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The use of Auto or Manual Load Banks for Prevention of
Light load running and Wet-Stacking in Diesel Generator Sets
Wet-Stacking is a common problem with diesel engines which are operated for extended periods under
light or no-load conditions. When diesel engines operate without sufficient load they normally do not run
at the manufacturers designed optimum temperature. Diesel engines use a compression ignition system
and in order to operate at peak efficiency they must be able to provide fuel and air in the proper ratio and
at a high enough engine temperature to completely burn all of the fuel. Failing to operate at this
temperature will allow unburned fuel to accumulate in the engine, which can foul the fuel injectors,
engine valves and exhaust system, including turbochargers, and reduce the operating performance.
Wet-stacking can reduce the system performance and increase maintenance if not addressed at an early
stage.
The proper load can be achieved by adding additional building load or by providing supplemental load
through the use of a resistive load bank. Providing additional load from the building load may not be
practical particularly in situations where the generating set is supplying critical communication or life
support loads. Any interruption of power to these loads may cause a serious commercial loss or in
extreme situations even jeopardize life or personal safety.
The effects of wet-stacking can be alleviated if additional load is applied to allow the engine to achieve
the correct operating temperature. By applying an increasing load over a period of time until the excess
fuel is burned off and the generating set maximum load rating is reached usually can repair the
condition. This procedure can take several hours to burn off the accumulated unburned fuel and for
permanently installed generating sets create localised air pollution that may breach local environmental
regulations
Resistive load banks, either portable or permanently installed can provide an ongoing solution for wet
stacking issues.
 Portable load banks can be used at site on a regular basis to prevent wet-stacking problems
however they do require a large supply of fresh air for cooling and must be ventilated to remove
the exhaust heat from the load bank area. These are practical for infrequent testing but do
require additional labour for transportation, set up and connection of the load bank to the
generator under test.
 A purpose designed resistive load bank can be permanently installed either indoors with
ventilation ducting or externally in a weatherproof housing, to provide a readily available solution
to the wet-stacking problem. Two type of control systems are available for these units.
o A manual operating system so that the building operators can perform regular, periodic
testing under load without disruption to the building power supply. This type normally has
a switched or PLC based control system that will allow application of load in a number of
steps until the required load is running on the generating set. This system provides a
solution to the problem but requires on-site or regularly attending maintenance staff to
carry out this testing with the associated costs and inconvenience.
o The second and by far the best solution is a totally automated PLC control that monitors
generating set load and automatically applies the required resistive load steps to maintain
a pre-set percentage of the generator set full load rating. This type of control system
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eliminates the need for maintenance staff to carry out load testing and also ensures that
the generator set is always running at optimum performance and load levels.
Before installing a permanent load bank or carrying out preventative load banking it is advisable to
consult the manufacturer of the power generation equipment to be tested for their specific requirements
and recommended loading. Also, reviewing the appropriate operator manuals is important to fully
understand the maintenance and testing requirements. In general we would suggest a minimum load
of 30 - 40 percent of the nameplate rating to prevent wet stacking.
The proper installation and use of a load bank can prevent loss of capacity and increased maintenance
caused by wet-stacking. Viking Power Systems can provide resistive load bank solutions with either
manual or auto controls.
Copyright Viking Power Systems Co., Ltd
March 2014
May be reproduced for
non-commercial purposes
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