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Transcript Document

THERMOFLEX

THERMOFLEX Steam Cycle Tutorial

© Thermoflow, Inc., 2011

THERMOFLEX

Features illustrated in this tutorial

 Copying & pasting groups of components to save time (Topic 1)  Using the “Rubber Components” feature (Topic 1)  Setting up feedwater heaters with or without drain coolers and desuperheating sections (Topic 1)  Defining pipe losses & sizing pipes with PEACE (Topic 2)  Changing PEACE pipe diameter in OD mode and piping cost/performance tradeoff (Topic 2)  Defining steam turbine design-point details, including the leakage circuit, the SSR (Sealing Steam Regulator), and the GSC (Gland Steam Condenser) - (Topic 3)  Using the “Fix-a-pressure” feature (Topic 3)  Throttling connections into Mixers and from Splitters (Topic 3)  Using the Thermo Boiler component in design and off-design modes (Topics 4 & 5)  Modelling steam turbine pressure controls, exhaust loss curve, and GSC at off-design (Topic 5)  Using the macro feature (Topic 6)  Manipulating feedwater heaters at off-design (Topic 7) April 20 © Copyright Thermoflow, Inc., 2011 SC-2

THERMOFLEX

Steam Cycle Tutorial - Topic 1

1. Build a basic steam cycle

2. Include piping losses 3. Introduce additional steam turbine details 4. Use more elaborate Thermo Boiler model 5. Setup & run the model at off-design 6. Create part-load performance curves 7. Bypass top heater for peak-load

April 20 © Copyright Thermoflow, Inc., 2011 SC-3

THERMOFLEX

1-1. Example to build: A basic Rankine cycle plant.

 Non-reheat, straight-condensing, single-casing turbine, 60 MW-class.

 Throttle conditions 950 psia/950 ºF (65 bar/510 ºC).

 Condenser pressure 1 psia (69 mbar).

 Five feedwater heaters: Two LP, D/A, and two HP.

April 20 © Copyright Thermoflow, Inc., 2011 SC-4

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1-2. Build a steam cycle module April 20

Steam Turbine Splitter Connector

(For a more complete model, include a Pipe)

Feedwater Heater

THERMOFLEX

Optional: Replace ST with alternate larger visages for visual effect.

1-3. Copy & paste the module 5 times April 20 Two HP Heaters Leave a gap for the boiler feed pump This heater will be replaced by a deaerator © Copyright Thermoflow, Inc., 2011 Two LP Heaters SC-6

THERMOFLEX

Package Boiler

1-4. Complete the simple steam cycle then Check Drawing

ST Visage 5 - resized numerically to 118% height, 100% width Water-cooled Condenser Makeup/ Blowdown

Hints: Boiler Feed

Pump Deaerator Feedwater Heater with pump

Condensate

Pump

1) A Makeup/Blowdown component should always be included in a closed loop to ensure mass conservation.

2) A Package Boiler is used for simplicity, but more elaborate boiler models are available.

3) The Water-cooled Condenser includes a simple, intrinsic pump, so an external pump is not installed in the CW circuit. For more elaborate models, install a separate pump and an external hydraulic resistance and/or hydrostatic head, and April 20 © Copyright Thermoflow, Inc., 2011 SC-7

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1-5. Rubber component declaration Right-click each Steam Turbine group

Turn Rubber ON

(except the first), then select from the context menu. This tells the program to ignore the pressure input for the inlet of the ST group, and to determine it based on the connected heater’s feedwater exit temperature and terminal temperature difference. The

blue highlighting

which then appears on the component indicates its ‘Rubber’ status.

Hints: 1) If you wish to dictate the extraction pressures, rather than the feedwater temperatures, don’t set the Steam Turbines to “Rubber” and instead, set the inlet pressure of each.

2) Make sure the Splitters at the turbine extractions don’t have April 20 © Copyright Thermoflow, Inc., 2011 SC-8

THERMOFLEX

1-6. Set ST inlet pressure Double-click on the first ST group to open its menu. Set its inlet pressure to 950 psia. Leave other defaults in place.

Hints: 1) By default, all steam turbine groups are set on the same shaft (Shaft 1 in this case). Thus, they all drive one large generator.

2) You need to scrutinise the stage efficiency for each ST group, and adjust it as needed. In our example, we will accept the default of 85% for the initial runs.

April 20 © Copyright Thermoflow, Inc., 2011 SC-9

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1-7. Set ST inlet temperature and flow rate ST inlet temperature and mass flow rate are dictated by the network. Double-click on the Package Boiler, accept its default delivery temperature of 950 ° F, and set its mass flow rate to 150 lb/s, which should yield, very roughly, 60 MW net (about 400 kW per lb/s for non-reheat, conventional steam cycles). Leave its other defaults in place.

