'Wet' Water Mist Conversion to Pre Action

[Messy]

I have been asked to comment on proposals to alter a large water mist system from a wet system to a pre action double knock type operation.

This is early days and I haven't had a look at the (office) building's fire strategy yet, but I understand that the water mist was originally installed to allow protect a large atrium as there is some fire loading within. The building also uses a system of horizontal evacuation from one side of the building and across the atrium to the other as part of a phased evacuation plan. The water mist system is now installed in virtually every space and void.

I intend to download the relevant NFPA standard that the system was installed to as part of my research, along with reading the fire strategy document - when they can find it!

I will be honest and admit I am on the edge of my competence here and am anxious not to let others - such as the system installers - take advantage and install unsuitable kit, or indeed, allow any changes which will reduce the effectiveness of the system. So is there anything obvious I should be asking or considering? Any tips would be gratefully received!

[Phoenix]

Are they proposing open heads that only require the detection system to operate in order to discharge the water or will it be more akin to a pre-action sprinkler system where the detection system charges the pipes but the system does not discharge water until a head operates?

If it is the latter then you would want to know the length of the delay until water discharges.  There will be a delay unless they can guarantee that the entire system will be purged of air prior to discharge.  You would also want to know what the effects of the worst case delay might be.  They may claim that the delay is so short that the fire cannot grow significantly in the additional time to discharge and therefore it makes no difference to the performance of the system.  That may be true depending upon the fire loading.

I would not think that they are proposing the former as this set up is vulnerable to false alarms.

I'm not sure what the NFPA code says about the testing and servicing of the linked fire alarm system but you must bear in mind that this system has an additional role to play and so must perform as required at all times.  If the fire alarm system is down for some reason there must be a contingency measure in place which could be to return to a wet system for the duration of the fire alarm problem.

From your description it sounds like people have to escape through an atrium that contains some fire loading.  This is the situation in hundreds of shopping centres in the country so it can be done safely.  I don't need to tell you that you have to consider the whole package of fire safety measures together.  Presumably they have some ventilation in the atrium.  You should find out all you can about that.  And why do they evacuate across the atrium?  Why don't they just go down the stairs?  Are there none? 

The use of water mist in an atrium sounds pretty unusual.  The discharge points have to be close to the burning material and if they require the actuation of a fusible link or glass bulb then they must be immediately beneath a soffit or ceiling.  I guess it could work if all the different fuel locations have low ceilings immediately above them.

[Mike Buckley]

I would very much doubt that the proposal would be for a system with open heads that is activated by the fire alarm system for two reasons, first even if a double knock or a coincidental alarm system is in operation there will still be unwanted fire alarms although at a very reduced level and you don't want the mist system going off, second if the change is from a system where the system starts when a single head operates to one where a multitude of heads operate there will be implications for the amount of water available and the capacity of the pumps. Bear in mind that often water mist systems require their own supply of demineralised water to avoid the problem of the finer discahrge nozzles becoming blocked by particles in the water.

If the idea is to precharge the system when the fire alarm operates, this will have the advantage that the standing pressure in the system can be kept lower than the operating pressure which will reduce the standing strain on the system, however it may introduce another problem as the system will expand every time the fire alarm operates due to the pressure increase which may in time lead to further leaks. I would not anticipate any change in the discharge time for the system as the discharge is controlled by the bulb in the head which is a basic head detector and the detection time for smoke detection.

[Messy]

Thank you for the replies. The proposal is indeed to convert the wet system to a pre action system where two detector heads need to be actuated and the bulb fractured before the mist will be produced.

I hadn't considered considering an alternative system where the fire detection system is defective - but I will now. However, I am not happy with converting it to a wet system every time the AFD goes down. The time, cost & disruption of draining the system in order to convert it back to a pre action system would not be acceptable

The staff movement from across the atrium is an early part of the phased evacuation system, where staff are moved horizontally away from the suspected fire. If the fire detection system moves to a subsequent stage, the evacuation message is changed to proceed downstairs and outside.

I haven't seen the premises yet as the preliminary work has been via drawings only, but I should know more in a fortnight or so after the 1st site visit. Thanks

[Fishy]

Some issues immediately occur:

- would this alteration result in a change from the mist being smoke detection operated to heat detection operated?  If so, this might compromise the basis of design for the suppression/extinguishing system;
- if discharge is delayed then might this potentially allow the fire to grow beyond the size which the system was originally designed to suppress / extinguish?

[wee brian]

Sound slike an engineering nightmare.

If they are bog standard fusible link/bulb mist heads then introducing a pre-action element won't/shouldn't delay operation as the system will be charged before the heads pop.

Thats assuming you've got the right AFD in place (atriums can be fiddly).

The preaction does reduce the risk of accidental discharge when somebody wacks a head with a ladder. But as has already been said, you stress the system whenever the AFD trips and that can be a bit hairy.

