compressors

DELIVERING KNOWLEDGE. DEVELOPING COMPETENCE.

®

Compressors

Section 7

© 2011 Dr. Omar Barkat & PetroSkills, LLC. All rights reserved.

Field Compression Station


Compressors

Increase Pressure of Gas

By Decreasing Volume

T V

P

T

P V


Compression Ratio

RT = Overall Compression Ratio

Pd = Discharge Pressure, psia

Ps = Suction Pressure, psia

r = Compression Ratio Per Stage

(Sometimes Denoted as R or Rs)

n = Number of Stages

T d sR = P / P

1/n

d sr = P / P


Temperature Increase

k = Isentropic Coefficient (Heat Capacity Ratio)

Td = Discharge Temperature R

Ts = Suction Temperature R

MCp = Molar Heat Capacity

Constant Pressure, BTU / # mol / R

a - Depends on the Type of Compressor

a = 1.0 for Reciprocal (Positive Displace)

a = 1.25 for Centrifugal

a k-1

k

d sT = T r

p

p

MCk =

MC - 1.99

Thermodynamic

Property of Gas


Molar Heat Capacity: MCp

Gas

Chemical

formula

Mol

wt 0F 50F 60F 100F 150F 200F 250F 300F

Methane CH4 16.042 8.233 8.414 8.456 8.651 8.947 9.277 9.638 10.010

Ethyne (Acetylene) C2H2 26.036 9.683 10.230 10.330 10.710 11.130 11.540 11.880 12.220

Ethene (Ethylene) C2H4 28.052 9.324 10.020 10.160 10.720 11.400 12.080 12.750 13.140

Ethane C2H6 30.068 11.44 12.170 12.320 12.950 13.770 14.630 15.490 16.340

Propene (Propylene) C3H6 42.078 13.63 14.690 14.900 15.750 16.800 17.850 18.870 19.890

Propane C3H8 44.094 15.64 16.880 17.130 18.170 19.520 20.890 22.250 23.560

1-Butene (Butylene) C4H8 56.104 17.96 19.590 19.910 21.170 22.710 24.250 25.700 27.150

cis-2-Butene C4H8 56.104 16.54 18.040 18.340 19.540 21.040 22.530 24.000 25.470

trans-2-Butene C4H8 56.104 18.84 20.020 20.500 21.610 22.990 24.370 25.720 27.060

iso-Butane C4H10 58.120 20.40 22.150 22.500 23.950 25.770 27.590 29.390 31.110

n-Butane C4H10 58.120 20.80 22.380 22.710 24.070 25.810 27.540 29.230 30.900

iso-Pentane C5H12 72.146 24.93 27.160 27.610 29.420 31.660 33.870 36.030 38.140

n-Pentane C5H12 72.146 25.64 27.610 28.010 29.700 31.860 33.990 36.070 38.120

Benzene C6H6 78.108 16.41 18.380 18.750 20.460 22.460 24.460 27.080 29.710

n-Hexane C6H14 86.172 30.17 32.780 33.300 35.360 37.910 40.450 42.910 45.360

n-Heptane C7H16 100.198 34.96 38.000 38.610 41.010 43.970 46.930 49.770 52.600

Ammonia NH3 17.032 8.516 8.5180 8.519 8.521 8.523 8.525 8.527 8.530

Air 28.966 6.944 8.9510 6.952 6.960 6.973 6.990 7.009 7.033

Water H2O 18.016 7.983 8.0060 8.010 8.033 8.075 8.116 8.171 8.226

Oxygen O2 32.000 6.970 6.9970 7.002 7.030 7.075 7.120 7.176 7.232

Nitrogen N2 28.016 6.951 6.9540 6.954 6.956 6.963 6.970 6.984 6.998

Hydrogen H2 2.016 6.782 6.8560 6.871 6.905 6.929 6.953 6.965 6.977

Hydrogen sulfide H2S 34.076 8.000 8.0910 8.109 8.180 8.270 8.360 8.455 8.550

Carbon monoxide CO 28.010 6.852 6.9570 6.958 6.963 6.975 6.986 7.007 7.028

Carbon dioxide CO2 44.010 8.380 8.6980 8.762 9.004 9.282 9.559 9.810 10.050


Too Hot is Too Bad

Pressure Rating of Steel Derated at T > 250F

Non-Metallic Compressor Parts (Packing /

Seals) Fail at 250 to 300F

Steel in Compressor OK to 350F

Limit Temperature to < 300F

Zulkeefal Dar

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Fin – Fan Cooler

TIN

TOUT

t IN

t OUT


Fin – Fan Coolers

Ti = Gas Inlet Temperature To = Gas Outlet Temperature

ti = Ambient Air Temperature to = Outlet Air Temperature

Approach Temperature: Discharge Gas Temperature Versus

Inlet (Ambient) Air Temperature

Designed to Get Within 10F

Usually Can Reach Within 20

– If Air Temp < 100F, Gas can be Cooled to ± 115 – 120F so Comp Ratio 3.5

OK

Sophisticated Coolers

– Variable Air Exhaust Vanes (Louvers)

– Operated Manually or Pneumatically Based on To

– Water Cooling Towers Sometimes Added

Hot Climates: Tair > 105F to 140F: Consider Limiting

Compression Ratio to 2.5 – 3.0


Suction Scrubbers and Interstage Scrubbers

Liquid Must be Removed Prior to

Compression

– Close Clearance Between Piston and Cylinder

– Cooling Liquids: Add Scrubber

Scrubber:

– Single Phase Separator

– No Internal Devices

First Scrubber: “Suction Scrubber “

Others: “Interstage Scrubbers”


Compressor Flow Diagram


Solution: Compressors Exercise

Produced Gas Compressed from Suction P of 25 psia

to a Sales Line Pressure of 1000 psia.

