Fire engine pump

[gregpaul]

Fire truck pump can have a negative inlet pressure, since it can take water in a lake with a water level below the pump.

Some literature I read state that it is different when the pump is connected to a hydrant with hoses, and that the pump inlet pressure should not be lower than the positive pressure of 20 psig.

Could you direct me to an article or a book that would explain the technical reason behind this recommandation ?

Thank you in advance for your help.

 

[hydrae]

Backflow

When a fire truck connects to a fire hydrant the residual pressure is limited to prevent any cross connection from drawing a containminate into the system.
There are many type of cross connections. The case histories of events like this are enormous. One case the containminate was propane, turn on the faucet and lite up the water...
another case, pesticide was drawn into the water main in a subdivision. The pesticide was sticky and would not flush out, all the piping in the houses and street were replaced to get rid of the poison. These are not urban myths!

Many utilities have programs to install backflow prevention devices at hazards but this does not get everthing. By ensuring a minimum pressure of 20 PSI there is resonable safety in keeping the water system under positive pressure. In hilly areas it may be required to have even higher residual pressures, and water system operator should evaulate their system for such locations and notify the fire department (before the fire).

This link shows some of what has happened

http://www.treeo.ufl.edu/backflow/casehist.asp

you can google "backflow case histories" to get more.

Hydrae