WHY PUMP SHAFT FAILURES HAPPEN
Heinz P. Bloch *
Technical publications thrive on questions from reliability engineers. These questions
tend to inform editors and readers about prevailing level of knowledge. Also, the
questions often reflect training-related issues confronting industry. In any event, finding
answers may require practical knowledge and editors of trade and professional
publications must often match-up answer seekers with answer providers. Next, and to
the extent possible, editors fold the resulting discourse into presentations or articles of
interest to a wider spectrum of readers. This is one of these articles.
We were recently asked to provide guidance as a reader pondered over the possible
causes of catastrophic pump shaft and impeller failures at his plant. “In the course of my
work as a Pump and Vibration Specialist” the reader said, “I have encountered a number
of such failures. One root cause of such failures that has been suggested is a defective
check valve, which allows the process pump to freewheel in reverse due to back flow.
However, I have found no documentation that supports or explains this failure mode. In
conversations with various pump and field service technicians, it has been suggested that
when a pump is started while spinning in reverse, the starting torque exceeds the shaft
strength and catastrophic shaft failure occurs. But this explanation is counter to my
understanding of electric motor starting torque.”
Impellers and Reverse Rotation
Our initial reply highlighted first what our reader, of course, knew: There are many
cheap pumps and a few well-designed, more expensive pumps. When there was still an
abundance of common sense (decades ago, perhaps), some wise man wrote that we
always get what we pay for. Of course that is still true today and it applies to entire
machines, their component parts, services, employees, contractors, educators,
consultants---whatever.
Back to the point: User experts Ed Nelson and John Dufour (Ref. 1) noted that nearly all
impeller thread arrangements (left hand versus right hand) for single-stage end-suction
pumps are such that the impeller fit gets tighter if the pump shaft rotates in the asdesigned direction. Conversely, the impeller gets loose (or spins off the shaft) if the
pump shaft is rotated in the opposite, or unintended, direction of rotation. Impellers
installed with close-fitting keys (Figure 2, left representation) properly mated to the
shaft will not spin-off in case of inadvertent reverse rotation. Properly designed keyed
fits make it near-impossible for an impeller to come off in the event of a malfunctioning
check valve causing reverse pump rotation.
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
Figure 1: Screw-on (dead-end threaded) impeller hub (Ref. 2)
But pumps are occasionally running in the wrong direction and if they do, the reasons
are quite easy to find. Pump and steam turbine discharge check valves have been known
to leak. Metal distortion, seat erosion and hinge friction issues can occur over time.
Auto-start or standby equipment is more vulnerable because block valves are
deliberately left open. In another scenario an uninformed pump installer may decide to
test for motor direction of rotation after the driver is already coupled up to the pump
shaft. That’s a risky way to verify direction of rotation which, of course, should have
been checked before coupling the driving to the driven shaft. Yet, someone may have
decided “to save time” by installing the motor and pump n the as-shipped (generally
mounted on a base plate and coupled to an electric motor) condition. In that case the
chances of a non-keyed impeller coming off are, of course, 50%. The damage potential is
then several orders of magnitude greater than the hoped-for savings in time..
Know the Impeller Fastening Method
No pump manufacturer has a universal impeller securing method suitable for all pump
sizes and service environments. In fact, the fastening method is not usually shown on
the manufacturer’s standard drawings. Also, relatively few user-purchasers include
process pumps in thorough MQA, which stands for up-front machinery quality
assessment (Ref. 3). Some impellers are fastened to shafts by standard acorn nuts or
similar components which the pump manufacturer buys in bulk quantities from a costcompetitive supplier. Wanting to save money up front, some users join pump
manufacturers by purchasing parts from the lowest-cost sub-vendor or third-party
supplier. That’s why a good pump specification usually contains a clause requiring pump
cross-section views and parts lists which the purchaser reviews during the bid
evaluation
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
Figure 2: Different impellers and bores give clues regarding their attachment
methods. The bore on the left impeller has a keyway.
process. The commercial parts a pump manufacturer obtains from third parties must be
identified in exact detail. The purpose is not a secret: Years later, an equipment owneroperator may have cause to buy replacement parts (“buy-out parts”) directly from their
respective manufacturers or from entities that produce superior, upgraded,
components.
