HIoPE
4. Part Load Operation of Steam Turbines
Fully Open
#1
#1
Partially Open
#2
Closed
Closed
Stop V/V
(1.5% p)
Steam Turbine
Steam
Flow
#2
#3
#4
Control V/V
(1.5% p @ VWO)
4. Part Load Operation
Nozzle Bucket
First stage
shell pressure
1 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
11
3
Full Throttling
29
4
Partial Arc Admission
37
5
Sliding Pressure Operation
57
6
Hybrid Operation
64
7
Startup System of Steam Power Plants
73
8
Load Changes
76
Steam Turbine
4. Part Load Operation
2
2 / 81
Steam Turbine Control
HIoPE
 hin  hout 
Pm
Change power by changing steam flow
 or steam inlet enthalpy (h ).
rate (m)
in
Steam Turbine
4. Part Load Operation
3 / 81
Heat Balance - Design Condition
HIoPE








Steam Turbine
4. Part Load Operation
4 / 81
Heat Balance – Part Load Condition
HIoPE







Steam Turbine

4. Part Load Operation
5 / 81
A Basic Concept for Part Load Operation
MS
C/V
R
HP Turbine
Pressure
HIoPE
LP Turbine
100% Power
75% Power
50% Power
25% Power
Steam Turbine
4. Part Load Operation
6 / 81
HIoPE
Output and Efficiency at Part Load
Output and Efficiency at Part Load
Example: 460 MW, supercritical power plant
500
440


 Efficiency

 Power



Power, MW
410

380


350

320
290


260
230 
200
45




48.3
47.6
46.9

46.2

45.5


44.8

Efficiency, %
470
49.0
44.1

43.4

42.7

42.0
50
55
60
65
70
75
80
85
90
95 100
Load, %
Steam Turbine
4. Part Load Operation
7 / 81
HIoPE
Throttling Process
Nozzle Row
p0
h
p1
p1’
T0
100%
Bucket Row
U
p0: Inlet pressure
p1: Throttle pressure
100% load
75% load
Design-flow expansion line
50% load
[ Velocity Diagram at Various Loads ]

A turbine has different expansion lines as the load is
decreased.

But the part load expansion lines are generally parallel
to the full load expansion line.

This means that the internal efficiency under part load
conditions is very close to that under full load
conditions. That is, design efficiency of the turbine
blades is maintained during part load operations by
using the control valve.

However, the cycle efficiency is reduced under part
load conditions.
Steam Turbine
1′
4. Part Load Operation
Partial-flow expansion line
Available Energy
U
25%
1
25% load
Expansion lines are
essentially parallel
pc
2′
2
[ Effect of Throttling on Non-Reheat
Steam Turbine Expansion Line ]
s
8 / 81
Control of Steam Flow in HP Turbines
HIoPE
4 Methods Used in Steam Flow Control
1)
Full throttling (= Single admission )
• Constant pressure mode
• Throttling by pressure reducing valves
• All control valves are activated at the same time
• This is the simplest way to control the power, but this gives a large throttle
(pressure) loss because of using the pressure throttle valves
2)
Partial arc admission (Throttling by a control stage)
• Constant pressure mode
• Divided the first stage nozzle arc into several segments having its own control valve
• Lower throttle loss
• The control valves are activated in a sequential mode
3)
Sliding (or variable) pressure operation
• Variable pressure mode
• Controlling throttle flow by varying boiler pressure
4)
Hybrid operation
• A combination of partial arc admission and sliding pressure operation
Steam Turbine
4. Part Load Operation
9 / 81
HIoPE
Exhaust Loss during Part Load
m  1 0.01Y 
3600 AAN
 = saturated dry specific volume
AAN = annulus area
Y = percent of moisture at ELEP
[Exercise 4.1]
부분부하운전 시 배기손실 크기 변
화를 비교하시오. 아울러 그 결과를
heat balance에서 확인하시오.
Steam Turbine
139
93
47
0
Exhaust Loss, Btu/lb
VAN = annulus velocity
m = steam mass flow rate
80
Thermodynamic
Optimum
Exhaust Loss, kJ/kg
VAN 
186
60
Total
Exhaust
Loss
Axial
Leaving
Loss
Economic
Optimum
40
VAN proportional to:
rating
1/exhaust pressure
1/exhaust area
20
0
0
1000
500
1500
Annulus Velocity (VAN), ft/s
2000
0
305
152
457
Annulus Velocity (VAN), m/s
610
4. Part Load Operation
10 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
11 / 81
HIoPE
Valves
Main Steam
Steam
Generator
Crossover
Stop V/V
Control V/V
Front
Standard
Cold
Reheat
HP
IP
LP
Gen
Exciter
Ventilation
V/V
Reheat
Stop and
Intercept
V/V
Reheater
Condenser
Hot Reheat
[ A Typical Power Plant Steam Flow Diagram ]
Steam Turbine
4. Part Load Operation
12 / 81
Main Steam Valves
HIoPE
Siemens
Steam Turbine
4. Part Load Operation
13 / 81
HIoPE
Typical Individual Stop and Control Valve Assembly
GE
Generals
Valve 개수(표준화력 500MW 기준)
- Stop v/v : 2
- Control v/v : 4
Stop valve = on-off valve
Control valve = throttle valve라고도 불리며,
load 연동
MSV
Actuator
Typical closing time during emergency
- Stop v/v : 0.09초 10%
- Control v/v : 0.11초 10%
MCV
Actuator
Steam Turbine
4. Part Load Operation
14 / 81
Main Stop Valves

High-pressure steam is admitted to the main turbine
through two parallel main stop valves.

The primary function of the main stop valves is to
quickly shut off main steam flow to the turbine under
emergency conditions.


[1/4]
GE
Steam
Inlet
The stop valves also provide a second line of defense
against turbine overspeed in the event the control
valves fail.
The main stop valve bypass valves are also used for
full arc operation during startup and shutdown of the
turbine.

