Application of Heat

[Geoff]

Hi,

I'm not sure if this the right section for this question but here goes anyway.

Does the direct application of relatively low levels of heat (say from hot water pipes temp. approx. 70 - 90 deg C) to combustible material such as carboard, bed linen, etc, over a long period of time (say over 4-6 months or longer) lead to a reduction of the materials ignition point?  I've been told before that this chemical reaction is called pyrolosis, but I had only heard of it in relation to high temperatures on organic compounds.

I have a situation where new boilers have been put into one of our sheltered schemes and now they suddenly have hot water running through the pipes and believe me these pipes are very hot.  The issue is that all the pipes are in the small storage cupboards that are in the flats and these are used as wardrobes, general storeage areas, etc. Some of them are packed full and some material is very close to, if not touching these pipes.

I have asked for these things to be lagged/boxed/shielded in some way,  but our asset management people are proving a little resistant to the idea.

So if anyone can help with this and point me in the direction of some evidence/reports/statistics on this I would be grateful.

Many thanks in andvance.

Geoff

[Phoenix]

Hi Geoff,

The auto-ignition temperature for the items you're talking about (at which the material will burst into flames) is much higher than will be reached adjacent to hot water pipes.  You're right to think that this auto-ignition temperature will reduce because of the continuous heating - the material will dry out completely and, depending upon what it is, degrade to a certain extent.  But it is extremely improbable that the auto-ignition temperature will drop anywhere near as low as 100 degrees C.  Pyrolysis, as commonly conceived, will not occur but you may have slightly increased oxidation (technically slow combustion) of the material due to the heat (newspaper may go yellow or brown and get thinner over a long period, for example).

Insulation can make a difference and I imagine this is one of your concerns.  It is theoretically possible for the temperature in the material surrounding the pipes to become hotter than the pipes themselves but you'd have to have near perfect insulation and extremely thick and dense insulating material.  This tends to self regulate because the pipe itself, although very hot, becomes a heat sink as well as the outside.  I really do not think this is going to be a problem in terms of an ignition source.

I might still be unhappy about the pipes because they might pose a burns threat to the occupiers - but probably not if they're no hotter than, say, radiators.  You can make a judgement on that.

You could point out to the occupiers that materials in close contact to these pipes will be damaged over time by that contact and encourage them to leave an air gap around the pipes.

It would be different if they were storing paints or thinners or aerosols or similar near these pipes - if there is any risk of this happening then you should insist on the shielding/boxing that you referred to.

Might also be worth checking any fire stopping around the pipes as this is a classic situation for compartmentation to be breached.

I have come across fires starting because combustible materials have been in contact with incandescent light bulbs but they are much hotter than your pipes.

Stu

[CivvyFSO]

Just to add to Stu's comments;

If something has much of the water taken out of it then it will usually combust quicker. Look at www.nist.gov for the xmas tree fires, in particular the comparison between a dry xmas tree and a properly watered one. (They had difficulty setting fire to the properly watered one even with a blowtorch) However, its auto ignition temperature won't usually change, the quicker speed of combustion is because the energy supplied to the material isnt used turning the water into steam/vapour and is used to raise the temperature of the material instead.

[Fishy]

Might be relevant...

http://www.kingfell.com/~forum/index.php?topic=1809.0

[Mr. P]

I have seen socks and other underwear which have been 'dried' on hot water pipes. They have discoloured due to heat/pyrolysis, and some small areas had charred. Lucky it did not become a serious incident.

So, Yes, it can happen.

[Owen66]

As Pheonix pointed out - the client would also have a duty (particularly in an elderly care establishment) to limit access to unsafe temperatures - personally, I would want to see the exposed pipework enclosed so it couldn't be touched (along with LST radiators and control of HWS temperatures) - get the pipes boxed in (and query why a new heating system uses such high flow and  return temperatures anyway - not very green is it)

Regards

Owen

[Mike Buckley]

Another point that needs consideration is that although the flammablity of the original material may have been established, once it has been used this may change.

By how much is a difficult question, it depends on what has contaminated the material and how well (or poorly) it has been cleaned (if it has been cleaned). A clean shirt is one thing, an old oily rag is something else.

[Martin]

legionella prevention requires water in pipes to be 50 c at delivery point. Thermostatic mixer valve at point of use to ensure max of 41 for bath (full body immersion) 43c for hand wash.

Most calorifiers are running at 60c. Can't uderstand need for temperatures described.

