HRSG is a device used in power plant to recover waste heat from exhaust gas of a Gas Turbine (GT), Gas Engine Generator (GEG), or other combustion system. The HRSG will convert the heat energy from exhaust gases and uses it to produce the steam, which can further be used to drive a Steam Turbine (ST) to generate additional electricity.
Main Components
A HRSG is a complex piece of equipment designed to recover waste heat from the combustion system. It typically consists of Evaporator, Superheater, Economizer, Steam Drum, Feed Water System, Auxiliary Burner, Stack, Exhaust flue gaseous, and Control System. The purpose of each component is explained below.
Evaporator: The evaporator is a component which is used to generate steam. It contains a series of heat exchange surfaces, usually in form of tubes.
Superheater: The superheater is responsible for further increasing the temperature of the steam produced by the evaporator to a superheated level which is suitable for driving a steam turbine.
Economizer: Economizer is positioned at the inlet of the HRSG to preheat the feedwater before it enters the evaporators. Thus, this unit helps to improve the overall efficiency of the HRSG.
Steam Drum: The steam drum is a vessel where the generated steam is collected before it is distributed to the downstream process. It also serves as a separator to remove any remaining water droplets from the steam.
Auxiliary Burner: In some HRSGs are part of a combined cycle power plant, the burners are installed to provide additional heat input when required such as during maintenance of gas turbine.
Hazards of HRSG
Overpressure: At the downstream of the HRSG, there is the Main Steam Stop Valve or MSSV which its play role as to control the rate of steam flow into the steam header. Hence, in case of the mis-operate failure of opening the MSSV during normal operation, the HRSG can experience with excessive pressure.
Overheating and damage: Without a proper water supply to the HRSG or low level in the steam drum, HRSG may experience of the deterioration of materials such as tubes and other components within the HRSG. This will lead to cracking and leakage.
Liquid carryover: In the opposite scenario, if lack of level control and high liquid inside the steam drum, foaming can be generated, and liquid can carryover the downstream equipment. This will lead to the water hammer in piping system and damages the steam turbine.
Fire and explosion: Unburnt exhaust gas can accumulate inside the exhaust stack if insufficient of performing the purging activity. It may lead to the internal fire and explosion if ignited.
Corrosion and Fouling: Also, unburnt exhaust gas can contain corrosive elements and particular matters. If these gases accumulation or contact with the low surface temperature part, they can contribute to the formation of corrosion and fouling.
Corrosion and Fouling: In the waterside, if lack of control the water control qualities such as fails to open blowdown system. It will lead to the accumulation of dissolved solids, sediments, scale formation and other impurities in the boiler water. Potentially causing of promote the corrosion.
Emission Compliance Issues: Unburnt exhaust gases may contain pollutants such as carbon monoxide, which can contribute to air quality concerns and environmental compliance issues.
Design Code
ASME Boiler and Pressure Vessel Code (BPVC): ASME BPVC includes not only design code but also safety standards for the operation of boilers, pressure vessels, including HRSG.
- PG-70: Capacity of Pressure Relief Valve;
- PG-72: Operation of Pressure Relief Valve
- PG-73: Minimum Requirement for Pressure Relief Valves
Specific section of the ASME BPVC, such as Section I (Rules for Construction of Power Boilers), and Section VIII (Rules for Construction of Pressure Vessel)
- UG-126: Pressure Relief Valve
- UG-128: Liquid Pressure Relief Valve
NFPA 85 – Boiler and Combustion System Hazards Code
- This code may be applicable to HRSG safety, particularly regarding combustion safety and fuel system.
PROcess Safety TASK 