Compressor Mechanical Design

This document describes the mechanical design calculations for centrifugal compressors in NeqSim, implemented in the CompressorMechanicalDesign and CompressorCasingDesignCalculator classes.

Overview

The mechanical design module provides sizing and design calculations for centrifugal compressors based on API 617 (Axial and Centrifugal Compressors) and industry practice. The calculations enable:

Design Standards Reference

Standard Description
API 617 Axial and Centrifugal Compressors and Expander-compressors
API 672 Packaged, Integrally Geared Centrifugal Air Compressors
API 692 Dry Gas Sealing Systems
API 614 Lubrication, Shaft-Sealing and Oil-Control Systems
ASME Section VIII Div. 1 Pressure Vessel Design (UG-27, UG-34, UG-99)
ASME Section II Part D Material Properties (allowable stress tables)
ASME B16.5 Pipe Flanges and Flanged Fittings (NPS 1/2–24)
ASME B16.47 Large Diameter Steel Flanges (NPS 26–60)
NACE MR0175 / ISO 15156 Materials for Sour (H2S) Service

Design Calculations

1. Number of Stages

The number of compression stages is determined by the total polytropic head and the maximum allowable head per stage:

numberOfStages = ceil(totalPolytropicHead / maxHeadPerStage)

Design Limit: Maximum head per stage = 30 kJ/kg (typical for process gas centrifugal compressors)

The actual head per stage is then:

headPerStage = totalPolytropicHead / numberOfStages

2. Impeller Sizing

Tip Speed Calculation

The impeller tip speed is derived from the head requirement using the work coefficient:

tipSpeed = sqrt(headPerStage [J/kg] / workCoefficient)

Where:

Design Limit: Maximum tip speed = 350 m/s (material limit for steel impellers)

Impeller Diameter

From the tip speed and rotational speed:

impellerDiameter [mm] = (tipSpeed × 60) / (π × speedRPM) × 1000

The design verifies the flow coefficient is within acceptable range (0.01-0.15):

flowCoefficient = volumeFlow [m³/s] / (D² × U)

3. Shaft Diameter

Shaft diameter is calculated from torque requirements and allowable shear stress:

torque [Nm] = power [kW] × 1000 × 60 / (2π × speedRPM)
shaftDiameter [mm] = ((16 × torque) / (π × allowableShear))^(1/3) × 1000 × safetyFactor

Where:

4. Driver Sizing

Driver power includes margins per API 617:

Shaft Power Driver Margin
< 150 kW 25%
150-750 kW 15%
> 750 kW 10% 
driverPower = (shaftPower + mechanicalLosses) × driverMargin

5. Casing Design

Design Pressure and Temperature

designPressure = dischargePressure × 1.10  (10% margin)
designTemperature = dischargeTemperature + 30°C

Casing Type Selection

Design Pressure Casing Type
> 100 bara Barrel
40-100 bara Horizontally Split
< 40 bara Vertically Split

Casing Wall Thickness (ASME VIII Div. 1 UG-27)

The CompressorCasingDesignCalculator computes the required casing wall thickness for the cylindrical shell under internal pressure:

\[t = \frac{P \times R}{S \times E - 0.6 \times P}\]

where:

The minimum wall thickness includes:

Maximum Allowable Working Pressure (MAWP)

MAWP is back-calculated from the selected wall thickness:

\[P_{MAWP} = \frac{S \times E \times t_{eff}}{R + 0.6 \times t_{eff}}\]

where $t_{eff}$ = selected thickness minus corrosion allowance.

Material Selection

Materials are selected from a built-in database covering ASME II Part D properties:

Grade Type SMYS [MPa] SMTS [MPa] NACE Compliant Typical Use
SA-516-70 Carbon Steel 260 485 No Standard service
SA-516-60 Carbon Steel 220 415 No Low-pressure service
SA-266-Gr2 CS Forging 250 485 No Forged casings
SA-266-Gr4 CS Forging 310 550 No High-pressure barrel casings
SA-350-LF2 Low-Alloy 250 485 No Low-temperature service (to -46°C)
SA-182-F316L Austenitic SS 170 485 Yes Sour service / corrosive gas
SA-182-F304L Austenitic SS 170 485 Yes Cryogenic / sour service
SA-182-F22 CrMo Forging 310 515 No High-temperature service (>400°C)
Inconel-718 Nickel Alloy 1035 1241 Yes High-pressure sour / HP-HT

Automatic material recommendation via recommendMaterial() considers:

Temperature derating of allowable stress is applied per ASME II Part D Table 1A for design temperatures above 200°C.

