Flow of Water

[FF702]

Can someone talk me through:

The flow of water when open water pumping using an LPP (e.g what happens when priming lever is pulled ,priming valve, atmospheric pressure, exhaust gases etc)

Also the flow of water from the tank to the deliveries/hose reel? 

Thanks in advance

[OscarTango]

What are you actually asking here? How to get an LPP and main pump to work? 

Or a description of how the process works according to physics? 

It may help if you're more specific about what you're trying to understand then we can help you out.

[FF702]

Sorry the question was a bit vague.

I know how to work the pumps, but would like to know the physics of what happens when the LPP is primed, If that makes sense. Sorry I'm a probationer 

[OscarTango]

No worries at all pal. 

So it depends on what LPP you have. Some have hand pump primers, most have exhaust gas ejectors. But they both do the same job.

All a primer does is remove air from the pump casing (volute) and the hard suction.

It needs to do this because what is actually moving the water into the pump is the weight of the atmosphere itself. The maximum it can theoretically "push" water in is one bar. 

The hand pump primers version are literally a just a piston that pushes the air out. 

Exhaust gas ejectors work on the Venturi principle. High pressure air will draw out low pressure air as it moves. If you're in a moving car and someone farts, wind down the window a crack. The speed of the air whizzing past is at a higher pressure relatively speaking than the air inside the car, which will get pulled out through the window. 

Exhaust gas ejectors use the hot gas from the engine exhaust to do the same thing to the air in the pump casing. 

Hope this helps!

[C_Iain]

@OscarTango that might just be the best descriptIon of the Venturi principle I’ve ever heard 🤣

[HB2016]

Brilliant!

[Messyshaw]

Only a firefighter can start off using a scientific description to explain the laws of physics........... before verifying the lecture with a fart analogy 😊😊

I wonder if they use this method at Eton or Cambridge?🤔

So much has changed since I joined, indeed since I retired. But its refreshing to see standards remain toilet based 🙂 

[Nick417]

Have a look at this doc. You'll have to look dig through it to find what you need and hopefully learn a bit more whilst trying to find your answers.

Edited May 31, 2022 by Carl
Link formatted correctly, please see FAQ.

[Highlander]

The reason you need to prime a LPP is because the pumps we all use are centrifugal pumps. They are simple and reliable which is the primary function, but they have a gap between the impeller vane and the volute. Their output(flow rate) can change dependent on the pressure at the outlet.  It's other effect is that it can't effectively create a vacuum in a compressible fluid (air). So it is required to be positively fed or be prefilled with uncompressible fluid(water) to do this, you need either a positive displacement pump (hand piston pump) or a Venturi (fart method) to create a low pressure and suck the fluid up into the pump. 

The main pump... has an extra level of complexity called a water ring, that allows it to prime in air, im not going to pretend i can fully explain it, this is better.