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Natural gas compression performance calculations
Constantinos Hadjistassou, PhDAssistant Professor
Programme in Oil & Gas (Energy) EngineeringUniversity of Nicosia
Web: www.carbonlab.eu
Oct., 2015
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Overview
Two topics: Gas hydrates Sub-ambient flow in pipelines
Principles of gas compression
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Two topics
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Two topics
Gas hydrates Sub-ambient flow in pipelines
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Gas hydrates
Besides posing problems for flow assurance gas hydrates in the future could be a source of NG fuel
Several research programs target their economic extraction
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Sub-ambient pipeline flow
Is it possible for a pipe to transmit fluids at sub-ambient pressure ie below atmospheric pressure?
Pressure drop (Δp):
A pressure differential drives flow. Therefore, p2<p1
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Major pump needs in an LNG facility
Amine circulation
Scrub column & fractionation towers
LNG product pumps & loading pumps
Seawater pumps (if seawater cooled)
Hot oil pumps
2-4 pumps per LNG tank
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Gas compression performance
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Definitions
Pressure (P) is the direct force per unit area normal to the surface
Absolute pressure (Pabs)is measured relative to absolute vacuum (absolute zero pressure)
Gage pressure (PG) is the difference between absolute & atmospheric pressure
Vacuum pressure is below atmospheric
An ideal gas is an imaginary substance which obeys PV=RT
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FPA
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More definitions
Incompressible substance is the one whose specific volume (or density) remains constant when subject to a pressure .
A reversible process can be reversed w/o leaving any trace on the surroundings. Net heat & net work exchange btw stm&surroundings =0
Enthalpy is the thermodynamic potential of a system denoted by h (units: J or BTUs):
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Compression performance
Interested in calculating:– Compressor head (inlet & outlet pressure)– Discharge temperature – Compressor efficiency– Sizing of compressor (hp)
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Gas-turbine driven centrifugal compressor
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Reversible isothermal gas compression
Purpose of compression: mechanical work raises gas suction pressure to higher discharge pressure
Reversible shaft work of a compressor, ws, for open process for P1 to P2
(1)
where: h = enthalpy, h = u + pvv = specific volume (m3/kg or ft3/lb)
−ve sign indicates that work is being done on the compressor• Since V = RT/P, for an isothermal process the reversible shaft work of
an ideal gas is:(2)
where: R = gas constant (Nm/kgK or Btu/lbmolR), MW = molec. weight
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Reversible adiabatic heat transfer
Heat transfer from compression into fluid Thermal energy absorbed by the gas phase Polytropic process:
γ is the polytropic exponent; C = constant
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= (3)γPv C
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Work done on compressor
Adiabatic work done on compressor:
(4)
Ratio (P2/P1) is the compression ratio T1: inlet temperature. The specific heat ratio, γ, is:
(5)
Since R = CP – CV
Where CP is specific heat @ constant pressure & CV the specific heat @ constant volume [of the gas]
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γ= P P
V P
C CC C R
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Outlet compressor temperature
For a reversible adiabatic compression of an ideal gas, outlet temp:
(6) Empirically, T2 can be obtained from:
(7)
ηIS is the entropic efficiency with heat losses. Thus:
(8)
κ is the polytropic constant (empirically obtained)
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=κPv C
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Outlet compressor temperature
κ replaces γ in eqn (7):
(9)
If κ is not known, γ can be used. Assumption: all gas components behave like an ideal gas. Using eqns (1)-(9) we can determine:
Isothermal work to compress a gas from pressure (P1) into (P2) Adiabatic work to compress a gas from pressure (P1) into (P2) Discharge temp. for adiabatic compression
Example #1
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Multi-stage compression
Multi-staging: final pressure is attained by more than one stages/steps Reasons for multi-staging:
Cool the gas btw stages thereby increasing efficiency by decreasing gas volume & work. Material limitations. 150°C limit for construction materials, seals, lubricants. Recommended temp. decreases with high pressure
Limiting temps define compression ratios to btw 3:1 to 5:1 Min work when each stage does same amount of work (~press
ratio/stage) Pressure ratio (PR):
(10)
m = number of stages Example #2
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Compressor efficiencies
Reversible adiabatic (or isentropic efficiency), ηIS:(11)
Ws is determined from eqn (4) Centrifugal compressors’ efficiency given by polytropic efficiency, ηP:
(12)
Polytropic efficiency > adiabatic efficiency. Discharge conditions can also be determined from:
(13)
ηP is the polytropic efficiency Example #3
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Capacity & power calculations
Compressibility factor, Z, is:(14)
Z is due to gases deviating from ideal gas law. Z = Vactual/Videal
Power Requirements For isentropic compression & efficiency:
(15)
is the mass flow rate; zavg is the average of the inlet & outlet compressibility factors
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= PvZRT
m
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Reciprocating compressors
Power estimation:(16)
Brake HP is work delivered to compressor , F, correction factor: For single stage (m=1), F = 1.0 For double stage (m=2), F = 1.08 For three-stages (m=3), F = 1.10
PR = pressure ratio; HP = horsepower; m = # of stages Vendors rate compressors on 14.4 psi [for simplicity] than 14.7 psi Equation 16:
Developed for large, slow-speed compressors of 300-400 rpm Gases with SG=0.65 & PR>2.5 For 0.8<SG<1.0, use 20 in eqn (16), if SG<0.8 use 22
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Flow rate
Volumetric flow rate (Q) given by:
(17)
Where “1” & “R” denote inlet & reference conditions. Reference state: 14.7 psia @ 60°F (15.6°C); scfm standard cubic feet
Example #4
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Compressor efficiencies
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Thanks for your attention!
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