Double shaft deflection in a BB pump

[albertpet]

Shaft deflection shall be limited in each pump type to not have eccesive vibrations and/or internal wear due to contacts.
But in more than one occasion I have found the reccomendation that double shaft deflection can not be acceptable in any case. I can understand this suggestion is reasonable, but searching in the almost endless state of arts I haven't found any clear explanation.
I hope your deeper experience could help me.

 

[electricpete]

I see no responses, so I'll ask for some clarification?

BB pump = between-bearings?

double shaft deflection =? Is this a description of shaft bending shape? For example second critical modeshape?

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[albertpet]

Sorry for the cryptic message.
As you write, BB means between bearing.
With double shaft deflection I mean a fisical double bending due to the weight and the radial forces acting on the shaft.

 

[Bjegovic]

my guess is:

1) BB layout is much less succeptible to deflection than overhung.
2) BB shaft usually supports double impeller (double volute pump) which very vell balances hydraulic loads in complete range of operating conditions.

therefore, shaft deflection is not a big issue in BB pumps.

 

[Artisi]

There seems to be some confusion as to double volute and double suction impeller pumps in this thread and one other currently active thread 407-142821 "double volute vs single volute".
My understanding is and I would welcome other comments:
1. A double suction pump is usually an axially split case unit with a single inlet dividing into 2 branches to a single double suction impeller, ie, flow to both sides of the impeller.
2. A double volute pump is one in which there are 2 outlets diametrically opposed to each other in a single casing which discharge into a single outlet branch.
3. A double suction pump can be either double volute or single volute design.
4. A single suction pump could be either double volute or single volute design.
5. A unit with more than 1 volute would be designated a multi stage pump.

Naresuan University
Phitsanulok
Thailand

 

[Bjegovic]

I am sorry that my posts (both here and in thread 407-142821 ) were confusing, ignorant and written in a hurry.

I agree with in general with definitions that Artisi offered.

I also have tried to give specific codes reffering to API610 (8th, 1995) of Hydraulic Institute standards (14th edition) but no specific clasification is related to single/double volute or single/double suction pumps.

anyway, as for shaft deflection, API610 limits it to 0.002in at primary seal faces. this is a general provision in section 2.5 and as I took a glance in specific requirements for BB pumps there are no additional provisions.

also, as I mentioned above, shaft deflection at span L with radial force F will be proportional to 1/3 FL^3
for overhung impeller, and 1/48 FL^3 for BB layout with force acting in the middle of the span. theoretical simplification, but it may help.

when I mentioned double volute pump (this indeed reduces radial forces and shaft deflection) this was not a correct observation because DV can be also be overhung. I constantly had in mind ABS type Z 22 pump which is both double volute and double suction... sorry for confusion again!