Q&Ans:Tell me about the pressure measurement of water putting off trough nozzel of a fire vehicle

As water exits a nozzle, right at the tip there is a pressure that can be measured with a pitot gage. This pressure is known as nozzle pressure.

 All nozzles are designed to work at specific nozzle pressures. Example, a fog nozzle may be designed to deliver 100 gallons per minute (gpm) at a nozzle pressure of 100 psi. 
If the nozzle pressure is less then 100 psi then the flow will be less then 100 gpm. The three basic nozzle pressures used in the fire service are: Straight tip hand line = 50 psi Master stream with a smooth bore nozzle or stacked tips = 80 psi Fog nozzles (All) = 100 psi The nozzles pressures listed above must be committed to memory. Fog nozzle, 100 psi nozzle pressure Adjustable gallonage nozzle Engine Pressure (EP): Engine pressure is the holy grail of pump operators. This is the number in psi that we need to see on our gages at the pump panel. Engine pressure is the sum of the nozzle pressure + the friction loss + any elevation or devices. Based on the engine pressure formula EP = NP + FL if we need a nozzle pressure of 100 psi to flow 100 gpm then the engine pressure needs to be greater then 100 psi. 
We have to add extra pressure to 20 overcome the loss of energy as the water moves through the hose. 
Image result for fog nozzle

The bulk of this class will be structured to determine the friction loss requirement based on the types of hose and hose layouts. In order to determine friction loss we need to know three things:
 1. What is the flow, how much water in gpm is needed. 
2. Hose size (Diameter)
 3. Length of the hose line or hose lay. To find the gallons per minute we need to look at the nozzles. Nozzles are designed to flow specific amounts of water at certain nozzle pressures. A one inch tip nozzle will flow 210 gpm at a nozzle pressure of 50 psi. 

Pump Pressure Gages

We monitor the pump operation by using pressure gages. The two types of gages we
use are the compound gage and the discharge pressure gage. The compound gage is
connected to the intake or the suction side of the pump and measures intake pressures in psi if
the water supply is from a hydrant or other apparatus. The compound gage also measures
pressures below zero which are called vacuums. This measurement is in inches of mercury
(Hg). There is only one compound on the pump panel. However, some pump panels use
gages that all have the same face that might look like compound gages.
The discharge gage measures discharge pressures on the discharge side of the pump.
The main discharge gage measures the discharge pressure at the center of the pump. The
compound gage and the main discharge gage are the two largest gages on the pump panel.
Each discharge usually has its own discharge gage which is smaller in diameter then the main
pressure gage. These smaller discharge gages measure the pressure at the discharge outlet

between the ball valve and the pump panel cover.

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