chlorox On limit stat switches, it seems to me from doing some reading on the net on them, they seem to be basically thermal fuses. Is that so? If so, why would they be required if a PID is used instead of a mechanical pressure stat? Because if the pid goes down the whole machine goes down anyway so there should be no danger of an overheating situation. That may be why they are not required for CE certification unlike the case with a safety valve if I understand what dave is saying correctly?
In a coffee machine with twin boilers you have the service boiler which produces steam and hot water. This should have a safety valve to release excess pressure. Certification can be obtained with only a thermal breaker (but it’s not a great idea). It’s one of the reasons I insisted Crem (at the time) fit safety valves to the Crem 1B2LFPP and other models they wanted to sell in the UK via Bella Barista.
On the brew boiler there cannot be a safety valve, because these operate at 10+ bar. So the expansion valve can effectively become the route out of the system for excess pressure. Of course the expansion valve is not designed to vent steam, especially superheated steam at 184C. Any tube attached to the exit will pop off due to the heat and the pad mechanism may well degrade. The result could certainly be quite dramatic, especially as the exit hole is much larger than the normal safety valve (which remember is operating at 2.8 or 3 bar max usually. So now we have an electrically live machine full of steam and condensed water!
SSRs - the most common failure mode is shorted e.g. heating element on. No matter how good the PID is, it can’t turn off a shorted on SSR.
An electromechanical limit stat removes most of these concerns…they are a simple bimetal strip spring mechanism that pop open and directly disconnect the heating element, so can be reset, other types that have a meltable link, can’t.
One thing that will put this one to bed is if Tom could get photos showing the underside of the boilers as best he can.
Without a shadow of doubt, the Nurri Leva should have mechanical limit stats on the brew and steam boilers (and I hope it does). The cost is minimal to the factory (10 euro and a bit of wiring), it’s safer and good practice. There is no reasoned argument that holds water for not having them. If they are not on the bottom of the boilers (less than ideal), then Nurri should fit them ASAP in production units going forward and providing a retrofit kit for existing machines would be a very good idea…..Best placement is top of boiler.
If I did a detailed engineering review on any production machine for BB, this is something I would insist on!
P.S. Ideal temperature trip settings for the mechanical limit stats are 135C brew and 145C steam.
P.P.S. Pipes cannot explode because there are 2 routed out of the sealed system, one is the safety valve and the other the expansion valve…but it won’t be pretty and could be unsafe