April 20 © Copyright Thermoflow, Inc., 2011 SC-10

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1-8. Set condenser pressure Double-click on the condenser to open its menu. Accept its default pressure of 1 psia. Leave other defaults in place.

Hints: 1) The default CW range (Item 4) is 18 ° F (10 3) In our example, the CW source is at 59 ° ° C). This may need to be reduced, for environmental reasons, for many open-loop systems.

2) If you wish to model the CW pump in greater detail, letting it impose the flow rate at off-design for instance, you should set Item 6 to zero to eliminate the work of the condenser’s intrinsic pump, then introduce a separate pump and appropriate hydraulic resistances in the CW circuit, outside the condenser.

F (15 ° C), by default.

April 20 © Copyright Thermoflow, Inc., 2011 SC-11

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1-9. Set ST exhaust loss April 20 © Copyright Thermoflow, Inc., 2011 Double-click on the last ST group to open its menu. Ignore its inlet pressure, since it has been declared ‘Rubber’. Go to its Exhaust Loss & Miscellaneous tab and set its exhaust loss to some reasonable value. In the example, we have assumed 10 BTU/lb.

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1-10. Decide on FWH exit temperatures Hints: In a typical, well-designed steam cycle, the heaters will have roughly equal temperature rises. The condenser, at 1 psia, has a saturation temperature of roughly 100 ° F. We shall heat the feedwater to 400 ° F in five heaters, with a temperature rise in each of 60 ° F. We shall install drain coolers in each of the two HP heaters, but leave the LP heaters without drain coolers, the default.

April 20

400 340 280 220 FWH Exit Temperatures, in

F 160

THERMOFLEX

1-11. Set first & second heater exit temperatures Double-click on the first LP heater to open its menu. Set its exit temperature to 160 ° F. Leave other defaults in place.

Repeat for the second heater, setting its exit temperature to 220 ° F.

Hints: 1) By default, arbitrarily large values are set for the temperature differences of Items 4 and 5. A drain cooler approach exceeding the difference between heater condensing temperature and that of incoming feedwater implies no drain cooler. A residual superheat exceeding the actual superheat of the heating steam implies no separate desuperheating zone within the heater (which means that it cannot have a negative terminal temperature difference).

2) For the THERMOFLEX Feedwater Heater w/pump component, Item 2 defines temperature leaving the heat exchanger, before mixing with the pumped drain. After mixing, the FW will be slighlty warmer.

April 20 © Copyright Thermoflow, Inc., 2011 SC-14

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1-12. Set third heater (D/A) exit temperature Double-click on the deaerator to open its menu and set the Deaerator outlet temperature to 280 ° F. Note that the Deaerator design pressure is automatically updated to its saturation value for the input temperature.

April 20 © Copyright Thermoflow, Inc., 2011 SC-15

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1-13. Set fourth heater drain cooler & exit temperature Double-click on the fourth heater to open its menu. Set FW exit temperature, Item 2, to 340 ° F.

Install a drain cooler

by setting its approach,

Item 4

, to 12 ° F.

April 20 © Copyright Thermoflow, Inc., 2011 SC-16

THERMOFLEX

1-14. Set top heater drain cooler & desuperheating section, and set its exit temperature Double-click on the fifth (top) heater to open its menu. Set FW exit temperature, Item 2, to 400 ° F.

Install a drain cooler

by setting its approach,

Item 4

, to 12 ° F. Since we expect sufficient superheat in the extracted heating steam, we

install a desuperheating section

by setting residual superheat,

Item 5

, to 30 Item 6, left at the default 4.5 ° F.

° F. This allows a small, or even negative, TTD. Thus, we set Item 3 to zero, recognising that it may not be achievable, since it is still subject to the minimum pinch of April 20 © Copyright Thermoflow, Inc., 2011 SC-17

FILE: steamcycle.tfx

THERMOFLEX

1-15. Check inputs, compute & view overall results Hints: 1) To show the performance summary, add a Table of Variable Labels by clicking the toolbar button circled above and selecting the appropriate outputs.

2) The simple “black box” Package Boiler model has no fuel connections. We left its default LHV efficiency at 93%, commensurate with a natural-gas-fired package boiler. We also left its ratio of HHV/LHV at the default of 1.11, corresponding to CH 4 . You may wish to use a lower boiler efficiency and a lower HHV/LHV ratio if your plant will run on oil or coal.

