to fail i.e. rupture/explode unless it reached something like 20 - 50bar. But that isn't something you want to test at home, either.
Posted Wed Sep 15, 2010, 2:23pm Subject: Re: Safety valve maintenance for home machines
c4h5n2o Said:
Just to clarify, espresso machine boilers typically operate below 2bar and commonly closer to 1bar. It's only the group head or HX/group head that is pressurized to 9 bar. From a materials standpoint, the boiler isn't likely to fail i.e. rupture/explode unless it reached something like 20 - 50bar. But that isn't something you want to test at home, either.
I don't understand how the machine works in that case. How can a brew boiler not be pressurised to brew pressure when you are pulling a shot ? I'm not referring to the steam boiler which I know is only going to be around 1.3-ish.
In my opinion, responsibility is entirely with the manufacturer of the equipment. A failure of the boiler, by exploding and potentially causing injury or death should simply NEVER happen.
A runaway heater + a clogged safety relief valve = bomb?
Where is the next fail-safe element in the design? There should be at least one or more safety mechanism(s) to prevent explosive pressure failure of the boiler; plain and simple. A 'blow plug', a seal plate, a weak link somewhere in the system to relief the excess pressure of a runaway boiler.
The operating pressure of the boiler is 2 bar. This is so far lower than explosive pressures, there is no excuse. This is not a high pressure steam engine!
If this is all that is between you and a bomb in your kitchen, I would unplug immediately.
I do hope all those injured have a speedy recovery. I do find it amazing how blame gets pointed at without any facts. Just to note, there is also a safety thermal sensor on the heating element for any overheating. So, this is the secondary safety factor. In my opinion, there has to be a LOT of steam pressure to blow out a rupture in a tube AND force a heavy machine to move so much. If there was not a lot of steam pressure, there would be more of a hissing leak to start. My conclusion are any of the three failures
a) the boiler safety valve failed (my guess most likely due to limescale) b) the pressurstat stuck in the closed position allowing contact energy to the heating element - too many on/off clicks can cause to stick c) the thermal sensor inside the boiler right near the heating element was disconnected/bypassed or failed (a usual failure here is caused by a surge, but then it huts down the power to the heating element. An opposite failure is a surge and sends constant voltage), and/or d) the heating element was not submerged in water, melted down, and was powered like a race car through ground before any safety mechanism can catch it (meaning faulty hookup by installer).
So, when my wife's car squeals because it needs new brakes, can I have the feature to have it shut the car down so my wife can not drive itanymore?
Once again, I hope those injured have a speedy recovery.
Sincerely, Jim Piccinich Business Partner 1st-line Equipment, LLC www.1st-line.com
Posted Wed Sep 15, 2010, 2:58pm Subject: Re: Safety valve maintenance for home machines
c4h5n2o Said:
Just to clarify, espresso machine boilers typically operate below 2bar and commonly closer to 1bar. It's only the group head or HX/group head that is pressurized to 9 bar. From a materials standpoint, the boiler isn't likely to fail i.e. rupture/explode unless it reached something like 20 - 50bar. But that isn't something you want to test at home, either.
Different countries have different standards based on the testing organizations. I believe the minimum is 20. I know one time a home machine (no name will be mentioned) was ETL tested for a rupture at 50 bar. And, of course, the machine was shipped to us with a lawsuit threat. It came down to complete 1/4" thick scaling on the complete internals of the machine. Yuk!
Sincerely, Jim Piccinich Business Partner 1st-line Equipment, LLC www.1st-line.com
c4h5n2o Senior Member Joined: 19 Jul 2005 Posts: 234 Location: Vancouver Expertise: I live coffee
Espresso: LM Grinder: Bunn/Vario Drip: Swiss gold Roaster: No time
Posted Wed Sep 15, 2010, 3:05pm Subject: Re: Safety valve maintenance for home machines
__________ Said:
I don't understand how the machine works in that case. How can a brew boiler not be pressurised to brew pressure when you are pulling a shot ? I'm not referring to the steam boiler which I know is only going to be around 1.3-ish.
If the boiler was heated to 9bar, the corresponding temperature would be ~ 350F. Probably hotter than you like your espresso. Here is an over simplified description of how an espresso machine heats a shot. The brew boiler is pressurized to about 1bar (250F) and it's main purpose is really to heat the group head. The pump then pressurizes cold water to about 9bar and forces it into the group head. Since the group head has already been heated to something like 250F, the incoming pressurized, cold water ends up being heated to about 200F for the shot.
Posted Wed Sep 15, 2010, 3:25pm Subject: Re: Safety valve maintenance for home machines
c4h5n2o Said:
If the boiler was heated to 9bar, the corresponding temperature would be ~ 350F. Probably hotter than you like your espresso. Here is an over simplified description of how an espresso machine heats a shot. The brew boiler is pressurized to about 1 bar (250F) and it's main purpose is really to heat the group head. The pump then pressurizes cold water to about 9bar and forces it into the group head. Since the group head has already been heated to something like 250F, the incoming pressurized, cold water ends up being heated to about 200F for the shot.
I still do not understand your explanation. I have the full engineering, electrical and hydraulic schematics for this machine. It is the pump which pressurises the brew boiler to 9bar when pulling a shot, not the heater ! The water in the brew boiler is ~ 96c The water can only get into the brewhead as a result of coming out of the brew boiler (not the steam boiler which is quite separate). I think your machine must be a completely different design.
It is clear from the photograph that the boiler exploded. A detailed analysis of failure(s) leading to this event will point out the exact failure mechanism. My opinion (er blame) is based on the fact that the boiler exploded.
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