Design Density for Domestic Sprinklers

[GB]

Does anyone know of a recognised design density of water for a domestic sprinkler system

[FSO]

From a quick calculation in my head, I think it is about 2.8. I stand to be corrected though.

[deaconj999]

Chaps,

Calcs for domestic sprinklers do not use design densities.

[kurnal]

No they dont GFSM  but it is interesting for comparison purposes. It does work out at about 2.5mm/sqm/minute if you calculate the nozzle discharge divided by the area of coverage of the head. It becomes  relevant when you need to argue with developers who wish to install domestic sprinkers in commercial situations. And especially in view of the limited duration of the supply.

For example, in the same way ESFR sprinklers often do not refer to design densities but I recently asked for this information from the designer/installer when we were pushing the envelope in a warehouse. This was to cover a new risk in an existing warehouse in which ESFR was already installed. No risk  specific design data was available for an ESFR system but data was available for conventional systems. The D/D from the ESFR  was shown to be in excess of 90mm/min, BSEN12845 would have recommended HSS  with  12.5mm in this storage configuration so the insurers were satisfied.

[deaconj999]

I thought if there were ever such a DD for domestic sprinklers it was more like 4mm seeing as domestic flow requirements are 60 l/min for 1 head / 15 sqm.

But I'm sure I would benefit from a breakdown of how 2.8mm is achieved - after all one never stops learning in the dark arts.

[kurnal]

Yes thats the design criteria for the water supply but what actually comes out of the head is the product of the K factor and the square root of the pressure. A typical K factor would be 60.
The minimum pressure requirement at the most unfavourable head is a minimum of 0.5 Bar for a BS7259 system so the actual output may be as low as 15 litres per minute. In actual fact it is unlikely to be as low as this as most systems would be designed to better the minimum criteria.

[deaconj999]

Well I would work that out to be a pressure at the MUH of 1 bar assuming domestic req for 60 l/min. Twice the minimum requirement, but a BS 7259 system I've not heard of.

[kurnal]

Neither have I. Of course it should have been BS 7251

Must point out that I am pretty well self taught when it comes to sprinklers and the instructor was rubbish.

[deaconj999]

Kurnal,

You are wiser than me without doubt, in cases of 7251 read 9251 

[Phoenix]

This is where the 2.8mm/minute comes from:

Water flow requirement for domestic is 60 litres/min from one head or 42 litres/min from each of two heads in the same room.  So the system must be capable of delivering a flow of at least 84 litres/min at the most hydraulically unfavourable location (i.e. furthest from and highest above the main valve).  The coverage is 15m² max. 

Bear in mind that a lot of systems will deliver more than the minimum but let's suppose that a system is delivering the minimum of 84 litres/minute over the maximum area of 15m² x 2 (as two heads are operating) = 30m².  84 litres equals a volume of 0.084m³. 

0.084/30 = 0.0028m = 2.8mm

It's 84 litres per minute so the equivalent design density is 2.8mm/min.

If only one head operates then we expect 60 litres per minute to be delivered which will be at a density of 4mm/minute (0.06/15).  The system should be capable of satisfying both requirements.

Residential systems are slightly different.


Stu

[H2OFS]

This is where the 2.8mm/minute comes from:

Water flow requirement for domestic is 60 litres/min from one head or 42 litres/min from each of two heads in the same room.  So the system must be capable of delivering a flow of at least 84 litres/min at the most hydraulically unfavourable location (i.e. furthest from and highest above the main valve).  The coverage is 15m² max. 

Bear in mind that a lot of systems will deliver more than the minimum but let's suppose that a system is delivering the minimum of 84 litres/minute over the maximum area of 15m² x 2 (as two heads are operating) = 30m².  84 litres equals a volume of 0.084m³. 

0.084/30 = 0.0028m = 2.8mm

It's 84 litres per minute so the equivalent design density is 2.8mm/min.

If only one head operates then we expect 60 litres per minute to be delivered which will be at a density of 4mm/minute (0.06/15).  The system should be capable of satisfying both requirements.

Residential systems are slightly different.


Stu

Good answer, residential requirement is 4 heads in the same room at 42 litres/min running for 30mins.