April 20 © Copyright Thermoflow, Inc., 2011 SC-18

FILE: steamcycle.tfx

THERMOFLEX

1-16. View graphic ouputs for fifth (top) heater Click on the top heater to summon its expanded graphic output Click on profile

TQ Diagram

to summon the temperature

Desuperheating zone Drain cooling zone Condensing zone

April 20 © Copyright Thermoflow, Inc., 2011 SC-19

FILE: steamcycle.tfx

THERMOFLEX

1-17. View graphic outputs for fourth heater Click on the fourth heater to summon its expanded graphic output. It has a drain cooler but no desuperheating section April 20 © Copyright Thermoflow, Inc., 2011

Drain cooling zone Condensing zone

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FILE: steamcycle.tfx

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1-18. View graphic outputs for second heater April 20

Condensing zone

FILE: steamcycle.tfx

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1-19. View text outputs April 20 © Copyright Thermoflow, Inc., 2011 SC-22

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Steam Cycle Tutorial - Topic 2

1. Build a basic steam cycle

2. Include piping losses

3. Introduce additional steam turbine details 4. Use more elaborate Thermo Boiler model 5. Setup & run the model at off-design 6. Create part-load performance curves 7. Bypass top heater for peak-load

April 20 © Copyright Thermoflow, Inc., 2011 SC-23

2-1. Introduce pipe pressure drops

THERMOFLEX

Uncheck the drawing & insert pipes between each ST extraction port and the connected feedwater heater. Likewise, insert one between the boiler and the ST inlet. Check Drawing then edit the pipe inputs as shown below. Check Inputs & Compute.

4 4 4 6 8 10 Assumed pipe pressure drops, % Assumed enthalpy loss for all pipes=1 BTU/lb

Hints: 1) Pipes at low pressure need to be designed with higher percentage pressure drops than those at high pressure.

2) Since we have declared ST groups following each extraction as rubber, letting FW temperatures dictate extraction pressures, introducing the pipes will result in raising the turbine extraction pressures to attain the same, given FWH exit temperatures.

April 20 © Copyright Thermoflow, Inc., 2011 Double-click on each pipe and set its assumed pressure and heat losses. Remember, the input is decimal, 10% is 0.1!

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2-2. View output & note effect of pressure drops Note how output drops from 66224 to 65902 kW. The pressure drops have cost the plant 322 kW, about 1 / 2 % of its output. The throttle temperature, set at 950 ° F at the boiler, is now 946 954 ° ° F at the ST inlet due to the losses in the live steam pipe. To reinstate 950 F, then recompute.

° F at ST inlet, we return to inputs and set the Package Boiler exit temperature to April 20 © Copyright Thermoflow, Inc., 2011 SC-25

FILE: steamcycle_P.tfx

THERMOFLEX

2-3. Reinstate 950 °F at ST throttle and recompute April 20 After reinstating 950 ° F at ST inlet, by setting the Package Boiler exit temperature to 954 1 / 8 % of plant gross output.

° F, we recover 233 kW of the 322 kW lost due to piping. Thus, the effect of piping pressure losses, with constant ST throttle conditions, is only to forfeit 89 kW, i.e. about There is, however, some additional fuel input to the boiler, to raise its outlet steam temperature from 950 ° F to 954 ° F, so the plant gross efficiency gain due to reinstating throttle temperature, from 35.11% to 35.16% (0.05%) is less than its gross power output gain from 66046 to 66274 kW (0.35%).

THERMOFLEX

2-4. To size the steam piping, use PEACE pipes April 20 Uncheck the drawing and delete the THERMOFLEX pipes. Install new

PEACE pipes

in their place.

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2-5. Set the assumed TD mode pipe pressure drops

April 20

6 6 8

10 12

Double-click on the first heater’s steam pipe. Set its assumed TD mode pressure drop to

12%

. Since the program always rounds up the diameter to the next standard size, the actual pressure drop, after the pipe is sized in ED mode, will likely be less.

Likewise, set the pressure drops for the remaining pipes as shown in green Check Inputs & Compute

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2-6. From the output screen, select ED mode Selecting Engineering Design (ED) Mode will place all PEACE components which currently are in TD mode into their ED mode, and will revert to the input screen April 20 © Copyright Thermoflow, Inc., 2011 SC-29

THERMOFLEX

2-7. Edit Main tab for each pipe in ED mode Double-click on the first heater’s steam pipe. Set its length to 90 ft and its heat loss to 1 BTU/lb. Accept all other defaults.

Hint: The default sizing criteria is pressure drop for steam pipes and velocity for water pipes, but you can choose either method.

April 20 Set = 1 BTU/lb Set = 90 ft © Copyright Thermoflow, Inc., 2011

Likewise, set the length of all the other PEACE pipes to 90 ft each, and their heat loss to 1 BTU/lb each Click on the Fittings tab

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2-8. Edit Fittings tab for each pipe in ED mode Go to the Fittings tab of the first heater’s steam pipe. Assume 6 Long Elbows and 1 Globe Valve.

