8/6/2008 37th Turbomachinery Symposium 1
Start-up of Parallel Turbo Expander-Compressor Units OperatingIn
Hydrocarbon Processing Plants
37th Turbo machinery SymposiumHouston, Texas
September 7-11, 2008
Doug Bird bp Energy CanadaReza Agahi Atlas Copco Gas and ProcessBehrooz Ershaghi Mafi-Trench Co.
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•Capacity of hydrocarbon processing plants have increased since turbo expander-compressor technology was utilized in early 1960’s
EXPANDER FLOW DEVELOPMENT
0 . 0 0
2 0 0 . 0 0
4 0 0 . 0 0
6 0 0 . 0 0
8 0 0 . 0 0
1 0 0 0 . 0 0
1 2 0 0 . 0 0
1 9 6 1 1 9 6 2 1 9 6 7 1 9 6 8 1 9 6 9 1 9 7 0 1 9 7 8 1 9 8 2 1 9 9 2 1 9 9 6 1 9 9 7 1 9 9 9 2 0 0 0 2 0 0 2
Ye a r
Expander Fl ow Li near (Expander Fl ow)
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EXPANDER POWER
0
2 0 0 0
4 0 0 0
6 0 0 0
8 0 0 0
10 0 0 0
12 0 0 0
14 0 0 0
Ye a r
EXPANDER POWER DEVELOPM E NT Li near (EXPANDE R POWER DEVE LOPM ENT )
•Upper limit of installed experience for turbo expander-compressors poweris at 15,000 KW
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•Due to large capacity many plants have parallel turbo expander-compressor trains
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Operational Challenge:
•Simultaneous Start-up
Similar to a single train star-up
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Case Study Reference:
For this case study a bp gas plant with two parallel turbo expander-compressor trains with the following gas dynamics performance was considered:
14.70%Liquid Fraction
7,7257,876HP
615,000706,000Flow (lb./hr)
172- 96T2 (oF)
430350P2(Psia)
107- 6T1 (oF)
2901,080P1(Psia)
18.2319.38Mw
CompressorTurbo Expander
50
55
60
65
70
75
80
85
90
0.0 500000.0 1000000.0 1500000.0
EXPANDER FLOW, lb/hr
EXPANDER
EFFICIENCY,
%
110%100% SP EED90%
80%
70%
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.0 500000.0 1000000.0 1500000.0
EXPANDER FLOW, lb/hr
SHAFT
POWER, hp
110%
100% SP EED
90%
80%
70%
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.0 200000.0 400000.0 600000.0 800000.0 1000000.0
COMPRESSOR FLOW, lb/hrSHAFT POWER, hp
110%
100% SP EED
90%
80%
70%
Surg
e lin
e
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
0.0 200000.0 400000.0 600000.0 800000.0 1000000.0COMPRESSOR FLOW, lb/hr
PRESSURE RATIO, P2/P1
110%
100% SP EED
90%
80%
70%
Surg
e lin
e
cont
rol l
ine
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Case Study Reference:
An efficiency curve for parallel EC could be developed to guide operation for one or two trains in service for the maximum efficiency
Isentropic Efficiency of Expander
0102030405060708090
100
0 50 100 150
% Flow
% E
ffici
ency
1 unit2 units
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Operational Challenge:
Start-up of one unit when the parallel unit is in full load operation:
•Compressor cannot compress directly into discharge header and hence Requires recirculation.
% COMP-Recycle flow
020406080
100120
0 50 100 150
% Mass flow
% re
cycl
e flo
w
% COMP-Recycle
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Operational Challenge:
The following conditions are to be monitored during start up of an EC while recycle valve open:
• Compressor discharge process gas temperature•Axial Loads•Radial Loads
Development of the above conditions depends on pressure ratio of the compressor. Two scenarios will be examined:
•Low Pressure Ratio 1.2
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Operational Challenge:
Low Pressure Ratio Compressor P2/P1
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Operational Challenge:
Low Pressure Ratio Compressor P2/P1
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Operational Challenge:
How Pressure Ratio Compressor P2/P1 >1.20
14.70%Liquid Fraction
7,7257,876HP
615,000706,000Flow (lb./hr)
172- 96T2 (oF)
430350P2(Psia)
107- 6T1 (oF)
2901,080P1(Psia)
18.2319.38Mw
CompressorTurbo Expander
50
55
60
65
70
75
80
85
90
0.0 500000.0 1000000.0 1500000.0
EXPANDER FLOW, lb/hr
EXPANDER EFFICIENCY, %
110%100% SP EED90%
80%
70%
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.0 500000.0 1000000.0 1500000.0
EXPANDER FLOW, lb/hr
SHAFT POWER, hp
110%
100% SP EED
90%
80%
70%
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0.0 200000.0 400000.0 600000.0 800000.0 1000000.0
COMPRESSOR FLOW, lb/hr
SHAFT POWER, hp
110%
100% SP EED
90%
80%
70%
Surg
e lin
e
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
0.0 200000.0 400000.0 600000.0 800000.0 1000000.0COMPRESSOR FLOW, lb/hr
PRES
SUR
E R
ATI
O,
P2/
P1
110%
100% S P E E D
90%
80%
70%
Surg
e lin
e
cont
rol l
ine
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Operational Challenge:
High Pressure Ratio Compressor P2/P1 >1.20
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000
Thrust load, Lb.