The main stop valves are located in the main steam
piping between the boiler and the turbine control valve
chest.

The outlet of each stop valve is welded directly to the
valve chest.

The main steam stop valves are operated and
controlled by the turbines Electro Hydraulic Control
System in concert with the units DCS control system.

HIoPE
Valve Seat
Valve Disc
Pressure
Seal Head
Steam
Outlet
Valve Stem
Actuator
The bypass valve disk is fastened to the end of the
stem by special coarse threads strong enough to
withstand full closing force, yet designed to permit
freedom of disk movement relative to the stem so that
the valve will seat.
Steam Turbine
Steam
Strainer
4. Part Load Operation
Closing
Spring
[ Main Stop Valve ]
15 / 81
Main Stop Valves
HIoPE
[2/4]

The steam from the steam generator flows to the main steam stop or throttle valves.

The primary function of the stop valves is to provide backup protection for the steam turbine during turbine
generator trips in the event the main steam control valves do not close.

The energy contained in the main steam can cause the turbine to reach destructive overspeed quickly when
generator loose the load.

The main stop valves close from full open to full closed in 0.15 to 0.5 s.

The main stop valves are closed on unit normal shutdown after the control valves have closed.

A secondary function of the main stop valves is to provide steam throttling control during startup.

The main stop valves typically have internal bypass valves that allow throttling control of the steam from
initial turbine roll to loads of 15% to 25%.

During this startup time, the main steam control valves are wide open and the bypass valves are used to
control the steam flow.

Some recent and current designs do not have these bypass valves.

Initial turbine speed runup is controlled by the main stop valves.
Steam Turbine
4. Part Load Operation
16 / 81
Main Stop Valves
HIoPE
[3/4]
Bypass Valve



GE
The bypass valve is held in the valve disk by a
bolted cap. Holes are located in the cap for steam
entrance, and holes in the valve disk pass the
steam when the bypass valve is utilized.
When the stop valve is opened the bypass valve
opens first as the valve stem moves in the open
direction.
When the bypass valve is fully open it contacts a
bushing on the stop valve and pulls it open. When
the stop valve is fully open, a bushing seats on the
inner end of the valve stem bushing and prevents
steam leakage along the valve stem.
Bypass
Valve Disc
Main Stop
Valve Disc
Main Stop
Valve Disc
Seating
Surface
Bypass
Valve Ports
(8 ea)
Main Stop
Valve Stem
[ Stop Valve Bypass ]

Each stop valve has two steam leakoff points where the stop valve stem passes through the stop valve
casing.

The first leakoff point located closest to the stop valve is referred to as the high-pressure leakoff and is
routed to the steam seal header.

During startup or low loads steam is supplied to this leakoff to assure a seal. After the turbine is loaded,
steam is fed through this line from the stop valve stem into the steam seal header.

The second leakoff point is referred to as the low-pressure leakoff and is routed to the gland steam
condenser.
Steam Turbine
4. Part Load Operation
17 / 81
Main Stop Valves
HIoPE
[4/4]
Bypass Valve
Steam Turbine
4. Part Load Operation
18 / 81
Main Steam Control Valves



The steam from the stop valves flows to the
main steam control or governor valves.
Steam from
No.1 C/V
HIoPE
[1/3]
Snout Pipes
Steam from
No.3 C/V
Snout
Pipe
Seal
Rings
The primary function of control valves is to
regulate the steam flow to the turbine and thus
control the power output of the steam turbine
generator.
HP
Inner
Shell
The control valves also serve as the primary
shutoff the steam to the turbine on unit normal
shutdowns and trips.
180 Degree Nozzle Box
HP
Inner
Shell
Actuator
HP
Inner
Shell
Upper
Lower
HP
Inner
Shell
MSV
HP
Inner
Shell
180 Degree Nozzle Box
MCV
Siemens
Steam Turbine
Actuator
HP
Inner
Shell
Snout
Pipe
Seal
Rings
MHI
Steam from
No.2 C/V
4. Part Load Operation
Snout Pipes
Steam from
No.4 C/V
19 / 81
Main Steam Control Valves
HIoPE
[2/3]
GE


The control valves regulate the steam flow to the turbine to
control the main turbine speed and/or load. The four control
valves are mounted in line on a common external valve chest.
Steam is supplied to the external valve chest through the main
stop valves. The valve chest is separated from the turbine, and
individual steam leads from the valve chest are provided from
each control valve to the inlet of the HP turbine. Each control
valve is operated by a hydraulic power actuator which positions
the control valves in response to signals from the Electro
Hydraulic Control System.
During startup, the control valves are wide open (full arc), and
the stop valves’ internal bypass valves control the steam flow to
the turbine. Under these conditions, steam is admitted through
all four steam leads around the entire periphery of the HP
turbine inlet. The purpose of this full arc admission is to reduce
thermal stresses caused by unequal steam flow through the
nozzle sections. During full arc admission, throttling of the
steam occurs at the stop valve bypass valves only, and there is
uniform steam flow into the HP turbine. This also results in
lower steam velocities at the turbine inlet. Because of the lower
steam velocities the temperatures cannot change as rapidly.
Full arc admission is used until the high transfer point is
reached, at which time transfer to partial arc will occur.
Steam Turbine
4. Part Load Operation
Closing
Spring
Balance
Chamber
Valve
Seat
Steam
Valve Chest
Disc
[ Main Steam Control Valve ]
20 / 81
Main Steam Control Valves
HIoPE
[3/3]
GE

During normal operation, the main stop valves are wide open and the control valves control steam flow to
the turbine. The control valves operate sequentially to control steam flow to the turbine and the unit load.

All four control valves are never open the same amount for any given load up to full load with wide-open
control valves. This is referred to as partial arc admission.

Transfer to partial arc admission is normally automatically performed by the low transfer and high transfer
micro- switches but may also be initiated by the operator when the OK TO TRANSFER light comes on.