Determine Number of Stages Required

Compress Ratio of Each Stage = (40) 1/n

for 1 stage: n = 1 (40) 1 = 40

for 2 stages: n = 2 (40) ½ = 6.32

for 3 stages: n = 3 (40) 1/3 = 3.42

1000 psiaOverall Compression Ratio = 40

25 psia


Compressor Flow Diagram: Shrinkage

Using a 2.2 compression ratio results in the following pressures if No P between Stages:

(50 psig) 64.7 psia 142 psia 313 psia 689 psia 1515.7 psia (1500 psig)

If Shrinkage Due to Cooling and Scrubbing Decreases Volume 3% between stages:

(50 psig) 64.7 psia 142 psia 138 304 295 648 629 1383 1341 (1327 psig)

Final Pressure Too Low: Therefore INCREASE Compression Ratio + 3%

Illustrate Your Compression Flow Scheme:

Zulkeefal Dar

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Compressor Skid


Types of Compressors

Positive Displacement

Type

Dynamic Type

Continuous Flow

Thermal

Type

Reciprocating EjectorsRotary Axial FlowRadial Flow

(Centrifugal)

Mixed Flow HIGH VELOCITY

JET

Entrains

Inflow Gas

PISTON - CYLINDER CASE – Rotating Element

HIGH SPEED

SLOW SPEED

Straight Lobe

Helical Lobe

(Screw)

Sliding Vane

Liquid-Ring

Diaphragm


Reciprocating Compressor

POSITION 1 POSITION 2

POSITION 3 POSITION 4


Two Stage Compression

Volume Reduction due to

Cooling and Scrubbing

between stages


Integral Reciprocating Compressor

(Usually Slow Speed: 200 to 600 rpm)


Slow Speed Reciprocating Compressors

Slow Speed (200 to 600 rpm) Integral

Size:

– Common 2000 to 4000 BHP (Up to 15,000)

– (Very Slow Speed: 140 to 360 BHP Skid Mounted)

– 2 to 10 Cylinders are Common

– Flowrates up to 5 Mmacfd

– 20 psi to 30,000 + psi

– ( > 5000 psi Requires Special Design)

Advantages:

– Reliability 99% Up Time

– Efficient Over Wide Range (90%)

– Long Operating Life – 10 yrs Without Overhaul

30 to 40 + Year LIFE

– High Flowrates and Pressures

– Cheaper than Centrif if BHP < 2000

Disadvantages:

– High Initial Cost

– Large, Difficult to Move

– Large, Solid, Heavy Foundation

– Vibration and Pulsation Dampening

– Cannot Handle Liquids

Commonly Used in Plants and on Transmission Lines

Zulkeefal Dar

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Zulkeefal Dar

Highlight


High Speed Reciprocating Compressors

High Speed: 900 to 1200 RPM Separable

Size:

– 1 to 2000 BHP

– 2, 4, 6 Cylinders are Common

– 0 psi to 2000 psi Pd

– Rates up to 5 MMacfd

Advantages:

– Can be Skid Mounted

– Self Contained, Easily Moved

– Lower Initial Cost

– Flexible Capacity Range

Disadvantages:

– Only 95% Up Time

– Daily Maintenance

– 4 to 6 Years Between Overhauls

– 25 Year Life



Separable: Compressor Frame Coupled to Driver




Type

Dynamic Type

Continuous Flow

Thermal

Type


(Centrifugal)


JET

Entrains

Inflow Gas


HIGH SPEED

SLOW SPEED

Straight Lobe

Helical Lobe

(Screw)

Sliding Vane

Diaphragm


Straight – Lobe Rotary Compressor

Suction Pressure Discharge Pressure


Screw Compressor


Sliding Vane Rotary


Sliding – Vane Rotary


No Valves

Ports Sized When Made

Size: 50 to 500 BHP

– Mostly < 125 BHP

– Pd Up to 400 psi

– Usually < 200 psi)

– Rates Up to 4 Mmacfd

Advantages:

– Good Vacuum Service

– No Pulsation

– Smaller Space / Weight

– Inexpensive for Vapor Recovery

Disadvantages:

– Clean Gases Only

– Uses 10 x Oil vs. Reciprocating

(Use After – Cooler and Separator to Recycle)


Sliding Vane Rotary Compression

Excellent for Vapor Recovery Unit




Type

Dynamic Type

Continuous Flow

Thermal

Type


(Centrifugal)


JET

Entrains

Inflow Gas


HIGH SPEED

SLOW SPEED

Straight Lobe

Helical Lobe

(Screw)

Sliding Vane

Liquid-Ring

Diaphragm


Axial – Flow Dynamic Compressor

Gas Flow through the

Stators and Rotors


Axial Compressors Blades

SINGLE STAGE BLADES

MULTI-STAGE BLADES


Axial – Flow Dynamic Compression

Gas Flows Parallel to Shaft

Energy Transferred by Row of Blades

– One Set Rotates

– One Set Stationary

High Flowrate – High Speed

– (No Vortex Action)