Hint: If you install more fittings and instruct the program to size the pipe based on a pressure drop criterion, ED mode will tend to size the pipe with a larger diameter. If you instruct the program to size the pipe based on a velocity criterion, installing more fittings will not change the diameter found by the program, but will increase the pressure drop.

April 20 © Copyright Thermoflow, Inc., 2011

Likewise, assume the same set of fittings for each of the other PEACE pipes Check Inputs & Compute

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FILE: steamcycle_P-PCE.tfx

2-9. View results for overall plant and for each pipe

THERMOFLEX

With the pipes as sized, the pressure drops turn out to be lower than assumed for the original case with THERMOFLEX pipes. Thus, net output is higher by about 40 kW. Clicking on each pipe reveals its details, such as diameter, schedule, material selected, steam velocity and pressure drop.

The pipe nominal diameters computed by the program are shown in green.

April 20

5 6 10 12 20

FILE: steamcycle_P-PCE.tfx

2-10. View estimated cost for each pipe

THERMOFLEX

In the output graphic of each pipe, click on Specification to view its estimated costs

12 5 6 10 12 20

April 20 © Copyright Thermoflow, Inc., 2011 SC-33

2-11. Change a steam pipe to a different diameter

THERMOFLEX

Return to the input screen, still in ED mode. Double-click on Pipe [27] to open its menu.

From within the open menu of Pipe [27], select Off-design mode. This changes Pipe [27] to OD mode, but leaves the other components in design mode. OD mode allows you to explicitly dictate a pipe’s specs.

Change the nominal diameter of Pipe [27] from 10” to 8” then recompute.

Hint: Changing the mode from within a component’s menu will only change it for that particular component.

April 20 © Copyright Thermoflow, Inc., 2011 SC-34

FILE: steamcycle_P-PCE1.tfx

2-12. View results with the smaller steam pipe

THERMOFLEX

After computing, note that the system is in ED mode, but Pipe [27] is in OD mode. Its smaller diameter has saved about $15,600 in capital, but increased its pressure drop from 1.63 to 4.27 psi, causing an 11 kW reduction in plant output. The larger, original 10” pipe may not be justified, since it procures the 11 kW increment at a cost of $1415/kW.

April 20 © Copyright Thermoflow, Inc., 2011 SC-35

THERMOFLEX

Steam Cycle Tutorial - Topic 3

1. Build a basic steam cycle 2. Include piping losses

3. Introduce additional steam turbine details

4. Use more elaborate Thermo Boiler model 5. Setup & run the model at off-design 6. Create part-load performance curves 7. Bypass top heater for peak-load

April 20 © Copyright Thermoflow, Inc., 2011 SC-36

THERMOFLEX

3-1. Set more realistic ST section efficiencies

82 Assumed dry step efficiencies, % 85 85 86 87 88

The efficiency of a turbine section (group) is described via its “Dry Step Efficiency”. This is basically the efficiency of each stage (step) within the section, if the steam is dry (superheated). The section efficiency computed by the program includes correcting dry stage efficiency for wetness, and the benefit of compounding the several stages within each section.

The high-pressure end of a steam turbine normally has short blades with low reaction designs. Thus, its dry step efficiencies are lower than those for the low pressure end. If the HP inlet has a governing stage, which normally has a larger diameter, a high pressure ratio, a high steam velocity, and partial arc admission, the efficiency of the first section will be further reduced.

In our example, we select the dry-stage efficiencies shown above, entered in the dry step efficiency text box of the menu for each turbine group.

April 20 © Copyright Thermoflow, Inc., 2011 SC-37

FILE: steamcycle_PT1.tfx

THERMOFLEX

3-2. View output & note effect of revised ST efficiencies April 20 Using the section efficiencies assumed in the previous slide results in a gross power output of 66403 kW, rather than the 66135 kW obtained previously when a flat 85% was assumed, all other assumptions being the same. (The assumption of more efficient LP groups has outweighed the assumption of less efficient HP groups).

3-3. Introduce steam turbine leakage

THERMOFLEX

Uncheck Drawing, relocate the Package Boiler and its Pipe to make room for inserting a Leakage component representing the

main HP leakage

, from the middle of the labyrinth seal and valve stem seals, which goes to a medium-pressure destination.

April 20 Hint: The Leakage component is under the General tab. It has many transpositions, so drop it in an empty area and press F9 repeatedly, until you find a suitable orientation.

THERMOFLEX

3-4. Send the main HP leak to the deaerator Delete the connector and install a Mixer instead; then install a pair of tags to connect the main leakage to the deaerator.

