The Impact of Reciprocating Compressor Pulsations on the Surge Margin of Centrifugal Compressors By Dr. Klaus Brun – Southwest Research Institute Ms. Sarah Simons – Southwest Research Institute Dr. Rainer Kurz – Solar Turbines, Inc. Mr. Joseph Thorp – Aramco Services
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The Impact of Reciprocating Compressor Pulsations on the Surge Margin of Centrifugal Compressors
ByDr. Klaus Brun – Southwest Research Institute
Ms. Sarah Simons – Southwest Research InstituteDr. Rainer Kurz – Solar Turbines, Inc.Mr. Joseph Thorp – Aramco Services
Objective
• Many compressor stations have both recip and centrifugal compressors installed.
• There is still limited understanding of how pulsations, piping resonance, and impedance impact centrifugal compressor performance and surge.
• Analytical and computational predictions exist but no test data is available.
• Current design practices are limited because of lack of knowledge and data.
• Fundamental questions:– Can pulsations drive a centrifugal compressor into surge?– If so, what amplitudes and frequencies are required?
Unexpected periodic surge has been observed in the compressors when pulsations were present
Pulsations: What are They?
• A traveling compression wave in a fluid • Fluid particles (molecules) force interaction • Waves are composed of two components: Pressure and Velocity• Waves move at the speed of sound. (Flow does not.)
xx
y
y
A
y
A
Speed of sound, c, is function of fluid bulk modulus of fluid, B
c2 = 𝑩𝑩𝝆𝝆
Centrifugal Compressor Map and Surge
-6-4-202468
1012
0 1 2 3 4 5 6 7 8 9
Time [s]
CC
In
let
Flo
w [
m/s
]
Compressor Map
1
1.2
1.4
1.6
1.8
2
2.2
2.4
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Flow*10 (kg/s)
Pres
sure
Rat
io (P
2/P1
)
8000 RPM
14300 RPM
10000 RPM
12000 RPM
Flow pulsations can cause periodic surge in centrifugal compressor
Surge
Brun et al. 2010
Surge
Sources of Excitation in Compressor Piping System
• Strouhal excitation or vortex-shedding • Blade passing and blade vane interactions • Turbulence induced radial or other 3-D acoustic responses in large
vessels • External pulsation from reciprocating compressors or other positive
displacement machines • Unstable flow control valve control cycling (process valve dynamics)• Check valve or relief valve chatter• Surge cycles• Diffuser rotating stall and other stalls • Mismatch between the operating points of the compressors resulting
in periodic “hunting” for a stable operating point.• Other process and aero flow instabilities
All periodic excitations can be amplified by piping system acoustic resonances
Pulsation Decay
0 5 10 15 20 25 30Distance - Miles
Pres
sure
Dis
turb
ance
-ps
i
16141210
86420
Decay of a 33Hz pulsation in a pipeline.Pulsation amplitude at inlet was 1% inletpressure of 1500 psi. [Kurz et al., 2003]
Pulsations can propagate over long distances
What is Piping Acoustic Impedance?
Z = pU
= puA
= ρuA
~ ρcA
Z – Acoustic impedanceA – Pipe cross section areap – sound pressureU – acoustic volume flowρ – densityu – molecule particle bulk velocityc – local speed of sound
• Different than pipe friction or flow resistance• Applies only to transient flows/pulses (frequency typically > 1 hz)• Results in different behavior for steady-state and transient flows
Pressure rise perparticle velocity
Classic Compressor Dynamic Response Theory
(Sparks et al., 1983)
Provided explanation and analysis on how pulsations behave in centrifugal compressor systems
Numerical Analysis of Pulsations in Compressors (Brun et al., 2010/2014)
Pressure to Pipe Impedance for Steady Flow: Pipe Flow
Converging Nozzle
Flow
Pipe Impedance
Flow Velocity
Pressure
Z
V ~ Z2
P ~ 1/Z4
Z ~ 𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷 𝑹𝑹𝑷𝑷𝑹𝑹𝑷𝑷𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑷𝑷 (𝐑𝐑)
Pressure to Pipe Impedance for Short Pulse: Pipe Flow
Converging Nozzle
Pressure Pulse(travels at speed of sound)
Pipe Impedance
Pulse Velocity
Pressure of Pulse
Z
V ~ c
P ~ Z
c c c
Piping Impedance for Short Pulses: Centrifugal Compressor
c c
Pressure Pulse
MostlyClosed Valve
Open Valve
∞
c
Pressure Pulse
∞c c
c
High Impedance
Low Impedance
Pulse can be amplified or attenuated(Is this the same if the through flow velocity is high or low?)
