8/13/2019 Shaft Deflection in centrifugal pumps

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DECEMBER 2004 www.pump-zone.com PUMPS & SYSTEMS

Last month we discussed the cause of shaftdeflection and identified the fact that in anend suction, overhung centrifugal pump

with a single volute casing, there exists the potentialfor unbalanced radial forces to cause excessive shaftdeflection if the pump is run away from the bestefficiency point (BEP). So Lets Get Practical. How

do we combat that problem in the field?About 20 yearsago, a concept calledthe Shaft SlendernessRatio was introduced.It compares the dia-meter (D) of the shaftunder the shaft sleeve,

with the distance (L)between the impellerand radial bearingcenterlines.

Slenderness Ratio = L3/D4

While most mathematicians can relate the slender-ness ratio with a simplification of the complex for-mula given in last months column, it is fairly obvi-ous that the amount of movement at the end of theshaft depends on the diameter of the shaft and thelength of the overhang.

It is important to note that a shaft sleeve doesnot help in the rigidity of the shaft unless the sleeve

is shrunk onto the shaft. As this is not a commonpractice on end suction process pumps, the diame-ter of the shaft under the sleeve must be measuredto establish the slenderness ratio.

Accordingly, as the shaft diameter increasesand/or the length decreases, the slenderness ratiowill decrease, and the shaft will become more resis-

tant to deflection. Incontrast, a highervalue reveals a shaftthat is less resistant.Consequently, it willdeflect more easily.St ronge r sha f t srarely exhibit any real

shaft deflection prob-lems and, therefore,seals and packing ope-rate without any diffi-

culty, resulting in a very reliable pump. This relia-bility is al-most regardless of the operating condi-tions.

Pumps with shafts having higher levels of slen-derness ratio can also display high levels of reliabil-ity, but only when they are operating at, or close to,their BEP. If the operating point on these pumpsmoves away from the BEP, the radial loads canincrease beyond the point that can be effectively

resisted by the weaker shaft, and excessive shaftdeflection can take place.

Many pumps in the chemical process industry(and others) are subjected to excessive and inappro-priate hydraulic conditions, such as fluctuatingpressures or repeated cavitation or recirculationproblems. When faced with such conditions thepump shafts are susceptible to a degree of deflection(and frequently, vibration) that is sufficient to causerepetitive seal and packing failure. As a result, anypump that displays the symptoms of premature andrepeated failures of the mechanical seal is exhibitingthe classical symptom of excessive shaft deflection.

Considering all practical options to overcomethis condition, the simplest and most effectivemodification is to remove the shaft sleeve andincrease the shaft diameter to the same size as theoriginal sleeve. In some cases, this will cut the slen-derness ratio almost in half, thus making the shafttwice as capable of resisting deflection.

Lets Get Practical

Shaft Deflection Part Two:The Effect

Ross Mackay, Contributing Editor

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Figure 1. Radial forces in asingle volute case

Figure 2. Shaft and L/D

dimensions

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8/13/2019 Shaft Deflection in centrifugal pumps

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PUMPS & SYSTEMS www.pump-zone.com DECEMBER 2004 11

It should also be noted that this will require anon-fretting mechanical seal, but this seems to be asmall price to pay for increased pump reliability inthe most practical sense.

Vertical PumpsOwing to the distance from the impeller to the

first bearing on a vertical pump, the validity of theslenderness ratio has been brought into question.This will depend on the type of vertical pump beingconsidered. For example, a vertical turbine pumputilizes axial diffuser vanes in the bowl assemblythat minimize any consequential radial loads.Hence, the slenderness ratio is not a factor.

On a vertical inline pump that uses a singlevolute casing, however, the slenderness ratio

becomes a consideration as the radial loads will stillbe imposed at right angles to the pump shaft. It isof particular concern for the older inline pumpdesigns shown in Figure 3. These older designsdont incorporate a bearing in the pump, but mere-ly secure the impeller to an extension of the motorshaft. In some cases, this will create an increaseddistance from the bearing centerline to the impellercenterline, resulting in a higher value of the slen-derness ratio. This makes the shaft more susceptibleto deflection. This problem has been addressed bynewer inline pump designs, where a bearing hous-ing has been built into the pump in a manner sim-

ilar to that of the horizontal process pump.

Weight of Impeller and ShaftIt also has been suggested that the weight of the

impeller and shaft might be a factor, but it is evi-dent from Figure 1 that the bulk of the resultantout-of-balance radial force created in a horizontalpump is acting upwards and will be opposing the

weight of the impeller and shaft. So, a lighterimpeller, such as what might be found in a non-metallic pump, may even be detrimental to theresulting condition. To a lesser degree, this will alsobe the case in a vertical shaft pump, where the phys-

ical weight of the impeller and shaft is negated.

An Excellent ToolThe slenderness ratio is an excellent tool to

consider when excessively repetitive seal failures areevidentbut where it has been clearly establishedthat these failures are not an actual seal problem.This is a relatively safe consideration if a variety of

seal styles and types are exhibiting a similar failurepattern on the same pump.

The classic failure pattern is when all the dif-ferent seals that have been installed in the pump fail

within the same approximate time period. There isalso the wear pattern on the seal that can be repeat-ed, but this one depends on the actual design of theseal, where the barrel of the seal is within the clos-est proximity of the bore of the stuffing box. Under

these conditions, the shaft deflection brings thatpart of the seal into touch with the stuffing boxbore, which creates a rubbing mark all round thediameter of the seal.

So Lets Get Practical, again. The ShaftSlenderness Ratio should not be considered in iso-lation to the operating condition of the pump. Itonly becomes a factor when the pump is not run-ning at its BEP. On the other hand, how manypumping applications can boast that luxury? P&S

Ross Mackay specializes in helping

companies increase their pump reliability

and reduce operating and maintenancecosts through consulting and education. He

is the author of the new book The

Practical Pumping Handbook, thats now

available at www.practicalpumping.com.

He can be reached at 1-800-465-6260.

Figure 3. Vertical inline pump

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