Coker A.K. Fortran Programs for Chemical Process Design, Analysis, and Simulation

Подождите немного. Документ загружается.

Compressors 447

low TDH impellers have high specific speeds. At a particular head and

capacity, a pump of low specific speed will operate safely at lower NPSH

than one with a high specific speed. Designers should ensure that the

capacity or TDH is increased when recommending speeding up an

existing pump. This will enable that the new NPSH does not exceed

what is available.

DH0.25

D~ = Q0.5 (6-90)

Figure 6-4 shows the dimensionless parameters as presented by Balje.

The figure is the graphical combination of Equations 6-89 and 6-90.

Nomenclature

Bhp = brake or shaft horsepower, bhp

C = constant

D = impeller diameter or rotor, ft

del. pres. = pump delivery pressure, psia

D~- specific diameter, ft

EFF = pump efficiency

gpm = fluid flow rate through pump

H = head, (ft lb)/lb

HBhp = hydraulic brake horsepower required by pump (pump

efficiency = 1.0 for HBHP calculation)

L. loss = pressure drop due to piping, psi or ft

orig. pres. = vessel pressure on pump suction, psia

pump dish. pres. = pump discharge pressure, psia

Q = Volumetric flowrate, gpm

vap. pres. = vapor pressure of pumped fluid at pumping tempera-

ture, psia

suc. pres.- pump suction pressure, psia

S.Hd. = static head of liquid on pump suction or discharge, ft

total pump AP = pressure rise across pump, psi or ft

PROBLEMS AND SOLUTIONS

Problem 6.1

A centrifugal compressor is to be specified for a gas plant. The unit is to

compress 18,000 lbm/hr of gas mixture at 50 psia and 150~ to 200 psia.

The gas mixture consists of 30% hydrogen

(H2),

45% methane (CH4),

448 Fortran Programs for Chemical Process Design

u) c'-

~IJJ

Cj~

e-<

t-.O_

o'6

~rn

~ I-- --'-~.

• r-- .

448

Compressors 449

15% ethane

(C2H6) ,

7% propane

(C3H8) ,

and 3% n-butane (n-

C4H10 ).

Assuming a polytropic efficiency of 75%, and the average compress-

ibility factor, Zavg -- 0.95, calculate the polytropic head, total break horse-

power, and the discharge temperature. Table 6-4 gives the properties of

the gas mixture.

The mixture specific heat ratio, k, is defined by

k m

MCp

MC v

MCp -

1. 986

k m

10.317

8.331

k - 1.238

The average molecular weight, M w - 17.162

Solution

The computer program PROG61 calculates the polytropic and adia-

batic heads of a compressor. Table 6-5 shows the input data and results of

the gas mixture. The gas discharge temperature is 310.4~ and the com-

pressor polytropic head is 76874 ft with a total break horsepower =_ 960 hp.

Table 6-4

Properties of the Gas Mixture

Gas

H 2

CH4

C2H 6

C3H8

nC4Hl0

Mole

fraction

0.30

0.45

0.15

0.07

0.03

Molecular

weight

2.01

16.04

30.07

44.09

58.12

Mw =

yMw psia

0.603 188.1

7.218 666.0

4.511 707

3.086 616

1.744 551

17.162

MCp

Btu

lb,,,, mol. ~ R ~

Tc (~ @ 150~

60.2 6.94

343 8.95

550 13.78

660 19.52

765 25.81

MCp=

yMCp

@ 150~

2.082

4.028

2.067

1.366

0.774

10.317

450

Fortran Programs for Chemical Process Design

Table 6-5

Input Data and Computer Output of Polytropic

Compressor Calculations

DATA61.DAT

POLYTROPIC

18000.0 50.0 200.0

80.0 1.238 17.162

0.95 0.75

POLYTROPIC COMPRESSOR CALCULATION

TYPE OF SERVICE:

GAS FLOWRATE, ib/hr.:

SUCTION PRESSURE,

psia:

DISCHARGE PRESSURE,

psia:

COMPRESSION RATIO:

INLET TEMPERATURE,

oF:

RATIO OF SPECIFIC HEAT CAPACITIES, k:

MOLECULAR WEIGHT OF GAS, ib/ib-mole:

AVERAGE COMPRESSIBILITY FACTOR,

POLYTROPIC EFFICIENCY:

POLYTROPIC HEAD,

ft:

GAS DISCHARGE TEMPERATURE,

oF:

GAS HORSEPOWER,

hp:

MECHANICAL LOSSES,

hp:

TOTAL BRAKE HORSEPOWER,

hp:

WORK DONE, ib-mole/hr.:

POLYTROPIC

18000.000

50.000

200.000

4.000

80.000

1.238

17.162

.9500

.75

76873.770

310.404

931.803

27.954

959.757

735.634

Problem 6.2

Size a centrifugal pump with a 4-inch suction nozzle, and a 3-inch

discharge nozzle that handles a gas oil at normal flow rate of 250 gpm

through piping and component's system as shown in Figure 6-5, at the

following conditions, temperature t = 555~ specific gravity = 1.04,

and viscosity of the gas oil ~t = 0.6 cE Table 6-3 gives the details on a

pump calculation sheet. Other piping details are shown in Table 6-6.

