compressor basics
TRANSCRIPT
Los Angeles, June 1, 2007
Objectives
• Review basics of centrifugal compressors• Explore key terminology• Understand parameter relations• Performance capabilities of different
compressor technologies
Los Angeles, June 1, 2007
Basics of Compressors
• The principles of increasing pressure for pumps and compressors are the same.
• A PUMP moves an incompressible fluid - a liquid. The volume of a liquid does not change with pressure and temperature.
• A COMPRESSOR moves a compressible fluid - a gas. The volume of a gas changes with pressure, temperature, and gas composition.
Los Angeles, June 1, 2007
Centrifugal Compressor characteristic (performance) curve
Flow (ACFM)
Hea
d Surge
Stonewall
Los Angeles, June 1, 2007
Centrifugal compressors don’t make pressure ratio
Centrifugal compressors don’t make pressure ratio
They make
HEAD!
Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k - 1
M.W. k-1 P1
Los Angeles, June 1, 2007
… And they do Volume Flow, not Mass Flow
Q = w * T * z
M.W. * P
• Q = volume flow• w = mass flow• MW = mole weight• P = absolute pressure• T = absolute temperature• z = compressibility
Los Angeles, June 1, 2007
Flow
• Process Engineers are trained to calculate mass balances and therefore work in MASS flow (lbmm/hr, kgm/hr).
• Centrifugal compressors are designed to handle a given VOLUME flow.
• Difference between– “Standard” volume flow, and– “Actual” volume flow
Los Angeles, June 1, 2007
Flow
Standard Conditions = 0 psig, 60 °F
Normal Conditions = 0 BAR g, 0 °C
Example: 1,000 lb/min of pure Methane @ 1,000 psia & 100 °F correspond to 339.2 acfm, or 23,616 SCFM;
1,000 lb/min of pure Methane @ 500 psia & 50 °F correspond to 632.7 acfm, or 23,616 SCFM
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Key Terminology
• Volume Flow• Headrise (adiabatic and polytropic)
– Specific heat ratio (k)
– Compressibility (z)• Specific Speed• Mass Flow• Power• Mach Number• Surge, Stonewall or Choke
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Headrise
• Head is the energy in ft-lbff (N-m) required to compress and deliver one lbmm (kgm) from one energy level to another.Head H = ft-lbff / lbmm ( (N-m/ kgm)
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Headrise
• Reversible thermodynamic paths– Isentropic (adiabatic) = no heat loss– Polytropic = heat loss– Adiabatic and polytropic virtually the same for
single stage. Much different for multistage
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Headrise Calculation
• Required Headrise = z* R*T1(r m-1)/m
– z = Compressibility Factor (approx. 1.0)– R = Universal Gas Constant (1545/MW)
– T1 = Absolute Suction Temperature of Gas
– r = Pressure Ratio– m = [(k-1)/k– k = Specific Heat Ratio of Gas (Cp/Cv)
– = Polytropic Efficiency
Los Angeles, June 1, 2007
Headrise
The amount of energy required to compress a volume to the same pressure for a gas is much higher because the gas is at a much lower density than the liquid.
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Polytropic vs. Adiabatic (Isentropic) Head
Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k – 1
M.W. k-1 P1
Hp (ft-lbf/lbm) = z * 1545 * T1 (°R) * n * P2 (n-1)/n – 1
M.W. n-1 P1
n = polytropic exponent
T2 = P2 (n-1)/n p = k-1 * n Had = Hp
T1 P1 k n-1 ad p
Headrise
M
LIQUID
HEAD = 2.311 X P(Ft.) S.G.
Water P = 100 PSIHEAD = 231 Ft.P1 - 14.7 PSIA1T1 = 100°F
GAS
HEAD(Ft.) = 1545
M. W.(T1) K
K-1 -1
K-1 K P2
P1
( )
Nitrogen P = 100 PSIHEAD = 86,359 Ft.P1 = 14.7 PSIA1T1 = 100°F
231 Ft.
Pump
114.7PSIA
86,359 Ft.