Hint: The

Outlet Tag

and

Inlet Tag

are created automatically when you create a tagged connection. You can form this kind of connection by either holding the Ctrl key down when clicking on the nodes to be connected or by right-clicking on a connector and choosing Connect with tags on its context menu. Tags are useful in making connections on a busy screen. April 20 © Copyright Thermoflow, Inc., 2011 SC-40

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3-5. Set design-point leakage flow rates After Check Drawing passes, double-click on Leakage [31] to open its menu. Select the Pressure difference method in Item 4 (for computing the leakage flow rate at off-design). Set the leakage flow rate at 1.5% at the design-point (Item 2).

April 20 © Copyright Thermoflow, Inc., 2011 SC-41

THERMOFLEX

3-6. Throttle flashback before mixing with leakage (optional) Double-click on Mixer [31] to open its menu. Set the flow to the mixer from the flashback of FWH 4 to “Throttle”. This lets the 49.2 psia deaerator pressure dictate the leakage destination pressure. Otherwise, it would be set by the shell pressure of FWH 4.

Hints: 1) This step is optional and cosmetic. It has no effect on the result, since the leak flow is set by the Leakage component and its enthalpy is unaffected by throttling.

2) The pressure of the optional secondary steam inlet to the D/A can be any value higher than or equal to D/A pressure. It is only set equal to D/A pressure if it undetermined by any other significant pressure dictator.

April 20 © Copyright Thermoflow, Inc., 2011 SC-42

FILE: steamcycle_Leak.tfx

THERMOFLEX

3-7. Check Inputs, compute, and view results April 20 Note that gross output falls from 66403 kW to 65884 kW, a loss of 519 kW or about 0.8% when main HP seal and valve stem leakage is considered.

FILE: steamcycle_PT1.tfx

THERMOFLEX

3-8. Steam Turbine Assembly We could model additional steam turbine leakages by repeating steps taken above, and also could spend some time setting up additional components to model the Sealing Steam Regulator (SSR). However, there is an easier way. THERMOFLEX allows you to define a Steam Turbine Assembly in models that contain steam turbine groups and have been calculated at least once. This assembly, described in Chapter 3 of the THERMOFLEX manual, will: 1) Automatically estimate the efficiencies of its steam turbine groups 2) Automatically model the various common steam turbine leakage flows 3) Allow PEACE to generate the dimensions, weight, and cost of the steam turbine Reopen the file steamcycle_PT1.tfx and return to Edit Inputs mode. Open the Define menu and select ST Assembly.

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3-9. Adding a new steam turbine assembly Selecting Define > ST Assembly will open the ST Assembly Manager. Click on the Add New Assembly button to create the new assembly.

April 20 The ST Assembly Initialization window allows you to set up your physical steam turbine based on your desired turbine and cycle types. This model utilizes a condensing, non-reheat steam turbine, so make these choices in the Turbine Type panel. Accept the default, single-casing layout and press OK to continue.

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3-10. Affiliating ST groups with the ST Assembly Once you have initialized the Steam Turbine casing configuration, you must add your ST groups to the current assembly.

You can rename our ST Assembly by typing the new name in this box. This feature can be useful for models containing multiple steam turbines.

All of the ST groups in this model will be affiliated with the current steam turbine assembly, so select each of the unaffiliated ST components and insert them into our assembly by clicking the arrow button to the right.

April 20 When you are finished, click on the Edit Assembly Input button.

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3-11. ST Assembly Main Inputs When the ST Assembly Input form is loaded, the ST Main Inputs tab is displayed. Set the Number of governing stage rows to 1. Accept all other defaults on this tab and move on to ST Leakages.

Note: When ST Group Efficiency Estimate is set to one of the Automatic modes, the program will ignore the dry step efficiency input of the individual ST groups. If you would rather use these inputs, you must select User-defined here.

April 20 © Copyright Thermoflow, Inc., 2011 SC-47

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3-12. ST Assembly Leakages The ST Leakages tab allows you to define the destination and flow of rate of the various leakages available for your steam turbine. Click on the box next to the leakage name to open its menu. In this case, HP end leak 1 should be sent to the DA, as it was before, so its destination must be changed to allow the assembly to send the leakage flow elsewhere in the THERMOFLEX model.

April 20 Click on the box to open the menu for this leakage.

Choose TFX source for the Leak destination. This will allow us to link this flow to a Water/Steam Source component that will be placed in the model after the ST Assembly has been completely initialized.

THERMOFLEX

3-13. Steam Seal System Rather than simply discarding the SS packing exhaust (the default behavior of the ST Assembly), we will add a little more detail to our model by sending it to a Gland Steam Condenser (GSC). This component will be added to the model after the ST Assembly has been fully initialized.

Set the SS packing exhaust destination to To TFX Source.

Once this change has been made, click OK to accept all other defaults, and click OK again on the ST Assembly Manager to return to the flowsheet.

