HWC expansion valve question tricky question.

I have a mains pressure HWC with a low pressure coil inside hooked up to a wetback. The HWC also has a temperature gauge about half way up. I lit the fire and let it rip for a few hours. The temperature gauge reached about 75 degrees ( so it would have been a lot hotter at the top of the cylinder). Then the expansion valve opened and started to dump hot water. Valve looks like (http://www.rmc.com.au/product-
s/h-valves/). So my set up looks like https://apexvalves.co.nz/wp-content/uploads/Diagram-6-Wetback.png This continued for about 5 minutes until the gauge read 40 degrees. So my question is- is this normal for the expansion valve? Should it dump for this period of time or is it time for a new valve. Thanks in advance.

Not the same thing, but pressure release valves, in my field, need pressure to almost equalize, either side of valve, before they will reset. so that usually means pressure drops to near zero.
For a discharge release valve on big refrideration, reset can be done by blocking outlet(just for a brief moment, till reset), which allows pressure to equalize and if below trip pressure, reset.

Growing up i recall, you sure needed to dump a lot of hot water down sink, to stop wet back cylinder rumbling and banging away, spouting out on roof.So 5 minutes could be usual, considering all (water needed to lose to drop pressure, and wetback still cranking)

But Multiple trips do make them weaker, so if yours has tripped a lot, could well be getting tired.

not as simple as ones i know, found this on Crosby brand

The design of the control
chamber determines where the closing point will occur.
The difference between the set pressure and the closing
point pressure is called blowdown and is usually expressed
as a percentage of set pressure.

But did not see what percentage that is likely to be.
Pressure Relief Valve Engineering Handbook
http://www.isibang.ac.in/~library/onlinerz/resources/enghandbook3.pdf

If you "let it rip" for a few hours, there's a good chance the cylinder could have been boiling. Obviously it needed to let the pressure off for a while. Once it opens, the pressure needs to get quite a lot lower before it closes again.

The valve might be faulty - but it sounds more like it was just doing its job.

If you're going to "let it rip" then you should be using the hot water. That's what it's designed for.

It would be more dangerous if it hadn't opened.

If you don't want to waste hot water, ask a plumber if they can add a cold water expansion valve. That way, the cold will drain off instead of the hot. But I don't know if they will work properly with the wetback system you have. Especially if you "let it rip" all the time :)

Thanks for your advice people. I phone Apex who said the valves are designed to trip when the temperature or pressure rise above a certain point. So you are correct the water was near boiling and tripped the valve. They reset when the water is below a certain temperature.

Looks like a nice system. Bit pointless though to let the fire rip until the system dumps a huge amount of hot water. I suggest you back off the fire a bit, use less firewood and dump less hot water. The latter is effectively the waste cost of your over enthusiasm with the firewood. Had a similar system in a house we lived in late last century and I was also a bit inclined to overdo the fire until I worked out the result.