Spee
d, R
PM
Radial Load , Lb
050
100150200250300350400
0 10000 20000
RPM
Radi
al lo
ad
Radial Load , Lb
Estimated Recycle Gas Temperature rise
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35
Time, Minute
Tem
pera
ture
, F
P2/P1=1.4
Bearing thrust capacityStart up axial load
Valve close
Valve open
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Estimated Recycle Gas Temperature rise
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35
Time, Minute
Tem
pera
ture
, F
P2/P1=1.4
P2/P1=1.2
Alarm
Comparison of compressor discharge gas temperature for low and high pressure ratio cases.
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Therefore the main operational problem associated with start up parallel turbo expander-compressor units is process gas temperature at the compressor discharge for high pressure ratio compressors
The following remedies may be applied depending on circumstances. All these shall continue until the compressor discharge pressure approaches to compressor discharge header pressure:
•Diluting closed loop warm recycle gas with cold gas
•Venting compressor discharge to a lower pressure sink
•Venting /flaring compressor discharge flow
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Diluting Closed Loop Warm Recycle Gas with Cold Gas
Adding cooling gas from expander inlet to the recycle gas to maintain the discharge temperature below the Alarm Level
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Venting the recycle gas to expander discharge piping
Venting Compressor Discharge to a Lower Pressure Sink
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More common in Older plants
Venting /Flaring Compressor Discharge Flow
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Pros and Cons of the remedies:
• Diluting closed loop warm recycle gas with cold gas:
1. Loss of refrigeration2. Loss of condensate recovery3. Requires cold gas piping, valves, space, etc.
1. No loss of process gas2. No environmental consequences
• Venting compressor discharge to a lower pressure sink1. Loss of refrigeration2. Loss of condensate recovery3. Requires gas piping, valves, space, etc.
1. No loss of process gas2. No environmental consequences
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Pros and Cons of the remedies:
• Venting /flaring compressor discharge flow1. Loss of process gas2. Environmental consequences
1. No piping loop2. Suitable for older plant with no expansion provisions
• Because of loss of process gas and environmental consequences requires special attention
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Three start up alternatives will be evaluated:
Alternate -1 Maintain flow of the operating unit at 100%
Alternate-2 Increase flow of the operating unit to 130%
Alternate-3 Decrease flow of the operating unit 70%
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Turboexpander Performance parameters during start up
speed/design speed
020406080
100120
0 50 100 150
% Mass Flow
Spee
d
% full speed
Compressor pressure ratio
11.11.21.31.41.51.6
0 50 100 150
% Mass Flow
Pres
sure
Rat
io
Comp-P2/P1
Pow er- KW/ per unit
0
2000
4000
6000
8000
0 50 100 150
% Mass Flow
KW Pow er- KW
EXP- wt% liquid
0
5
10
15
20
0 50 100 150
% Mass flow
Wt %
Liq
uid
EXP- w t% liq
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-600
-400
-200
0
200
400
600
800
1000
1200
1400
0 5 10 15 20 25
Poly. (100% Flow)
Poly. (70% Flow)
Poly. (130% Flow)
Cumulative Economical Consequences
Duration of start up, min
$ 1,
000
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Conclusions:
• Capacity of cryogenic gas plants have increased over the last forty years • There are many cryogenic natural gas processing plants with parallel
turbo expander-compressor units• Start up of one train EC while the other is in operation is an operational
challenge• Compressor gas recycling is mandatory during start up• Several operational parameter of EC are to be monitored. Compressor
discharge gas temperature is the critical parameter to be controlled• Two scenarios were considered, low pressure ratio (1.2) compressor• The former scenario does not impose any operational challenge during
start up• The latter scenario demands special attention and procedures
1. The most convenient method is to vent the recycle flow into the expander discharge stream
2. The compressor recycle gas may be cooled down by diluting it with cold gas from the expander discharge
3. If neither of the above is possible, the only remedy is to vent/flare gas- The most economical approach is to increase flow of the
operating EC to 130% and then start up the parallel EC