Fasteners can be another source of problems. Hardness and metallurgy must be observed,
which brings us back to MQA. Usually, an impeller spins off only if it is not properly secured.
But even a keyed impeller fit jeopardizes reliability if the key is loosely fitted. And, whatever
their size and speed, pump impellers secured by castellated nuts or tab washers must retain
impellers in a manner that does not allow them to come off while operating in any
direction. That's why we should examine drawings before we purchase; we should also
know how parts or machines work before we buy parts. While this does not mean that one
needs 50 years of experience to buy a $10,000 pumps, it does mean that, in the interest of
reliability and safety, one must ask lots of relevant questions before buying plant assets.
The majority of superior pump designs use keys to secure impellers. A good key fit is a
“snug fit,” which means that hand-fitting is generally advantageous (Ref. 2). The more
vulnerable sharp-cornered keyways should be avoided. Half-keys are often superior to
full keys. Keyways with generous bottom fillet radii and so-called “sled runner
geometry” have low stress concentration and a more desirable (higher) shaft factor of
safety. Well-designed shaft ends also have a generous fillet radius at the shaft shoulder
(see Figure 4, later). As strength considerations prompt us to maximize the various radii,
their contours must not interfere with the mating radius at the bearing inner ring.
Verification takes time and is time well spent.
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
Shaft Deflection Induces Fatigue Failures
Hydraulic forces act on pump shafts; the magnitude of these forces determines shaft
deflection. Shaft diameter, hardness, metallurgy, fillet radius at shaft shoulder and
distance to nearest bearing also influence how much the shaft will deflect. Shaft
Figure 3: Force vectors (arrows) indicate how a bending load acts on the pump
shaft and how the magnitude and direction of this force changes at low flow (left
image) versus flow at design throughput conditions (right image).
deflection is greater when centrifugal pumps operate at throughputs below design point
or in excess of design point. Because the hydraulic force action (Figure 3) is of unequal
magnitude and the shaft rotates, reverse bending will take place and fatigue failures are
possible if the design is marginal. Examining the fracture surface and performing a
simple stress calculation will give focus to weaknesses and available remedies. Possible
risk reduction steps will present themselves, and future failures will be less likely when
we thoughtfully select one or more experience-based upgrade option.
Motor Torque and Shaft Stresses
For the record: The starting torque of many motors is as high as 7-times the full
“normal” running torque. Agreed, discharge check valves rarely leak to the point of
allowing substantial reverse flow. But “lean and mean” plants don't always install these
check valves--and if they do, they sometimes forget to include these valves in their
preventive maintenance scheduling. In any event, a pump reliability and/or failure
analysis review should include the piping and all systems in the pumping loop. Pumps,
controls, valves piping and other elements mutually interact and all must be considered.
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
All mechanical parts failures attributable to “FRETT”
Through diligent training and reading we learn about “FRETT.” We come to accept that
any and all mechanical parts can fail only due to one or more of the four cause
categories Force, Reactive Environment, Time and Temperature, “FRETT.” So, because a
pump shaft is a mechanical part it can only fail due to “FRETT.” An excessive reverse
bending force will exist if the shaft is too slender, if its allowable bending moment or
twist-inducing torque input is exceeded, if its metallurgy is not suitable for the fluid
environment, if it has been in service for an abnormally long time, or if it was operated
at an excessively high temperature. The reader who expressed his opinion about shaft
strength and motor torque would have to examine shaft shoulder radii (fillet radius
dimension) and the resulting stress intensification factors, Figure 4. He would eliminate
Figure 4: Stresses are intensified at higher D/d and lower r/d ratios (Source:
Peterson, “Formulas for Stress and Strain”)
some of the FRETT causes, perhaps T for Time and T for Temperature. It might be
evident to the reader that no Reactive Environment (”RE”) existed in the failed shaft
system and his investigative efforts for finding the root causes of the shaft failure might
come back to “F” = Force. Examination of motor starting torques may indeed show that
starting torques can reach seven times normal running torque. Applying an inrush
current while a shaft system spins in reverse can cause shaft breakage.