The control valves are throttled until they have control of steam flow and the stop valves then automatically
run full open.

Number l and 2 control valves are balanced type, with internal pilot valves. Number 3 and 4 control valves
are unbalanced single disk type.

The balanced type valves are equipped with an internal pilot valve connected to the valve stem. When
opening, the pilot valve is opened first to equalize the pressure across the main valve disk. Further opening
of the stem opens the main disk.

The disk of the unbalanced type valve is directly connected to the stem.

Each control valve is provided with two steam leakoff points where the control valve stem passes through
the external steam chest wall. The first leakoff point located closest to the external steam chest is referred to
as the high-pressure leakoff and is routed to the hot reheat steam line. The second leakoff point is referred
to as the low-pressure leakoff and is routed to the steam seal header.
Steam Turbine
4. Part Load Operation
21 / 81
Reheat Stop and Intercept Valves
HIoPE
[1/4]
[ Combined Reheat Stop and Intercept Valve, GE ]
Steam Turbine
4. Part Load Operation
22 / 81
Reheat Stop and Intercept Valves

Two combined reheat stop and intercept
valves are provided, one in each hot
reheat line supplying reheat steam to the
IP turbine.

As the name implies, the combined
reheat intercept valve is actually two
valves, the intercept valve (IV) and the
reheat stop valve (RSV), incorporated in
one valve casing.


Although they utilize a common casing,
these valves have separate operating
mechanisms and controls.
HIoPE
[2/4]
GE
Steam
Strainer
Balance
Chamber
Steam
In
Intercept
Disc
Intercept
Actuator
Reheat
Stop Disc
The function of the intercept valves and
reheat stop valves is to protect the
turbine against overspeed from stored
steam in the reheater.
Steam Out
Closing
Spring
[ Reheat Stop and Intercept Valves (SKODA) ]
Steam Turbine
4. Part Load Operation
Reheat Stop
Actuator
23 / 81
Reheat Stop and Intercept Valves
HIoPE
[3/4]

The intercept valve disk is located above the reheat stop valve disk, with its stem extending through the
upper head. The reheat stop valve stem extends downward through the below-seat portion of the casing.

Both valves share a common seat; however, the intercept valve is designed to travel through its full stroke
regardless of the reheat stop valve position, while the intercept valve must be in the “closed” position for the
reheat stop valve to open.

During normal operation of the turbine-generator unit, the intercept valves are fully open.

The purpose of the intercept valve is to shut off steam flow from the reheater, which, because of its large
storage capacity, could possibly drive the unit to overspeed upon loss of generator load.

The intercept valve is capable of reopening against maximum reheat pressure and of controlling turbine
speed during reheater blowdown following a load rejection.

The primary function of the reheat stop valves is to provide a second line of defense (backup protection)
against the energy storage of the reheater in the event of failure of the intercept valves or the normal control
devices.

However, note that the reheat stop valves also close upon a routine shutdown, or by operation of certain
boiler and electrical trips whenever the main stop valves are closed.

The reheat stop valve power actuators are sized so that the reheat stop valves are capable of reopening
against a steam pressure differential of approximately 15 percent of maximum reheat pressure.
Steam Turbine
4. Part Load Operation
24 / 81
Reheat Stop and Intercept Valves
HIoPE
[4/4]

The function of the reheat stop and intercept valves is similar to the main steam stop and control valves.

The reheat stop valve offer backup protection for the steam turbine in the event of a unit trip and failure of
the intercept valves to close.

The intercept valves control unit speed during shutdowns and on large load changes, and protect against
destructive overspeeds on unit trips.

The need for these valves is a result of the large amount of energy available in the steam present in the HP
turbine, the hot and cold reheat lines, and the reheater.

On large load changes, the main steam control valves start to close to control speed, however, energy in the
steam present after the main steam control valves may be sufficient to cause the unit to overspeed.

The steam after the main steam control valves could expand through the IP and LP turbines to the
condenser, supplying more power output than is required, causing the turbine to overspeed.

The intercept valves are used to throttle the steam flow to the IP turbine in this situation to control turbine
speed.

During unit shutdowns, a similar situation could occur, and the intercept valves are used to control speed
under these conditions as for the trip condition.

During unit trips, both the reheat stop and the intercept valves close, preventing the reheat-associated steam
from entering the IP turbine.

During normal unit operation, the reheat stop and intercept valves are wide open, and load control is
performed by the main steam valves only.
Steam Turbine
4. Part Load Operation
25 / 81
Ventilation Valve

During unit trips, the closure of the main stop and control valves and of the
reheat stop and intercept valves traps steam in the HP turbine.

During the turbine overspeed and subsequent coastdown, the HP turbine
blades are subject to windage losses from rotating in this trapped steam.

The windage losses cause the blades to be heated.

This heating, in combination with the overspeed stress, can damage the HP
turbine blades.

To prevent this, a ventilation valve is provided to bleed the trapped steam to
the condenser.

On a unit trip, the valve is automatically opened.

The bleeding action causes the trapped steam to flow through the HP turbine,
maintaining the HP turbine temperature within acceptable limits by preventing
heat buildup from the windage losses.
Steam Turbine
4. Part Load Operation
HIoPE
[ Ventilation Valve, CCI ]
26 / 81
HIoPE
Ventilation Valve
Main Steam
Crossover
Stop V/V
HP bypass
station
Control V/V
HP
IP
LP
Gen
Ventilation
Cold
Reheat V/V
Reheater
Reheat Stop and
Intercept V/V
Condenser
Hot Reheat
HRH bypass station
(HRH: Hot Reheat)
[ Turbine Bypass Diagram ]
Steam Turbine
4. Part Load Operation
27 / 81
HIoPE
Pressure Drop in Valves
The efficiency of a steam turbine is governed by the efficiency of the individual stages and the pressure
drop through valves, cross-over pipes, and exhaust hoods.