Rates Up to 1 BaCFD

Low Pd < 100 psig


Axial – Flow Dynamic Compressor




Type

Dynamic Type

Continuous Flow

Thermal

Type


(Centrifugal)


JET

Entrains

Inflow Gas


HIGH SPEED

SLOW SPEED

Straight Lobe

Helical Lobe

(Screw)

Sliding Vane

Liquid-Ring

Diaphragm


Centrifugal Compressors

Radial Flow


Ga

s O

UT

Gas IN

CompressorElectric Motor

Gear Box

Single-Stage Centrifugal Compressor


Centrifugal Surge


Centrifugal Choked Flow




Type

Dynamic Type

Continuous Flow

Thermal

Type


(Centrifugal)


JET

Entrains

Inflow Gas


HIGH SPEED

SLOW SPEED

Straight Lobe

Helical Lobe

(Screw)

Sliding Vane

Liquid-Ring

Diaphragm


Ejector Compressors

Forces High Pressure Gas Through Nozzle

Creates High Pressure Jet Across Suction

Kinetic Energy (Velocity) Converted to

Pressure Inside Diffuser

Main Use: Compress from Vacuum to Small

Positive Pressure


MAIN USE: Compress from Vacuum to Small Positive Pressure


Capacity Control – Reciprocating

Problem: Less Gas Available than Required

Control Mechanisms– Reduce Driver Speed

– Recycle a Portion of the Gas

(Often Used on Start-up)

– Valve Lifters (Unloaders)

– Alter Clearance

Variable Clearance Cylinder Heads

Cylinder Head Pocket (with Fill Plug)

Clearance Unloaders

Auto – Pockets

Mechanical Adjustment

– Automatic Start–Stop Control: < 100 BHP

Step Control: > 100 BHP


Reciprocating Compressor Control

Problem: Capacity More than Needed

Variable Volume

Clearance


Reciprocating Compressor Control

Auto-

Pocket


Volume Bottle Sizing Methods

1. Consult Manufacturer or Design Engineering Firm

2. Estimate by Following Rule of Thumb:

a) Calculate Displacement Volume of Single Stroke = Vd

b) Choose Suction Bottle Volume = (Vd) x (Suction Multiplier)

c) Choose Discharge Bottle Volume = (Vd) x (Discharge Multiplier)

Estimate Multipliers from Chart below

If More than One Cylinder is Connected to a Single Bottle: Sum

the Displacement Volumes


Compressor Brake Horsepower

Rule of Thumb Method

BHP = 22 (R Per Stage) (# of Stages) (MMsCFD) F

F Corrects for Interstage Cooling– = 1.00 for Single – Stage

– = 1.08 for Two – Stage

– = 1.10 for Three – Stage

Centrifugal Compressors: Add 18%

High Speed Compressors May Require Up to 20%

More BHP– (Check with Manufacturer)

r < 2.5 and SGg > .65 Will Require Less BHP

– Use 20 vs. 22 if SG > .8

– Use 17 vs. 22 if r < 2.0


Compressor Selection Guide


Centrifugal Advantages

Lower Installed Cost

Lower Maintenance Expense

Great Dependability and “Uptime”

Less Operating Attention

Greater Capacity for Size and Weight

Can Couple with High Speed Low

Maintenance Drivers

Not for Low Q and High ΔP


Reciprocating Advantages

Greater Range of Capacities and Pressures

– Low Q even with High ΔP

More Efficient

Lower Power Cost

Can Deliver Higher Pressure

– (30,000 psi vs. 10,000 psi)

Can Handle Low Flowrate

Flexible with Changing Gas Compositions


®

Appendices 7

Section 7


®

Appendix 7A

Reciprocal Compressor

Parts and Valves

Other Considerations


Reciprocating Compressors: TroubleShooting

Trouble Probable Cause(s) Trouble Probable Cause(s)

Compressor Will Not

Start

1. Power supply failure.

2. Switchgear or starting panel.

3. Low oil pressure shut down switch.

4. Control panel.

Packing

Overheating

1. Lubrication failure.

2. Improper lube oil and/or insufficient lube rate.

3. Insufficient cooling.

Excessive Carbon

on Valves

1. Excessive lube oil.

2. Improper lube oil (too light, high carbon residue).

3. Oil carryover from inlet system or previous stage.

4. Broken or leaking valves causing high temperature.

5. Excessive temperature due to high pressure ratio

across cylinders.

Motor Will Not

Synchronize

1. Low voltage.

2. Excessive starting torque

3. Incorrect power factor.

4. Excitation voltage failure.

Low Oil Pressure 1. Oil pump failure.

2. Oil foaming from counterweights striking oil

surface.

3. Cold oil.

4. Dirty oil filter.

5. Interior frame oil leaks.

6. Excessive leakage at bearing shim tabs and/or

bearings.

7. Improper low oil pressure switch setting.

8. Low gear oil pump by-pass/relief valve setting.

9. Defective pressure gauge.

10. Plugged oil sump strainer.

11. Defective oil relief valve.

Relief Valve

Popping

1. Faulty relief valve.

2. Leaking suction valves or rings on next higher

stage.

3. Obstruction (foreign material), rags), blind or valve

closed in discharge line.

