Vent cockup

[chrishutt]

Wouldn't this kind of drastic pressure escape up the (unrestricted) return?

I haven't actually tested this, but I imagine things would proceed something like this:-

Boiler stat fails, water reaches boiling point (100+C), steam bubbles form and rise up flow pipe. Initially these will collapse on contact with cooler water (with attendant banging noises) but as all temps continue to climb steam pushes up flow pipe and displaces water. But steam cannot escape to vent due to closed MV.

More steam is produced and more water displaced until the upper part of the boiler heat exchanger is dry. Temperature then climbs well above boiling point, but at some point condensing steam causes water to enter super-heated part of boiler. This water flashes to steam very violently and with huge increase in volume, causing sudden increase in pressure. Pipes back to feed tank are not large enough to relieve this sudden increase in pressure which then fractures boiler or pipework.

Also quite likely that sudden temp changes when metal at say 200C is doused with water will caused fracturing of cast iron heat exchanger. Once the boiler or pipework is fractured, the high pressures will be released, causing the boiling point of water to suddenly drop back to 100C (opposite of pressure cooker effect). Large volume of water at say 110C turns instantly to steam. Huge volume increase demolishes surrounding structures and causes severe skin burns to anyone in the vicinity.

Sleep well, Goodnight....

 

[PaulAH]

That's the last time I make a cup of tea. Chris, you're a genius.

 

[PaulAH]

So gentlemen, if I may trouble you once more. How does this look now?

 

[ChrisR]

Don't think so. If the MV etc were shut - etc, so steam built up in the boiler at say the 110º, the PRV wouldn't open so the dry boiler could still pop. This is working on PRV which opens at 3 bar - water boils at 156 or so at 3 bar. They don't let steam out if it's only at 2 bar!
In the good old bad old days when lip service was paid to safety these things were used
http://www.bes.ltd.uk/products/graphics/7512.gif
You can adjust the pressue by twisting the end cap - to gawd knows what pressure!

I believe it's inthe BS on CH that there must be an unrestrictable route from the boiler to the vent. If it doesn't go in a contiuous rise then you need a high-limit stat on the boiler as someone said above.

I'd pressurise the system, if you have a Hi limit stat (Honeywell do one if not). You only need say half a bar of excess pressure(above the highest point in the system). That's only a floor and a half, shouldn't be enough to worry about. You might get a leaky rad valve, but at £2 each (10 bar tested, BES) who cares!

 

[oilman]

If this was a sealed system, the PRV is the only release mechanism, so why is it suddenly so dangerous if it is on a vented system?

 

[chrishutt]

I think ChrisR may have a valid point abut the benefit of conversion to a fully sealed system. Consider what happens in each case in the very unlikely event of steam generation in the boiler.

Steam generation will cause large volume increases which cannot be accommodated with a sealed system, hence sharp increase in pressure to 3 bar when PRV operates and releases steam in a way which is safe but alerts householder to problem. If the householder fails to take action the discharge will continue until available water is used up and the boiler finally goes for meltdown.

However with the hybrid open system proposed, volume increase from steam generation will be accommodated either by venting through vent pipe, which is acceptable although not as safe as venting via PRV, or (if route to vent is closed off) by backflow of water into feed tank and discharge of water via overflow pipe (this water being displaced from top of boiler by trapped steam leading to catastrophic consequences like those imagined above).

In the latter case the pressure may not rise to 3 bar and so the PRV may not operate, except perhaps too late in the catastrophic process when the valve and discharge pipe will not be able to handle the volume of steam suddenly generated. This is why ChrisR mentioned the old style safety valves which can be adjusted to blow off at pressures only slightly higher than normal.

However it must be remembered that the existence of the overheat (OH) cutout is crucial. In the event of main boiler thermostat failure the OH will cutout and require manual resetting. This will alert the householder to the problem and the nuisance factor of constant resetting of the OH will oblige them to get it fixed.

Steam generation will only occur in the unlikely event of a failure of both the boiler stat and the OH cutout. OH cutout will be designed to fail safe, so a failure leading to steam generation is highly unlikely.

 

[ChrisR]

I'm really not sure what's best in theory. Perhaps the only wise course is just to stick to either/all of Brit Stds and Mfrs Instructions. You do need something to point to if something goes wrong!

 

[chrishutt]

Which in this case means going for a sealed system if the boiler manufacturer says it's OK. Unless a practical way can be found of making the vent pipe connection come before the pump. Thinking of which....