Throttle steam from the boiler first pass through the stop valve and then control valves.

The stop valve pressure
drop is 2% and the control
valves in the wide open
position also have a
pressure drop of 2%.
Therefore, total pressure
drop occurred in the
valves is 4%.

The total loss in overall
heat rate of these pressure
drops is about 0.4%.
Throttle to bowl pressure drop (%)

8
6
4
2
0.2
Steam Turbine
0.4
0.6
Equivalent throttle flow ratio
4. Part Load Operation
0.8
1.0
28 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
29 / 81
HIoPE
Concept of Full Throttling
All control valves have a same position in full throttling operation
#1
#1
Steam
Flow
#2
#3
#2
#4
Stop V/V
(1.5% p)
Control V/V
(1.5% p @ VWO)
Nozzle Bucket
First stage
shell pressure
Steam Turbine
4. Part Load Operation
30 / 81
HIoPE
Generals for Full Throttling
p0
h
0
p1
1
p2
2
Initial (Main Steam)
Throttling
Ahead of First
Stage Nozzles
(Bowl Pressure)
s
0
Steam condition entering the control valves
1
Steam condition entering the first stage
2
Steam condition entering the second stage
After First Stage
(Shell Pressure)
Load
Steam Turbine
4. Part Load Operation
31 / 81
HIoPE
Generals for Full Throttling

Full throttling is the simultaneous operation of all main steam control valves at the same time.

The main steam supplied during part load operation has a same pressure with that supplied during full load
operation.

Therefore, throttling loss is occurred whenever the plant is operated with part load.

The steam turbine output increases as the valves are opened and full load is reached when the valves are
wide open.

During the part load operation, full throttling is the least efficient of all control modes because the available
energy in the expansion process is reduced greatly by the throttling process. For this reason, the HP turbine
with full throttling has a greater entropy increase than that with partial arc admission.

This method is also called as ‘full arc admission’, ‘single admission’, or ‘throttling control’ because of the
steam admission to all portions of the control stage.

During the startup of some units, the control valves are wide open.

Steam is initially admitted to the turbine by throttling the steam flow by using the bypass valves internal to
the main steam stop valves.

This flow control method is used up to 15% to 25% load.

Above this load, the main steam control valves are used to control the steam flow and the main steam stop
valves are wide open.

The complex control stage design is not required.
Steam Turbine
4. Part Load Operation
32 / 81
HIoPE
Advantages of Full Throttling

For 50% reaction turbines with throttling control, the pressure ratio across the first stage is about 1.13 at all
loads.

This value of pressure ratio is too low to produce sonic velocity.

Thus, an advantage of throttling control is no solid particle erosion in the first stage.

The solid particle erosion would be in the control valves, which are less expensive to repair than the first
stage.

Another advantage of throttling control is lower stress levels due to low first stage velocities and the absence
of inactive arcs.
Steam Turbine
4. Part Load Operation
33 / 81
HIoPE

Stage pressure is roughly proportional to the mass flow
rate to the following stage, the throttle flow minus
leakages, and all extractions from the preceding stages
and stage in question, plus any steam returned to the
turbine ahead of this stage.

Therefore, mathematical relationship is
pi  pi ,d
pi
pi ,d
m i
m i ,d
m i
m i ,d
= steam pressure at the nozzle of stage i
= design steam pressure at the nozzle of stage i
= steam mass flow rate to the stage i
= steam mass flow rate to the stage i
Absolute steam pressure
Stage Pressure during Part Load

m1

m0
[ Variation of stage-shell pressure ]
Steam Turbine
4. Part Load Operation
34 / 81
HIoPE
Stage Pressure during Part Load
[Exercise 4.1]
아래 그림은 각각 설계조건과 부분부하운전조건에서 LP터빈 첫 번째 추기 지점에서의 조건이다. 부분부
하운전조건에서 추기압력을 계산하고 검토하시오.
3566457 lb/hr
2701136 lb/hr
LP Turbine
LP Turbine
131561 lb/hr
60.6 psia
105869 lb/hr
46.21 psia
[ Full load ]
Steam Turbine
[ Part load ]
4. Part Load Operation
35 / 81
Stage Pressure during Part Load
HIoPE
[Solution]
Design conditions are
pd = 60.6 psia
md = 3,566,547131,561 = 3,434,986 lb/hr
md means the steam mass flow rate to the next stage.
Under the part load conditions this mass flow rate becomes
m = 2,701,136105,869 = 2,595,267 lb/hr
Extraction pressure can be calculated
pi  pi ,d
m i
m i ,d
p = 606.6  2,595,267/ 3,434,986
= 45.8 psia
This calculated pressure is very close to extraction pressure shown in heat balance, 46.21 psia.
Steam Turbine
4. Part Load Operation
36 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
37 / 81
Concept of Partial Arc Admission
Fully Open
#1
#1
Partially Open
#2
Closed
Closed
Stop V/V
(1.5% p)
Steam Turbine
HIoPE
Steam
Flow
#2
#3
#4
Control V/V
(1.5% p @ VWO)
Nozzle Bucket
First stage
shell pressure
4. Part Load Operation
38 / 81
HIoPE
Nozzle Box
#1
#3
42
43
Turbine C.W.
Number of nozzle
42
43
#4
#2
[ 500 MW (3,500 psig, 1,000F) ]
Steam Turbine
4. Part Load Operation
39 / 81
Concept of Partial Arc Admission
Full Arc
Partial Arc
(Single Admission)
#3 v/v
#1 v/v
HIoPE
#3 v/v
#1 v/v
Inactive
arc
#4 v/v
#2 v/v
#4 v/v
#2 v/v
The dimensions of turbine blades and flow channels are primarily a function of the volumetric
flow rates passing through the machine
Steam Turbine
4. Part Load Operation
40 / 81
HIoPE
Pressure Variation
Initial (Main steam pressure)
Throttling
Ahead of First Stage
Nozzles (Bowl
Pressure)
Initial (Main
steam
pressure)
Ahead of
Nozzle
Box #2
Ahead of
Nozzle
Box #3
Ahead of
Nozzle
Box #1
After First Stage
(Shell Pressure)
After First Stage
Load
Load
Full
Arc
Steam Turbine
Ahead of
Nozzle
Box #4
Partial
Arc
4. Part Load Operation
41 / 81
HIoPE
Generals for Partial Arc Admission

The inlet annulus area of the first stage nozzle is divided into several segments (vary from 4 to 6 to 8) along
tangential direction.