DischargePressure (Pd)
Volume Flow (V)
Ps Pd
SurgeLine
Constant Suction PressureLines
Ps1
Ps2
Ps3
Compressor Map: Resistance versus Impedance
Compressor Map: Resistance versus Impedance
(Slow Transients)
DischargePressure (Pd)
Volume Flow (V)
Ps Pd
SurgeLine
Constant Suction PressureLines
Ps1
Ps2
Ps3
Resistance ~ V2
DischargePressure (Pd)
Volume Flow (V)
Ps Pd
SurgeLine
Constant Suction PressureLines
Ps1
Ps2
Ps3
Low Impedance p ~ Z∙V
Valve Open
Compressor Map: Resistance versus Impedance
(Fast Transients)
DischargePressure (Pd)
Volume Flow (V)
Ps Pd
SurgeLine
Constant Suction PressureLines
Ps1
Ps2
Ps3
High Impedance p ~ Z∙VValve Closed
Compressor Map: Resistance versus Impedance
(Fast Transients)
DischargePressure (Pd)
Volume Flow (V)
Ps Pd
SurgeLine
Constant Suction PressureLines
Ps1
Ps2
Ps3
Low Impedance p ~ Z∙V
Valve Open
Surge
Compressor Map: Resistanceversus Impedance
(Fast Transients)
What affects piping impedance?
• Pipe friction• Flow constrictions (valves, orifices)• Flow area changes (bottles, transition pieces)• Speed of sound changes (coolers)• Gas composition changes (side streams,
• A standing wave occurs there is coherence between a wave and its reflection.• Resonance occurs when the standing wave frequency coincides with a periodic excitation frequency.
f = Response frequency (Hz)c = Velocity of sound (ft/sec)L = Acoustic length of pipe span (ft)n = 1,2,3,…
Quarter-wave Acoustic Response
Open-closed configuration
Pressure minimum at open end
f = Response frequency (Hz)c = Velocity of sound (ft/sec)L = Acoustic length of pipe span (ft)n = 1,2,3,…
( )L4c1n2f −=
What Causes Piping Resonance?
(What can cause a pressure wave to reflect?)
• Open and closed ends• Rapid area changes (bottles, T’s, transitions)• Speed of sound changes (coolers)• Hard walls in the flow path (orifices, valves)• Rapid flow direction change (elbows)• Any surface projected normal into the flow path.• A rapid change of impedance.
Significant amplification of periodic excitation is possible
The theory is good. Numerical analysis and lots of anecdotal field evidence agrees with it.
But there was absolutely no test data available to validate it…
Surge Testing Project
Surge Testing Project
• Determine whether pulsations from reciprocating compressors (RC), vortex-shedding or other sources can cause surge in centrifugal compressors (CC) when operating at low surge margin.
• Develop understanding of the physical process that causes pulsation induced CC surge.
• Determine the amplitude and frequency of pulsations required to cause CC surge. (Develop simple physical relationship or rules for pulsation induced surge avoidance.)
• Determine impact of pulsations on performance.• Evaluate the impact and interaction of acoustic pipe resonances on
pulsation induced CC surge.• Evaluate the impact and interaction of pipe impedance on pulsation
induced CC surge.• Validate Compressor Dynamic Theory (Sparks et al., 1983) predictions
for pulsation amplification in centrifugal compressors.