High Discharge

Temperature

1. Excessive ratio on cylinder due to leaking inlet

valves or rings on next higher stage.

2. Fouled intercooler/piping.

3. Leaking discharge valves or piston rings.

4. High inlet temperature.

5. Fouled water jackets on cylinder.

6. Improper lube oil and/or lube rate.Noise in Cylinder 1. Loose piston.

2. Piston hitting outer head or frame end of cylinder.

3. Loose crosshead lock nut.

4. Broken or leaking valve(s).

5. Worn or broken piston rings or expanders.

6. Valve improperly seated/damaged seat gasket.

7. Free air unloader plunger chattering.

Frame Knocks 1. Loose crosshead pin, pin caps or crosshead

shoes.

2. Loose/worn main, crankpin or crosshead bearings.

3. Low oil pressure.

4. Cold oil.

5. Incorrect oil.

6. Knock is actually from cylinder end.

Excessive Packing

Leakage

1. Worn packing rings.

2. Improper lube oil and/or insufficient lube rate (blue

rings).

3. Dirt in packing.

4. Excessive rate of pressure increase.

5. Packing rings assembled incorrectly.

6. Improper ring side or end gap clearance.

7. Plugged packing vent system.

8. Scored piston rod.

9. Excessive piston rod run-out.

Crankshaft Oil Seal

Leaks

1. Faulty seal installation.

2. Clogged drain hole.

Piston Rod Oil

Scraper Leaks

1. Worn scraper rings.

2. Scrapers incorrectly assembled.

3. Worn/scored rod.

4. Improper fit of rings to rod/side clearance.

Gas Processors Suppliers Association Courtesy of Ingersoll-Rand Co.


Centrifugal Compressors: Trouble Shooting

Trouble Probable Cause(s) Trouble Probable Cause(s)

Low Discharge

Pressure

1. Compressor not up to speed.

2. Excessive compressor inlet temperature.

3. Low inlet pressure.

4. Leak in discharge piping.

5. Excessive system demand from

compressor.

High Bearing Oil

Temperature

Note:

Lube oil temperature

leaving bearings

should never be

permitted to exceed

180F.

1. Inadequate or restricted flow of lube oil to

bearings.

2. Poor conditions of lube oil or dirt or gummy

deposits in bearings

3. Inadequate cooling water flow lube oil

cooler.

4. Fouled lube oil cooler.

5. Wiped bearing.

6. High oil viscosity.

7. Excessive vibration

8. Water in lube oil.

9. Rough journal surface.

Compressor Surge 1. Inadequate flow through the compressor

2. Change in system resistance due to

obstruction in the discharge piping or

improper valve position.

3. Deposit buildup on rotor or diffusers

restricting gas flow.

Excessive Vibration

Note:

Vibration may be

transmitted from the

coupled machine. To

localize vibration,

disconnect coupling

and operate driver

alone. This should help

to indicate whether

driver or driven

machine is causing

vibration.

1. Improperly assembled parts.

2. Loose or broken bolting

3. Piping strain.

4. Shaft misalignment.

5. Worn or damaged coupling.

6. Dry coupling (if continuously lubricated

type is used).

7. Warped shaft caused by uneven heating or

cooling.

8. Damaged rotor or bent shaft.

9. Unbalanced rotor or warped shaft due to

severe rubbing.

10. Uneven build-up of deposits on rotor

wheels, causing unbalance.

11. Excessive bearing clearance.

12. Loose wheel(s) (rare case).

13. Operating at or near critical speed.

14. Operating in surge region.

15. Liquid “slugs” striking wheels.

16. Excessive vibration of adjacent machine

(sympathetic vibration).

Low Lube Oil

Pressure

1. Faulty lube oil pressure gauge or switch.

2. Low level in oil reservoir.

3. Oil pump suction plugged.

4. Leak in oil pump suction piping.

5. Clogged oil strainers or filters.

6. Failure of both main and auxiliary oil

pumps.

7. Operation at a low speed without the

auxiliary oil pump running (if main oil pump

is shaft-driven).

8. Relief valve improperly set or stuck open.

9. Leaks in the oil system.

10. Incorrect pressure control valve setting or

operation.

11. Bearing lube oil orifices missing or plugged.

Shaft Misalignment 1. Piping strain.

2. Warped bedplate, compressor or driver.

3. Warped foundation.

4. Loose or broken foundation bolts.

5. Defective grouting.

Water in Lube Oil 1. Condensation in oil reservoir.

2. Leak in lube oil cooler tubes or tube-sheet.

Gas Processors Suppliers Association


Comparison of Characteristics of Compressors for Oilfield Service

TYPE OF COMPRESSOR -- >> ROTARY

VANE

ROTARY

SCREW

RECIPROCATING CENTRIFUGAL

1 –STAGE MULTI –STAGE 1 –STAGE MULTI –STAGE

COMMON APPLICATIONS VAPOR

RECOVERY

VAPOR

RECOVERY

GENERAL

HIGH PRESSURE

GENERAL

HIGH VOLUME

DISCHARGE PRESSURE, PSIG

COMMON 5 TO 50 < 100 < 2,000 (a)

MAXIMUM < 400 < 250 < 200 < 15,000 < 1,500 < 8,000 (b)

ACTUAL CUBIC FEET < 1,000 < 20,000 < 1,000 < 2,000 < 100 < 500

PER MINUTE < 1,000 < 20,000 (b)