I 13 PSIG

14'

13.5

PSIG

35'

[

Figure 6-5. Gas oil transfer pump system.

Compressors

451

Table 6-6

Piping Details and Fittings

Schedule 40

Normal size, inch

Inside diameter, inch

Actual pipe length,ft

Fittings

Long radius 90 ~ elbows

Reducer, inch

Gate valve

Entrance

exit

Suction Line

6.065

4.026

Discharge Line

4.026

156

3.068

Solution

The line pressure drops for the suction and the discharge can be cal-

culated by the method described in Chapter Three.

The total suction pressure drop is:

Total suction AP~uct = 5.40 + 27.7 - 0.23

32.87 psi

Total discharge APdisch

"--

28.2 + 14.86 + 10.0 + 5.0 + 2.0 + 4.38

= 64.44 psi

Differential pressure AP - APdisch

APsuct

= 64.44 - 32.87

= 31.57 psi

The brake horsepower Bhp -

(Q, gpm)( AP, psi)

(1715)(EFF)

(250)(31.57)

(1715)(0.7)

= 6.57 hp

~ 7.0 hp

452

Fortran Programs for Chemical Process Design

The computer program PROG62 sizes the centrifugal pump for the given

flow rate and fluid characteristics. The program calculates the hydrau-

lic brake horsepower required by the pump and the actual brake horse-

power. In addition, the program computes the available net positive

suction head (NPSH) and the pump efficiency. Table 6-7 shows the

input data and results of the pump hydraulic design calculation. The

available NPSH is 64 ft, and the actual brake horsepower required for

the pump operation is 7.0hp, with a pump efficiency of 68%.

Table 6-7

Centrigugal Pump Sizing- Program Inputs and Outputs

DATA62.DAT

GAS-OIL

555.0 0.6 1.04

4.0 250.0

27.7 0.22 12.0

28.2 33.0 i0.0

5.0 2.O 0.0

4.38 0.0 0.0

CENTRIFUGAL PUMP SIZING

FLUID NAME:

FLUID OPERATING TEMPERATURE, oF:

FLUID VISCOSITY,

cP:

FLUID SPECIFIC GRAVITY:

FLUID VAPOR PRESSURE,

psia:

VOLUMETRIC FLOWRATE:,

gpm:

PRESSURE AT EQUIPMENT,

psia:

SUCTION LINE LOSSES, psi:

OTHER LOSSES (e.g. STRAINER/FILTER), psi:

SUCTION STATIC HEAD, ft.:

SUCTION STATIC HEAD, psi:

TOTAL PUMP SUCTION PRESSURE,

psig:

TOTAL PUMP SUCTION PRESSURE,

psia:

AVAILABLE NET POSITIVE SUCTION HEAD, ft.:

DELIVERY PRESSURE, psia.:

DISCHARGE STATIC HEAD, ft.:

DISCHARGE STATIC HEAD, psi:

CONTROL VALVE PRESSURE DROP, psi.:

EXCHANGER PRESSURE DROP, psi.:

ORIFICE PRESSURE DROP, psi.:

FURNACE PRESSURE DROP, psi.:

DISCHARGE LINE LOSSES, psi.:

OTHER LOSSES, psi.:

TOTAL PUMP DISCHARGE PRESSURE,

psig:

TOTAL PUMP DISCHARGE PRESSURE,

psia:

TOTAL PUMP PRESSURE DROP, psi.:

TOTAL PUMP PRESSURE DROP, ft.:

HYDRAULIC BRAKE HORSEPOWER REQUIRED BY THE PUMP,

bhp:

ACTUAL BRAKE HORSEPOWER REQUIRED FOR PUMP OPERATION

bhp:

PUMP EFFICIENCY, (%):

GAS-OIL

555.000

.6000

1.040

4.000

250.000

27.700

.220

.000

12.000

5.403

18.183

32.883

64.153

28.200

33.000

14.857

i0.000

5.000

2.000

.000

4.380

.000

49.737

64.437

31.555

70.O88

4.600

6.809

67.557

Compressors

453

Problem 6.3

A 6-inch pump delivers the following heads when rotating at 1750

rpm and at varying flow rates as shown in Table 6-8. For a geometri-

cally similar 8-inch pump running at 1450 rpm, calculate"

1. the heads developed by the 8-inch pump.

2. the brake horsepower for the flow rates.

3. the pump efficiencies for the 6- and 8-inch pumps.

4. develop the characteristic curves for the 6-inch and the 8-inch

pumps.

