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Positive Dispacement Pump definition
Delivers a measured amount of water on each stroke or cycle of the pump.
Positive Displacement Slippage
No slippage. If discharge is stopped, pump will break.
Types of Positive Displacemnt Pumps
- 1. Piston
- 2. Rotary Gear
- 3. Rotary Vane
Rotary Gear and Vane uses
Driven by starter motor. Used as primer pump to exhaust air in a centrifugal pump.
Centrifugal Pump advantage/disadvantage
- 100% slippage (can run without discharge)
- Can not exhaust air
Centrifugal Pump Nomentclature
- Volute- housing around impeller
- Impeller- spinning part that creates flow
- Eye- takes in water to center of impeller
- Diffuser Vane -water thrown out of impeller here
- Wear rings- take bulk of wear in spinning impeller
- Freedom pump- Mechanical seal replaced packing gland for cooling the shaft
Pump Rated Capacity PSI
Rated at 150 psi net pump pressure. Above this output is diminished.
Net Pump Pressure
- Pump Pressure
- Intake pressure.
- 100% at 150 psi
- 70% at 200 psi
- 50% at 250 psi
- Occurs when impeller tries to thow more water than intake can provide.
- Rapidly creates steam in pump and causes damage
- Sounds like rocks in the pump
Possible causes of cavitation
- Diminished supply
- Throttle too high
- Clogged intake
Positive Displacement pump lowers atmospheric pressure in the volute forcing water into the eye of the pump.
Anti Siphon Hole
- Top of priming system
- Size of a broom straw .04"
- Small intake to release suction on oil tank when priming is done
Two Stage centrifugal pump
- Has two impellers
- independate (parallel or volume)
- one feeds the other (series or pressure)
- Used in two stage pump to change from parallel to series
- Lower pressure to 50 psi before changing from volume to pressure, because pressure will double
When to use parallel or volume
When ever you pump more than 1/2 your rated capacity
Intake relief or Dump Valve
Takes excessive intak water pressure and dumps it on the ground.
Tank to Pump
Opens tank water to gravity feed into the pump.
Tank to pump notes
- 500 gallons per minute
- NFPA requires 80%
- 3- 31/2 inch line
Tank Fill valve
- Fill the tank from the pump
- 1" line
- Helps to cool the pump
3/8 inch discharge line to cool the engine
- Used on older engines to relieve excessive back pressure created by closing a discharge
- Feeds water and pressure back to intake side of pump
PSG (Pressure Sensor Governor)
- Electronic control to maintain Master Gauge pressure when there are changes in the the hose lines
- Used in stead of relief valve
- Changes the fuel to the engine
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