Flexible Operation
Sliding pressure Operation
Sliding Pressure Operation: Some Related
Facts………..
• Sliding pressure operation improves the heat rate
and APC: extent and how…..
• Heat rate deteriorates with throttle pressure
reduction
• Boiler metal temperature (Superheater) increases
with sliding pressure operation
• SH sprays increases with sliding pressure operation
• Sliding pressure operation results in lower MS
pressure and there is a probability of steaming in
economizer
• Extent of throttle pressure reduction: limiting factor
• Test Studies carried out in 210 MW and 500 MW unit
• Sliding pressure operation study: 210 MW unit
Sliding pressure operation study: 210 MW unit
Description
Unit
Desig
n
Test 1
Test 2
Load
MW
210
207.4
207.2
Throttle pr
Kg/cm2
150
151.2
138.1
HRH temp
C
537
540.8
542.7
SH spray
T/hr
0
34.2
48.2
RH spray
T/hr
0
1.4
7
HPCV position
%
65
~75
FG temp - AH out
C
137
122
119
GTCHR
Kcal/kwh
1985
2028
2026
BFPpower cons
KW
5396
5130
Draft power cons
KW
4784
4771
Boiler maximum metal temp
C
552.3
551.7
Unit heat rate
Kcal/kwh 2274
2384
2382
APC
%
7.73
7.6
Net Heat Rate
Kcal/kwh
2584
2577
•Unit with adequate margin in HPCV at
full load
•Study carried out at full load at
reduced throttle pressure
•Throttle Pressure reduced by 13 ksc
•No change in GTCHR and unit heat
rate
•APC reduction by 0.13% primarily
due to reduction in BFP power (266
KW)
•Net HR improved by 7 kcal/kWh
•SH spray increased by 14 T/hr
•No change in boiler metal temp
excursion
Sliding pressure operation study: 500 MW unit
Description
Unit
Test 4
349.5
347.8
152.5
170.3
114
540.
539.9 6
563.
565.9 0
115.6
116.2
540.3
541.8
566.0
563.3
0
117
108
113
72.6
67
53.3
60.5
75.9
1932
2103
56.7
211
1
2117
2120
31.6
37
40.4
48.7
341.2
350
352.3
370.8
7
16.2
2408
2412
MW
Load
500
Kg/cm2
Throttle pr
1st Stg
Pressure
170
Kg/cm2
114.5
C
MS temp
537
C
HRH temp
SH spray
TDBFP extn flow
537
T/hr
T/hr
Corr GTCHR
Kcal/kwh
Temp gap: Sat temp-Eco outlet
C
Desuperheater outlet temp
C
Temp gap: DS outlet – Sat
temp
Unit heat rate
C
Test
Test 1 2
347.
350.6 1
139.
131.7 7
Test 3
Design
4.8
Kcal/kwh
2274
2392
8
240
2
•Throttle Pressure reduced by 40
ksc during part load
•GTCHR improved by 17
kCal/kWh and unit heat rate
improved by
20kcal/kWh
•SH spray increased by 44 T/hr
•No change in RH spray
•TDBFP extraction steam flow
reduced by 23T/hr
•Desuper heater outlet temp
reduced and temp gap with sat
temp also reduced
Improvement in Heat rate by Sliding Pressure Operation
HR deterioration due to reduced throttle pr increase in
latent heat
•HR improvement due to reduction in HP control valve
throttling losses
•Improvement in APC (with MDBFP) due to reduction in
BFP power consumption
•Improvement in HR (with TDBFP) due to reduction in
extraction steam flow to TDBFP reduction in BFP
power consumption
•Exact impact: interplay of all the above factors
•Exact impact assessment through performance tests
and through thermodynamic simulation
•Other Impacts:
Almost constant temp at HPT inlet; With constant throttle
pr, throttling increases and MS temp at turbine inlet
decreases
Sliding Pressure Operation
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Sliding Pressure Operation: Limitations…….
SH spray increases with throttle pressure reduction (Reduced steaming
capacity of furnace - increased latent heat of evaporation - reduced
steam flow through SH – increased SH temp - higher SH spray)
Increased SH spray: Lower MS temp at desuperheater outlet –
possibility of two phase flows in platen SH
Lower drum pressure – lower saturation temp – possibility of steaming in
Economiser
Need of keeping margin for increasing the load by
5% at any time: full sliding pressure operation
(HPCV wide open) not permitted
Gradual reduction in throttle pressure (in steps) monitoring closely DS
outlet temp
Limiting throttle pressure: Reduced throttle pressure with margin to
increase the load by 5% (~Operating Throttle pressure: 1st stage
pressure
plus 15 KSC for 500 MW)
Lower thermal stresses in the turbine during load changes.
• Control range of RH temp is extended.
• Reduced pressure level at lower loads prolongs the life span of the components.
• Overall reduction in power consumption and improved heat rate.
•