< 100,000 (a)

HORSEPOWER

COMMON < 125

MAXIMUM < 500 < 400 < 12,000 < 20,000

SMALL for HC 50 TO 150 TO 350 STARTS AT 500

LARGE < 250 2,000 TO 4,000 PARITY AT 2,000

CAN HANDLE LIQUIDS MODERATE ABSOLUTELY NOT

CAN HANDLE DIRTY GAS NO YES

LUBRICATION COOLER YES YES

EFFICIENT -20% DP < 50 STANDARD FOR EFFICIENCY LOWER EFFICIENCY

FLEXIBLE LOW TO OPERATING CHANGES LIMITED FLEXIBILITY


Reciprocating Compressor Gas Throughput

EV = Volumetric Efficiency (%)

PD = Piston Displacement (cfm)

Where C = Cylinder Clearance (%)

Piston Displacement: One of the Following

(Configuration)

– Single Acting Cylinder (Head End Displacement)

– Single Acting Cylinder (Crank End Displacement )

– Double Acting Cylinder ( Crank and Head Ends)

Where dc = diameter of cylinder, inches

dr = diameter of rod, inches

MMCF v s s s

Q = .051 E PD P / T Z

1/k

v d s d s s dE = 96 - P / P - C P / P Z / Z -1

2

c in rpmPD = d L S / 2200

2 2

c r in rpmPD = d - d L S / 2200

2 2

c r in rpmPD = 2d - d L S / 2200


®

Appendix 7B

Compressor Exercises and Calculations

Discharge Temperatures

Volume Bottle Sizing


Number of Stages


Approximate Heat – Capacity Ratios of Hydrocarbon Gases

(MW = 28.96

Specific Gravity)



(MW = 28.96 x

Specific Gravity)

What is the

heat-capacity ratio (k)

for 0.69 gravity

Natural gas at 250oF?


Approximate Heat – Capacity Ratiosof Hydrocarbon Gases

(MW = 28.96 x

Specific Gravity)

What is the heat-

capacity ratio (k) for

0.69 gravity Natural

gas at 250oF?

MW = 28.96 x 0.69 = 20

k = 1.21

Most Oilfield Applications


Compressor Discharge Temperature: Exercise #1

Overall Problem:

Methane is Compressed from 250 psig to 600

psig, using a reciprocal compressor.

If the Suction Temperature is 100F, What is

the Discharge Temperature?

a k-1 /k

d sT = T r


Compressor Discharge Temperature #1

Methane Compressed: 250 psig to 600 psig. Ts = 100F

What is the Td ?

Problem Broken Down Into Steps:

What is the Comp Ratio? (Remember: psia = psig + 14.7)

Determine k from “k vs. Mol Wt” Graph: FIGURE A page 10 – 2

(Mol Wt of Methane = 16.042)

Alternate Method: Determine k from Formula (MCp = 8.651)

What is (k-1) / k?

What is Suction Temperature in R?

Finally, Calculate Discharge Temperature if (2.32).23 = 1.21

a k-1 /k

d sT = T r

p pk = MC / MC - 1.99


Step 1: Discharge Temp – Calc Compression Ratio

Methane Compressed: 250 psig to 600 psig.

Ts = 100F

What is the Td?

What is the Comp Ratio? (Remember: psia =

psig + 14.7)

T

Pd 600 + 14.7R = = =

Ps 250 + 14.72.32

k-1 /ka

d sT = T r


Step 2: Determine k (from Chart)


Ts = 100F

What is the Td?

Comp Ratio = 2.32

Determine k: From Chart

From “k vs Mol Wt” Graph: Figure A (page 10 – 2)


Using MW = 16.042 and T = 100F:

Find k = ??

k-1 /ka

d sT = T r



Mol Wt of C1

= 16.042

Ts = 100F



Mol Wt of C1

= 16.042

Ts = 100F

1.3


Compressor Discharge Temperature #1


Ts = 100F

What is the Td?

Comp Ratio = 2.32

Determine k: From Chart

From “k vs Mol Wt” Graph: Figure A (page 10 – 2)


Using MW = 16.042 and T = 100F:

Find k = 1.3

k-1 /ka

d sT = T r


Alternate Step 2: Determine k (from Equation)


Ts = 100F

What is the Td?

Comp Ratio = 2.32

Determine k: from Equation

Find k = ? ?

k-1 /ka

d sT = T r


Alternate Step 2: Determine k (from Equation)


Ts = 100F

What is the Td?

Comp Ratio = 2.32

Determine k: from Equation

8.651k = =

8.651-1.991.3

p p

p

k = MC / MC - 1.99

MC = 8.651 from TABLE

k-1 /ka

d sT = T r


Molar Heat Capacity: MCp

Gas

Chemicalformula

Molwt 0F 50F 60F 100F 150F 200F 250F 300F

Methane CH4 16.042 8.233 8.414 8.456 8.651 8.947 9.277 9.638 10.010

Ethyne (Acetylene) C2H2 26.036 9.683 10.230 10.330 10.710 11.130 11.540 11.880 12.220

Ethene (Ethylene) C2H4 28.052 9.324 10.020 10.160 10.720 11.400 12.080 12.750 13.140

Ethane C2H6 30.068 11.44 12.170 12.320 12.950 13.770 14.630 15.490 16.340

Propene (Propylene) C3H6 42.078 13.63 14.690 14.900 15.750 16.800 17.850 18.870 19.890

