Very trick problem with pump

[marcioeq]

In the company I work for there's a centrifugal pump for pure acetic acid unloading from a truck container. The pump data is 60 m3/h x 60m. Since the flowrate is a little bit high, when the container is almost empty (i.e. when acid level in container drops) there´s a vortex formation inside the container and air (in fact N2) enters the pump succion pipeline so that it is necessary to prime the pump ( remove the N2 from a sample valve at discharge). This operations is very dangerous since the fluid is acetic acid. Using a priming pump would solve the problem? what would you do in this scenario? Please need a help!

Thanks a lot

[Nabilia]

Have a valve on the suction end so that when you are near the level that causes the suction vortex, you can close the valve some to decrease the flow which will stop the vortexing

[hermes]

marcioeq:
Your problem is very common. Almost always when you are near to empty a vessel, a vortex can begin.
You should go in the way to break the vortex. There are a lot of designs about vortex-breaking devices, mostly for vessels, but there are some also for piping. It will increase a little bit the pressure drop on the suction line, but I guess it wouldn´t be a problem for the pump.
Have success.
Hermes

[superandy]

Dear friends,

because pump has a very high capacity, it is never a good choice to close suction valve because you can cause problems concerning cavitation and so on.

It will be better to close partially the valve on the pump discharge.

This will reduce pump rate at the end of your operation and so the vortex formation. Another possibility is to manage a bypass between discharge and suction line, in order to recirculate in the suction line a little portion of the flow rate.

Regards
Superandy

[marcioeq]

Dear Superandy, I can't figure out why having a portion of fluid recirculated to the suction line would help with this problem. Could you explain that please?
Thanks a lot,

marcioeq

[superandy]

Recirculation from 5 to 10% of total flowrate from pump discharge to suction line, reduces total flowrate delivered by the pump and helps to have suction line completly full of product, so avoiding vortex formation caused by the progressive reduction of the liquid level in the tank (NPSHd lowering). Another way is to increase the size of suction line and to install some device (baffles) inside the pipe to break vortex formation: so at the end of the operation will be necessary to use a gear pump to evacuate residual product from tank and line.

Regards
Superandy

[mohamedbadawy]

Dear in such case normally we use a dry run protection system, which its function to stop the pump at a critical level, a float with electric signal is used in this process to send a feedback signal to a local control box (LCB) to give an action to the pump to stop, but ...... in your case, the pump is not permanently connected to the tank but you can prepare this system in a shape of mobile or ready use for such case , i hope this info is helpful, and for such cases don't hesitate to contact me

[marcioeq]

I thank all of you for your helpfull answers. But I'd like your opinion about getting a self-priming pump...do you consider replacing the existing pump for a self priming one a good idea? Do you see any technical inconvenient ( let's forget the cost for a while!)?

Mr Mohamed, the succiont line has already a FSL ( low flow switch) to prevent damage to pump due to cavitation.

[superandy]

A self priming pump may be a good choice: you could install such a pump (having a little capacity but the same head) in parallel to centrifugal one. So when the level in the tank reaches the lower limit for centrifugal pump, you can start the little pump to evacuate the tank.

Regards
Superandy