Centrifugal Compressors Performance Curves Factors that Affect Compressor Performance
Topics p
H How aC Centrifugal t if l works k /E Energy C Conversion i
Performance Curves
Operation Limits: Surge & Overload
Factors Affecting Compressor Performance
Operational Iss Issues es – Optimizing Optimi ing Compressor Efficiency
How A Centrifugal Works Centrifugal Stage
Return Bend
Return Bend
Diffuser Reduces Velocity Increases Static Pressure
Return Channel
Guide Vanes
Impeller p Increases Velocity Increases Static Pressure
How A Centrifugal Works
How A Centrifugal Works
Energy Conversion
P4,V4,T4
P5,V1,T5
P3,V1,T3
P5,V1,T5 P2,V4,T2
P1,V1,T1
P3,V1,T3
Head Concept Mechanical: The “work” (energy) developed to raise a weight of 1 pound by a distance of one (1) foot. Expressed in foot-pound (or equivalent Kgm or Nm);
Gas Compressors: “ work” done by the compressor / amount of gas. The head expressed in feet, is the height to which hi h the th gas could ld b be lift lifted d
Head Concept The height to which the gas is lifted depends on the velocity of the gas
For any given RPM, the head developed by the compressor is fairly constant, constant independent of the gas nature nature.
Head is depending upon: • Compressor C geometry t (i.e. (i no off stages, t impeller i ll di diameters) t ) • Compressor speed
Z: Compressibility Factor R: Gas Constant = 1545 / MW Ts: Suction Temperature (°R)
M:
r: Pressure Ratio (Pd / Ps) M Polytrophic M: P l hi E Exponent
Head Concept – Example
Performance Curves
Basic Components
Fixed/Variable Speed p
Surge/Overload
Effects on Performance
Compressor p Performance Curves illustrates the operating range and flexibility of a given compressor
% Head, P Pressure, P Pressure R Ratio
120%
110% 100% 105% 100% 95% 90%
90% 85%
80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Compressor Performance Curves There are two types of curves that are generally required required, section and overall:
• section refers to an impeller or sequence of impellers between two nozzles such that there is no pressure drop or temp reduction between impellers
• overall refers to a complete compressor or compressor train Note: a back-to-back unit with a crossover may often be considered a two section compressor; but with respect to performance curves two-section curves, it is a single section since no pressure drop or cooling is introduced between the impellers
For single section compressors, the section curves and overall curves are one in the same
Design Point is the point at which usual operation is expected t d and d optimum ti efficiency ffi i iis . It iis th the point i t att which hi h the vendor certifies that performance is within the tolerance % Head, P Pressure, P Pressure R Ratio
120%
110% 100%
90% 80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Rated Point is intersection on the 100 % speed line corresponding di to the h hi highest h flflow off any operating i point i
% Head, P Pressure, P Pressure R Ratio
120%
110% 100%
90% 80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Stability: the percent of change in capacity between the rated (design point) capacity and surge (limit) point point, all at constant speed speed, is measured as the stability of the centrifugal compressor. Indicates the capability of the centrifugal compressor to operate at less than design flow
% Head, P Pressure, P Pressure R Ratio
120%
110% 100%
90% 80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Turndown: the percent of change in capacity between the rated
% Head, P Pressure, P Pressure R Ratio
(Design point) capacity and the surge (limit) point, all at constant head or pressure is measured as turndown of the centrifugal compressor. Indicates the capability of the centrifugal compressor to operate at less than design flow 120% 110% 100%
90% 80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Rise to Surge: Surge: the percent of change in discharge pressure between the rated point and surge limit at constant speed speed. High RTS means the compressor can accommodate a modest increase in discharge pressure with a little change in flow
% Head, P Pressure, P Pressure R Ratio
120%
110% 100%
90% 80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Surge g Phenomenon At any given speed, there is minimum flow, below which, the compressor cannott be b operated t d in i a stable t bl condition. diti Thi This minimum flow value is called “surge “ point.
Surge is oscillation of the entire flow of the compressor system and this oscillation can be detrimental to the machine.
