Fire Water Pressure Question

[doug223]

Hi everyone

Sorry if this is a silly question but I am unsure and dont want to get it wrong.

I am involved in a project that is designing a fire pump system. A jockey pump maintains pressure at around 7 bar. If a hydrant is opened (350m away) supplied from a 10" main, will the drop in pressure only come from friction losses from a firefighters hose?

I have read up on static and residual pressure and am I right in thinking that pressure supplied by a pump does not drop when a valve is opened as opposed to pressure supplied by gravity?

I know this is basic but its not really my area. Thanks for any help

 

[bimr]

The jockey pump probably supplies less than 50-100 gpm. The hydrant has a flow capacity of 600-800 gpm.

When the hydrant is opened, the water pressure will drop, because of the 600-800 gpm outflow and the jockey pump will not be able to maintain the hydrant flow and system pressure. The drop in pressure will cause the rire pumps to run.

The drop in pressure is caused by the release of the water, not friction losses. The pressure at the fire engine will be less because of the friction losses.

I understand that the fire pumper will actually draw suction on the water system as it pumps the water.

 

[cvg]

your question is poorly worded at best, so the answer is likely to be wrong, but here goes...

pressure drop is caused by friction losses in the 350m pipe and then minor losses in the hydrant nozzle, regardless of whether the water is supplied by gravity or a pump.

The water flows through the pipe due to the pressure differential from the pump to the open nozzle. More pressure differential equals more flow.

Pumps operate on a pump curve, pressure varies inversely with discharge. So pressure and flow supplied by the pump will vary depending on your system characteristics. Therefore pressure will also tend to drop when you open the hydrant because the pump operating point changes.

 

[doug223]

Hi

Thanks for replying and sorry for the poorly worded question

The jockey pump is rated at 250 l/min. I guess what I am asking is that what should I be factoring in to the losses to determine if opening a hydrant valve will cause the fire pump to start?

The jockey pump should be able to supply the hydrant flow and pressure requirements without the fire pump starting but it seems this is not likely to happen.

Is there a reputable website for civil engineering formulas such as water main flow calcs etc? Its just that I have seen a couple of different equations and not sure which one is the correct one.

Thanks Again

 

[bimr]

The jockey pump will not be able to maintain pressure when the hydrant opens. Jockey pump typically supply 50-100 gpm. You jockey pumps supplies 250 l/min. A normal modern hydrant has a flow capacity of 600-800 gpm.

Jockey pumps are used only to maintain pressure, not to supply water for hydrants. Water pressure is lost due to minor leakage.