April 20 © Copyright Thermoflow, Inc., 2011 SC-49

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3-14. Send the main HP leak to the deaerator Once you have returned to the flowsheet, go back to Edit Drawing mode. As before, we need to add a Mixer component between FWH4 and the DA to accept the leakage flow. Delete the connector and install a Mixer instead; then install a Water/Steam Source to act as the destination for the main leakage flow coming from the ST Assembly. This source will be linked to the ST Assembly in Edit Inputs mode.

April 20 © Copyright Thermoflow, Inc., 2011 SC-50

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3-15. Install the Gland Steam Condenser (GSC) Delete this connector and install a Mixer between the ST exhaust and the condenser. Connect one of the Mixer’s inlets to the Drain outlet of the GSC to send the condensed SS packing exhaust to the condenser.

Delete the connector and install a Shell-Tube Water Heater before the first FWH to act as the GSC. Attach a Water/Steam Source to its Heating steam inlet node to accept the ST Assembly’s SS packing exhaust flow.

April 20 Once these steps have been completed, check the drawing and proceed to Edit Inputs mode.

THERMOFLEX

Go to Item 6 (Link to GTP/GTM/STM or ST assembly) and select 2 – ST assembly. This will open the ST Assembly Stream to Water/Steam Source selection menu.

Set the ST assembly steam pressure (Item 17) to 49.2 psia to match the deaerator’s pressure. This will prevent a mandatory pressure spec change message when we go to compute.

Double-click on Water/Steam Source [31] to open its menu.

3-16. Link Water/Steam Source [31] to HP leak 1 Select HPT HP leak 1 of Steam Turbine and click OK.

April 20 © Copyright Thermoflow, Inc., 2011 SC-52

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3-17. Linked Water/Steam Sources Once the link has been completed, you will notice that the Source’s icon has changed. This identifies a Water/Steam Source that has been linked to a ST Assembly, allowing you to identify such sources at a glance.

Repeat the above process to link Water/Steam Source [33] to SS packing exhaust of Steam Turbine.

April 20 © Copyright Thermoflow, Inc., 2011 SC-53

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3-18. Set the GSC’s inputs Ensure that the Shell Heating Fluid Phase is set to Condensing.

Set the GSC’s Shell pressure to 12 psia to match that of the SS packing exhaust stream.

April 20 © Copyright Thermoflow, Inc., 2011 SC-54

THERMOFLEX

Double-click on Mixer [30] to open its menu. Set the flow to the mixer from the flashback of FWH 4 to “Throttle”. This lets the 49.2 psia deaerator pressure dictate the leakage destination pressure. Otherwise, it would be set by the shell pressure of FWH 4.

3-19. Throttle mixer inlet flows Perform the same steps to throttle the 3 rd clockwise inlet flow of Mixer [34]. This step is not strictly necessary, but is good practice as it will clearly define where the 12 psia SS packing exhaust is throttled to the condenser pressure.

April 20 © Copyright Thermoflow, Inc., 2011 SC-55

THERMOFLEX

3-20. Set ST inlet pressure control Double-click on the first ST group to open its input menu. Set the Inlet Pressure Control model to MVL (Mean of Valve Loops). Accept the default Control pressure drop of 2.5%.

April 20 © Copyright Thermoflow, Inc., 2011 SC-56

FILE: steamcycle_STAssembly.tfx

THERMOFLEX

3-21. Check Inputs, compute & view results

Original η ds , %: 82 New η ds , %: 88.7

85 91.0

85 90.8

86 90.0

87 88 92.0 92.4

Note that gross output climbs from 66403 kW to 68169 kW, a gain of 1952 kW (over 2.6%)! This gain is due to the new, higher group dry step efficiencies calculated by the ST Assembly (shown above in red) compared to the values that we had input earlier (shown in blue), which more than compensate for the additional losses due to leakages and valve pressure drops accounted for by the assembly. As mentioned earlier, if the ST Assembly’s ST Group Efficiency Estimate is set to Automatic, then it will ignore and overwrite any previously-defined efficiency and will block the user from editing this value on the component’s input screen. Compare the dry step April 20 the previous slide.

THERMOFLEX

3-22. ST Assembly Outputs Now that the model has been computed, the Assemblies output view has been enabled. Clicking on this button will display the outputs for the only assembly in our model – the Steam Turbine. These outputs include summaries of the thermodynamic performance of both the steam turbine as a whole and each of its constituent groups, steam turbine and leakage schematics, cost and sizing estimates, and drawings of the physical steam turbine genset.