In the reader’s repeat failure example, it is also possible that several seemingly small
deviations had combined. One can easily get away with one or even two deviations, but
one rarely gets away with four or five. And what does this tell us as we ponder that,
next to an electric motor, a pump is the simplest machine used by modern industry? Say
a pump typically has 40 parts and yet fails relatively often. An aircraft jet engine has
more than 7,000 parts and fails rarely. Why? We believe the jet engine and aircraft
manufacturers strive for perfection; they disallow every known deviation. In contrast,
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
pump engineers strive for low cost because that’s what the pump purchaser often
seems to prefer over pump reliability.
But top manufacturers and good engineers know that normalization of deviance can
cause disasters. Their quest to find root causes of failure and not tolerating known
deviances requires training and discipline, strict adherence to checklists and procedures,
and allocating the time needed to do things right.
In this instance we were not given enough information to accurately determine (from
behind our desk) why the reader’s pump shafts failed. We can only vouch for the greatly
increased probability that a few seemingly minor deviations from best practice combine.
Together, these deviations cause trouble. Their collective safety factors will vanish,
impellers come off, and shafts break. There will never be a good substitute for following
Figure 5: Sharp corners and edges negatively affect a machine’s factor of safety
procedures and for uncovering what happened in this instance. Replacing parts and restarting the rebuilt machine without addressing and eliminating the true root cause will
set owners up for repeat failures. Finding root causes of failure and implementing sound
remedial steps is the common sense course of action. That’s exactly what aircraft jet
engine manufacturers do, and with great success.
From our response, the reader will have inferred that any and all verbal hints at what
may have caused impellers to spin off or shafts to fail must be supported by factual
observations and evidence. There will always be a cause-and-effect relationship and it
should be our task to uncover this relationship. For the benefit of the reader asking the
original question, numerous texts show motor torque curves and explain why starting
torques and inrush currents can differ greatly from normal operating torques and
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057
currents. Check valve action and condition may indeed be relevant. Impeller attachment
is always important. Finally, pump shafts have safety factors which range from none to
considerable. Safety factors are affected by shaft stresses and high stresses can exist in
shafts with discontinuities such as steps, keyways, etc. Radiused corners (Figure 5) will
reduce stress intensification in keyways; these and other upgrades can be found in
sturdy machinery and are readily duplicated by diligent reviewers.
We hoped that our response prompted the original reader to expand his horizons. He,
and others, should start by considering better specifications. They should become
familiar with keys or half-keys, keyways and how these blend back into the shaft with a
“sled runner contour.” Failure risk can be affected by fillet radii, stress intensification
factors and the many other well documented contributors to safe pump design and
pump failure avoidance.
References
1.
2.
3.
4.
Dufour, J. W. and Nelson, W.E., “Centrifugal Pump Sourcebook,” (1992)
McGraw-Hill, New York, NY
Bloch, H. P., “Pump Wisdom: Problem Solving for Operators and Specialists”,
(2011) John Wiley & Sons, Hoboken, NJ
Bloch, H. P. and Budris, A. R., “Pump User’s Handbook: Life Extension”, 4th
Edition (2014), The Fairmont Press, Lilburn, GA
N L Pedersen - Stress concentrations in keyways and optimization of keyway
design – Dept. of Mech. Engineering, Solid Mechanics, Univ. of Denmark,
(2010)
Heinz P. Bloch resides in Westminster, Colorado. His professional career began in 1962
and included long-term assignments as Exxon Chemical’s regional machinery specialist
for the US. He has authored over 600 publications, among them 19 comprehensive
books on practical machinery management, failure analysis, failure avoidance,
compressors, steam turbines, pumps, oil-mist lubrication and practical lubrication for
industry. He holds BS and MS degrees in mechanical engineering, is an ASME Life Fellow
and maintains registration as a Professional Engineer in New Jersey and Texas.
Sent by Heinz P. Bloch, at January 07, 2016, for publication at www.tecem.com.br
TI057