Compressor surge may be evidenced by the following: a) Excessive rotor vibration b) Increasingly higher process gas temp c) Rapid changes in axial thrust d) Sudden changes in load e) Audible sounds (if surge is severe)
Surge g – Damage g of Compressor p Internals High g axial displacement p
Deformation due to high temperature
Resistance to Flow Causes
Surge Description
Pressure to Rise Which Causes Flow to Decrease
Sudden Reversal of Flow Slams Thrust Disc Against
% Head, P Pressure, P Pressure R Ratio
120%
Inactive Thrust Bearing
110% Pressure Builds along the
100%
Design Curve Back to the Design Point
90% 80% Pressure Ratio Drops Low Enough
70%
for Flow to Instantaneously Build Back to the Design Curve
60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Surge The frequency of the surge cycle varies inversely with the volume of the system
If the check valve is located near compressor discharge nozzle, the frequency will be much higher than of a system with a large g volume in the discharge g upstream of the check valve
The higher g frequency q y of the surge, g , the intensity y will be lower (i.e. few cycles / minute up to more than 20 cycles / sec)
The intensity of the surge increases with gas density , pressure and lower temperature
Surge - Effects of Gas Composition Best Efficiency point E% Heavy Gas (propane, propylene) Medium Gas (air, nitrogen, natural gas) Light Gas (Hydrogen reach gases, i.e. hydrocarbon processing plants)
Surge points
Q
Surge - Effects of Gas Composition Observations made in respect to the heavy gas:
Th flow The fl at surge iis hi higher; h
The stage produces more head than corresponding to medium gas / light gas
The right Th i ht side id off the th curve tturns d downward d (approaches stonewall) more rapidly
The curve is flatter in the opening stage (small RTS)
Surge Control IInputt Signals Si l R Required i d 1 - Suction Flow 2 - Suction Pressure 3 - Discharge Pressure Suction
Flow Element
Flow Transmitter FT
Pressure PT Pressure PT Transmitter Transmitter PC SP U
Recycle Valve I/P
G D A AC G E B I I OO B T C MCH
Surge Control In the PLC
Discharge
Surge g Control Surge Controller Performance Map
External Causes and Effects of Surge g
Restriction in suction or discharge g of system y
Process changes in pressure, temperatures, or gas MW
Internal plugging of flow passages of compressor (fouling)
Inadvertent loss of speed
Instrument or control valve malfunction
Operator error
Misdistribution of load in parallel operation
Improper assembly of compressor (impeller overlap)
Restriction in Suction / Discharge
Parallel Operation Typically, for parallel operation, the flow is not split evenly and one section or compressor handles more flow than the other, but both sections ti are required i d tto make k th the same pressure ratio ti
Careful analysis of the pressure ratio curves is required to insure satisfactory operation and suitable overall range
“similar pressure ratio curves” • At the design flow flow, section (1) is much more flow than of section (2)
• If the total flow is reduced 10%, the compressor slows down to maintain the same pressure ratio
• The Th flow fl to t each h section ti iis reduced 10% (dashed line) since the pressure ratio curves have a approximately the same rise
“different pressure ratio curves” (section 2 pressure ratio curve is steeper p than section 1)) • If the total flow is reduced 10% the compressor slows down to maintain pressure ratio • Section (1) reduces more than 10% ( about 12.5% - the dashed line) since its curve is shallower • Section (2) reduces less than 10% (about 5% - dashed line) since its curve is steeper • The two sections are now operating at significantly different portions of the curve and are now handling a different percentage of the total flow than they were at the design point point.
• Section (1) is nearing surge. Further reduction in flow would force section one into surge
• The difference in the curve shape results in a reduced overall range for parallel operation
Impeller p Overlap p with Diffuser
Impeller Overlap with Diffuser
Positive overlap
Nominal
Non Desirable
Limited
Desired
Limited
Impeller Overlap with Diffuser
It is preferable that no impeller shall have negative overlap
The negative overlap is limited to 5% of the impeller tip
Instrument / Control System y Malfunction
Overload
% Head, P Pressure, P Pressure R Ratio
120%
110% 100% 105% 100% 95% 90%
90% 85%
80%
70% 60% 60%
80%
90%
% Inlet Capacity or Flow
100%
120%
Choke Limit Choke is the maximum flow that a centrifugal compressor can handle at a given speed speed. At that point point, the compressor is unable to produce any net overall pressure ratio.