Typically, the first stage divides the annulus into four to eight segments (arcs) having different angles
depending on the guarantee points.

Partial arc admission (PAA) is the sequential operation of the main steam control valves.

PAA varies the output of the steam turbine by increasing or decreasing the arc of admission of steam flow to
the turbine control (first) stage. (The first admission stage in steam turbines often referred to as governing or
control stage)

Each control valve feeds a separate segment of the control stage, and the amount of arc in use is
determined by the number of valves open.

The valves are opened in a particular order that is determined by the allowable stresses on the control stage.

PAA is also called as governing control.

Rated throttle conditions are used throughout the load range to the extent allowed by the steam generator.
- to be continued Steam Turbine
4. Part Load Operation
42 / 81
HIoPE
Generals for Partial Arc Admission

The first stage is of impulse design because it gives only a small circumferential pressure gradient after
nozzle so the spreading of the jets circumferentially may be attenuated. For the 50% reaction turbine, the
first stage is the same as for the impulse turbine.

Partial arc admission can cause high impulse loads on the nozzle and buckets, possibly leading to high cycle
fatigue failures.

If the partial arc admission is applied for small turbine, the blade height can be increased in order to increase
stage efficiency.

Normally, entropy production becomes higher for small turbines than large machines. This is because of
higher endwall losses for shorter blades, and the substantial part of the endwall losses caused by the
secondary flow formed in both nozzle and bucket row.

One way to prevent this is to increase the dimension of the turbine blades and adopt partial admission. Even
though extra losses due to the employment of partial admission is introduced, it might be beneficial due to
the decreased endwall losses.
Steam Turbine
4. Part Load Operation
43 / 81
HIoPE
Secondary Vortices in Short and Long Blades
Tip
Tip
Vortex
Vortex
Vortex
Hub
Hub
Bucket efficiency
(a) Short Blades
Steam Turbine
High
Efficiency
Radial height
Radial height
Vortex
Bucket efficiency
(b) Long Blades
4. Part Load Operation
44 / 81
HIoPE
Advantages of Partial Arc Admission



PAA is more efficient than full throttling because the throttling
process loss is minimized by reducing the number of control
valves throttling at any one time.
#3 v/v
If the PAA is employed, the inlet flow rate can be controlled
and a high inlet pressure and temperature can be maintained
as high as for the fully admitted arcs, even for low flow rates.
Considerably less pressure is lost due to throttling by PAA so
that more pressure is available to produce power in the first
stage, with a corresponding improvement in overall heat rate.
Steam Turbine
4. Part Load Operation
#1 v/v
Inactive
arc
#4 v/v
#2 v/v
45 / 81
HIoPE
Comparison of Throttling Methods
h
h
p0
0
p0
p1
1
p1
1
0
p2
p2
2
2c
2b
2a
s
0
1
2
s
01
Flow in the control valves
1  2b
Steam condition entering the first
stage
Flow across the first stage in an
arc segment
0  2a
Steam condition entering the second
stage
Flow across the first stage in
the fully opened segments
2c
Steam condition into the
second stage
Steam condition entering the control
valves
(a) Full throttling
Steam Turbine
(b) Partial arc admission
4. Part Load Operation
46 / 81
HIoPE
Turbine Section Efficiency
100
90
80
Efficiency [%]
70
60
50
40
IP Turbine
30
LP Turbine
20
HP 2nd Stage to Cold Reheat
10
HP 1st Stage
0
0
0.2
0.4
0.6
Throttle Flow Ratio
Steam Turbine
4. Part Load Operation
0.8
1.0
(VWO)
47 / 81
HIoPE
HP Turbine Efficiency
90
85
HP Turbine Efficiency [%]
Partial Arc
Admission
80
75
Full Throttling
70
65
60
55
0
0.2
0.4
0.6
Throttle Flow Ratio
Steam Turbine
4. Part Load Operation
0.8
1.0
(VWO)
48 / 81
HIoPE
Heat Rate
Actual turbine cycle performance is shown on a
valve loop basis heat rate curve.

This curve reflects the steam throttling effect as
the steam passes through a partially closed
steam admission.

The throttling pressure drop reduces the
available energy of the steam as the throttled
admission steam expands across the control
stage.
Valve Loop Basis (True Curve)
Heat rate

Mean of Valve Loop Basis
Valve Point Basis
(Locus-of-valve best points)
20

An alternative method of representing turbine
heat rate impact due to turbine valve losses at
part load is by a mean of valve loop method.

This method is an approximation of the heat rate impact illustrated on the valve loop basis curve and
represents a mean of the turbine heat rate and passes through the valve loop curve.

