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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.

 

fin / tube temps


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.