A small system can transfer the weight of your car in one minute!

Have you ever seen a fire nozzle arc of water thrusting from a tug, workboat or similar? I’ll bet you saw a few during the news coverage of the BP oil spill in the Gulf.

These streams may not seem too impressive from a distance, but the amount of water flowing through each of those nozzles may surprise you. A nozzle flow could be around 500 gpm (gallons per minute) or upwards of 5000 gpm or more.

Take 500 gpm for example. In one minute of flow, 500 gallons of water is transferred. 500 gallons of water weighs a little more than 4000 pounds… That’s the weight of a standard car!

So you can imagine the power needed to propel these quantities of water.

Here’s a quick guide to estimate the horsepower: If you know the flow and head needed at the pump, just multiply the flow in gpm times the head in psi, then divide by 1714 for the hydraulic power. This is the raw power, so you would then need to divide that by pump and motor efficiencies. A good rule of thumb for this application would be 0.5 if using a centrifugal pump with no self-priming. That will get you in the ball park so your electrical guy can accommodate for all the sparky stuff.

For example, if you needed 500 gpm at 100 psi at the pump, the estimated horsepower would be:

Eq1

By comparison, if your car is 200 hp, this pump will need nearly 1/3 of that power to operate. Multiply that by 10 for a 5000 gpm pump at the same head.

I’ve put together a two-part video series for the highlights of a functional design for a fire monitor system. A few typical steps are needed before a functional design makes it to CAD.

  • Part I describes how to set a basic system up.
  • Part II progresses through the pipe and pump sizing on a theoretical system.

The videos are fairly short so they will only highlight the broad brushes, but I hope this will be helpful to you.

Part I:

Part II:

 

Log in for the free Darcy pressure drop spreadsheet mentioned in the video’s:

 

Please leave a comment and let me know your thoughts.

Thanks,

Dave

 

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