Surge Testing ProjectTest Setup
Recip mounted upstream of centrifugal:- 700 Hp Clark 2 stage CC with VFD (0-
14,000 rpm)- 50 Hp Ariel single stage RC with VFD
(50 -1000 rpm)- Dynamic pressures, temperatures, and
flow measurements throughout piping and machinery flanges
Actual Test P&ID
HWXXXHot Wire
Anemometer
Spool PieceSuction
Spool PieceDischarge Flow Control
CoarseFlow ControlFine
Key Measurements
- Pressure and temperature measurements per ASME PTC-10- Transient flow measurement using high speed hot wire
anemometer near centrifugal suction flange- Steady flow measurement from orifice plate meter and PV
card mass balance- Dynamic pressure measurements from recip discharge and
centrifugal suction and discharge- Centrifugal power from shaft torque meter- Uncertainties (% from operating point):
Surge Line Validationand Surge Identification
(No Recip)
Discharge & Suction Pulsations(7,000 RPM, No Surge, No Stall)
No peaks at ~1/3 running speed
Pressure pulsation at ~ 1/3 running speed, indicates onset of stall
Discharge & Suction Pulsations(7,000 RPM, Stall, No Surge)
4 Hz pressure pulsation indicating onset of surge
Discharge & Suction Pulsations(7,000 RPM, Surge)
No 4 Hz Peak
Axial Vibration(7,000 RPM, No Surge, No Stall)
4 Hz Peak
Axial Vibration(7,000 RPM, Surge)
Surge at 4 hz axial vibrations and pulsations is consistent with previous measurements (McKee 2003)
Axial Vibration(7,000 RPM)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Mils
acfm
4 Hz Axial Vibration vs. Flow (no Recip)
Surge Line (at 100.5 acfm)
Discharge Pulsations and Flow(7,000 RPM)
70.00
75.00
80.00
85.00
90.00
95.00
100.00
105.00
110.00
115.00
120.00
0
0.05
0.1
0.15
0.2
0.25
0.3
acfm
PSI P
k-Pk
Time
4 Hz Discharge Volute Pulsation and Flow vs. Time (no Recip) Surge Line
4 hz axial vibrations and discharge pulsation provide best indication of onset of surge
Surge Line(based on 4hz axial vibrations & discharge pulsations)
1.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
1.2
1.22
0 50 100 150 200 250 300
Pres
sure
Rat
io
Flow (acfm)
7000 RPM
6000 RPM
5000 RPM
4000 RPM
Testing on 7,000 rpm Speed Line
Surge (at 100.5 acfm)
Surge Line Testing with RecipRunning to Determine Impact of Pulsations
• Flow through centrifugal compressor is controlled using recycle line to near surge line
• Recip compressor is then swept to decrease speed until surge
Discharge Pulsation(7,000 RPM, Recip Running, No Surge, No Stall)
Consistent with Compressor Dynamic Response Theory
Comparison of Transient Analysis to Test Results 8 Hz recip
TAPS Model of the System
Pulsation Analysis can accurately predict suction/discharge pulsation into a centrifugal compressor and thus define the
operating map ellipse of the compressor
Approx. 30% of time spent across surge line
Portion of Pulse from Recip that Takes the Flow Across the Surge Line
Surge was consistently identified when 30% of the area of the operating map ellipse crosses the surge lines for all suction/
discharge pulsation frequency orders below 75 Hz
Conclusions
• External pulsations applied to the suction or discharge flange of a centrifugal compressor reduce its surge margin significantly.
• The geometry of the piping system immediately upstream and downstream of a centrifugal compressor can have significant impact on the surge margin reduction (surge margin differential).
• The reduction of surge margin due to external pulsations is a function of the pulsation’s amplitudes and frequencies at the compressor suction and discharge flange. High suction flange amplitudes at low frequencies significantly increase the risk of surge. Surge margin reductions (differentials) over 40% were observed during testing.
• Utilizing the transient operating map ellipse of the centrifugal compressor to identify whether induced pulsations can result in the operating point temporarily crossing the surge line is a useful tool to identify the potential onset of surge. From the operating map ellipse surge margin differential can be calculated for various orders of pulsations.
Conclusions – cont.• If the upstream piping system impedance curve is flat, pressure pulses are
converted to high volume flow pulses which increase the centrifugal compressor pulsation induced surge margin differential. On the other hand, steep piping impedance curves of the downstream piping reduce the surge margin differentials.
• Surge was consistently identified when approximately 30% of the area of the operating map ellipse had crossed the surge lines for all suction/ discharge pulsation frequencies orders under 75 Hz.
• Test result and trends were consistent with predictions from CDR (Spark, 1983) and numerical predictions (Brun, 2014) for pulsation amplification and attenuation across a centrifugal compressor.
• A transient time domain 1-D Navier-Stokes pipe network analysis model was able to accurately predict suction/ discharge pulsations into a centrifugal compressor and thus, its operating map ellipse. Using the above described basic design rule (30% of the operating map area across the surge line for all pulsations below 75 Hz), these pressure/ flow pulsation amplitude predictions can be related to surge margin differential.
Thanks!Any Questions?
Dr. Klaus BrunContact info:Southwest Research InstituteTel: [email protected]
The 2nd Law of Thermodynamics Does Not Violate the 1st Law!