THERMOFLEX

Steam Cycle Tutorial - Topic 4

1. Build a basic steam cycle 2. Include piping losses 3. Introduce additional steam turbine details

**4. Use more elaborate Thermo Boiler model**

5. Setup & run the model at off-design 6. Create part-load performance curves 7. Bypass top heater for peak-load

THERMOFLEX

4-1. Substitute

Thermo Boiler

for

Package Boiler

Uncheck Drawing and delete the Package Boiler. Install a Thermo Boiler and provide it with a Fuel Source, a Gas/Air Source (fresh air for combustion) and a Gas/Air Sink (stack), as shown. The Thermo Boiler has, intrinsic to its icon, an air preheater; a water/steam circuit with economiser, evaporator and superheater sections; as well as forced-draft and induced-draft fans. These features are activated and specified from within its input menu. It also has optional reheaters, unused in our example.

THERMOFLEX

4-2. Define the fuel as Pennsylvania Upper coal Click on Solid then on Coal, then on American Coals April 20 Double-click on the Fuel Source to open its menu, then click on its Define Fuel button.

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Double-click on the Thermo Boiler to open its menu. Set steam flow rate (Item 4) to 150 lb/s @ 954 (Item 22) to 20% and minimum stack temperature (Item 26) to 300 ° F. Set fuel delivery power other defaults in place.

° F (Item 6). Set minimum excess air (Item 35) to 20 kWh/ton. Leave all 4-3. Define

Thermo Boiler

THERMOFLEX

4-4. Check Inputs, compute & view message A “Remark” (benign) is generated when we compute. We can read this message in the Text output view. It advises us that our “desired” boiler efficiency (93% on LHV in Item 3 of the Thermo Boiler inputs) could not be achieved in view of the other inputs, viz the excess air and minimum stack temperature. Actually, we hadn’t bothered to input a “desired efficiency”, but merely left the rather high default value in place, expecting that it will be overridden by the other constraints we entered.

FILE: steamcycle_STA_TB.tfx

THERMOFLEX

The results for the coal-fired Thermo Boiler show a drop of about 400 kW in net plant output, compared to the equivalent case using the simple, original Package Boiler. This is largely due to the higher boiler auxiliary load (along with minor changes in assumed pressure drops affecting feedpump power). Gross output is essentially unchanged. The Thermo Boiler turns out to be 91% efficient on LHV, vs. 93% for the original Package Boiler. However, due to the HHV/LHV ratio for coal being much lower than the 1.11 assumed for the Package Boiler, the HHV efficiency of the Thermo Boiler (88.2%) is higher than that for the Package Boiler (83.8%). Thus, plant net LHV efficiency falls, but HHV efficiency increases slightly for the present case.

THERMOFLEX

4-6. Further boiler modelling April 20 A more detailed boiler model may be built from components, with the Radiant Boiler for the furnace, separate convective HXs for the various convective elements, and a Rotary Air Heater for the air preheater. Please see sample file S(1-10) for an example, depicted here.

THERMOFLEX

Steam Cycle Tutorial - Topic 5

1. Build a basic steam cycle 2. Include piping losses 3. Introduce additional steam turbine details 4. Use more elaborate Thermo Boiler model

5. Setup & run the model at off-design

6. Create part-load performance curves 7. Bypass top heater for peak-load

THERMOFLEX

5-1. Select off-design mode at the main output screen Selecting “Universal Off-Design” sets all components to their default OD mode and reverts to the input screen.

April 20 Hint: If Off-design Mode cannot be chosen in your model, you may need to run an Engineering Design Mode calculation first.

THERMOFLEX

Double-click on the first ST group to open its input menu. Ensure that the Inlet Pressure Control model is set to MVL (Mean of Valve Loops). Leave all other defaults.

The shorter brown arrows point to items left at default in our example, but which will usually require scrutiny.

THERMOFLEX

5-3. Set steam turbine exhaust loss curve Launch the ST Assembly Manager (Define > ST Assembly). Open the ST Assembly Edit Inputs menu and navigate to the Exhaust End Design tab. Click on Edit

Thermoflow exhaust loss model

parameters, and then Plot Exhaust Loss Curve to view the default exhaust loss curve. Accept all defaults and return to the flowsheet.

THERMOFLEX

5-4. Define boiler part-load curves Double-click on the Thermo Boiler to open its menu. The Performance Map button summons a table in which you may define part-load boiler efficiency. The Excess Air button invokes a table in which you may define the change in % excess air, relative to its design-point value, as a function of load. Accept the default tables for the present example.

FILE: steamcycle_STA_TB_OD.tfx

THERMOFLEX

5-5. Check Inputs & compute The results of a properly setup off-design model should match their design-point counterparts to within the convergence tolerance, if the equipment sizes and all operating conditions are the same. In this case, the difference is 1 kW (0.0015%).

THERMOFLEX

Steam Cycle Tutorial - Topic 5

1. Build a basic steam cycle 2. Include piping losses 3. Introduce additional steam turbine details 4. Use more elaborate Thermo Boiler model 5. Setup & run the model at off-design

6. Create part-load performance curves

7. Bypass top heater for peak-load

THERMOFLEX

6-1. Create 10-case part-load macro Click on the Multiple Runs button. Once the Thermoflow Macro program starts, select “Boiler outlet steam flow” as the sole input parameter to be varied. (Note: If needed, you can select multiple input parameters to vary).

