The LWF Blog
Fire Engineering Design and Risk Assessment – Sprinkler Installation Design – Part 37
March 16, 2018 10:55 amIn LWF’s Fire Engineering blog series for Architects and others in the building design business, the use of sprinklers as a method of fire suppression is being explored. In part 36, the sprinkler system grid design was discussed along with the graphs to be produced which contain data on demand curves. It was ascertained that the point at which the most favourable curve intercepts the pump curve at its highest point is known as Qmax, which is the value used to calculate necessary tank size and pump duty. Part 37 continues from that point.
In order to determine the tank capacity requirement, the flow of Qmax must be allowed for the duration of demand, as per the hazard classifications – light = 30 minutes, ordinary = 60 minutes and high hazard = 90 minutes.
If the tank infill source is reliable then it may be possible permit the adjustment of the tank capacities to reflect that. The basis of such a concession in the design codes is that the reduction in tank capacity must be made up by the rate of infill into the tank during the discharge period.
It is important that sprinkler pumps are set to start in response to a drop in pressure and will run until they are switched to off manually.
Depending upon the water level’s relationship to the pump centre line, the conditions under which the pump operates can be defined as either flooded lift or suction lift. Flooded suction applies when 2.0m or less, or one third of the water capacity (whichever is the smaller amount) falls below the centre line of the pump. When considering natural and unlimited supplies such as rivers, canals, lakes etc. the pump line must be 0.85m below the lowest known or expected water level within the tank.
In the case of suction lift pumps it is important that full priming facilities which include priming tank and pipework are provided. Each pump must have separate suction pipes and as a result of decreased velocity, the pipe size is likely to be larger because of the decreased velocity limit.
In many cases, the main sprinkler pumps are supplemented by smaller capacity pumps known as ‘jockey’ pumps which are also used to make up small losses in the trunk main and prevent the operation of the main pumps in such circumstances. The jockey pump, unlike a main pump, will automatically switch off when the set cut-out pressure is reached. In effect, the jockey pump maintains system pressure when sprinkler activation has not occurred, then supports the main pump on activation.
Where a number of buildings on site require sprinkler protection, consideration must be given to common water supplies, in cases where water is a valued commodity. A common water supply to all buildings should be capable of delivering the flows and pressures required by the building with highest risk.
In part 38, the commission and testing of sprinkler systems will be discussed. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.