Propane C3H8 44.094 15.64 16.880 17.130 18.170 19.520 20.890 22.250 23.560

1-Butene (Butylene) C4H8 56.104 17.96 19.590 19.910 21.170 22.710 24.250 25.700 27.150

cis-2-Butene C4H8 56.104 16.54 18.040 18.340 19.540 21.040 22.530 24.000 25.470

trans-2-Butene C4H8 56.104 18.84 20.020 20.500 21.610 22.990 24.370 25.720 27.060

iso-Butane C4H10 58.120 20.40 22.150 22.500 23.950 25.770 27.590 29.390 31.110

n-Butane C4H10 58.120 20.80 22.380 22.710 24.070 25.810 27.540 29.230 30.900

iso-Pentane C5H12 72.146 24.93 27.160 27.610 29.420 31.660 33.870 36.030 38.140

n-Pentane C5H12 72.146 25.64 27.610 28.010 29.700 31.860 33.990 36.070 38.120

Benzene C6H6 78.108 16.41 18.380 18.750 20.460 22.460 24.460 27.080 29.710

n-Hexane C6H14 86.172 30.17 32.780 33.300 35.360 37.910 40.450 42.910 45.360

n-Heptane C7H16 100.198 34.96 38.000 38.610 41.010 43.970 46.930 49.770 52.600

Ammonia NH3 17.032 8.516 8.5180 8.519 8.521 8.523 8.525 8.527 8.530

Air 28.966 6.944 8.9510 6.952 6.960 6.973 6.990 7.009 7.033

Water H2O 18.016 7.983 8.0060 8.010 8.033 8.075 8.116 8.171 8.226

Oxygen O2 32.000 6.970 6.9970 7.002 7.030 7.075 7.120 7.176 7.232

Nitrogen N2 28.016 6.951 6.9540 6.954 6.956 6.963 6.970 6.984 6.998

Hydrogen H2 2.016 6.782 6.8560 6.871 6.905 6.929 6.953 6.965 6.977

Hydrogen sulfide H2S 34.076 8.000 8.0910 8.109 8.180 8.270 8.360 8.455 8.550

Carbon monoxide CO 28.010 6.852 6.9570 6.958 6.963 6.975 6.986 7.007 7.028

Carbon dioxide CO2 44.010 8.380 8.6980 8.762 9.004 9.282 9.559 9.810 10.050

55


Step 3: Calculate (k-1) / k


Ts = 100F

Comp Ratio = 2.32 k = 1.3

Calculate (k-1) / k:

(1.3 – 1) / 1.3 = 0.23

a k-1 /k

d sT = T r


Step 4: Calculate Td


Ts = 100F

What is the Td?

R = 2.32 k = 1.3 (k-1) / k = 0.23

Calculate Td

a k-1 /k

d sT = T r

(k-1)/k .23sd

T = T (r) = ( 560 )( 2.32 )

= ( 560 )( 1.21 ) = 678 R = 218 F


Compressor Discharge Temperature: Exercise # 2

If this same gas was compressed to 1600

psia (In a Single Stage) what would be the

Discharge Temperature?


Compressor Discharge Temperature: Solution #2

If this same gas was compressed to 1600

psia (In a Single Stage) what would be the

Discharge Temperature?

Td = Ts (r)a(k-1)/k

Td = 560 (6.04).23

Td = 847R = 386F

Pd 1600r = = = 6.04

Ps 250 + 14.7


Compressor Discharged Temperature: Exercise #3

For a compression ratio of 3.5 (good operating practice), what is the

maximum suction temperature that assures that the discharge

temperature remains below 250F?

k = Isentropic Coefficient (heat capacity ratio)




constant pressure, BTU / lb-mol / R

a – Depends on the Type of Compressor

a = 1.0 for reciprocating (positive displacement)

a = 1.25 for centrifugal

a k-1 /k

d sT = T r


Compressor Discharged Temperature: Solution #3

For a compression ratio of 3.5 (good operating practice), what is the

maximum suction temperature that assures that the discharge

temperature remains below 250F?

k = Isentropic Coefficient (heat capacity ratio)




constant pressure, BTU / lb-mol / R

a – Depends on the Type of Compressor

a = 1.0 for reciprocating (positive displacement)

a = 1.25 for centrifugal

a k-1 /k

d sT = T r

1.0 1.26-1 /1.26

s

s

250 + 460 = T 3.5

T = 548°R = 88°F


®

Compressor Volume Bottle

Sizing Exercise



1. Consult Manufacturer or Design Engineering Firm


a) Calculate Displacement Volume of Single Stroke = Vd

b) Choose Suction Bottle Volume = (Vd) x (Suction Multiplier)

c) Choose Discharge Bottle Volume = (Vd) x (Discharge Multiplier)

Estimate Multipliers from Chart below

If More Than One Cylinder is Connected to a Single Bottle: Sum

the Displacement Volumes


Exercise: Volume Bottle Sizing

Ps = 500 psia

Pd = 1500 psia

Cylinder Bore = 6 inches Diameter

Cylinder Stroke Length = 18 inches

Choose: Suction and Discharge Bottle Size


Solution: Volume Bottle Sizing

Ps = 500 psia Pd = 1500 psia



Displacement Volume = Vd = r2 L = 32 (18)

= 509 Cubic Inches

Suction Multiplier from Chart at 500 psia?

