RobsVette
Mechanical
- Apr 15, 2009
- 94
Hi Guys, I was hoping some of the experts here can educate me on some practices I follow but dont know the specific reasons why.
As the title states I have some questions on the different kinds of centrifugal pumps, specifically in regards to inertia bases and inertia bases using a VFD. Here are my questions.
So here is my understanding of when you use an inertia bases and why you would need one.
Inertia bases are used when you wish to vibration isolate a horizontal split case. The technical reason you need one is because on a horizontal pump, there will be a bow in the shaft (due to the weight of the impeller and the large distance from bearing to bearing with the pump casing and 2 mechanical seals in the middle,) that upon starting will cause unbalanced forces (similar to critical speed vibrations in a steam turbine) so to counter act these forces you utilize an inertia base to prevent them from pulling the pump all over the place and breaking things. I also always assumed that this is why these pumps utilize a soft coupling (to take account for unbalanced start-up forces) Is this correct and if not, why do you need an inertia base. Examples here are the Bell and Gossett VSX series, the Paco KP or the Armstrong 4600 series pumps.
On a vertical inline pump, you don't need to use an inertia base because the deflection from the weight of the impeller runs with the shaft not perpendicular to it like the horizontal pump. This makes good sense to me. Even in an "across the line start" you wouldn't have any of the unbalanced forces you would have in the horizontal split case. Plus the weight of the pump is directly in line with the piping so you can support the pump through the piping without any excessive torque or moments created on the piping. Examples of these kind of pumps as the Armstrong 4300 series (single and double suction versions,) the Paco VLS and the Bell and Gossett Series 80.
What about when you have a vertical mounted horizontal split case? I have always seen these pumps installed on inertia bases and assumed that even though the shaft deflection is vertical, since the pump connections are not in line with the vertical center of gravity, the inertia base is used to support the weight of the pump. I would assume they wouldn't need to be as large since there really wouldn't be much in the way of unbalanced forces to deal with upon start-up.
Last question is with the application of soft starting VSDs as the pump starter. For instance, if you utilize a VFD on a horizontal split case pump, with its soft starting ability, I would think the inertia base plays less of a role in dampening unbalanced forces and is really only used for vibration isolation, so perhaps the inertia base could be smaller and just has to hold the pump stiff.
Hopefully you guys can help. I never understood the full technical reasons for using an inertia base and I am hoping you guys can help me learn more about why to use them.
Thanks,
Rob
As the title states I have some questions on the different kinds of centrifugal pumps, specifically in regards to inertia bases and inertia bases using a VFD. Here are my questions.
So here is my understanding of when you use an inertia bases and why you would need one.
Inertia bases are used when you wish to vibration isolate a horizontal split case. The technical reason you need one is because on a horizontal pump, there will be a bow in the shaft (due to the weight of the impeller and the large distance from bearing to bearing with the pump casing and 2 mechanical seals in the middle,) that upon starting will cause unbalanced forces (similar to critical speed vibrations in a steam turbine) so to counter act these forces you utilize an inertia base to prevent them from pulling the pump all over the place and breaking things. I also always assumed that this is why these pumps utilize a soft coupling (to take account for unbalanced start-up forces) Is this correct and if not, why do you need an inertia base. Examples here are the Bell and Gossett VSX series, the Paco KP or the Armstrong 4600 series pumps.
On a vertical inline pump, you don't need to use an inertia base because the deflection from the weight of the impeller runs with the shaft not perpendicular to it like the horizontal pump. This makes good sense to me. Even in an "across the line start" you wouldn't have any of the unbalanced forces you would have in the horizontal split case. Plus the weight of the pump is directly in line with the piping so you can support the pump through the piping without any excessive torque or moments created on the piping. Examples of these kind of pumps as the Armstrong 4300 series (single and double suction versions,) the Paco VLS and the Bell and Gossett Series 80.
What about when you have a vertical mounted horizontal split case? I have always seen these pumps installed on inertia bases and assumed that even though the shaft deflection is vertical, since the pump connections are not in line with the vertical center of gravity, the inertia base is used to support the weight of the pump. I would assume they wouldn't need to be as large since there really wouldn't be much in the way of unbalanced forces to deal with upon start-up.
Last question is with the application of soft starting VSDs as the pump starter. For instance, if you utilize a VFD on a horizontal split case pump, with its soft starting ability, I would think the inertia base plays less of a role in dampening unbalanced forces and is really only used for vibration isolation, so perhaps the inertia base could be smaller and just has to hold the pump stiff.
Hopefully you guys can help. I never understood the full technical reasons for using an inertia base and I am hoping you guys can help me learn more about why to use them.
Thanks,
Rob
