Modification of Bearing Pedestals

[metengr]

I recently reviewed a consultants report concerning our steam driven boiler feed pumps at one of our generating stations. We have had vibration problems with these pumps over the years. The pumps were supplied by Byron Jackson and are 14X14X16 4 stage boiler feed pumps.

The recent evaluation consisted of gathering vibration/impact data to generate an operating deflection shape (ODS) under operating speed conditions.

The main conclusion from their study is that the pump pedestals are twisting from a natural frequency imparted by the operating speed range of the pump. To correct for this problem, it was recommended to stiffen the pedestal support legs by welding stiffener plates or filling the cavity in the hallow legs with concrete.

My question is... what type of concrete should be used because I am concerned regarding shrinkage and flexing strength. Is their a recommended concrete for this type of specific application?

 

[micjk]

I would personally go the steel plate route. Pouring concrete/using grout can and will work but I have seen situations where over time the problem returned. Reason being the vibration, though reduced, over time caused the structure to act like a pulverizer, reducing the concrete/grout bond to the structure. What I mean is thhe fill turned to powder and problem resurfaced. Now you have a big chunk of fill that prohibits any further modifications.
This is only my opinion,

Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA

 

[sms]

I am with Roy, these are large high energy pumps, I would not mess with grout. Stiffen it by welding the plates.

 

[magicme]

another vote for micjk's solution...."steel is forever"

daveleo

 

[Tmoose]

I have been around a few machines including pumps that responded nicely to experiments with angle iron and c-clamps. I sure like playing from deck with cards I've marked.

I'd be real careful not to interfere with any of the manufacturer's provisions for thermal expansion.

I doubt you can get the manufacturer to bless any design change. Have you checked to see if they offer any special service revisions you can buy?

 

[fredt]

The reason that concrete or sand in the plinth can work is that it adds mass and effectively detunes the system. As other correspondents have observed it does little to stiffen the structure. The issue is fundamentally a mass, spring, damped system.

Having said that, my experience has been minimal effect from filling the pedestal with concrete or other material. Stiffening with steel is probably better but keep in mind that the stiffness value is dominated by the most flexible member in the system and stiffen accordingly.

 

[Tmoose]

There is a message board at Reliability Magazine monitored by folks who may have gone thru this process before on your exact machines.

 

[TPL]

If you can rig up some form of temporary stiffening device, using a car jack (or something more suitable such as a scaffold jacking screw) to add stiffness to the housing whilst the machine is running, you can monitor vibration whilst applying increasing stiffness (admittedly uncontrolled)and check out whetehr or not this approach has any merit

 

[Hemish]

I would greatly appreciate if you let me know which plant this problem occured (vibration). We are currently considering a spring supported foundation for steam driven boiler feed pumps (variable speed) for a 650 mW power plant. I am also interested in a list of US and or UK power plants where springs were used to support boiler feed pumps.