Q&A from 'smoke control versus pressurisation' webinar

Posted by Paul Compton on 16/02/16 12:00

Coltshaft-224636-edited.jpgI received some excellent questions during the Q&A section during the recent webinar that I presented. Here you can see my answers to these questions, slightly edited for clarity.

There is also a recording of the webinar available.

What’s the maximum acceptable velocity for smoke extract ductwork?

There isn’t actually an absolute maximum when looking at smoke extract ductwork. It’s really a case of balancing the requirement to keep the ductwork down to a relatively small size against the extra fan power that you need if you go for a smaller duct and a higher velocity. We generally find that the optimum is around 10m/s, but I would stress that this is just a simple rule of thumb: there will be applications where we need to keep the power down, and therefore will go for a lower velocity, and there will be applications where we are really squeezed for space and allow a higher velocity.

Looking at the size of the actual mechanical shaft, then generally we would design in a lower velocity than what is theoretically desirable. The reason for this is that the shaft is generally of a fairly rough construction: it often has obstructions in it, because there may be problems with making the openings in the floors, or because the dampers will impinge upon the area slightly. So generally we would go for a lower velocity in the shafts so that we can keep the pressure drop down the shaft to a minimum and therefore ensure that the volume flow rates that we get - whether extracting from the upper floor or lower floors - are reasonably constant. Again, typically as a rule of thumb something like 5m/s is fairly normal sizing for a typical average height building.

With pressurisation systems, is the vent at the top of the shaft required to be operated automatically?

Yes, in a pressurisation system pretty much everything has to operate automatically so that the whole system will work because if one part of the system isn’t operating then the whole system fails. So when we operate a pressurisation system, we’ll generally open any ventilator that is protecting the pressure relief damper on the stair; we’ll also start the fans, open any termination ventilator on the supply system and we will also operate any AOV or shaft dampers for the accommodation air release, but only on the fire floor.  So we need separate detection on each floor, so that we can tell which floor the fire is on for us to open the right ventilators. 

In some cases you need a de-pressurisation shaft in the lobbies if pressurisation release isn’t possible in the fire cell or the apartment. Which method do you use to calculate the size of this shaft, or the size of the pressure relief in the apartment, for that matter?

Where we are looking at the apartment itself, it’s the accommodation air release. If it’s a mechanical system, then we will design it on 10m per second. If it’s a natural system, then EN 12101-6 recommends that if it’s a shaft system you should have a maximum velocity of 2m/s and if you are using an AOV then you should have a maximum velocity of 2.5m/s through the AOV.

How do you determine the flow rate of the fan for the mechanical shaft system?

That flow rate is generally manufacturer-specific, in that different manufacturers will have slightly different design flow rates. Typically these flow rates all tend to be in the range of something like 2 – 5m3/s at the damper. Obviously we also have to allow for leakage in the system in terms of sizing the fan, but in terms of deciding exactly which flow rate we need, this is generally done by CFD analysis.

What this means is that where we are looking at a compliant building, then most manufacturers will have done CFD analysis to prove that their system works on a generic basis. Where we are looking at extended travel distance corridor systems, then generally CFD is done to prove the flow rate on a project-specific basis.

How many floors can a mechanical system cope with?

There’s not really a limit. The limitation tends to come in the following situation: if we are extracting simply from one location, then the taller the building is, the bigger the shaft has to be, and obviously generally the taller the building is, the more pressure there is on space anyway.

So you can go as tall as you want but generally if you are going for a really tall building it will be helpful to have mechanical extract into the shaft at a number of levels, perhaps every 80 or 100m or something to keep the shaft size down to a reasonable size.

Should stack effects be taken into account?

In general such effects are not taken into account in the design. The fact is that there may well be a stack effect when the system starts due to perhaps a warmer temperature in the stair than within the accommodation or indeed outside. However once the system starts running and starts the ventilation, then that stack effect very quickly dissipates. It has been shown in practice in some quite tall buildings that the stack effect is pretty negligible once the system is up and operating.

Do evacuation lifts impact upon the design principles associated with stairwell pressurisation?

I think the only way the impact might be felt is that if there is an evacuation lift then that lift certainly has to be protected. Therefore you have to pressurise either the lift or the lobby to ensure that the lift is protected but that’s the only impact.

Has Colt implemented such a system in a commercial building taller than 30m?

The answer is yes, we have implemented both pressurisation systems and mechanical shaft systems in tall buildings. In London it’s becoming quite common to have buildings 150m or taller and we have done both pressurisation and shaft systems in those sorts of buildings. So it’s really not a problem to design.

How far can we extend travel distances with pressurisation systems within residential corridors? 

I do want to make it quite clear that extended travel distances can’t be achieved with pressurisation systems. They can only be achieved with mechanical shaft systems. In this instance there is guidance that the maximum extended travel distance should not exceed 30m. This guidance is given in Revision 2 to its ‘Guidance on Smoke Control to Common Escape Routes in Apartment Buildings (Flats and Maisonettes)’ by The Smoke Control Association (SCA), which is available for download from www.feta.co.uk/smokecontrol. This was written by a wide cross section of people, from Building Control, Fire Consultants and Manufacturers and the general recommendation is not above 30m. Please see our blog for further info.

The NFPA Standards allow for lower pressures of 12.5 Pascals for sprinklers. Has this been used in the UK as part of a PB (“Pressurised Building”) approach, perhaps?

Obviously what I have been talking about is the European Standards. By contrast NFPA has indeed been used in some buildings.