Hydrostatic Test Pressure (ASME VIII UG-99)

\[P_{test} = 1.3 \times MAWP \times \frac{S_{test}}{S_{design}}\]

With the additional requirement that the test pressure is at least 1.5 × design pressure per API 617. The test stress must not exceed 90% of SMYS.

Flange Rating (ASME B16.5 / B16.47)

Flange class selection follows ASME B16.5 (NPS ≤ 24) or B16.47 (NPS > 24) pressure-temperature tables with Group 1.1 carbon steel derating:

Flange Class Ambient Rating [barg]
150 19.6
300 51.1
600 102.1
900 153.0
1500 255.0
2500 425.0

Ratings are derated for temperatures above 38°C.

Nozzle Load Analysis (API 617 Table 3)

Allowable nozzle forces and moments scale with nozzle size:

\[F_{allow} = F_{ref} \times \left(\frac{D}{D_{ref}}\right)^{1.5} \times k\] \[M_{allow} = M_{ref} \times \left(\frac{D}{D_{ref}}\right)^{2.5} \times k\]

where $k$ = 1.85 (API 617 amplification factor), $D_{ref}$ = 200 mm (8 inch reference nozzle).

Thermal Growth and Differential Expansion

\[\Delta L_{casing} = L \times \alpha \times (T_{op} - T_{amb})\]

Differential expansion between casing and rotor is monitored to ensure seal clearances remain adequate. An acceptable limit of 2 mm is applied; values beyond this trigger a design warning.

Split-Line Bolt Design (Horizontally-Split Casings)

For horizontally-split casings, the split-line bolts must resist:

Bolt material is SA-193 B7 with allowable stress of 172 MPa at ambient. The calculator iterates through standard metric bolt sizes (M16–M48) and selects the smallest configuration that meets the stress requirement with minimum bolt spacing of 2.5–4× bolt diameter per API 617.

Barrel Casing Design

For barrel casings, the calculator also sizes:

NACE MR0175 / ISO 15156 Sour Service

When sour service is flagged or H2S partial pressure exceeds 0.3 kPa, the calculator:

6. Rotor Dynamics

Critical Speeds

maxContinuousSpeed = operatingSpeed × 1.05
tripSpeed = maxContinuousSpeed × 1.05

The first lateral critical speed is estimated using simplified Rayleigh-Ritz formulation based on shaft geometry.

API 617 Requirement: Separation margin from critical speed ≥ 15%

Bearing Span

bearingSpan = numberOfStages × (impellerDiameter × 0.8) + impellerDiameter

7. Weight Estimation

Rotor Weight

impellerWeight = numberOfStages × 0.5 × (impellerDiameter/100)^2.5
shaftWeight = bearingSpan/1000 × 7850 × π × (shaftDiameter/2000)²
rotorWeight = impellerWeight + shaftWeight

Casing Weight

casingThickness = max(10mm, designPressure × impellerDiameter / (2 × 150))
casingWeight = π × casingOD × casingLength × casingThickness × 7850 × 1.2

For barrel-type casing, add 30% additional weight.

Total Skid Weight

Component Estimation Method
Casing As calculated above
Bundle (rotor + internals) rotorWeight + stage internals
Seal system 100 × (shaftDiameter/100) kg
Lube oil system 200 + driverPower × 0.1 kg
Baseplate casingWeight × 0.3
Piping emptyVesselWeight × 0.2
Electrical driverPower × 0.5 kg
Structural steel emptyVesselWeight × 0.15

8. Module Dimensions

moduleLength = compressorLength + driverLength + couplingSpace + auxiliarySpace
moduleWidth = casingOD + 3.0m (access each side)
moduleHeight = casingOD + 2.0m (piping and lifting)

Minimum dimensions: 4m × 3m × 3m

Integration with CompressorMechanicalLosses

The mechanical design integrates with CompressorMechanicalLosses for:

When setDesign() is called, the mechanical losses model is automatically initialized with the calculated shaft diameter.