Solution

Using the affinity law (Equation 6-76), the corresponding values for

the head at varying flow rates are shown in Table 6-8. The computer

program PROG63 calculates both the brake horsepower and pump effi-

ciencies of the 6- and 8-inch pumps. Tables 6-9 and 6-10 show the input

data and the computer results of these pumps. The characteristic curves

are shown in Figure 6-6.

Table 6-8

Total Heads and Flow Rates for the 6-inch

and 8-inch Centrifugal Pumps

6 inch @ 1750 rpm 8 inch @ 1450 rpm

e (gpm) H (ft) Q (gpm) H (ft)

0 160 0

100 158 100

200 155 200

300 152 300

400 146 400

500 140 500

600 132 600

700 118 700

800 104 800

900 84 900

1000 60 1000

195.3

192.8

189.2

185.5

178.2

170.9

161.1

144.0

126.9

102.5

73.2

454 Fortran Programs for Chemical Process Design

200

180

160

140

120

100

0 100 200 300 400 500 600 700 800 900 1000

FLOWRATE

gpm

A 8" EFFICIENCY

8" HEAD

BHP

6" HEAD

,~ 6" EFFICIENCY

~6" BHP

Figure 6-6. Characteristic curves of the 6-inch and the 8-inch centrifugal pumps.

Table 6-9

Efficiency and Brake Horsepower Calculations for the 6-inch

Centrifugal Pump Program Inputs and Outputs

DATA63.DAT

0~ 160.0

i00.0 158.0

200.0 155.0

300.0 152.0

400.0 146.0

500.0 140.0

600.0 132.0

700.0 118.0

800.0 104.0

900.0 84.0

i000.0 60.O

EFFICIENCY AND BRAKE HORSEPOWER CALCULATIONS

FLOWRATE gpm HEAD ft. EFFICIENCY % BRAKE HORSEPOWER

Bhp

.0 160.0 .0 .0

i00.0 158.0 52.4 7.6

200.0 155.0 56.3 13.9

300.0 152.0 59.4 19.4

400.0 146.0 62.2 23.7

500.0 140.0 64.4 27.4

600.0 132.0 66.2 30.2

700.0 118.0 67.8 30.8

800.0 104.0 69.0 30.5

900.0 84.0 70.4 27.1

1000.0 60.0 72.3 20.9

Compressors 455

Table 6-10

Efficiency and Brake Horsepower Calculations for the 8 inch

Centrifugal Pump Program Inputs and Outputs

DATA63.DAT

0.0 195.3

i00.0 192.8

200.0 189.2

300.0 185.5

400.0 178.2

500.0 170.9

600.0 161.1

700.0 144.0

800.0 126.9

900.0 102.5

i000.0 73.2

EFFICIENCY AND BRAKE HORSEPOWER CALCULATIONS

FLOWRATE gpm HEAD ft. EFFICIENCY % BRAKE HORSEPOWER Bhp

9 0 195.3 .0 .0

I00.0 192.8 49.6 9.8

200.0 189.2 53.8 17.8

300.0 185.5 57.3 24.5

400.0 178.2 60.3 29.8

500.0 170.9 62.7 34.4

600.0 161.1 64.5 37.8

700.0 144.0 66.2 38.5

800.0 126.9 67.4 38.0

900.0 102.5 68.9 33.8

i000.0 73.2 71.0 26.1

References

1. Lapina, R. E, "Compressor Performance, How Changes in Inlet Con-

ditions Affect Efficiency,"

Chem. Eng.,

July 1990, pp. 110-113.

2. Engineering Data Book,

Vol. 2, Gas Processors Suppliers Associa-

tion, 10th Ed., 1987.

3. Perry, R. H.,

et. al., Perry's Chemical Engineers'Handbook,

6th

Ed., McGraw-Hill Book Company, 1984.

4. Branan, C.,

The Process Engineer's Pocket Handbook,

Gulf Pub-

lishing Company, 1976.

5. Escoe, A. K.,

Mechanical Design of Process Systems,

Vol. 2, Gulf

Publishing Company, 1986.

6. Garay, E N.,

Pump Application Desk Book,

The Fairmont Press,

Inc., 1990.

7. McGuire, J. T.,

Pumps for Chemical Processing,

Marcel Dekker

Inc., New York, 1990.

456

Fortran Programs for Chemical Process Design

8. McNaughton, K. J. and the Staff of Chemical Engineering, The

Chemical Engineering Guide to Pumps, McGraw-Hill Publications

Co., N.Y., 1984.

9. Schiavello, B., "Troubleshooting Centrifugal Pumps," Chem. Eng.

Prog., Nov. 1992, pp. 35-42

10. Hawks, J., "Process Improvement and Pumps," Chem. Eng. Prog.,

Nov. 1992, pp. 26-30.

11. Hughson, R. V., "Programs Predict Pump Performance," Chem. Eng.

Prog., Nov. 1992, pp. 43-46.

12. Stepanoff, A. J., Centrifugal and Axial Flow Pumps, John Wiley &

Sons, Inc., New York, 1948.