[Mike Buckley]

Messy,

Normally the system is left full of water, the only thing that happens with a pre action system is that the pumps engage when the fire alarm goes and the system is pressurised ready to go. This is in contrast to the non preaction system where the pumps only cut in when the pressure drops. I would expect the system to have a residual pressure which if it drops below a predetermined level the pumps would kick in whether or not the fire alarm has activated.

[Tom Sutton]

Hi-Fog says "The pre-action water mist system uses automatic sprinklers attached to a piping system containing air, with a supplemental detection system installed in the same areas as the sprinklers. System activation requires both a signal from the detection system and the heat activation of a sprinkler. Pre-action systems are used to protect areas where the risk of false discharge or leakage must be kept to an absolute minimum" definitions seem a little confused.

http://www.marioff.com/fire-protection/hi-fogr-suppression-system-types/pre-action-suppression-system

[wee brian]

yes, thats the type of pre-action (sprinkler) I've worked with. The range pipes are pressurised with air. Rather like a dry system that you might use where there's a freezing risk.

I suppose it acts as a leak detector too.

I guess there may be other ways of doing it with mist - given that mists systems are all a bit bespoke.

[Messy]

Thanks for the replies. I am most grateful.

What concerns me most is the lack of a suitable BS and the need to rely on the installation company to set their own standard and design criteria. As the time of this post shows, its giving me some sleepless nights

I understand that there is a NFPA 750 guide, but at this time I am unsure if this covers pre action systems. There's also talk of a prEN 14972 - but I am struggling to find any detail on this standard - or even if it ever got past a consultation process. Then theres a German Vds standard - are they printed in English?? I am the first to accept I am at the end of my technical understanding  here and may require a competent 3rd party assessment of the proposals to convert this wet system to pre action system.

I am aware that many of you here have knowledge of a variety of experts in various fire safety fields, but can anyone please point me towards an independent & competent water mist design expert. S/he would need to be able to audit the proposals by examining the drawings and a possible site visit to be able to write a report with recommendations - with references to international standards. I appreciate that this person may not be UK based.

I must stress that this is an enquiry at this stage as I am yet to meet the design team. But my gut feeling is I want a second professional opinion as there are special circumstances which dictate this conversion must deliver a 100% resilient system (ok, 99.9%!)

Please PM me if necessary

[kurnal]

I can relate to where you are messy and the reason you feel how you do is because there are currently too many unknowns so there is too much research in unchartered waters.
it's not really your problem at this time. I hope I am not stating the obvious but by breaking it down to logical steps of progression may help.

There is an existing system, what is the design standard?
What is the problem the with it as it stands?
Who is proposing to change it and why?
What standard and performance factors are proposed for the modified system, VDS standards are fine and for sprinkler systems integrate with the 12845
What will be the proposed effect on the fire time line and life/ property considerations?

You are entitled to and indeed require such answers from the designer and it's not your problem to assess a blue sky proposal or comment on the design of the system until they know what they are proposing to do and tell you why they want to do it.

[Fishy]

The draft British Standards for water mist are available as the DD 8489 series (and have been for some time).  In my view they're likely to be issued as full standards some time in the next 6 months or so, which will mean they will pre-empt the ENs (that my contacts tell me are some way away from being published).

As this is an issue around actuation of the system, then I would recommend referring to BS 7273, in particular Part 3 - "Code of practice for the operation of fire protection measures - Electrical actuation of pre‑action watermist and sprinkler systems" & Part 5 - "Code of practice for the operation of fire protection measures - Electrical actuation of watermist systems (except pre-action systems)".

I'd endorse all of what Kurnal has said - unless one is confident that one understands the basis of design for a particular fire protection measure then I'd not recommend that anyone considers endorsing its alteration!

[Messy]

Many thanks once again.

I have heard about (and ordered) this DD8489 today and have secured a 3rd party competent assessor if one is required later. I wasn't aware of BS7273 so am grateful for that info and the examples of how to break this problem into bite size chunks.

The customer is in a rush to spend some cash he has been allocated and is gobsmacked I am expressing concern and the need for a slower methodical approach. But thankfully he is now on board.

On an associated matter, why on earth have the BSI been dragging their feet over a water mist standard as this kit has been around for years? They've managed to get a hypoxic air fire suppression system standard out, which surely is a more contemporary method of fire suppression (well on land anyway) than water mist? 

[Dinnertime Dave]

On an associated matter, why on earth have the BSI been dragging their feet over a water mist standard as this kit has been around for years? They've managed to get a hypoxic air fire suppression system standard out, which surely is a more contemporary method of fire suppression (well on land anyway) than water mist? 

Now if I was cynical I would suggest that the sprinkler industry is being a little protective of their product.

[Fishy]

 

On an associated matter, why on earth have the BSI been dragging their feet over a water mist standard as this kit has been around for years? They've managed to get a hypoxic air fire suppression system standard out, which surely is a more contemporary method of fire suppression (well on land anyway) than water mist? 

Now if I was cynical I would suggest that the sprinkler industry is being a little protective of their product.