NPSHa calculation if pipe flows back down

[flow.help.]

I'm working on changing a system my company currently uses to offload a railcar from underneath the tank to the top of it. I'm trying to confirm that there is enough NPSH available, but I can't find anything online that relates to the setup we'll be using - I'll attach a picture and describe it as best as I can.

From the top of the railcar there is a dip tube, submerged in liquid, extending to the bottom of the railcar. We will be designing piping to attach to the top of the dip tube, then run down to a centrifugal pump that's center line is below the railcar. In my NPSHa calculations, would the height difference be the lowest liquid level to the heighest point the liquid will reach in the piping before the pump, or would it be from the lowest liquid level to the pump centerline (i.e. does the flow downwards cancel the height change up)?

Any help would be greatly appreciated.

 

[MartinLe]

yes, NPSHa would be between lowest level in the tank and pump centerline, minus the head loss in the pipe.

However you need to prime the system, either the pump is self priming or you need to create a sufficient vacuum in the pipe some way.

Also be mindful of the pressure at the highest point in the line, this will be the lowest pressure and depending on medium and temperature you may have an evaporation issue.

 

[Artisi]

NPSHa is atmospheric pressure plus the distance from the lowest level in the tank to the pump centreline less all friction losses in the pipework and vapour pressures /temp considerations.
For this arrangement you shouldn't have any NPSHa problems, you start out with approx 10m NPSHa - just deduct all inlet losses etc to establish what is available.
As the previous poster pointed out - initial priming could be a bit of a problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

The issue shouldn't be the pump when flowing but how you prime the pump and also avoiding vapour formation at low tank level at the high point, especially if you're pumping high vapour pressure liquid like gasoline.

Getting the last dregs of liquid out can also be difficult unless you've got a sump but even then you can draw vortex in when the level gets low

Why do you want to change system?

There's a few downsides to doing what you're doing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.