Usage Example

Basic Mechanical Design

// Create and run compressor
SystemInterface gas = new SystemSrkEos(300.0, 10.0);
gas.addComponent("methane", 1.0);
gas.setMixingRule(2);

Stream inlet = new Stream("inlet", gas);
inlet.setFlowRate(10000.0, "kg/hr");

Compressor comp = new Compressor("export compressor", inlet);
comp.setOutletPressure(40.0);
comp.setPolytropicEfficiency(0.76);
comp.setSpeed(8000);

ProcessSystem ps = new ProcessSystem();
ps.add(inlet);
ps.add(comp);
ps.run();

// Calculate mechanical design
comp.getMechanicalDesign().calcDesign();

// Access design results
int stages = comp.getMechanicalDesign().getNumberOfStages();
double impellerD = comp.getMechanicalDesign().getImpellerDiameter(); // mm
double driverPower = comp.getMechanicalDesign().getDriverPower(); // kW
double totalWeight = comp.getMechanicalDesign().getWeightTotal(); // kg

// Apply design (initializes mechanical losses)
comp.getMechanicalDesign().setDesign();

// Get seal gas consumption
double sealGas = comp.getSealGasConsumption(); // Nm³/hr

Casing Design with Material Selection and NACE

// Configure casing-specific options before calcDesign()
CompressorMechanicalDesign design = comp.getMechanicalDesign();
design.setCasingMaterialGrade("SA-516-70");        // or "SA-182-F316L" for sour
design.setCasingCorrosionAllowanceMm(1.5);
design.setNaceCompliance(true);                    // enable NACE sour-service check
design.setH2sPartialPressureKPa(3.0);             // for NACE assessment
design.calcDesign();

// Access casing calculator
CompressorCasingDesignCalculator casingCalc = design.getCasingDesignCalculator();

double wallThk    = casingCalc.getSelectedWallThicknessMm();   // mm
double mawp       = casingCalc.getMawpBarg();                  // barg
double hydroTest  = casingCalc.getHydroTestPressureBarg();     // barg
int    flangeClass = casingCalc.getFlangeClass();               // e.g. 300, 600
String naceStatus = casingCalc.getNaceComplianceStatus();       // COMPLIANT / NON_COMPLIANT

// Get automatic material recommendation
String recommended = casingCalc.recommendMaterial();

// Full JSON report including all casing analysis
String json = design.toJson();

Standalone Casing Calculator

The CompressorCasingDesignCalculator can also be used independently of a process simulation:

CompressorCasingDesignCalculator calc = new CompressorCasingDesignCalculator();
calc.setDesignPressureBara(150.0);
calc.setDesignTemperatureC(180.0);
calc.setCasingInnerDiameterMm(500.0);
calc.setCasingLengthMm(1800.0);
calc.setMaterialGrade("SA-266-Gr4");
calc.setCorrosionAllowanceMm(1.5);
calc.setJointEfficiency(0.85);
calc.setCasingType(CompressorMechanicalDesign.CasingType.BARREL);
calc.setSourService(true);
calc.setH2sPartialPressureKPa(5.0);

calc.calculate();

// Results
System.out.println("Wall thickness: " + calc.getSelectedWallThicknessMm() + " mm");
System.out.println("MAWP: " + calc.getMawpBarg() + " barg");
System.out.println("Hydro test: " + calc.getHydroTestPressureBarg() + " barg");
System.out.println("Flange class: " + calc.getFlangeClass());
System.out.println("NACE status: " + calc.getNaceComplianceStatus());
System.out.println("Barrel OD: " + calc.getBarrelOuterODMm() + " mm");

// Full JSON with all sections
String jsonReport = calc.toJson();

Design Output Parameters

Process & Sizing Parameters (CompressorMechanicalDesign)

Parameter Method Unit
Number of stages getNumberOfStages() -
Head per stage getHeadPerStage() kJ/kg
Impeller diameter getImpellerDiameter() mm
Tip speed getTipSpeed() m/s
Shaft diameter getShaftDiameter() mm
Bearing span getBearingSpan() mm
Design pressure getDesignPressure() bara
Design temperature getDesignTemperature() °C
Casing type getCasingType() enum
Driver power getDriverPower() kW
Max continuous speed getMaxContinuousSpeed() rpm
Trip speed getTripSpeed() rpm
First critical speed getFirstCriticalSpeed() rpm
Casing weight getCasingWeight() kg
Bundle weight getBundleWeight() kg
Total skid weight getWeightTotal() kg
Module dimensions getModuleLength/Width/Height() m