Turbine heat balance developed on the basis of this assumption are considered to be on a locus-of-valve
best points basis. This heat balances describe heat rates assuming an infinite number of small valves
having a 3% pressure drop.
Steam Turbine
0
4. Part Load Operation
60
40
Generator output
80
100
49 / 81
HIoPE
Partial Admission
The control stage should be designed with
impulse turbine in order to avoid circumferential
flow of steam after passing through the first
stage nozzle
Steam Turbine
4. Part Load Operation
50 / 81
Efficiency at Partial Arc Admission
HIoPE
80
HP Turbine Efficiency [%]
A
B
C
D
70
A
B
E
C
F = 85/170
60
D
E
50
0.3
0.4
0.5
0.6
Velocity Ratio [U/C]
Steam Turbine
4. Part Load Operation
51 / 81
HIoPE
Partial Admission Losses
Stagnation Region
Stagnation
Region
Tangential
Ventilation Loss
Steam Turbine
Sector-End Loss
4. Part Load Operation
Axial
52 / 81
Unsteady Forces
HIoPE
t=0
The large unsteady forces acting on the buckets in tangential
direction are produced when the buckets enter and leave the
admission jets
t=1 Tref
Tangential Blade Force under Partial Arc Admission
over Time
0.30
t=2 Tref
Force Acting on a Blade in
Tangential Direction
0.25
0.20
t=6 Tref
0.15
0.10
0.05
t=7 Tref
0.00
- 0.05
t=8 Tref
- 0.10
0
2
4
6
8
10
Time
Steam Turbine
4. Part Load Operation
53 / 81
HIoPE
Partial Arc vs. Single Admission
Partial Arc Admission
Single Admission
• It can employ longer blades which give better
aerodynamic turbine efficiency.
• It gives smaller entropy increase during part
load operation.
• Therefore, the HP turbine efficiency is better
during part load operation.
• The better part load performance for partial
admission must be balanced against the
increased potential for high cycle thermal
fatigue in the first stage.
• As load is decreased on the single admission
unit, an increasing amount of throttling takes
place in the control valves.
• It gives better efficiency than partial arc
admission at VWO because there is no inactive
arc.
• Many of the large nuclear units are designed full
throttling.
• The first stage is called as governing stage or
control stage.
Steam Turbine
4. Part Load Operation
54 / 81
HIoPE
Comparison of HP Turbine Performance
The HP turbine efficiency is better during part
load operation with sequential valve operation
than throttling condition. However, at valve wide
open, the throttling controlled turbine gives better
performance. This is because single admission
has no inactive arc.
Valve-loops are the result of changes in HP
turbine efficiency as the valve goes from closed to
fully open.
Steam Turbine
4. Part Load Operation
55 / 81
HIoPE
Effect of Admission Modes
2400 psig/1000/1000F
+4
Single Admission (Full Throttling)
4valves, first 3 valves open together
+3
2 Admissions
4valves, first 2 valves open together
+2
3 Admissions
+1
4 Admissions
0
1st
1
2nd
3rd v/v pt.
Base Line is Locus of “Valve
Best Point” (Partial Arc
Admission)
2
20
30
40
50
60
70
VWO
Valve Loop
80
90
100
% VWO Load
Steam Turbine
4. Part Load Operation
56 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
57 / 81
Sliding Pressure Operation
HIoPE
Comparison with Full Throttling
Main Steam
Main Steam
Ahead of First
Stage Nozzles
Ahead of First
Stage Nozzles
After First Stage
After First Stage
Load
[ Full Throttling ]
Steam Turbine
Load
[ Sliding Pressure Operation ]
4. Part Load Operation
58 / 81
HIoPE
Generals for Sliding Pressure Operation

In a sliding pressure operation mode, which is also called as variable pressure operation, the steam flow is
controlled by varying boiler pressure with the main steam control valves are always fully open. Therefore, no
throttling occurs and control stage is not needed.

The steam pressure is proportional to the load, not only within the turbine but also as the inlet to the turbine
(main steam line), and in the steam generator.

Main steam pressure is controlled by the boiler firing rate.

The main advantage of sliding pressure operation is that the main steam temperature remains relatively
constant across the load range which shortens startup times and increases turbine rotor life.

The disadvantages of sliding pressure operation are poorer thermodynamic efficiency and limited load
response capability.

It may require a forced circulation boiler to have a fast response capability.

A sudden load increase is not possible because all control valves are always fully open.

The lower main steam pressures of this operating mode result in less available energy than in the partial arc
admission operation, but more than in the full throttling operation.

Both feed pump power and throttling losses in the turbine control valves can be reduced during the sliding
pressure operation because of lower pressure.
Steam Turbine
4. Part Load Operation
59 / 81
HIoPE
Generals for Sliding Pressure Operation


Sliding pressure operation has a reduced
thermodynamic efficiency during part load
operation because of reduced available energy
caused by reduced boiler pressure.
However, there are two important facts to be
considered in terms of efficiency of plant. Firstly,
boiler feedwater pump power at low load
operation with variable pressure can be
reduced significantly. Secondly, the moisture
loss at low load operation with variable pressure
can be reduced. These two facts contribute to
increase efficiency of the plant.
T
3
c
decrease in wnet
2
increase in wnet
2
1
4
4
increase in qout
a
Steam Turbine
3
4. Part Load Operation
b
b
s
60 / 81
HIoPE
효율변화 분석
결과
이유
효율변화
밸브 교축손실

모든 출력에서 VWO상태이기 때문에 교축손실 매
우 작음

HP Turbine 효율

HP 1단에서 full arc 운전이며, 밸브에서 교축손실
작음

Rankine cycle 효율

부분부하운전에서 사이클 압력 저하로 available
energy 감소


부분부하운전에서 펌프 동력 절감 (출력이 낮아질
수록 보일러 운전압력이 낮아지기 때문에 펌프 동
력 절감 크기 증대)

Items
급수펌프 동력
Steam Turbine
4. Part Load Operation
61 / 81
HIoPE
First Stage Shell Temperatures


