Question:

Does the property quality in "Flow and Geometry variables" of the results tab not indicate "dryness level" rather than humidity? The value is often minus or more than 1. If the property is quality, should it not be between 0 and 1?

Answer:

According to the traditional thermodynamic definition, it is correct to assume that Quality should be between 0 and 1, because it is an indication of the quality in the two-phase region. What Flownex does in the case when a fluid is super-heated (e.g. all the water turned into steam), then the Quality is indicated as a value above 1. When the fluid is fully saturated (e.g. only liquid water is present), then Flownex indicates a negative Quality value. Therefore, as implemented in Flownex, the thermodynamic definition of quality has been extended to indicate when the fluid is not in the two-phase region.

This case study demonstrates the modeling of a Combined Cycle (CC) power plant of a High Temperature Gas-Cooled Reactor (HTGR). The nuclear reactor is not modeled in detail as the focus of this case study is not on the reactor but on the CC Power Conversion Unit (PCU).

This Case Study describes how Steinmüller was able to do what-if studies and to make decisions such as whether to remove the evaporative platens to optimize the plant in terms of reliability, plant availability and overall plant efficiency. Flownex proved valuable in the investigation by providing the new flow distribution in the water walls and super heaters based on design changes and also the steam condition that is provided to the turbines.

Building Blocks of Heat Recovery Steam Generator Systems (HRSG)

Each heat recovery steam generators (HRSGs) has three pressure natural circulation units that use the gas turbine exhaust to produce superheated steam for the steam turbines.

Included in the HRSG exhaust gas path are modules consisting of feedwater preheater, economiser, evaporator, superheater and reheater heat transfer elements.  These elements have been arranged and sized for effective absorption of the residual heat from the exhaust gas.  The exhaust gas passes horizontally through the top supported, vertically hung heat exchanger elements.

Each HRSGs has three pressure, natural circulation units that use the gas turbine exhaust to produce superheated steam for the steam turbine.

The high pressure (HP) section contains the economiser, evaporator, and superheater heat transfer elements.

The economiser is designed to heat the inlet feedwater to approach a temperature below the saturation temperature in the steam drum while minimizing the potential for phase change in the economiser tubes during any HRSG operating mode.  The feedwater pumps deliver feedwater to the HP economisers.

The evaporator section includes a steam drum.  The steam drums are designed to accommodate surges during unit starts and operating transients and to ensure proper steam purity by efficient removal of moisture and dissolved impurities.  Evaporator downcomers are configured for natural circulation within the evaporator.  The evaporator sections, including the associated steam drum, are designed to generate the specified steam capacity of dry, saturated steam at full load.  Each drum is protected from over pressurisation by relief valves.

Saturated steam generated in the steam drum enters the superheater section to increase the energy in the steam prior to final delivery to the steam turbine.  Relief valves protect the superheater sections and piping from over pressurisation.  The HP superheater is provided with an attemperator for steam temperature control.  HP feedwater, from the feedwater system, is supplied for spraywater.

Library Components Used:

• Control  Valve (HP feedwater control valve, HP feedwater bypass control valve)
• Non-return Valve (Check valve)
• Control Valve (HP feedwater isolation valve, HP intermittent blowdown valve, Auxiliary steam to HP isolation valve, Auxiliary steam to HP control valve, HP continuous blowdown valve, Drain valves)
• Two Phase Tank (HP steam drum)
• Pressure relief Valve (HP drum pressure relief valve, HP drum pressure relief valve)
• Heat Transfer (HP steam Desuperheater)
• Heat Exchangers (HP superheater, HP economisers, HP evaporators)
•  Pipes

Each HRSGs has three pressure, natural circulation units that use the gas turbine exhaust to produce superheated steam for the steam turbine

The intermediate pressure (IP) section contains the economiser, evaporator, and superheater heat transfer elements.

The economiser is designed to heat the inlet feedwater to approach a temperature below the saturation temperature in the respective steam drum while minimizing the potential for phase change in the economiser tubes during any HRSG operating mode.  The feedwater pumps deliver feedwater to the IP economisers.  A side stream of heated IP feedwater discharging from the IP economiser is sent to the fuel gas heat exchanger to heat the fuel gas; this improves the gas turbine performance.  The remaining IP feedwater enters the IP drum.

The evaporator section includes a steam drum.  The steam drums are designed to accommodate surges during unit starts and operating transients and to ensure proper steam purity by efficient removal of moisture and dissolved impurities.  Evaporator downcomers are configured for natural circulation within the evaporator.  The evaporator sections, including the associated steam drum, are designed to generate the specified steam capacity of dry, saturated steam at full load.  Each drum is protected from over pressurisation by relief valves.

Saturated steam generated in the steam drum enters the superheater section to increase the energy in the steam prior to final delivery to the steam turbine.  Relief valves protect the superheater sections and piping from over pressurisation.