How about putting the pump and the DHW motorised valve onto the return pipe? That would free up the boiler - vent route of any possible blockages and might be done without any major pipework alterations.

 

[ChrisR]

Exactly! Sometimes the old ways....

 

[PaulAH]

You guys are stars. A few other observations:

Unfortunately, placing the pump on the return would involve major surgery (MV less so). The layout of pipework is more convoluted than my diagram suggests and there's physically nowhere to put the pump without losing circulation on the rad circuit.

Not keen on going sealed if it can be avoided - the original part of the system is about 30 years old and I don't trust it to take a lot of extra pressure.

Boiler has thermostat, overheat cutout (110deg according to manufacturer) and external fire valve with internal sensor. So we have a fair amount of protection already.

Interesting point, Chris & Chris, about the blowoff theory with a sealed system. ie it would trigger quicker than a vented one and so (possibly) do less damage. But here's a 1.5bar PRV

http://www.plumbworld.co.uk/343-4216

Still not good enough?

Edit: Didn't read it closely enough. This is a reducing valve by the looks of it, not a blowoff, right?

 

[chrishutt]

Yeah, just to confuse the amateurs PRV can mean Pressure Release Valve (aka safety valve or blow-off) or Pressure Reducing Valve (aka water governor). The latter can't be used as the former (and vice versa).

Are you sure about not being able to fit the pump on the return? Can you post some pics of the pipework around the boiler so we can see if it's possible or not.

As for sealed systems, the pressure may not have to be that much more than existing system. For example if the existing head at the boiler is 5 metres = 0.5 Bar, then the sealed system could be set for say 0.7 Bar.

 

[PaulAH]

Can you post some pics of the pipework around the boiler so we can see if it's possible or not.

Kind of you to offer but you've hit a sore point. The digital camera went on the blink this very afternoon, just when I needed it. Perhaps it overheated

So until Christmas Day (is there a smiley for fingers crossed?) I'll describe. The boiler is in a solid walk-in cupboard with a clearance of only about 4ins each side, and all three returns meet within the gap on the right. Obviously the pump must sit in the last leg to the boiler, which lies entirely within this gap - no room for a pump there. Perhaps there's a way of linking them higher up without moving too much pipework. I'll have another beard-stroking session (would help if I had a beard).

Getting a little confused here. Are you saying that if we go for sealed there would be no need to move any of these components? And, excuse my ignorance, where is the best place for the expansion vessel and relief valve?

Moving the pump to the return: Isn't that frowned upon because of cavitation risks?

Something else bothers me. There must be countless examples of gravity systems being converted to fully pumped. What is the "normal" solution?

ChrisR: Your relief valve - I can't trace a link between the graphic and the stock data. Where did it come from? That BES site looks extremely good value - is their stuff any good?

Just hit on another solution. Since it's in a cupboard we could line the whole thing with armour plate, fit a submarine-style door, and if the whole thing goes bang no-one will even notice.

 

[oilman]

Your boiler has got two thermostats, and is made of 5 or 6 mm high carbon steel. It has a fairly low water content, and so the possibility of an explosion as described is somewhere near effall. I see dozens of systems where the feed has become blocked and the system is running out of water, the boilers don't go bang. The problem with steam explosions and unvented cylinders is because of the large volume (150 litres and more) of water under pressure AND at possibly high temperature. Overheating and a quick release here IS a problem, but the vessel is only made from sub 1mm stainless steel. You are allowed to have a 10 litre unvented cyl without all the requirements for the larger cylinders and this is considered "safe" (legally). Your return pipe will provide sufficient relief. So stop worrying and have a good christmas.

 

[chrishutt]

I see dozens of systems where the feed has become blocked and the system is running out of water, the boilers don't go bang.

I take you points about low water content, but in the situation referred to by this quote the two thermostats will still be operating, even in a dry boiler, and will still cut off the burner when the set temps are reached.

My explosion fantasy was based on the very unlikely failure of both thermostats. I suspect it would be a pipework joint or tube that would fail rather than the boiler casting, but I don't really know. ChrisR is a metallurgist I think, so might shed some light on this.

 

[PaulAH]

Oilman, your earlier mention of the boiler's second port (thanks for the reminder) has given me another wheeze. As I may have mentioned we are about to convert to a conventional flue. This involves sticking a 4in liner up a 6in diameter chimney. So clap you eyes on this and weep.

Possible snags
1. Heat transfer from liner to vent
2. Inaccessibility of venting stub in chimney stack
3. Ingress of dirt
3. Looks wierd
4 ....?
Any thoughts, anyone?