6-2. Define part-load macro cases

THERMOFLEX

6-3. View computation messages The computation process has finished successfully, but the first three cases report that they have messages for us. If we right-click on one of the columns of the results and choose Show Messages from the context menu, a window will open and display the computation messages. These cases have an Advisory to let us know that the deaerator pressure is above its steam supply pressure. This seems like it could warrant some further investigation to ensure that our model is behaving correctly, so right click again and choose Show Program Output. This will launch THERMOFLEX in a special output viewing mode that will allow us to examine the results of that particular case in more detail.

Note: Choosing Export this Heat Balance File as… will create a TFX file from the selected Macro case. This TFX file can then be edited just like any other THERMOFLEX model.

FILE: steamcycle_STA_TB_OD.mtf

THERMOFLEX

6-4. View Case 1 deaerator details Click on the D/A to see its expanded graphic. It turns out that the flashback from the next heater, along with the main HP leakage steam, have raised the D/A temperature, hence the corresponding saturation pressure, to a level above the pressure of the connected heating steam port. Hence the heating steam flow is shut off, just as it should be.

FILE: steamcycle_STA_TB_OD.mtf

THERMOFLEX

6-5. Select parameters to tabulate at part-load Select the General – 1 tab to create a custom macro table. When first opened, this table will be mostly empty. Click the Add/Remove Variables button to launch to Display Variable Selector. Choose the variables you are interested in, then press OK to display them in the table.

FILE: steamcycle_STA_TB_OD.mtf

THERMOFLEX

6-6. Plot heat rate at part-load Proceed to the X-Y Plots topic. Choose Net power as the X-Axis Variable and Net heat rate (LHV) as the Y-axis Variable. Once you have made your selections, click Show Plot to display the chart.

FILE: steamcycle_STA_TB_OD.mtf

THERMOFLEX

Steam Cycle Tutorial - Topic 5

1. Build a basic steam cycle 2. Include piping losses 3. Introduce additional steam turbine details 4. Use more elaborate Thermo Boiler model 5. Setup & run the model at off-design 6. Create part-load performance curves

7. Bypass top heater for peak-load

7-1. Bypass top heater to increase output (Trick 1)

THERMOFLEX

To bypass a heater, you can alter your drawing to include a bypass. To reduce tedium, one trick that accomplishes the same result is to simply lower its set point shell pressure.

Click on the top heater to open its menu. Set its desired shell pressure to lower than saturation pressure at the incoming water temperature. This will turn off its steam supply. The incoming feedwater in this case is at about 340 ° F, so we input 100 psia (T sat =328 ° F).

Hint: If you wish to reduce or eliminate its water-side pressure drop, reduce Item 18.

FILE: steamcycle_OD1.tfx

THERMOFLEX

7-2. View result of bypassing top heater (Trick 1) Bypassing the top heater increases net output, since its heating steam will not be extracted, but will expand through the turbine. In our example, the increase is about 2.7 MW (from 68196 to 70924 kW). Net efficiency gets worse, though, due to the increase in boiler heat input to compensate for the cooler feedwater. In our example it falls from 32.77% to 32.18% (HHV).

THERMOFLEX

7-3. Bypass top heater (Trick 2) Click on the top heater to open its menu. Set “UA” for all its sections to zero. This eliminates its area.

Write down your UA’s before you delete them, or better still, to preserve all decimals, save your file first before zeroing your UA’s !

Hint: If you wish to reduce or eliminate its water-side pressure drop, reduce Item 18.

FILE: steamcycle_OD2.tfx

THERMOFLEX

7-4. View result of bypassing top heater (Trick 2) Results are identical to those obtained via “Trick 1”, to within convergence tolerance.

FILE: steamcycle_OD2.tfx

THERMOFLEX

7-5. By default, the shell pressures of active heaters do not exceed their design-point values The THERMOFLEX feedwater heater is equipped with a valve that can reduce the heating steam to a desired, maximum shell pressure, and by default, this set point is the design-point shell pressure. When the top heater is bypassed, more steam expands throughout the turbine, raising the pressures at all its extraction ports. If the only pressure drop in each extraction line is the pipe, then the shell pressures would increase for all heaters. Thus, if you wish to ensure that the heating steam valve is wide-open at off-design, to allow shell pressures higher than the design values, you should raise the set point (Item 9) for each heater. An increase of about 20 % is adequate for this example.

FILE: steamcycle_OD3.tfx

THERMOFLEX

FILE: steamcycle_OD3.tfx