Library Components Used:

• Control  Valve (IP feedwater control valve, IP feedwater bypass control valve)
• Non-return Valve (Check valve)
• Control Valve (IP feedwater isolation valve, IP intermittent blowdown valve, Auxiliary steam to IP isolation valve, Auxiliary steam to IP control valve, IP continuous blowdown valve, Drain valves)
• Two Phase Tank (IP steam drum)
• Pressure relief Valve (IP drum pressure relief valve, IP drum pressure relief valve)
• Heat Transfer (IP steam Desuperheater)
• Heat Exchangers (IP superheater, IP economisers, IP evaporators)
• Pipes

Each HRSGs has three pressure, natural circulation units that use the gas turbine exhaust to produce superheated steam for the steam turbine.

The low pressure (LP) section contains the feedwater preheater, evaporator, and superheater heat transfer elements.

The feedwater preheater is designed to heat the inlet feedwater to approach a temperature below the saturation temperature in the steam drum while minimizing the potential for phase change in the economiser tubes during any HRSG operating mode.  Condensate is supplied to the feedwater preheater by the condensate system.  To prevent cold end corrosion, the feedwater preheater recirculation pump and control valve operate to maintain feedwater temperature above the exhaust gas dew point temperature.

The evaporator section includes a steam drum.  The steam drum is designed to accommodate surges during unit starts and operating transients and to ensure proper steam purity by efficient removal of moisture and dissolved impurities.  Evaporator downcomers is configured for natural circulation within the evaporator.  The evaporator sections, including the associated steam drum, are designed to generate the specified steam capacity of dry, saturated steam at full load.  The steam drum is protected from over pressurisation by relief valves.

Saturated steam generated in the steam drum enters the superheater section to increase the energy in the steam prior to final delivery to the steam turbine.  Relief valves protect the superheater sections and piping from over pressurisation.

Library Components Used:

• Control  Valve (LP feedwater control valve, LP feedwater bypass control valve)
• Non-return Valve (Check valve)
• Control Valve (LP feedwater isolation valve, LP intermittent blowdown valve, Auxiliary steam to LP isolation valve, Auxiliary steam to LP control valve, LP continuous blowdown valve, Drain valves)
• Two Phase Tank (LP steam drum)
• Pressure relief Valve (LP drum pressure relief valve, IP drum pressure relief valve)
• Heat Transfer (LP steam Desuperheater)
• Heat Exchangers (LP superheater, IP pre heater, LP evaporators)
• Pipes

The reheater sub-system accepts cold reheat steam from the HP steam turbine exhaust combined with steam flow from the IP superheater steam flow.  This aggregate steam flow then passes through the reheater sections, taking the steam further into the superheated domain.  The reheater is provided with an attemperator for steam temperature control.  IP feedwater from the feedwater system is supplied for spraywater.

Library Components Used:

• Control  Valve s
• Non-return Valves
• Control Valve s
• Pressure relief Valves
• Heat Exchanger (Reheater)
• Pipes

This sub-system provides the sink for drain and vent flows. 

Water chemistry is also maintained by continuous and intermittent blow down of the steam drums to the boiler blow down tank.

Library Components Used:

• Two Phase Tank (Blow down tank)
• Variable Speed Pump (Blow down water sumps)
• Pipes

Building Blocks of Steam Turbine Generator Systems

The sub-system safely admits and removes hydrogen in conjunction with carbon dioxide from and to the generator.  Hydrogen is supplied and circulated within the generator casing to cool the stator and rotor of the generator.  Carbon dioxide is supplied to purge the generator.

A hydrogen storage truck trailer bulk supply system provides hydrogen gas for the gas turbine generators and steam turbine generator.  Hydrogen bottles with a manifold are also provided to assure a continuous supply of hydrogen during truck trailer change-out. 

The sub-system maintains the gas pressure in the steam turbine generator at a desired value and indicates to the operator at all times the condition of the machine with regard to gas pressure, temperature, and purity.

Library Components Used:

• Valves (Manual Open/Close)
• Heat Exchangers (Steam turbine generator hydrogen cooler)
• Fan or Pump (Gas dryer blower)
• Pipes

The generator seal oil sub-system is designed to provide turbine lube oil from the bearing oil header to the generator shaft sealing rings at the collector end and turbine end of the generator.  The oil film in the constricted area between the sealing rings and the generator shaft forms a seal that prevents the hydrogen from leaking past the end seals along the shaft.  Higher oil pressure than hydrogen pressure at the seals ensures efficient sealing of the generator.

Library Components Used:

• Control Valves
• Pipes

The generator converts mechanical energy from the turbine into electrical energy for distribution within and outside of the unit.  The generator consists of two circuits, an electric circuit and a magnetic circuit.  The generator rotor is coupled to and rotated by the turbine.  The rotation of the rotor produces a rotating magnetic field inside the armature.  The magnetic field of the rotor is due to a DC current supplied to the rotor windings.  This DC current is supplied by an excitation system.

