what can cause pressure relief valve to activate on steam kettle?
Over pressurized steam jacket.
Could be the pressure A.K.A the heat in the jacket got too high and so the pressure was raised above (50 PSI?) and so the valve blows open. Could be caused by a defective thermostat (if its adjustable and electric).
When I calibrated my 3 KGL100 kettles I would run into having turned the set point a tad too high and the valve would pop because the pressure in the jacket went to 50 PSI.
Its unlikely but it could also be a defective valve, if your kettle has a pressure gauge which I believe it does, then watch it as it goes up while the unit is heating, you should have the kettle empty and then heat it to 10, if it blows make sure you are watching the gauge and compare the pressure when it blew to what the valve is rated for, that will tell you if its a valve problem or an internal fault in the kettle.
That is a self-contained, gas heated unit. As olivero said, it's rated to 50 psi. It's obviously exceeding that and blowing off.
You've probably got air leaking into the jacket overnight after it has cooled off. Usually this occurs if the pressure relief valve not seating properly. That happens over time when its seal in it gets fouled.
Here's what may be happening. As the kettle cools down over night and goes into a vacuum, it shouldn't suck in any air. So before the kitchen staff come in first thing in the morning, go there first and take look at the pressure gauge. It should be in a vacuum by reading well into the GREEN range (on some types gauges) or be reading around 30" of mercury (if the gauge shows that gradient). That simply means there's no air in it - which is perfect. That's what it's designed to be.
It the gauge reads 0 psi instead of a vacuum, then air leaked into the jacket until it equalized to atmospheric pressure. Atmospheric pressure is 14.7 psia. With that extra pressure within the jacket, when they turn it on the next day and it heats up, that extra pressure is in addition to the designed operating pressure. The designed operating pressure is around 35 psi when it's operating at max. temperature setting. That 35 psi + atmospheric pressure = probably enough to set off the pressure relief valve - usually as water. After several times of that, it's then low on water.
I suggest replacing that pressure relief valve. Like I said, it's most often the culprit for letting the air in. There really isn't anywhere else it can come from, since everything else is factory sealed.
Some other things. You'll need to restore the water level in the jacket. Use ONLY distilled water. Cleveland also recommends adding a rust inhibitor.
Lastly - after you've sealed the kettle back up (with a new safety relief valve), you'll need to burp the jacket to get the air out. This is required after breaking the jacket's air-tight seal and before placing it back into service. It only needs to be done one time, then it should be good until the NEXT time the pressure relief valve starts showing its age.
Begin reading on page 20 of this manual for details on these procedures:
Lastly, I don't know you, so I must provide fair warning by say this:
Do BE the technician that your profile says you are. If you do refrigeration, then this should all make sense.
If you're actually NOT a technician, then be warned that if you bumble this repair by being a hack, then some people can be fatally injured.
Here locally, a handyman restaurant manager decided to stop the leak from a safety relief valve on their kettle by putting a pipe plug up in it. The kettle blew up. Several employees subsequently went to the hospital for their injuries.
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