Vertical Turbine - Bearing Spacing

[alexcmmi]

My question is w. regards to max bearing spans for use in a VT pump

Application is raw water through a Ranney collector well (so screened and river bed filtered). 40 to 65 deg F. Should be very good quality raw water.

Pumps are 300 HP, design operating point is 2604 GPM at 328' TDH. Most of head is static.

Pumps bowls will be set at ~ El. -20
Discharge head will be set at ~ El. 28.5
Therefore total pump length is 48'

Pumps will be VFD driven. 1800 RPM w. ~ min 1400 RPM

Critical speed analysis performed by pump manufacturer had indicated that bearing spacing at either max 5' and 10' will avoid shaft criticial speeds and manufacturer 'prefers' using 10' max spacing. We spec'd 5' max spacing.

Is there any reason why 10' would be better? Every reference seems to indicate reducing bearing spans to 5' max is better, especially in SHORT SET applications.

However, I am not sure if this is considered a SHORT SET application? What is the cut off on VTs where they are considered Short Set vs LONG pumps?

Any info / insight would be VERY APPRECIATED.

MANY THANKS!

 

[alexcmmi]

Well, through further research it seems the cut off for short set vs deep well applications is 20 feet. However, I am not sure how reliable the source is. Is this 'cut off' specified in HI standards?

If my application is indeed 'deep well' it seems the 10' spacing may be more applicable

Any input would be greatly apprciated. THANKS.

 

[Artisi]

You will find it is based on critical speed of the shaft. This is where you should be looking for answers.

 

[TenPenny]

A 10 ft spacing will typically require a larger diameter shaft (all else being equal); however, the shorter spacing also means more bearing retainers = more friction losses up the column. Also, for maintenance, more bearings = more work required for repair/rebuilds.

 

[alexcmmi]

Many Thanks for the responses thus far.

The critical speed analysis has indicated that with 5' spacing, we will be WELL below the 1st mode shaft critical speed.

With 10' spacing, our operating range is between the 1st and 2nd mode shaft critical speeds. However, we WILL avoid them altogether based on our presumed operating speeds (ie 1400 - 1800 rpm...)

Therefore, to me, the conservative approach would definitly be the 5' spacing b.c the first critical speed is around 2600 rpm and we have an 1800 rpm...We will NEVER approach or be in a critical speed scenario. IN the 10' spacing we will pass through the 1st mode critical speed on ramp up, but it will likely be for a split second (nanosecond?) and we will NEVER operate in this range for any period of time.

However, the maintenance issue indicated by TenPenny makes sense. Will being conservative utlitmately be non cost effective and detrimental?

Any thoughts appreciated...leaning towards 10' spans though....

 

[Artisi]

I would go with 5' - the small additional costs etc will more than pay for themselves so you can sleep at night not worrying about critical speed problems.
I have run water lubed line shaft bearing pumps between 1st and 2nd C.S. but occasionally we had problems so made the decision to remain below 1st C.S to be on the safe side.

Whether water lubed cutless rubber or lubed bronze bronze, if properly selected and lubricated bearing life depending usage should be many years.

 

[TenPenny]

Based on what you've said, I second the 5 ft decision. Me, I don't like pumps operating above 1st critical speed, if there's a simple alternative.