Discharge Multiplier from Chart at 1500 psia?




– 509 Cubic Inches

Suction Multiplier from Chart at 500 psia? ?

Discharge Multiplier from Chart at 1500 psia? ?

Estimate Multipliers from Chart Below




– 509 Cubic Inches

Suction Multiplier from Chart at 500 psia? ? ± 7

Discharge Multiplier from Chart at 1500 psia? ? ± 9.5

Estimate Multipliers from Chart Below


Solution: Volume Bottle Sizing

Ps = 500 psia Pd = 1500 psia



Displacement Volume = Vd = r2 L = 32 (18)

= 509 Cubic Inches

Suction Multiplier from Chart at 500 psia 7

Discharge Multiplier from Chart at 1500 psia

9.5

Suction Bottle Size = 509 x 7 = 3563 cubic in.

Discharge Bottle Size = 509 x 9.5 = 4836

cubic in.


®


BHP


Calculate: Compressor Brake Horsepower

1. Rule of Thumb

2. Katz et al Equation

3. Charts and Figures Method

4. Computer



Rule of Thumb Method

BHP = 22 (R per Stage) (# of Stages) (MMCFD) F

F Corrects for Interstage Cooling

– = 1.00 for Single – Stage

– = 1.08 for Two – Stage

– = 1.10 for Three – Stage

Centrifugal Compressors: Add 18%

High Speed Compressors May Require Up to 20%

More BHP

– (Check with Manufacturer)

r < 2.5 and SGg > .65 Will Require Less BHP

– Use 20 vs 22 if SG > .8

– Use 17 vs 22 if r < 2.0



Equation Method: KATZ et al

Equation for Low Speed Recip Compressors

– Add 4% for High Speed Reciprocating

– Add 18% for Centrifugal

k-1 /k

MMCF s s d s

BHP =

.124 Q T Z k / k -1 P / P -1



Chart and Figure Method

1. Determine K of Mixture: Use Figure A page 10 – 2

– Guess Tavg Based on Ts

2. Determine Td for First Stage: Use Figure B page 10 – 3

– Check Tguess by Averaging Ts and Td

– If Too Much Error, Repeat Steps 1 and 2

– Repeat Steps 1 and 2 for Each Stage

3. Determine Zs for Each Stage: Charts 1 and 2 pp 10 – 8 and 10 – 9

4. Determine BHP/MMCF: Figs BHP1A and BHP1B pp 10 – 6 and 10 – 7

– Repeat for Each Stage

– Multiply by the Flowrate in MMCFD

5. If Needed, Correct for “LOW Ps” Figure C page 10 – 4

6. If Needed, Correct for “High Speed” Table page 10 – 4

7. If Needed, Correct for “Low Pd /Ps” (<2.2) Fig D page 10 – 5


Exercise: Compressor BHP Chart Method

From Figure A (Heat Capacity) page 11-2, Determine K (guess avg. Temp = 150F)

From Figure B page 11-3, Determine discharge Temperature from first stage:

– read r to K to ts to td

If interstage cooling lowers ts for second stage to 120F, what is td for second stage?

Extra Given Bit Of Information Obtained From These Temperatures:

– First Stage avg. Z .975

– Second Stage avg. Z .940

Z given here, but you could have obtained them from Chart 1 page 11-8 and Chart 2 page 11-9

From Figure BHP – 1A page 11-6 or BHP – 1B page 11-7 Determine BHP / MMscfd for each Stage:

– BHP / MMscfd for Stage 1 = ______________ Stage 2 = _____________

– Do we need to correct for “Low Ps” (Figure C)? page 11- 4

– Do we need to correct for “High Speed and S.G.” (Table below Figure C)? page 11- 4

– Do we need to correct for “Low Comp Ratio vs. S.G.” ( Figure D )? Page 11- 5

– How much Horsepower do we require (Total)?

NOTE:

– Tables were prepared using Volume at 14.4 psia and intake Temp (Ts ).

– Sales contract is for volumes calculated at 14.65 psia AND 60F.

– We have BHP for 2MMCF at 14.4 psia, Zavg and Ts of each stage. We want @ 14.65 psia, 60F and Z=1

Correct the BHP For These Changes:

– BHPACTUAL = ( BHPCHARTS ) (14. 4 / PCONTRACT ) ( TCONTRACT / TCHART = Ts) (1 / ZCHART)

Compare Your Answer With This Rule of Thumb:

– BHP = (22 BHP) (Ratio / Stage) (Stages) (MMcfd) (F)

Where F corrects for interstage cooling = 1.00 for single-stage

– 1.08 for 2 stages



Exercise: Compressor BHP Chart Method

What total horsepower is required to compress 2 MMscfd of

gas measured at 14.65 psia and 60F into a 900 psia sales line

from an intake pressure of 100 psia, and intake temperature of

100F?