Compressor
114.7PSIA
Los Angeles, June 1, 2007
Los Angeles, June 1, 2007
Headrise from the Impeller’s Point of View
• Head = C * N2 * D2
– C = Unit conversion constant
– N = Speed
– D = Impeller Diameter
– = Impeller head coefficient (.4 to .7)
Los Angeles, June 1, 2007
Volume Flow
• Actual Flow – volume flow rate entering the suction flange– acfm, m3/hr
• Standard Flow – volume flow rate referenced to an established set of P, T conditions– scfm, Nm3/hr, MMSCFD
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Specific Speed
• Ns = N * Q1/2
H 3/4
– N = speed– Q = flow– H = headrise
• Specific Speed drives impeller geometry and efficiency
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Impeller EfficiencyImpeller Efficiency
SPECIFIC SPEED - Ns
EF
FIC
IEN
CY
- (
%)
Full Emission Impellers
Partial Emission Impellers
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Power
• Gas Horsepower (GHP)GHP = Head * Mass Flow
33,000 * Eff
• Brake Horsepower (BHP)
BHP = GHP + FHP (seal + gearbox losses)
Los Angeles, June 1, 2007
Mach Number
• Acoustic Velocity
a = 223 * T11 * Z11 * k MW
• Relative Mach Number
MnRelRel = Inlet Velocity a
• Machine Mach Number
MnMachineMachine = U = D * Na 229*a
• Affects curve shape and range. Practical limit = 1.3
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Surge & Stonewall
Flow (ACFM)
Hea
d Surge
Stonewall
Los Angeles, June 1, 2007
Surge
• Surge is a system phenomena that is the result of flow separation caused by low gas velocity anywhere in a compressor stage.
• Surge is an oscillation of backflow and forward flow.
• Left to continue, Surge is a bad thing!
Los Angeles, June 1, 2007
Stonewall or Choke
• Stonewall or choke flow is the maximum flow a given stage can handle.
• This value occurs when the ratio of the relative gas velocity to the acoustic velocity of the process gas is equal to 1.0, or Mach 1.
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Parameter Relations
Had (ft-lbf/lbm) = z * 1545 * T1 (°R) * k * P2 (k-1)/k - 1
M.W. k-1 P1
Q (acfm) = ώ (lb/min) * 10.729 * T1 (°R) * z
P1 (psia) * M.W.
BHP (hp) = GHP + losses = Had (ft-lbf/lbm) * ώ (lb/min) + losses
33,000 * ad
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Parameter Relations
• Assuming No Hardware Changes• Assuming Hardware Can Be Altered
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No Hardware Changes
VARIABLE CONSTANT CHANGED CONDITIONS
P1 ACFM, T1, H, MW P2 , T2 , w , HP
MW P1, T1, H, ACFM P2 , T2 , w , HP
T1 P1, H, MW, ACFM P2 , T2 , w , HP
ACFM P1, T1, MW H , P2 , T2 , w , HP
Los Angeles, June 1, 2007
With Hardware Changes
VARIABLE CONSTANT CHANGED CONDITIONS
P1 P2, w, T1, MW H , T2 , ACFM , HP
MW P1, P2, T1, ACFM H , T2 , w , HP
T1 P1, P2, T1, ACFM H , T2 , w , HP
ACFM P1, P2, MW, T1,
T2, H
w , HP
Los Angeles, June 1, 2007
Affinity LawsQ2 = N2 H2 = N2
2
Q1 N1 H1 N1
GHP2 = N2 3
GHP1 N1
GHP (hp) = ώ (lb/min) * Had (ft-lbf/lbm) = ώ (lb/min) * Hp (ft-lbf/lbm)
33,000 * ad 33,000 * p
Los Angeles, June 1, 2007
New Inquiry: Required info: Q & H
– Flow (Q)– Suction pressure (P1)– Suction temperature (T1)– Compressibility (z)– Specific heat ratio (k)– Mole weight (MW) or Gas Analysis– Discharge pressure (P2)
Los Angeles, June 1, 2007
Compressor Technologies
• Positive Displacement– Reciprocating (piston & diaphragm)– Screw (oil flooded and dry)– Rotary (liquid ring, sliding vane, lobe)
• Dynamic (Turbo)– Centrifugal– Regenerative– Axial
Compressor Types
Multi-st.Axial
Multi-st.Recip.
200
20
102 103 105 106
PR
ES
SU
RE
RA
TIO
Multi-stagecentrif
2
Integrally GearedCentrifugal
104
Sundyne Sundyne
Rotary
Single StageRecip.