The maximum flow region of the compressor performance curve is where the gas speeds approach Mach 1
Gas compression G i iis no llonger occurring i iin th the compression i channels. This region of the curve, as it becomes almost vertical at the choke limit, limit is also know as “Stonewall” Stonewall
Stonewall is usually not detrimental to the compressor, it simply limits the maximum flow flow. If choke occurs at an off design condition, the maximum volume flow can be increased byy increasing g the rotational speed p
API 617 7th Edition – Performance Curves
Performance Curves – Inlet Gas Condition Effects
Performance Curves – Inlet Gas Condition Effects
Factors Affecting Compressor Performance MW & Head - If MW increases, increases the head for a given ratio will decrease in direct proportion
Temp e p & Head ead - If tthe e Tss increases, c eases, tthe e head ead for o ag given e ratio at o will increase in direct proportion
Zave & Head - If the average compressibility increases, the head will increase in direct proportion
N and Head - If speed increases, the head will increase in direct proportion
Flow
and Speed - If the speed increases, the flow will increase in direct proportion
Factors Affecting Compressor Performance N & BHP - If the speed increases, increases the BHP will increase in proportion to the cube of the speed. (Because flow increases directlyy as speed p and head increases as the square q of the speed and BHP is the product of head X mass flow)
Density e s ty - The eo only y tthing g a co compressor p esso impeller pe e sees is s inlet et capacity. Thus to get more capacity out of an existing compressor it is necessary to change the density of the inlet by: • decreasing the suction temperature • increasing the suction pressure • increasing the MW of the gas
Compressor Off - Design Performance Performance curves for axial and centrifugal g compressors are usually based on constant inlet conditions (Ps, Ts, MW). In actual service, these compressors rarely l see th these b base curve conditions diti exactly
If the field inlet conditions deviate more then 5% from the curve inlet conditions then the field data can not be accurately plotted on the curve without converting the field data to curve conditions
To properly T l evaluate l t th the compressor (running ( i off ff design), the performance parameters shall be corrected to the design conditions
Operation Limitations Compressor
Driver
Power
Process
Compressor Operation IssuesIssuesEfficiency Drop
Internal recycle U t Un-tuned d Surge S Control C t lS System t Leakage via by-pass valve(s) in process Compressor operated out of “guaranteed performance envelope” p p Impeller & Diaphragm erosion Fouling
Internal Recycle – Gap at the diaphragm / guides splits
Internal Recycle – Gap at the diaphragm / guides splits
Labyrinth Leakage Leakage proportional to: • P • Clearance • Diameter •1 / (No.Laby Teeth)0.5 Eye laby leakage is approx. 10 times spacer laby leakage
Eye Laby Leakage Spacer Laby S L b Leakage
Internal Recycle – Labyrinth Clearance
Process labyrinths can be p plugged gg by y wet particles in the gas flow
Internal Recycle – Labyrinth Clearance Shaft Spacer
Impeller Cover
Internal Recycle – Labyrinth Clearance
Impeller Cover
PEEK Labyrinth
PEEK Physical Properties GRADE
Arlon CP T l Torlon 4340 Fluorosint 500
COEF. THERMAL EXPNSION (F) 17 x 10 /-6 18 8 18.8
TENSILE STRENGTH (PSI)
ELONGATION (%)
SPECIFIC GRAVITY
11,080 12 900 12,900
2.0 66 6.6
1.45 1 44 1.44
19.4
1,100
10.0
2.32
Un-- tuned Surge Control System Un
Recycle valve shall be calibrated at every planned S/D
• • •
fast opening ( < 1 sec) total travel 0-100 %; 4 – 20 mA mechanical h i l stop t tto coincide i id with ith 100 % close l
Valve positioner shall match the command
FT instrument shall be calibrated at every planned S/D
Flow calculation block – correct constants constants, correct range
Fouling … is the deposit and the non –uniform accumulation of debris in the gas
Occurs due to carryy over of liquids q and debris from the inlet suction scrubber
Polymerization may occur in wet gas and cracked gas compressors applications if the temperature exceeds the critical point beyond the polymerization process occurs (235 F)
Fouling g build up p occurs usually y on the impeller p hub and shroud. There is also a build up on the blades ( on the pressure side)
Fouling
IGV partially clogged
1st stage impeller – hard deposits
Fouling g Effects – Charge g Gas
3M7 – Eroded Sleeves
Fouling g Effects
April 25 '99
NPC Thai Fouling
Abrasive Scoring due to Fouling
9
Fouling g Effects – Charge g Gas
3M7 - Deterioration of stage clearances
Techniques q to Prevent Fouling g Condition monitoring, both aerodynamic and mechanical parameters
Process control
Online solvent injection
Coatings of Impellers and Diaphragms
Fouling - Condition Monitoring (aerodynamic and mechanical parameters)
Monitor and trend the information regarding process conditions • MW • Pressure • Temperature
Vibration monitoring • On O line li system t • Off line system
ONLINE CONDITIONING MONITORING
Condition Monitoring – DR CPM Online System
Condition Monitoring – DR CPM Online System
Condition Monitoring – DR CPM Online System
Condition Monitoring – DR CPM Online System
Condition Monitoring – DR RECON Online System
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