pump understanding 2

[usman74]

charging pump and other safety pumps having huge pressure difference across them have inlet and outlet connect via small pipe (short). could anybody here please guide me why we have that line?????

Regards

 

[Duwe6]

Typically it is to allow 'priming' - release of air, filling pump with liquid - during start-up of system.

 

[usman74]

but here one of my colleague told it is for pump protection against thrust due to huge pressure difference. Although, he failed to give convening argument.
the line i'm talking about is passing over the pump and directly connected suction & discharge line, how i could be used for priming of pump????

 

[usman74]

Duwe6 uploading pic.

 

[byrdj]

would you have a cross sectional drawing of the pump? I am not familiar with that pump but fist impression would be sealing water supply.

I notice a line comes off the "T" at high pressure end. where does that go as it might be an indication of the purpose of the short line

 

[racookpe1978]

Need a P&ID for that system to begin guessing (er, making assumptions).

 

[usman74]

dear byrdj
centrifugal multistage pump of nuclear safety class. there are separate sealing line. this is simple connection from pump suction side to discharging side.

 

[usman74]

dear racookpe1978
it is called pressure balancing line as there is huge pressure difference between suction & discharge. so there will be thrust on pump impeller. to mitigate thrust, this protection applied. but i'm not understanding how it can cope with that thrust.

 

[byrdj]

A cross sectional drawing is needed. If it is called a "balancing line", I would expect to see a rotating "piston" section
search for pump balancing piston/drum/disk

 

[racookpe1978]

OK. Given the name, here's how it works.

See diagram above: The original design tries to balance the axial thrust caused by the difference in pressure from one end to the other. (In the LP turbines, this is done by forcing the steam to enter in the middle of the turbine, and exhaust equally in both directions through equally-sized blades and vanes in both directions to equal regions of low pressure (the condenser below the LP turbine.)) You can't do it exactly in every LP turbine, so the turbine shaft thrust bearing takes up the differential pressures for the whole shaft system. The HP and IP turbines use similar counterflow paths to reduce imbalance, but both add forces tot he shaft as well.

You can't do that with a HP pump, so the original design of the pump casing ports high pressure from the outlet to one side of a piston area (the small side of the piston), and LP water at pump suction pressure to the other (larger) side of the same piston.

high discharge pressure x small area + thrust across pump ~~ lower suction pressure x larger area. Result is almost no force, which can be dealt with by a smaller, less expensive and more effective regular pump thrust bearing elsewhere on the shaft.