Hydrocarbon Processing - November 2021 - 46

Maintenance and Reliability
bearing seals, etc., must be
considered. External lubrication
systems or oil mist system
components, such as filters,
pumps, coolers, lubricators, oil
bottles, etc., on a dedicated lube
system for the pump must be
considered in the pump boundary.
* Cooling water supply lines to the
pump and/or heat exchangers are
excluded from the pump boundary.
* Mechanical seals and packing are
considered in the pump boundary.
Number of installed pumps. All the
pumps that are normally present in an
oil refinery must be counted as installed
pumps. It is recommended to calculate
MTBR based on pump type, production
unit and production complexes. It may be
helpful to compare the pumps installed in
one refinery to the pumps present in other
refineries within a single organization.
Definition of repair. The tricky part
of calculating MTBR is the definition
of a failure and/or the definition of repair.
Before calculating MTBR, the definition
of the pump repair must be first
understood. A repair includes all work
performed on a pump to maintain it in
satisfactory operating condition.
Repair work on a pump starts when
the pump is isolated from the piping system,
handed to the mechanical millwright
and is no longer available for use by plant
operations. A pump repair is considered
to be completed when all isolations for
mechanical work have been removed,
all mechanical locks have been removed
and the pump is handed to operations for
commissioning. If another failure is experienced
during commissioning or restart
of the pump, the resulting repair must be
counted as a different, additional repair.
TABLE 1 includes activities that can be
considered and counted as " repair. "
A case study provides better understanding
of the previous explanation. On
December 31, 2019, a fire occurred in
the centrifugal pump of a refinery alkylation
unit due to mechanical seal leakage.
The fire reached the hot surface of the
steam pipes, causing the alkylation unit
to shut down.
The alkylation unit has 158 different
types of pumps. Each pump has one
standby pump. Eight pumps out of 158
are indefinitely shut down, preserved due
to lack of operational need. Two different
pumps were impacted by the fire, five
motors were damaged and required total
replacement, and the power cabling to 10
pumps skids was burned; however, the
pumps were not damaged.
After inspection, management decided
to do a detailed check of all spare
pumps (Tag B) in hot standby mode.
The other pumps (Tag A) would be run
during the unit restart.
The alkylation unit was out of service
for 4 weeks and was restarted on January
31, 2020. Gussets were added to the
small-bore drain and vent from all the
pumps, based on company specifications
revised during this period. Some autonomous
maintenance was planned during
the shutdown period. Oil changes of the
Tag B pumps were also carried out during
this time. Furthermore, pump vibrations
for all Tag A pumps were checked
during restart.
For the 4 weeks the alkylation plant
was out of service, the pump failure/repair
count amounted to only two in January
2020. After unit commissioning and
restart in February 2020, 16 of the main
pumps (Tag A) failed within 2 weeks after
unit restart.
The root causes of the failure were
found to be mechanical seal failure, coupling
failure, bearing failure and shaft
high vibration. Therefore, the pump
failure count jumped from only two in
December 2019 to 16 in February 2020.
A note should have been added in the
MTBR report for December 2019 that
75 spare pumps were inspected and repaired
after the fire; however, they should
not have been counted for repair.
FIG. 2. Repair work visualization.
TABLE 1. Countable and non-countable repairs
Count as repair on pump boundary
Turnaround overhauls
General overhauls
Maintenance interventions require lockout/tagout (LOTO)
Upgrading to improve reliability
Onsite repair work, such as packing replacement or packing
ring addition by millwright
If the root cause analysis indicates that the failure was
caused by operator fault (such as running the pump dry)
46 NOVEMBER 2021 | HydrocarbonProcessing.com
Mean time between repairs. MTBR is
usually applied to a group of similar equipment
such as pumps. The industry accepted
formula shown in Eq. 1 is used for
calculating the MTBR of the pump:
Do not count as repair on pump boundary
Result of external factors (e.g., natural phenomena, flood, earthquake,
tornado, sabotage/terrorism, unit or plant fire, lack of utility, etc.)
Predictive maintenance (vibration analysis) or troubleshooting not requiring
a pump shutdown
Maintenance interventions do not require LOTO
Upgrading due to environmental and/or safety requirement
Any work outside of the pump boundary
Any work on the driver, gearbox and variable frequency drive
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