If you read the UK Regulations, you are permitted to take a fire engineered approach, and therefore if you can persuade Building Control that an NFPA system is sufficient, then the NFPA approach can be used. Certainly we have been asked to look at NFPA designs for buildings which are being used or built for American companies where they want to use the same standards around the world and then use NFPA.

If you design to NFPA standards, it’s quite a different design approach, and the pressure differentials do vary quite a bit from the UK requirements. Generally we find that you will end up with a different type of system. Whether the flow rate’s higher or lower depends upon the exact configuration but yes, it is possible to do it in the UK as part of a fire engineered approach.

Can an enhanced mechanical shaft be fitted to a listed building?

I think that this is very dependent upon the layout and type of building. Certainly if you are having to fit a new system into a listed building, then a mechanical shaft system will be the least disruptive, assuming you can’t use AOV’s. So I think that the answer normally is yes, there is normally a way, but it generally requires often quite a lot of design work and discussion between the design team to come up with a solution that is going to work.

What fire scenario do you typically use?

That will depend upon the type of building. Again there is guidance in the Smoke Control Association document for Residential buildings referred to above. Typically if you are looking at a sprinklered building, then you are probably looking at about 1 MW fire and if it is a non-sprinklered building you are looking at around a 2.5 MW fire. If it is a commercial building then the fire size will depend upon the particular application, and typically we are looking at somewhere between 1 and 12 MW, depending upon the specific project.

Let’s assume we have an existing building with a pressurisation system in the stairs for fire-fighting purposes and a refurbished stair in the lobby space. How would a supplier assess if an increase in stair or lobby space is feasible in terms of re-balancing the system before the works takes place?

This question is about the impact of making changes to the stairs and lobby where we have an existing system.

Basically the way you would normally deal with this is first of all to conduct a survey, so that you find out what flow rate you have got from the existing systems. Then you would need to do a set of design calculations based upon the proposed layout, and you will simply then do a comparison as to whether what there is at the moment will be enough.

If it isn’t, then you are looking at either perhaps just changing the fans, if it is a small change that is required. If it’s a big change, it may well be that you have to change the entire system. Again, it’s going to be very project specific, but the process will be the same: survey to find out what you have got, make the calculation for the proposed design to see what you are going to need, and then it’s engineering to provide the right solution.

Is there information on the Colt website about AOV’s?

The answer to that is yes, within the smoke control section – see http://www.coltinfo.co.uk/smoke-control-residential-buildings.html as a starter, but also please note that any smoke ventilator could be used as an AOV. Out natural vents are listed here: http://www.coltinfo.co.uk/smoke-control-products.html.

Please tell us more about the issue that Approved Document B still refers to BS 5588 even though it has been withdrawn.

This is simply a question of timing. BSI wanted to introduce the new “99xx” series of standards and when they did that the UK Government decided they didn’t want to update Approved Document B, and therefore Approved Document B still refers to BS5588. It is permitted to use the new “99xx” series of standards as long as the Fire Authorities and Building Control Authorities are content with that, and generally in the UK there’s not normally an issue, as long as you use either BS 5588 or one of the 99 series of standards and not try to mix and match the two.  No doubt when Approved Document B is next updated, which I believe is going to be in 2017, then there will be reference to the new standards.

Can a mechanical shaft system be used for extended travel in a dead end corridor for a residential building where there is only a single non-lobbied stair?

The answer to that is yes, that’s probably the main use for mechanical shafts in extended travel distances, so no problem with that at all.

How reliable or durable are the pressure sensors used in a shaft design?

Obviously I can only speak for for the pressure sensors that we use, but yes they are very durable and generally very reliable. They do drift slightly in use, so as part of annual maintenance it’s worth just doing a quick check on the sensors and re-calibrating them if necessary. In general we find that the drift is very small and it doesn’t normally cause any issues with performance of the system, because we are not trying to achieve any specific de-pressurisation as long as we don’t drift too far either way, and so that’s generally perfectly acceptable.

How many pressure sensors would you expect to locate in a tall building with variable fans?

I presume this is refers to a mechanical shaft system. In that case there would be a pressure sensor on every floor.  The reason for that is that we are only going to operate the system on the floor where the fire is, and a pressure sensor on another floor would give a false reading, so we do need a pressure sensor every single floor.

Can advice be provided on non-standard smoke extraction systems within an existing building?

The answer is yes. Colt or other manufacturers generally are happy to provide advice on a project specific basis. Please provide details of what you want to achieve.

If roof space is limited with a mechanical shaft system, can the fans be located at an intermediate level?

Generally there is no problem putting plant on intermediate levels or basement levels. If you are providing extract, you need to be sure that your discharge is going to be in a location where it is not going to affect either people escaping from the building or allow re-circulation of smoke back into the building. So within those two limitations then yes there is no problem putting plant at intermediate levels or at low level. As I have intimated earlier, in some cases it can actually be beneficial in keeping the size of the shaft down.

Do we consider sprinkler failure in terms of design fires?

The answer to that is no: generally sprinklers are pretty reliable and it is normal practice that if sprinklers are provided and they are compliant with British Standards it is accepted that the smoke ventilation system will be designed on the basis that the sprinkler system will work. If it doesn’t work, then yes, you will have worse conditions, but the smoke vent system will still provide some protection.

Whitepaper

We’ve also written a whitepaper to accompany this webinar, which can be downloaded from this link.


Paul Compton Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Webinar, Pressurisation, Smoke shafts, CPD