– The gas has a Specific Gravity of 0.8

Overall compression ration is:

Number of stages required is:

Increase the compression ratio, r , by 3% for interstage

cooling:

Calculate first stage Pd:

Calculate second stage Ps:


Solution: Compressor BHP Chart Method

From Figure A (Heat Capacity) page 11-2, Determine K (guess avg. Temp = 150F)– Convert Specific Gravity into Mol Wt: .8 x 28.96 = 23.2

– From Fig A at MW of 23.2 and T = 150F: Read k = 1.215

– (We are guessing that the average Temperature will be about 150F during Compression)

From Figure B page 11-3, Determine Discharge Temperature from first stage: – (read r to K to ts to td)

– r = 3.09, K = 1.215, ts = 100F: Read td 223F

– (Actual Average Temperature 161.5 which doesn’t change K enough to matter; so, our guess of using 150

F to determine K from Chart A was alright)

If interstage cooling lowers ts for second stage to 120F, what is td for second

stage?

Using Fig B page 11-2 again with r = 3.09 ts = 120F, K = 1.215: Read td 248F

Extra Given Bit of Information Obtained from these Temperatures: – First Stage avg Z .975

– Second Stage avg Z .940

Z given here, but you could have obtained them from Chart 1 page 11-8 and

Chart 2 page 11-9

From Figure BHP – 1A page 11-6: Determine BHP / MMscfd for each Stage:

BHP / MMscfd for Stage 1: 65.7 BHP / MMscfd Stage 2: 65.7 BHP / MMscfd– Do we need to correct for “Low Ps” (Figure C) page 11-4? NO

– Correct for “High Speed and S.G.” (Table below Figure C) page 11-4? NO

– Correct for “Low Comp Ratio vs. S.G.” (Figure D) page 11-5 ? NO



How much Total Horsepower do we require?– Stage 1: 65.7 BHP / MMscfd x 2.0 MMscfd = 131.4 BHP

– Stage 2: 65.7 BHP / MMscfd x 2.0 MMscfd = 131.4 BHP

– Total: 131.4 + 131.4 = 262.8 BHP

Note: Tables were prepared using Volume at 14.4 psia and intake Temp (Ts).

We are measuring volumes calculated at 14.65 psia AND 60F.

We have BHP for 2MMCF at 14.4 psia, Zavg and Ts of each stage. We

want at 14.65 psia, 60F and Z = 1

Correct the BHP for these Changes:– BHPACTUAL = (BHPCHARTS) (14.4 / PCONTRACT) (TCONTRACT / TCHART = Ts) (1 / ZCHART)

– Stage 1: (131.4) (14.4 / 14.65 ) (520R / 560R) (1/ .975) = 123.0

– Stage 2: (131.4) (14.4 / 14.65 ) (520R / 580R) (1/ .940) = 123.3

– Total BHP needed: 123.0 + 123.2 = 246.2 BHP

Compare Your Answer With This Rule of Thumb:

BHP = (22 BHP) (Ratio / Stage) (Stages) (MMcfd) (F)

Where F corrects for interstage cooling = 1.00 for single – stage



BHP = (22) (3.09) (2) (2) (1.08) = 294 Total BHP



What total horsepower is required to compress 2 MMscfd of

gas measured at 14.65 psia and 60F into a 900 psia sales line

from an intake pressure of 100 psia, and intake temperature of

100F?

The gas has a Specific Gravity of 0.8.

Overall compression ration is: r = Pd / Ps = 900 / 100 = 9

Number of stages required is: The square root of 9 is 3, which

is an acceptable compression ratio; therefore 2 Stages are

sufficient .

Increase the compression ratio, r, by 3% for interstage cooling:

3 x 1.03 = 3.09

Calculate first stage Pd = 100 x 3.09 = 309 psia

Calculate second stage

Ps = 309 psia x 97% (cooling, liquid removal) 300 psia


Determination of Heat Capacity Ratio, K

(MW = 28.96 x

Specific Gravity)


Reciprocating Compressors

Theoretical Discharge

Temperatures

Single – State Compression

Read r to k to ts t td


Super – Compressibility Factor: Z


BHP Required Per MMSCFPD


Brake Horsepower Correction Factors


Table: BHP Correction for High Speeds

GAS SPECIFIC

GRAVITY

PERCENT HORSEPOWER INCREASE

FOR HIGH SPEED UNITS

0.5 – 0.8 4

0.9 5

1.0 6

1.1 8

1.5 and propane

refrigeration units

10

Bottom of Page 10 - 4


®

Compressors Number of Stages

Exercise


Compressors: Number of Stages – Excercise

Produced gas is to be compressed from a Suction

Pressure of 50 psig to a Sales Line pressure of 1500

psig.

Assume ambient air temperatures reach > 140F

so the Compression Ratio should not exceed 2.5

Determine Number of Stages Required

– (maintain the same compression ratio in each stage)



Compressors: Number of Stages – Solutions

Produced gas is to be compressed from a Suction Pressure of 50 psig

to a Sales Line pressure of 1500 psig.

Compression Ratio should not exceed 2. 5

Determine Number Of Stages Required

– (maintain the same compression ratio in each stage)

Overall Compression Ratio = (1500 + 14.7) / (50 + 14.7) = 23.4

Compression ratio of each stage = (23.4)1/n

for 1 stage: n = 1 (23.4)1/1 = 23.4

for 2 stages: n = 2 (23.4)1/2 = 4.89

for 3 stages: n = 3 (23.4)1/3 = 2.86

for 4 Stages: n = 4 (23.4)1/4 = 2.2

THEREFORE: Need 4 compressor stages, 3 interstage-coolers plus 1

after-cooler, 1 suction scrubber and 3 interstage-scrubbers


Solution: Number of Stages



Compressor Flow Diagram

What is the Overall Compression Ratio?

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