Library Components Used:

• Steam turbine electrical generator
• Exciter

Building Blocks of Steam Turbine Systems

The HP turbine sub-system can be modelled with high-fidelity. Using standard library components. It is modelled as a steam and water flow network, taking into account:

• The presence and flow of steam.
• The presence and flow of water.
• Phase changes between water and steam.
• The presence and flow of air, for example during shutdown conditions when air ingress into the turbine can occur via the condenser and the glands.  This will ensure that the pressure inside the network remains atmospheric.

Library Components Used:

• Pipes
• Restrictors with discharge coefficient
• Pressure relief valve
• Control valve
• Turbine (HP Chart Assigned)

The LP turbine sub-system can be modelled with high-fidelity. Using standard library components it is modelled as a steam and water flow network, taking into account:

• The presence and flow of steam.
• The presence and flow of water.
• Phase changes between water and steam.
• The presence and flow of air, for example during shutdown conditions when air ingress into the turbine can occur via the condenser and the glands.  This will ensure that the pressure inside the network remains atmospheric.

 Library Components Used:

• Pipes
• Restrictors with discharge coefficient
• Pressure relief valve
• Control valve
• Turbine (IP Chart Assigned)

The LP turbine sub-system can be modelled with high-fidelity. Using standard library components it is modelled as a steam and water flow network, taking into account:

• The presence and flow of steam.
• The presence and flow of water.
• Phase changes between water and steam.
• The presence and flow of air, for example during shutdown conditions when air ingress into the turbine can occur via the condenser and the glands.  This will ensure that the pressure inside the network remains atmospheric.

 Library Components Used:

• Pipes
• Restrictors with discharge coefficient
• Pressure relief valve
• Control valve
• Turbine (LP Chart Assigned)

HP Turbine Bypass Sub-System

Library Components Used:

• Control valves
• Pipes

Library Components Used:

• Control valves
• Pipes

Library Components Used:

• Control valves
• Pipes

The auxiliary steam sub-system supplies motive steam to the air ejectors (hogging jet and steam jet air ejector) and sealing steam for the gland steam seals.  Auxiliary steam is supplied by three sources: the auxiliary boiler, the cold reheat steam line, and the high pressure steam line.

Library Components Used:

• Control valves
• Ppes
• Pipe with Check Valves
• Pressure Relief Valves

The gland steam seals sub-system provides steam to the turbine shaft packings to prevent the leakage of air into, or steam out of, the turbine casing along the rotor shaft.  The system also removes gland steam and air from the turbine glands and sends it to the gland steam condenser.  Air is exhausted from the gland steam condenser to atmosphere and the steam is condensed and drained to the condenser hot well.  The gland steam condenser maintains a partial vacuum in the glands to prevent steam leakage.

Library Components Used:

• Pressure relief valves
• Control Valves (Dump Valve, Feed Valve, Condensate Isolating Valve)
• Pipes
• Heat Exchanger (Gland steam emergency Desuperheater)

The atmospheric drain tank collect HP steam drains, cold and hot reheat steam drains, LP steam drains, and HP stop valves before seat drains.  Drains are routed to the atmospheric drain tank.

Library Components Used:

• Open container
• Pipes
• Valves

The shaft model includes the steam turbine shaft.  Turbine metal temperatures are also included in this sub-system.

Library Components Used:

• Shaft
• Gearbox
• Bearings (Thrust bearing ,front journal bearing, LP Bearings, Generator turbine end bearing, Generator exciter end bearings)

The function of the turbine lubrication oil sub-system is to receive, store, condition and supply clean lubricating oil to the steam turbine generator bearings, turning gear and generator seals.

Library Components Used:

• Reservoir (Steam turbine lube oil tank)
• Fan or Pump
• Pipe
• Pipe with Check Valve
• Control Valve

The Hydraulic oil sub-system includes all electro-hydraulically controlled (EHC) oil pumping, conditioning and transmission equipment necessary to hydraulically operate the steam turbine main steam valves and all the trip devices and valves in the trip-and-over-speed protection systems.  This equipment, along with the necessary control, monitoring, and protective devices, is skid mounted as a single assembly called the hydraulic power unit (HPU).  The high-pressure hydraulic oil system utilises a fire-resistant fluid as the working medium.

Library Components Used:

• Open Container
• Positive Displacement Pump (EHC oil pump)
• Script (EHC oil filter)
• Fan or Pump
• Pipe
• Pipe with Check Valve
• Control Valve
• Pressure Relief Valve
• Heat Exchanger (EHC oil cooler)

In Flownex the two-phase flow modelling functionality can typically be used to model:

Key issues:

Methodology:

Solution: