Hydrocarbon Processing - November 2021 - 45

Maintenance
and Reliability
S. ZARDYNEZHAD, Turbotech Consulting Corp.,
Calgary, Alberta, Canada
Calculate the MTBR of pumps in an oil refinery
Mean time between repairs (MTBR)
is an index for judging the performance
of a specific piece of equipment. In other
words, it is a measure of reliability-how
long an asset typically works until it fails.
It helps reliability engineers and managers
make data-driven decisions on maintenance
scheduling, safety, budgeting,
inventory management and equipment
design without relying on subjective
observations. MTBR is best used for discerning
whether an operating scenario is
improving or worsening.
This article addresses how to calculate
the MTBR of pumps in an oil refinery
based on the author's experience.
Definition of pump. The first step in
calculating MTBR is to define what is included
(and not included) in the pump
and its envelope. Pumps are equipment
used to increase the pressure of a liquid
from a suction to discharge pressure and
include all necessary support systems,
such as lubrication systems and seal support
systems (FIG. 1).
In calculating MTBR, several points
must be considered in defining the pump
boundary:
* Any equipment and/or instrument
in the vicinity of the pump that
is not included in the pump base
plate. The pump foundation, base
plate, skid, support pedestals and
anchor bolts should be included in
the pump boundary.
* Shaft coupling that is not mounted
on the pump shaft, such as the
coupling between the driver and
the gearbox when the pump has
an external gearbox. Coupling
mounted directly on the pump
shaft, including the flex/rubber
elements, half couplings, spacer,
keys, guards, etc., should be
included in the pump boundary.
* Piping flange gaskets and all piping
components after the pump flanges,
unions or valve installed on the
pump casing must be considered in
the pump boundary. Pump casing,
including all flanges cast with or
welded to the pump casing, is also
included in the pump boundary.
Small-bore piping connected
to the pump casing up to and
including the first flange, union or
valve (including the valve) must be
considered in the pump boundary.
This includes drain, vent, cooling,
external flushing and warmup lines.
Mechanical seal barrier/buffer fluid
lines between the reservoir and seal
gland are also included.
* Steam jackets/passages that are not
cast into the pump casing must be
considered in the pump boundary,
along with steam supply, quench
and tracing systems connected to
the pump but beyond the first block
valve, flange or connection from the
pump casing. Heat tracing placed
on the outside of the pump casing
and body must also be considered.
* The gasket between the main
suction/discharge flange and
piping system outside the pump
baseplate must be included.
* Separate gearboxes, clutches
and drivers with their own tag
numbers must be included. Integral
gearboxes built and supplied by
the pump vendor with the pump
must be considered in the pump
boundary. This includes all internal
components such as gears, bearings,
seals, oil pumps, etc. Accessories
mounted on the outside of the
casing, such as filters, shall be
considered in the pump boundary.
* Separate drivers such as motors,
turbines, turboexpanders and
internal combustion engines with
their own tag numbers are not
considered in the pump boundary.
* Lubrication system components
inside the pump, such as bearing
housings, oil rings, oil flingers,
FIG. 1. Pump boundary (dotted lines) for calculating MTBR.
Hydrocarbon Processing | NOVEMBER 2021 45
Hydrocarbon Processing - November 2021

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