Sliding pressure operation decreases the
potential for low cycle thermal fatigue in the
turbine during load changes as compared to
constant initial pressure operation.
In sliding operation, first stage exit
temperature is almost constant over the load
which reduces thermal stress.
First Stage Exit Temperature
Sliding Pressure
100% adm.
75% adm.
62.5% adm.
50% adm.
Sliding Pressure
Partial Arc Admission
Throttle Flow
Steam Turbine
4. Part Load Operation
62 / 81
HIoPE
Comparison of Heat Rate
+4
+3
Single Admission (Full Throttling)
Constant Pressure
Sliding Pressure
2 Admissions
+2
3 Admissions
+1
4 Admissions
0
1st
1
2nd
3rd v/v pt.
Base Line is Locus of “Valve
Best Point” (Partial Arc
Admission)
2
20
30
40
50
60
70
VWO
Valve Loop
80
90
100
% VWO Load
Steam Turbine
4. Part Load Operation
63 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
64 / 81
HIoPE
Hybrid Operation
2400
2000
3
2
1
1000
Variable Pressure Operation Mode
600
1. Full variable pressure with all control valve wide open.
2. Variable pressure with one control valve closed.
3. Variable pressure with two control valves closed.
20
40
60
80
100
% VWO Load
With a partial admission unit, it is attractive to close one or two valves and then vary
pressure with one or two valves closed. This is called as hybrid operation.
Steam Turbine
4. Part Load Operation
65 / 81
HIoPE
Korea Standard 500MW Fossil Power
분류
VWO
MGR
NR
Constant Pressure Operation
75
50
30
Sliding Pressure Operation
출력 (kW)
550,000
(110%)
541,650
(108.3%)
500,000
(100%)
375,000
(75%)
250,000
(50%)
150,000
(30%)
유량 (lb/hr)
3,757,727
(112.7%)
3,684,046
(110.5%)
3,335,116
(100%)
2,389,835
(71.7%)
1,564,131
(46.9%)
980,271
(29.4%)
복수기 압력
(in.Hga)
1.5
1.5
1.5
1.5
1.5
1.5
주증기 온도 (F)
1000
1000
1000
1000
1000
1000
주증기 압력
(psia)
3514.7
(100%)
3514.7
(100%)
3514.7
(100%)
2860.2
(81.38%)
1870.2
(54.47%)
1152.6
(32.79%)
1st STA Bowl P.
(psia)
3409.3
(100%)
[97.00%]
3409.3
(100%)
[97.00%]
3409.3
(100%)
[97.00%]
2774.4
(81.39%)
[97.00%]
1814.7
(54.49%)
[97.03%]
1118.0
(32.79%)
[97.00%]
1st STA Shell P.
(psia)
2630.8
(113.9%)
2573.8
(111.5%)
2309.0
(100%)
1683.5
(72.9%)
1128.0
(48.9%)
723.9
(31.4%)
FWPT 동력 (kW)
18,755
(3.41%)
18,390
(3.40%)
16,611
(3.32%)
9,622
(2.57%)
4,125
(1.65%)
1,523
(1.02%)
Steam Turbine
4. Part Load Operation
66 / 81
HIoPE
Hybrid Operation

The hybrid operation adopts advantages of both sliding pressure operation and partial arc admission through
the combination of partial arc admission operation and sliding pressure operation.

Sliding pressure operation has the advantage of no throttling loss, and partial arc admission operation has
the advantage of fast load response.

At low loads, some of the main steam control valves are wide open and steam flow is controlled by sliding
pressure operation.

The main steam pressure is increased with steam turbine load until the main steam pressure rated
conditions.

The steam turbine load is increased further by maintaining the rated main steam pressure and sequential
opening of the remaining main steam control valve as in the partial arc admission operation.

Starting from full load, load reductions are initially achieved by closing control valves sequentially and
maintaining constant initial pressure.

When a particular valve point is reached, further load reductions are achieved by holding valve position
constant and decreasing initial pressure.

The best point for the change from constant to sliding pressure depends on the boiler characteristics and the
value of design initial pressure.

The optimum point usually occurs at about 50 to 60% load.
Steam Turbine
4. Part Load Operation
67 / 81
Hybrid Operation
Steam Turbine
4. Part Load Operation
HIoPE
68 / 81
HIoPE
Comparison of Heat Rate
+4
+3
Single Admission (Full Throttling)
Constant Pressure
Sliding Pressure
2 Admissions
Hybrid Operation
+2
3 Admissions
1
+1
4 Admissions
2
0
1st
1
2nd
3
3rd v/v pt.
Base Line is Locus of “Valve Best
Point” (Partial Arc Admission)
VWO
Valve Loop
2
20
30
40
50
60
70
80
90
100
% VWO Load
Steam Turbine
4. Part Load Operation
69 / 81
HIoPE
Comparison of Heat Rate

The full throttling has the worst overall heat rate because of throttling losses.

The sliding pressure operation has a slightly better heat rate than full throttling operation, but still has poor
performance because of the lower main steam pressures.

The partial arc admission operation shows the best heat rate at higher loads because of the high main steam
pressures and minimized throttling losses resulting from throttling with only one control valve at a time.

The hybrid operation gives the most efficient operation because the unit life is extended by maintaining
relatively constant temperatures at low loads, reducing cycling effects.

Heat rate for the partial arc admission is typically plotted as a “locus of best valve points,” that is, the line
passing through the heat rate points where any valves open are wide open.

The actual heat rate curve for the partial arc admission is represented by a valve loop that incorporates the
throttling losses associated with a valve throttling between full closed and full open.

Theoretically, the more control valves, the smaller valve loop, the greater the possibility of operating without
throttling, and the better the heat rate.
Steam Turbine
4. Part Load Operation
70 / 81
HIoPE
Modified Sliding Pressure Operation
Modified sliding pressure operation uses throttle valve reserve so the valves are slightly closed at 90% to
95% load and some boiler stored energy can respond more quickly to rapid load change near full load.
• Today, modern power plants are operated in natural
sliding pressure mode or modified sliding pressure mode.
100
Fixed Pressure
Mode
Main Steam Pressure, %
• Primary electrical power response (additional
electrical power within seconds) is produced by
condensate throttling.
75
50
Fixed Pressure Mode
25
• At a certain part load, the control valves can to be
throttled to improve the primary response capacity
0
0
20
40
60
80
100
Steam Turbine Load, %
Steam Turbine
4. Part Load Operation
71 / 81
HIoPE
Effect of Operating Mode on Steam Turbine Design
Steam Turbine
4. Part Load Operation
72 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
73 / 81
Startup System of Steam Power Plants
HIoPE
Boiler-internal startup systems and unit startup system

The purpose of boiler-internal startup systems is to drain off the excess water discharged from the
evaporator and to ensure flow through once-through evaporators.

