Operational Performance of HRSG's
Heat recovery steam generators [HRSG] are a component of combined heat and power
[CHP] systems whereby the "waste" heat from a gas turbine - generator [GTG] set
is used in a heat recovery boiler to generate high pressure steam for either heat or power
generation from a steam turbine - generator set. HRSG's are especially favoured in
heavy oil production facilities to generate sub-saturated steam for the
cylic heating of in-situ oil formations. Oil recovery is directly proporational to the steam
sent to field.
The true capacity limit of this equipment is determined from consideration of a number of
limits in the materials of construction for the system train from turbine outlet through the
HRSG and to the discharge of the flue gas stack.
A little explored limit is the heat transfer duty of the HRSG design. This can be as much as
15% of nameplate duty. Existing and potential owners can better ensure they are operating to
the true physical limits of the equipment by reviewing the OEM's design. A 3rd party consultant
specializing in detailed knowledge of heat transfer engineering will be able to assist owners
in reviewing their equipment.
Thermal Capacity of HRSG
The steam generation capacity of an HRSG is governed by convective and radiant
heat transfer between gas turbine outlet. To reduce seasonal variations, duct burners often
augment the energy available in the gas turbine inlet gas stream.
Many of the "tubes" in an HRSG use extended surfaces. Stated steam generation capacity
may be impaired by failure to recognize actual capacity of the extended surface or loss of
normal capacity by failure of the extended surfaces ("burned fins"). In either case, steam
production is reduced for lengthy times, due to turnaround schedules and
expensive repairs are occassioned.
Oxidation and creep limits of metal components also need to be considered.
Evaluation of HRSG Steaming Capacity
A detailed review of pressure components and heat transfer mechanisms
can result in identification of additional capacity potential in existing equipment
or planned equipment of as much as 15% of calculated nameplate capacity.
When completed earlier in the design process, simple material changes may contribute to
greater reliability with reduced investment.
John Aumuller, P. Eng., Ph. D.