World Pumps
November 2013
When a hot strip mill experienced failures in its Worthington 10-stage split case diffuser style pumps, the results were costly: each rebuild was put at approximately $85,000 when failure of seals, bushings and shafts made it necessary to rebuild the pumps every 18 to 24 months. Additionally, difficulties in running the two pumps supplying hydraulic pressure to the coiler resulted in reduced production and made for a higher scrap rate.
So the mill’s engineering team looked for a new repair provider and selected Hydro Inc, the engineering pump rebuilding company from Chicago, to apply new technology to the pumps, using GRAPHALLOY®, a self-lubricating graphite/metal alloy bearing material manufactured by the Graphite Metallizing Corporation. Hydro implemented engineered upgrades including A and B gap modifications, rotor centralization and manufacture of new impellers and diff users. Following its work at the steel mill, it is estimated that the mill has saved over $250,000 in potential rebuild costs.
Prior to the steel mill selecting the Chicago based company, it was found that the seals of the Worthington WT-810 split case pumps, which are designed to produce up to 1,400 PSI fluid pressure (with two split mechanical seals were originally used in the coupling and thrust ends of the stuffing box) would wear
prematurely and would have to be replaced every six months.
The bronze bushings used on each of the ten
stages failed every 18 to 24 months, and required a full element rebuild. The steel mill
also experienced four broken shafts in one
pump over the course of two years. Hydro’s
engineers determined that this most likely
occurred because the seals ran dry under
various operating scenarios, and when this
happened, they failed quickly. As a result of
these failures, the two pumps that supplied
hydraulic pressure to the coil ran inconsistently,
resulting in lower than optimal production
and a higher scrap rate.
A new approach was required to address the
problem’s root cause, and after the mill
contacted it, Hydro performed an inspection
referred to as ‘rotor condition analysis’. This
involved a review of field operating conditions
including vibration, temperature, fl ow
and past repair history.
This was followed by a thorough dimensional
study to determine the physical condition of
the pump. This analysis provided sufficient
evidence to understand the pump’s problems and would serve as the basis for solving
its performance issues.
Engineers from the Chicago company
worked with the mill’s maintenance staff to
identify the root causes of the pump failures,
and it was found that the previous material
used for the stuffing box seals and diffuser
bushings performed well when fully lubricated
but failed quickly under run dry conditions.
Because there was no way to prevent
the pump from running dry under certain
operating scenarios, a new bearing material
was key to solving this problem.
Hydro’s engineers also noted that the pump
was being operated off its best efficiency
point, close to shutoff, causing axial shuttling
and hydraulic instability. The report additionally
noted that the previous repair shop had
provided a shaft manufactured from incorrect
material and had not used accepted manufacturing
techniques. This caused stress risers
to occur in the threaded areas leading to shaft
failure. After years of operation, there was also
deterioration in the pump’s base plate and foundation as well as leaks throughout the
piping system causing pressure drops which,
in turn, contributed to the overall instability of
the pump while in operation.
Hydro recommended upgrading the stuffi ng
box and diff user bushings to Graphalloy
because of its run dry capabilities. Graphalloy
is a graphite/metal alloy with enhanced
chemical, mechanical and tribological properties.
This self-lubricating bearing material
survives dry starts, flashing and loss of
pumpage for prolonged periods without
damage and allows immediate restarts.
The material provides a constant, low coeffi
cient of friction rather than just a surface
layer, helping to protect against catastrophic
failure. Graphalloy wear components also
improve reliability under conditions such as
low-speed operation, frequent starts and
stops, and switching from standby to continuous
running.
Hydro’s engineers recommended changing
the A gap (between the impeller vane tip and
diff user shroud) and B gap (between the
impeller shroud and impeller vane tip) to
reduce turbulent fl ow in the pump, increase
rotor stability, and reduce axial shuttling and
vibration.
The company’s team performed
rotor centralization, a process they pioneered
involving centering the rotating impellers
within the stationary diff users and volutes.
Hydro also manufactured new impellers and
diff users using upgraded materials. The impellers
were upgraded from bronze to high-grade
special stainless steel. The diff users and return
channels were upgraded from cast iron to
high-grade special stainless steel.
The company also manufactured a new shaft
using the proper grade of stainless steel. The
maintenance department at the mill worked
with its application engineering team to
develop recommendations for structural
changes to the pump’s mounting and to
relieve piping strain.
Rebuilding the pump
Hydro utilized stringent practices in
rebuilding the pump. Tighter fits and tolerances
were achieved to improve pump efficiency. Precise manufacturing and balancing
techniques were used to achieve a rotor
balance of less than 1W/N (W = component
weight and N = pump rotating speed) to
reduce vibration thereby extending the life
of the pump. The maximum shaft total indicator
reading (TIR) was held to 0.001 in which
is more stringent than the industry standard
of 0.003 in TIR.
Because Graphalloy is non-galling, pumps
can be designed with tighter clearances.
Hydro was able to reduce the clearances
between the bushings and shaft sleeves to
0.005 in or 0.0025 in per side. Pump elements
were assembled in the vertical position to
accommodate the much tighter assembly
tolerances. It took checked the balancing the
coupling with the rotor. Since even a perfectly
rebuilt rotor cannot operate as intended if
there are issues with the pump’s casing,
Hydro inspected the casing and performed
split line and line boring to restore the entire
casing fits.
The project has surpassed the expectations
of the mill’s management team, and the
Graphalloy stuffing box and diff user bushings
have lasted through two six-year pump
rebuild cycles without any failures. When the
pumps were disassembled during the scheduled
rebuilds, only minimal wear was
observed on the seals and bushings. The
bushings and seals were replaced although
they could have easily lasted many more
years.
The mill has estimated the design improvements
have saved a quarter of a million
dollars by avoided three rebuilds over six
years, and that it has achieved improved reliability
that eliminated lost production time.
The rebuilt pump is now considerably more
efficient resulting in reduced energy
consumption. The pump requires substantially
less maintenance, providing additional
cost savings.
www.hydroinc.com
www.graphalloy.com
Click here to learn more about GRAPHALLOY.
