Residual pressure in a fire sprinkler system

[TSN79]

I'm aware that the term means the remaining pressure at a point while there is flow. Say the distribution pipes downstream the sprinkler valve have a calculated total head loss of 3 bars during the designed flow. Does that mean that the residual pressure at the sprinkler valve needs to be at least 3 bars to achieve the designed flow? It just seems so unintuitive to me. After all, if I get the designed flow at a lower residual pressure, why would that be an issue?

 

[anorlunda]

I'm not 100% sure where the sprinkler valve is in your system, or where you measure residual pressure. But your questions all seem to relate to the relationship between pressure drop and flow.

Consider the following simple piping system. Water flows in the direction of the arrow. The box represents any collection of pipes, elbows, open or partially open valves that you like. The pressure difference Pa-Pb is the pressure loss across the whole collection.

Now the basic principle is that the more flow we have, the larger Pa-Pb becomes. Usually is is proportional to the square root of Pa-Pb, but that doesn't matter. What does matter is more flow means more pressure loss. It does not matter which words you use to label Pa and Pb, nor does it matter how big Pa is or Pb is. Only the difference Pa-Pb matters. You can not get more flow without increasing Pa-Pb.

I'm trying to help you. Does that answer your question?

 

[TSN79]

The sprinkler valve represents the beginning of the system, and is also where the residual pressure is measured. Let's say point Pa represents this location. Once there is flow, the pressure reading at Pa will start to fall. Is there a need for Pa not to fall below 3 bars (if the calculated loss is correct) in order to achieve the desired flow rate? Appreciate the help :)

 

[CWatters]

Each sprinkler head will deliver the specified flow at a specified pressure. You or the designer has to ensure that pressure is maintained under worst case conditions or the flow will be lower.

If you provide 3 bar at the inlet valve and the loss in the pipework is 3 bar then the pressure at the sprinkler will be low so the flow might be inadequate.

I think you need to find out what pressure is needed at the sprinkler heads and add the 3bar pipe loss to that.

The supply must be able to deliver that pressure at the required flow rate (eg not just the static pressure).