[Owen66]

Are you sure Martin

I recall the HSE guidance is 41 C at the WHB and 44 C at the bath

Regards

Owen

[Phoenix]

I have seen socks and other underwear which have been 'dried' on hot water pipes. They have discoloured due to heat/pyrolysis, and some small areas had charred.

This is an example of the oxidation that occurs in the organic material of the fabric.  It is slow combustion and that's why it has 'charred.'  But it would never catch fire if only heated by hot water pipes.

query why a new heating system uses such high flow and  return temperatures anyway - not very green is it.

That's a very good point, if it were an airing cupboard I could understand the desired heat emmission from the pipes but if it's not an airing cupboard it must be a massive waste of energy - presumably the pipes are not lagged anywhere if they're not lagged here.  What are they thinking of?  Does this come under part L of the Building Regulations, does anyone know?

A clean shirt is one thing, an old oily rag is something else.

This emphasises another good point.  If there is no control over what the residents put in these cupboards, then I guess you should assume the worst and make recommendations based on the assumption that they will be storing highly flammable and fairly volatile substances next to the pipes.  Such storage would demand separation from the heat source.  Lagging and boxing in.

Stu

[Mike Buckley]

Another point with the oily rags which should not apply to the scenario above (I hope) is the possibility of spontaneous combustion. This can occur when the pile of rags is big enough to insulate itself and begin to self heat. Something that should not occur here but it is worth looking for in a more industrial environment.

[Owen66]

That's a very good point, if it were an airing cupboard I could understand the desired heat emmission from the pipes but if it's not an airing cupboard it must be a massive waste of energy - presumably the pipes are not lagged anywhere if they're not lagged here.  What are they thinking of?  Does this come under part L of the Building Regulations, does anyone know?

Well it could be argued that the heat lost by the pipe is a net gain to the occupied space(indirectly) so it's useful (thermostat doesn't care where the heat is emitted just that its in the space). I was thinking perhaps more in terms of those temperatures precluding any form of condensing on the (assumed) new boilers.

It is a controlled service under Part L so you would think the mech services designer would have taken that into account.

Coupled with at least guidance on limiting water temperatures to high(er) risk groups I would have thought that both a lower temperature (for energy purposes) and gaurding (from a burn/scald poit of view) would address most of the concerns of the potential of the pipe to initiate a fire

Regards

Owen

[CivvyFSO]

Some interesting points raised, but has any one of us here answered the initial question with any sort of commitment yet?   

My opinion is: The materials are not going to ignite at the temperatures discussed here. The increased temperature might aid oxidation, but this is not pyrolysis and this would not change the auto-ignition temperature to any significant extent. (If at all) The only comparison I think you need to make is the use of hot water tank cupboards as airing cupboards. It was historically simply the place to put your towels to dry them out, (And still is where they exist) and if this led to auto-ignition of towels etc then we would have had lots of house fires attributed to this cause and we would have had very specific advice regarding this. (Bear in mind that this would have been commonplace well before the days where people started worrying about the temperature of the water in their tank, so the pipe temperature was probably considerably higher too.)

I would however pay attention to; Pheonix's comments regarding thinners/paints etc, and Mikes comments regarding spontaneous combustion. Both of these potential problem areas could be exacerbated by higher temperatures.

[Mike Buckley]

Re reading the intial post, although the debate has been about clothing, the post does state that the cupboards are used as general storage areas, which puts a new light on the matter.

We have been concentrating on the hot pipes but where are the boilers and what type of boiler are they? As a worst case scenario if there is a gas boiler inside the cupboard combined with flammable liquids and hot pipes there is a major risk.

Another consideration is electric cabling. I carried out a fire investigation where the cause of the incident was a power cable which had been trapped between a wall and a pile of photocopy paper. The extra insulation allowed the cable to heat up and eventually short.

[Geoff]

Hi,

Many thanks for all the replies, they have certainly made things clear about the pyrolisis (or lack of) and thank you for that.  Those that mentioned the bit about water temperatures at the outlets would be correct if this was a care scheme, but being classed as domestic (this is a sheltered scheme (should have mentioned that at the beginning! )) there is no requirement to fit TMVs.

The boilers are situated in a plant room and are condensing units.  The pipes in the flats are not lagged at all and there is also a legionella control issue due to the pipes, and  positioniong of cold and hot water tanks, which I wont go into here! 

There are also electrical fuseboards (with associated wiring) in these cupboards, as basically they have all the controls for the services in there.  It is a common issue across a lot of these older sheltered accommodation schemes.

Anyway thanks all for the posts they have been really helpful.

Geoff