Casing Design Parameters (CompressorCasingDesignCalculator)

Access via comp.getMechanicalDesign().getCasingDesignCalculator():

Parameter Method Unit
Required wall thickness getRequiredWallThicknessMm() mm
Selected wall thickness getSelectedWallThicknessMm() mm
MAWP getMawpBarg() barg
Hoop stress getHoopStressMPa() MPa
Stress ratio getStressRatio() -
Hydro test pressure getHydroTestPressureBarg() barg
Hydro test acceptable isHydroTestAcceptable() boolean
Flange class getFlangeClass() -
Flange rating getFlangeRatingBarg() barg
Flange rating adequate isFlangeRatingAdequate() boolean
Suction nozzle allowable force getSuctionNozzleAllowableForceN() N
Suction nozzle allowable moment getSuctionNozzleAllowableMomentNm() Nm
Discharge nozzle allowable force getDischargeNozzleAllowableForceN() N
Discharge nozzle allowable moment getDischargeNozzleAllowableMomentNm() Nm
Casing axial thermal growth getCasingAxialGrowthMm() mm
Differential expansion getDifferentialExpansionMm() mm
Thermal growth acceptable isThermalGrowthAcceptable() boolean
Split-line bolt count getSplitLineBoltCount() -
Split-line bolt diameter getSplitLineBoltDiameterMm() mm
Split-line bolts adequate isSplitLineBoltsAdequate() boolean
Barrel outer OD getBarrelOuterODMm() mm
Barrel end cover thickness getBarrelEndCoverThicknessMm() mm
NACE compliance status getNaceComplianceStatus() String
Material SMYS getSmysMPa() MPa
Material SMTS getSmtsMPa() MPa
Recommended material recommendMaterial() String
Applied standards list getAppliedStandards() List
Design issues list getDesignIssues() List

Limitations and Assumptions

  1. Single-shaft configuration - Does not handle integrally geared or multi-body compressors
  2. Empirical correlations - Weight and dimension estimates are approximate; vendor data should be used for detailed design
  3. Steel impellers - Tip speed limit assumes conventional steel; titanium or composites allow higher speeds
  4. Backward-curved impellers - Work coefficient assumes standard backward-curved blade geometry
  5. No intercooling - Multi-stage calculations assume adiabatic compression; intercooled designs require separate handling
  6. Simplified flange derating - Linear approximation of ASME B16.5 pressure-temperature tables; rigorous interpolation should be used for final design
  7. Bolt sizing - Gasket load estimated at 30% of pressure force; consult ASME PCC-1 for detailed gasket analysis

JSON Output Structure

The toJson() method returns a comprehensive JSON report including all design sections. The casing design is nested under the casingDesign key:

{
  "equipmentName": "export compressor",
  "designStandard": "API 617",
  "casingDesign": {
    "inputs": { "designPressure_MPa": 5.0, "materialGrade": "SA-516-70", ... },
    "materialProperties": { "SMYS_MPa": 260, "SMTS_MPa": 485, ... },
    "wallThickness": { "selectedThickness_mm": 16.0, "MAWP_barg": 55.2, ... },
    "hydrostaticTest": { "testPressure_barg": 82.8, "acceptable": true },
    "flangeRating": { "class": 600, "rating_barg": 102.1, "adequate": true },
    "nozzleLoads": { "suctionAllowableForce_N": 24789, ... },
    "thermalGrowth": { "differentialExpansion_mm": 0.45, "acceptable": true },
    "splitLineBolts": { "boltCount": 42, "boltDiameter_mm": 24, ... },
    "naceAssessment": { "status": "NOT_APPLICABLE" },
    "appliedStandards": ["API 617 8th Ed.", "ASME Section VIII Div. 1", ...],
    "designIssues": []
  }
}

References

  1. API 617, 8th Edition - Axial and Centrifugal Compressors
  2. Bloch, H.P. - “A Practical Guide to Compressor Technology”
  3. Japikse, D. - “Centrifugal Compressor Design and Performance”
  4. Lüdtke, K.H. - “Process Centrifugal Compressors”