TJB Genset : Underloading and its Solutions
Abstract—This document discusses in detail the problem of
underloading faced during the commissioning of the new diesel
generator at Taru Jabba facility, explores why underloading is
a problem in diesel generators and the various solutions that
were explored to resolve this issue.
I.
UNDERLOADING AND WHY IS THIS A
PROBLEM
Operating a diesel generator below 30 % of its rated output
is known as “underloading”. The consequence of
underloading is wet stacking or exhaust manifold slobber.
Underloading prevents the engine from reaching its optimum
temperature which prevents incomplete combustion of fuel.
The unburnt fuel combines with the soot to create a black
slobber. This slobber can leak from the exhaust of a generator
when it runs in a low-load scenario for a prolonged period.
Long periods of light loading result in carbon deposits
inside cylinders and injectors. These conditions result in
power losses, reduced efficiency and increased wear and tear
of generator components. Hence, the diesel generator at TJB
has an in-built mechanism which trips the generator when the
load falls below 30% of the rated output.
A correctly sized diesel generator will have a rated output
that will closely match the expected load requirement of the
facility. The correct size of the diesel generator is determined
from a load analysis of the site. This pre-requisite, however,
had been missed out at Taru Jabba terminal.
The probable solutions that we looked at to resolve this
problem at the terminal have been described in detail in the
following paragraphs.
yearly maintenance cost at least and a reduction in the useful
life of the generator.
3) Overhauling of the older generator: This included an
overhaul and repair of the older generator present at Taru
Jabba terminal. This solution w`ould have helped us to extend
its use during power outages at the terminal. The absence of
the tripping mechanism in the older generator would have
meant that underloading would no longer be an issue.
However, the repairs would have cost us 4 million PKR plus
the additional cost of a rented genset for the overhauling
duration alongside idling of the newly procured equipment.
4) Utilising a load bank: A load bank is a device that
simulates eletric load to test and verify the performance of
generators. Its key components include resistors, cooling
system and a control panel. A resistive load bank would
provide an artificial load in incremental steps of usually 5
kW/10 kW. This would ensure that the generator operates in
its ideal performance range and the threat of wet-stacking is
eliminated.
The cost of a 100 kW load bank is approximately PKR 2.5
million which will help us to operationalize the generator
safely. However, the only drawback is the additional
preventive maintenance cost that will be incurred for the
upkeep of the load bank.
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Probable Solutions
1) Using the electric fire pump: The two electric fire
pumps available at Taru Jabba are rated at 35 and 75 kW. This
workaround involved using the recirculation line provided
within the fire network system at the facility. The
recirculation valve was to be opened and the main fire water
header was to be isolated. Switiching on the electric fire
pump would have supplemented the load on the genrator and
the water would be rerouted into the firewater tank. This
solution would have allowed us to utilise our existing electric
fire pumps. On the contrary however, the terminal fire
fighting’s ability would have been compromised for a certain
period of time where the recirculation line would have been
used.
2) Bypassing the tripping mechanism: This solution
involved bypassing the in-built tripping mechanism provided
by the OEM. This workaround allowed us to operate the
diesel generator below 30% of its rated output. The
drawbacks, however, included invalidation of the
manufacturer’s warranty, an immediate cost of 1.7 million
PKR in lieu of procuring spare injectors, quadrupling of the
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II.
CONCLUSION
Keeping in mind the technical safety (HSSE) and
economical (cost) concerns, the most effective solution to
eliminate this problem of underloading is the utilization of a
load bank.
However, it should be noted that for any future project
which entail installation of a diesel generator at any facility
across the Wafi terminal network, it is necessary that a load
analysis study be conducted to determine the appropriate
size of the generator to be installed.
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