This week I have been looking at VDS CEA 4001 guidance for sprinkler systems especially the hydraulic calculation section. I came across an interesting section that has not seen another standard, the ability to have a section a pipe in the hydraulic most favourable area of operation which is smaller than the supply pipe. The purpose of this pipe is to help you to balance the most favourable area with the most remote area by producing a very high-pressure loss in a short section of pipe. By bringing the 2 areas hydraulically closer together will give you a better QMax calculation and a saving in the required tank capacity.
There are some limitations the maximum velocity is limited to 20 m/s which of course is much higher than the 10 m/s are allowed anywhere else in the pipe. The length is limited to 1m and you can only reduce by 3 pipe sizes, there is a limit to the smallest nominal diameter which is dependent on the hazard classification and density of discharge. Having said that if you had a 150mm supply main you could have a 1 m pipe which is only 65 mm nominal bore, that is quite a reduction.
It's not the first time I've heard of such practice I have come across someone doing something very similar with BS 9251 residential and domestic sprinkler system in this instance a number of elbows were been introduced to increase the pressure loss to the most favourable area.
This method will not work with all systems for instance if you have a looped or gridded system it would be completely pointless but presumably, you could balance the roof and the rack systems by introducing the smaller pipe?
If you are designing fire sprinkler systems to VDS CEA 4001 guidance for sprinkler systems then you will be pleased to know that our FHC software will allow you to incorporate the 'throttling' pipework.