How to Measure Water Pressure for Sprinklers
We are often asked by our customers: what are the fire sprinkler system pressure requirements and, what would be the maximum pressure for the fire sprinkler system be?
So, this simple article has been compiled as an aid to understanding the importance of the static sprinkler system pressure and the water flow combined with the dynamic pressure when considering the possibility of utilising the existing town mains water supply to the building or the buildings existing boosted cold-water supply.
The maximum water pressure is generally less important than assessing the minimum pressure for fire sprinkler system in the town main or, the duty of the booster pump that is providing the pressure and flow for the fire sprinkler system pressure requirements. A high pressure rarely causes a problem for a designer whereas a low fire sprinkler system pressure frequently does.
It is important to appreciate the fact that the dynamic water pressure and flow is generally not as important a factor when considering a Mist Sprinkler System for a residential property and that a Mist fire sprinkler system pressure can be obtained from a 25mm shared incoming water supply.
Residential Fire Sprinkler System Pressure
A typical residential fire sprinkler system pressure for a 4-bedroom house could be around 3 bar when measured at the point of entry within the dwelling and a typical fire sprinkler system pressure for a high-rise block of flats could be around 2.5 bar on each floor.
While the fire sprinkler system pressure requirements for a small 2 storey townhouse close to the town main could be as little as 2 bar measured at the point of entry however there are often very high dynamic pressure losses introduced by water companies like domestic water meters and stop cocks etc which reduce the maximum pressure for fire sprinkler system availability.
Consequently, while there may be 4.5 bar static pressure in the town main, the dynamics of the system between the town main and the entry point in the house will be considerable.
It is important to understand the difference between dynamic pressure and static pressure when deciding how much pressure for fire sprinkler system design is required. The simple explanation of static pressure of a system equates to the latent pressure in the system when no fluid is flowing. For example, a static pressure reading can be taken using a pressure gauge connected to an outside tap when there is no running water within the premises. However, this has little to do with the dynamic sprinkler system pressure.
The dynamic fire sprinkler system pressure for a residential sprinkler system is measured at a given flow of water typically, around 120 litres per minute (LPM) for a category 1 or 2 system and 240 litres/minute for a category 3 system. It is important to understand that these figures are irrelevant without knowing what the dynamic pressure equates to at the given flow of water.
Dynamic pressure could be thought of as a function of the flow of water and the losses that the flow encounters.
How to establish the minimum water pressure for the fire sprinkler system to work correctly?
The maximum water pressure requirement for the fire sprinkler system will be dictated generally by the water pressure requirement at the most remote point of the system.
This point is generally established when the hydraulic calculations have been produced for the proposed systems sprinkler system pressure loss due to height combined with the friction losses due to pipe size, pipe length and changes in direction due to the number of fittings used. There will always be an additional 0.5 bar added which equates for the pressure requirement for the last head on the system in order to operate properly.
When the sprinkler system pressure losses for the most remote head have been established, this will be documented as the minimum pressure for fire sprinkler system requirement, and this will be always required when carrying out an equivalence test from a pre-determined test point at the start of the system.
How to measure water pressure for sprinklers?
When the minimum water pressure for fire sprinkler system has been calculated and how much water pressure for sprinkler system has been established, the existing flow and dynamic water pressure within the town main will need to be established at the system test point positioned close to the point of entry to the property. This is usually done by means of a flow gauge and pressure gauge or can be carried out by running water at the calculated system dynamic pressure into a vessel for a given time duration and then measuring the quantity of water in the vessel.
So, for example, suppose that your fire sprinkler system pressure requirements are 100 litres per minute at 2.5 bar and you have 140 litres per minute flowing from the open end of the incoming supply (this will equate to 140 litres at atmospheric pressure or 0 bar) how to measure water pressure for sprinklers at the system requirement:
1st step: install a ¼ way lever valve,
2nd step: install a pressure gauge upstream of the lever valve,
3rd step: run the supply with the lever valve fully open then close the valve until the required pressure is showing on the pressure gauge behind the valve (in this case 2.5 bar)
4th step: maintain the flow, then divert into the vessel for 10 seconds (say) then multiply by 6.
If you measure 17 litres, then you have achieved your sprinkler system minimum requirement (i.e. 6×17 = 102 litres at 2.5 bar.
Please note that there are other factors that dictate the flow quantity like for example, shared domestic/sprinkler supplies, operation of a priority demand valve, relative position of the domestic take-off to the sprinkler take-off etc.
For more help and advice, find more on our dedicated blog, and if you’re considering installing a fire sprinkler system, or just need some honest advice, one of our engineers will be happy to take your call or inquiry via our contact page.