EndSuction pump with Plan 11 1

[653jon]

Is it possible to operate an Endsuction suction single stage centrifugal pump with fully enclosed impeller , with out balance holes and back wear-ring, on API seal plan 11?

 

[JJPellin]

Unless you do something else to lower the seal chamber pressure, you probably will not get good flush flow with Plan 11. You could use back pump-out vanes to lower the pressure. Or, you could use a different plan. I like Plan 11 in this situation. The flush fluid is drawn down the backside of the spinning impeller. This can tend to act as a centrifuge to remove some of the solid particles. If this is a problem, I might use Plan 14 which is a combination of 11 and 13.

It is easy to prove. Run the pump with no seal flush and measure the seal chamber pressure. Compare this to the discharge pressure to see how much differential you have.

Johnny Pellin

 

[653jon]

Thanks for the reply.

The intention is to increase the seal chamber pressure. API 682 6.1.2.14 recommends to raise vapour pressure margin by "raising seal chamber pressure by removing the back wear ring and plugging impeller balance holes".
The fluid is clean and we have seal failure due to flashing and dry running.

 

[JJPellin]

Removing the back wear rings and plugging the balance holes will increase the seal chamber pressure. It will also increase the thrust load. You need to account for that and make certain that your thrust bearing can take it. I would suggest a Plan 11 flush at that point. You can size the orifice in the flush so that you don't give back the higher pressure you created. I might also slot the throat bushing or increased the throat bushing clearance to be sure I didn't get excessive pressure drop across the throat bushing which would also work counter to your desire for higher pressure.

Johnny Pellin

 

[653jon]

Great Thanks,

yes. The extra axial thrust load will be taken care of. When discussed with a pump manufacturer, they indicated that this might lead to dead-heading the seal chamber? Is it industry practice to do this type of modification?

 

[JJPellin]

I have made this modification and the opposite. I have added back wear rings and balance holes and I have removed them. It is not common. It must be done with care to avoid problems with thrust loads or seal flush flows. But, it is not unheard of. Back in the day, some pump models were offered with both configurations depending on the suction pressure, vapor pressure margin, etc.

You do have an alternative if you just want to build more seal chamber pressure to avoid flashing. You can add a close clearance floating bushing at the bottom of the seal chamber and let your Plan 11 flush build additional pressure. This is less risky in terms of thrust loads. We have done this successfully on pumps with a tendency to flashing in the seal chamber. The bushing can be a separate assembly installed in the bottom of the box. Or, it can be built onto the seal cartridge. If we decided to go this way, you might want to add a gauge for seal chamber pressure and ask your operators to monitor this pressure. If the bushing opens up and the seal chamber pressure drops, you could be right back into the same boat with flashing. Removing the balance holes and back wear rings is a more certain way to increase chamber pressure.


Johnny Pellin

 

[653jon]

Thanks for the valuable comments.