Multistage Centrifugal Pump Balancing line

[Shafayat Ahmmed]

Hi all,
I am new to this community.
I want to know why an orifice is placed in the balancing line of Boiler Feed Water Pump.

 

[JJPellin]

The only possible reason is to measure the pressure differential across the orifice to determine the balance line flow rate.

Johnny Pellin

 

[Shafayat Ahmmed]

Thanks JP.
In some discussions in the internet i have seen that this orifice is used for regulating the flow in the balancing line and consequently, pressure in the balance chamber i.e after the balance disk.
That is why i am confused and want to know if there is any pressure control or flow restriction is performed by this orifice.
If yes then why the restriction done in the balance chamber by this orifice.

 

[JJPellin]

No, I have never seen an orifice used in an attempt to control the balance chamber pressure. It would only be used to measure the balance line flow, nothing else. It might be displayed as a balance line differential pressure. The equipment manufacturer may have provided a table to convert this differential pressure to flow rate.

Johnny Pellin

 

[Shafayat Ahmmed]

Thank You JP for your kind response.
"To ensure
proper balancing-disk operation, the change in back pressure must be of
an appreciable magnitude. This is accomplished by introducing a
restricting orifice in the leakage return line."
This line is quoted from a book named "Marks' Standard Handbook for Mechanical Engineers".
I want to know how the orifice controls the back pressure.
Searched a lot but couldn't find the principle.

 

[JJPellin]

I have never seen this done. That does not mean it has never been done. I work in a large oil refinery. We have about 20 large pumps that use balance lines to control the thrust in conjunction with a balance piston or balance disk. There is an FAQ on balance pistons / balance disks that you should read. The balance piston counters the cumulative thrust of the impellers by imposing a pressure drop across the cross-sectional area of a piston. Area times pressure differential equals force. If you change the pressure on the low pressure side of the balance piston by installing an orifice, you will change the differential pressure across the balance piston which will change the thrust force it provides. But, this creates a new problem. As the clearance in the balance piston increases, the flow through the balance line increases. The pressure drop across the orifice increases. The back pressure on the balance piston increases which reduces the thrust contribution of the balance piston. So, if you are relying on an orifice to adjust the thrust, you may have to change the orifice diameter as the clearance increases. We would not want to do that. For our pumps, the balance piston is designed to adequately balance out the thrust with new clearances and to still adequately balance out the thrust if the clearances in the balance piston are doubled. Beyond that, the pump must be overhauled to restore the clearances.

Johnny Pellin

 

[LittleInch]

The key here is "leakage return line".

Does this mean that the balancing line has a return path to the inlet/suction manifold?

If so then yes you need to restrict flow otherwise this would turn into a pump bypass line and reduce efficiency.

Most balancing drums heave a certain flow in order to blow any air out and allow the pressure to vary as the discharge pressure varies. But not too much....

I'm pretty sure the restriction orifice noted in the picture below is the orifice you're referring to.

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

 

[Shafayat Ahmmed]

Thank You LittleInch.
Yes this is the line and orifice I am referring to.
Does this orifice plays a role in building up a back pressure in the back side of balancing disk??
Because i got this information from a book, "To ensure
proper balancing-disk operation, the change in back pressure must be of
an appreciable magnitude. This is accomplished by introducing a
restricting orifice in the leakage return line."
If there was no restricting orifice and the balance chamber pressure is higher then suction than water will travel straight to the suction line without giving any back pressure-is this the case??

 

[Nutzman]

Good day Shafayat,

I worked for a pump manufacturer that manufactured high pressure multi stage pumps. Typically multi stage pumps with a discharge pressure <300m, would be fitted with double row deep groove bearings (non-drive end) and cylindrical roller bearing drive end (to allow for shaft growth in the event of high product temperature and shaft growth). The double row deep groove bearing could accommodate axial thrust generated during operation/start/stop.
Multi stage pumps >300m head are fitted with cylindrical roller bearings both DE and NDE. This is to allow the entire rotor to float during operation/start/stop. They are fitted with balance disc's or balance drums.
In terms of balance disc, the design principle was high pressure ("discharge pressure" from last stage impeller), over a small area, versus "lower" pressure (back of balance disc)over a larger/bigger area in the balance chamber.
The balance chamber was piped back to the pump suction. Personally, I have never seen an orifice fitted in the return line. Our engineers calculated the diameter coming out the balance chamber, to serve as the "orifice". My only thought for fitting an orifice is that there is a fluctuating suction pressure and the orifice serves to buffer the pressure fluctuation (back) into the balance chamber?