Start-up of Parallel Turbo Expander-Compressor Units Operating In Hydrocarbon ... · 2018. 12. 1. · • Capacity of cryogenic gas plants have increased over the last forty years

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.


•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)


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


•Due to large capacity many plants have parallel turbo expander-compressor trains


Operational Challenge:

•Simultaneous Start-up

Similar to a single train star-up


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


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


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



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



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



Low Pressure Ratio Compressor P2/P1



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 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


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



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


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.


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


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


Venting the recycle gas to expander discharge piping

Venting Compressor Discharge to a Lower Pressure Sink


More common in Older plants

Venting /Flaring Compressor Discharge Flow


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


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


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%


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


-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


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

Documents

Start-up of Parallel Turbo Expander-Compressor Units Operating In Hydrocarbon ... · 2018. 12. 1. · • Capacity of cryogenic gas plants have increased over the last forty years