VOLUME FLOW
Los Angeles, June 1, 2007
Positive Displacement vs Dynamic
Positive Displacement Dynamic
Volume Volume
Pre
ssu
re
Pre
ssu
re
Los Angeles, June 1, 2007
Positive Displacement Compressors
• Types Include– Reciprocating– Screws– Sliding Vane & Liquid Ring– Rotary Lobe
Los Angeles, June 1, 2007
Reciprocating Compressor
• Similar to an automobile engine
• Compresses a given volume of gas through
the use a reciprocating piston
• Positive displacement compressors increase
the pressure of a gas by operating on a fixed
volume in a confined space.
Los Angeles, June 1, 2007
Horizontal, Balance Opposed, Double Acting, Reciprocating Compressor
PISTON & RIDER RINGS
VALVE DESIGNMOTOR-OVER MOUNTING
SEGMENTED TEFLON PACKING
EXTERNAL VALVES
VARIABLE CLEARANCE HEADS
FORCE FEED CYLINDER LUBRICATION
CRANKCASE LUBRICATION
CROSSHEAD
CYLINDERS
Los Angeles, June 1, 2007
Diaphragm Compressor
Los Angeles, June 1, 2007
Reciprocating Performance
• Piston Type: flow to 3,000 cfm, pressure to 30,000 psi, compression ratio to 20:1 (3:1) per stage , power to 15,000 HP, efficiency +/- 90%– special designs to nearly 20,000 cfm at
low suction pressures• Diaphragm Type: flow to 100+ cfm, pressure
to 30,000 psi, compression ratio 20:1 per stage, power to 150 HP
Los Angeles, June 1, 2007
Screw Compressor Hierarchy
SCREW COMPRESSORS
Oil Free Oil Flooded
Dry LiquidInjected
Medium Pressure Air & Gas
High Pressure Gas & Refining
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Screw CompressorTop View
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Screw CompressorSide View
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Rotary Liquid Ring
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Rotary Sliding Vane
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Rotary Lobe (Roots) Blower
Four distinct “pockets” of gas are moved from the suction todischarge in each revolution of the driving shaft.
Operating Principle
Los Angeles, June 1, 2007
Rotary Compressor Performance• Screw
– Pressure to 350 psid, 4:1 compression ratio dry, 15:1 compression ratio flooded, flow to 10,000 cfm, max efficiency 75%
• Liquid Ring
– Pressure to 175 psig (29” Hg Vacuum), 5:1 compression ratio, flow to 17,000 cfm, max efficiency 50%
• Sliding Vane
– Pressure to 50/100 psid, 4:1 compression ratio, flow to 6,000 cfm, max efficiency 70%
• Lobe (Roots Type)
– Pressures to 20 psid, 2+:1 compression ratio, flow to 25,000 cfm, max efficiency 70%
Los Angeles, June 1, 2007
Centrifugal Compressors
• Dynamic Machines
• Impeller uses centrifugal force to add
velocity to gas
• Diffuser reduces the velocity changing the
energy from velocity to pressure
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Single Stage, Overhung, Centrifugal
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Multistage Centrifugal Compressor
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Regenerative Compressor
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Axial Compressor
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Competitors
• Centrifugal– AC, Atlas Copco, Cooper, Demag, Dresser-Rand
Man Turbo (Sulzer + Borsig), York
• Reciprocating– Ariel, Dresser, GE, Neuman & Esser, Sulzer
Burckhardt
• Diaphragm / Regenerative– Burton-Corblin (Periflow)
• Screw– Mycom, Howden, Kobelco, Roots
Los Angeles, June 1, 2007
Centrifugal Compressors
• Dynamic Machines
• Impeller uses centrifugal force to add
velocity to gas
• Diffuser reduces the velocity changing the
energy from velocity to pressure
Los Angeles, June 1, 2007
Sundyne MultistageMulti-pinion
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IG Compressor Staging Arrangements
Los Angeles, June 1, 2007
Integrally Geared TechnologyBenefits and Features
• Almost 40 years experience• 1600+ process gas installations• Compact designs-reduced space• Lower installation costs• Fewer rotating components• API 617 specification• Participation on API sub-committee• Proven and accepted equipment• Optimized specific speed-higher efficiencies• Proven centrifugal reliability
Los Angeles, June 1, 2007
Applicable API Standards
• 613 Special Purpose Gear Units• 614 Lubrication, Shaft Sealing ...• 617 Centrifugal Compressors• 618 Reciprocating Compressors• 619 Rotary Type PD Compressors• 670 Vibration, Axial Position and ...• 672 Packaged Integrally Geared,
Centrifugal Plant & Instrument Air