Excess water is defined as the quantity of water which must be removed from a water-filled heat-exchange
system due to increased volume as a result of steam formation and increased temperature.
Steam Turbine
4. Part Load Operation
74 / 81
HIoPE
Startup System of Steam Power Plants

Natural-circulation boilers only have equipment for removing the excess water from the drum.

During startup of once-through boilers, a circulation flow is combined with the once-through flow in the
economizer and evaporator until the minimum evaporator flow level.

The water that does not evaporate is separated from the steam in HP separators and then fed either to a
recirculation pump, a drain heat exchanger, the feedwater tank, or an atmospheric flash tank.

The unit startup system ensure cooling of the superheater heating surfaces and supplies steam to the
turbine at the required startup pressure and temperature.

In modern power plants, separate turbine bypass systems for the HP and IP/LP casings of the turbine,
respectively, are seeing increased use.

Essential advantages of this system are the result of the uniform heatup of the superheater heating surfaces
– preventing flaking of the protective coating on the inside of the tubes caused by thermal shock and
carryover of corrosion products into the turbine (solid particle erosion) – short startup time, and the same
basic startup procedure for cold, warm and hot starts.

If bypass systems are dimensioned appropriately ( 60% steam flow at full load), the steam generator can be
kept in operation even when there is a turbine-generator load rejection to the station auxiliary power level,
thus in most cases preventing a unit trip.
Steam Turbine
4. Part Load Operation
75 / 81
HIoPE
1
Basics for the Control of Steam Flow
2
Valves
3
Full Throttling
4
Partial Arc Admission
5
Sliding Pressure Operation
6
Hybrid Operation
7
Startup System of Steam Power Plants
8
Load Changes
Steam Turbine
4. Part Load Operation
76 / 81
Load Changes

HIoPE
Sudden loss of other power plants or grid disturbances require step increase of the load in those power
plants still on line of about 2 to 5% within seconds to maintain a stable grid frequency. Since it takes 2 to 3
minutes for increased firing in coal-fired units to achieve the desired electric power output, the storage
capacity of the water/steam system is used for making such step load changes. The various methods
applied vary with regard to dynamic behavior and economics:
1) Opening the control valves: A drop in pressure
upstream of the turbine activates the storage capacity
of the main steam line and the steam generator. The
magnitude of the possible load step increase
depends on the throttle setting on the turbine valves,
while the duration of the load step change depends
on the storage capacity. The storage capacity of a
drum boiler is about 2 to 3 times that of a oncethrough boiler. On the other hand, the permissible
pressure transient is only 6 to 8 bar/min as apposed
to 20 to 30 bar/min for a once-through boiler. The
curve given in the figure depicts throttling
corresponding to 5% of the main steam pressure at
100% load.
Dynamic impact of various measures
taken to activate load reserves
Steam Turbine
4. Part Load Operation
77 / 81
HIoPE
Load Changes
2) Opening of multistage valve: With respect to the load step change, the behavior of the multistage valve is
similar to throttling of the turbine control valves. Compared to throttling of the turbine control valves,
however, the implementation of the multistage valve requires additional investment cost, but on the other
hand provides a lower heat rate in steady-state operation.
3) Condensate throttling: Reducing the condensate flow reduces the extraction-steam flow and therefore
increases the steam turbine output. A steam-side shutdown of the LP feedwater heaters is also possible
and makes the additional output available somewhat more quickly than does the throttling of the
condensate. The heat rate in steady-state operation is not affected by this measure, and only a small
additional investment is required.
4) Shutdown of HP feedwater heaters: This measure can be applied without duration restrictions. Initially,
however, it causes considerable thermal stress in the thick-walled components of the steam generator due
to the large feedwater temperature transients. In normal operation, the heat rate is not affected.
5) Increased injection flow into the steam generator: This measure primarily makes use of the thermal
storage capacity of the heating surfaces and the headers based on a reduction of steam temperature. The
heat rate in steady-state operation is not affected.
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4. Part Load Operation
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Load Changes

HIoPE
During load changes, temperature changes occur in the turbine and in the steam generator which cause
temporary thermal stresses in the thick-walled components and which thus limit the load transients. In the
HP turbine, the temperatures change on constant-pressure mode. In Fig. 3.49, the values are plotted
downstream of the first stage. In sliding-pressure mode, in which no throttling takes place, the temperatures
are nearly constant. Sliding-pressure mode is therefore the more favorable operating mode for the steam
turbine.
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4. Part Load Operation
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Load Changes
HIoPE

The temperature in the steam generator remain nearly constant in constant-pressure mode but change in
sliding-pressure mode due to the pressure-dependent saturated-steam temperature in the evaporator and
the primary superheater region. Moreover, a load increase in sliding-pressure mode as a result of increased
pressure stores energy in the system, which also reduces the load transients. For the drum-type boiler with
its thick-walled drum and the large storage capacity of the evaporator system, constant-pressure mode is
therefore the more favorable operating mode. The once-through boiler is well suited for either operating
mode.

Unit behavior under continuous load changes is determined by the two main components the boiler and the
turbine as well as the control system on the one hand, and by the operating mode on the other. Table 3.11
shows the feasible load transients for these two main components and the resulting unit values.
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4. Part Load Operation
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HIoPE
질의 및 응답
작성자: 이 병 은 (공학박사)
작성일: 2015.02.11 (Ver.5)
연락처: ebyeong@daum.net
Mobile: 010-3122-2262
저서: 실무 발전설비 열역학/증기터빈 열유체기술
Steam Turbine
4. Part Load Operation
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