Machine Foundation Design

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MACHINE FOUNDATION ANALYSIS-ONLY PRACTICAL VIEW A.PAVAN KUMAR

AGENDA :•

Objective of machine foundation analysis

•

Types of machine foundation

•

Codes available –DIN 1024,IS 2974,VDI Guidelines,ACI 351

•

Machine foundation analysis

•

Modelling options –Solid element,Shell Element

•

Softwares Available –ANSYS,SAP 2000, etc

•

Real Problem -2*125MW Turbo Generator Foundation

2

AGENDA :•

Objective of machine foundation analysis

•

Types of machine foundation

•

Codes available –DIN 1024,IS 2974,VDI Guidelines,ACI 351

•

Machine foundation analysis

•

Modelling options –Solid element,Shell Element

•

Softwares Available –ANSYS,SAP 2000, etc

•

Real Problem -2*125MW Turbo Generator Foundation

2

DESIGN OVERVIEW •

Design Criteria: The basic goal in the design of a machine foundation is to limit its motion to amplitudes that neither endanger the satisfactory operation of  the machine nor disturb people working in the immediate vicinity. (Gazetas 1983)

Performance Criteria

4

Possible options of foundations

5

Possible options of foundations

6

STRUCTURAL DRAWING OF TG BUILDING

7

Schematic diagram of machine foundation system

8

MODELLING OPTIONS FOR FOUNDATIONSOLID SHELL,PLATE

9

10

MODELLING OPTIONS FOR SOILSPRINGS,CONTINUM

11

12

ISOLATION PRINCIPLE and TRANSMISSIBILTY

13

14

REAL PROBLEM-TABLE TOP FOUNDATION-TG FOUNDATION-NAGAI PROJECT

15

The objective is to study the dynamic behavior of Turbine Generator  (TG) pedestal under normal operating conditions and also emergency conditions for 2X150 MW Nagai Thermal Power Plant located at Nagapattinam (Dist), Near Okku & Venkidanathangal Villages, Tamilnadu State, India.

The following checks with relevant structural analysis have been carried out to accomplish the above object. Natural Frequency check – Modal analysis is carried out in ANSYS software to elicit the natural frequencies of machine-foundation system for all significant modes of vibration. The natural frequencies are checked with relevant provisions of DIN 4024 Part1. Vibration amplitude check – The absolute maximum amplitudes are obtained by performing steady state harmonic analysis of STG foundation in ANSYS and checked according to VDI-guideline 2056, Machine group „T‟

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DOCUMENTS WE RECEIVE,CODES NEED TO REFERRED Project Reference Drawings / Documents 1 Design Basis Report for Civil, Structural and Architectural W orks

Machine Manufacturer’s Drawings 2 2165-T-1-VVG-C-501

Turbine Foundation Loads

2165-T-1-VVG-C-502

Turbogenerator Acoustic Enclosure Foundation Loads

2165-T-1-UMP-C-501

Turbogenerator Foundation Drawing Plan View & Sections

2165-T-1-VVB-M-501

Turbogenerator General Outline Plan View & Sections

3 4 5 CODES FOR DESIGN OF BASE RAFT 6 DIN 4024 (Part1)

Machine Foundations - Flexible structures which supports machines with rotating elements

DIN ISO 1940-1

Balance Quality Requirements for Rotors in a constant (rigid) State

IS 2974 (Part 3)

Design and Construction of Machine F oundations – Foundations For Rotary Type Machines (Medium and High Frequency)

7 8

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MATERIAL DETAILS Material

Concrete, C40

Property

Value

Units

Density

25

kN/ cum

Characteristic Strength

40

N/ Sq mm

Modulus of Elasticity

32500 (Dynamic)

Remarks

IS-456 (2000)

N/ sq mm

IS-2974 (Part 3)

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LOADS WE RECEIVE FROM MECHANICAL PEOPLE LOAD POINT G1 G2 G3 Generator  G4 G5 G6 T1 T2 T3 T4 T5 Turbine T6 T7 T8 T9 T10 T11 I1 I2 Interceptor Valve I3 I4 R1 Non return valve R2 P1 Throttle and P2 Regulation valve P3 P4

FY -650 -650 -650 -650 -268 -122 -392 -122 -268 -392 -141 -542 -30 -30 -30 -30 -30 -30 -51 -51 -51 -51

DEAD LOAD MX -22 29 -22 29

MZ 75 86 75 86

Normal operation Vacuum Load Friction load due to expansion Generator F FY FY FX FZ FY -89 135 135 -1790 89 135 135 1790 -89 135 135 -1790 89 135 135 1790 49 88 49 81 -343 213 213 -196 111 111 -196 205 205 -196 111 111 -81 -343 213 213 -196 205 205 -98 49 49 189 189 300 75 300 75 162 648 0 0 0 0 0 0 0 0 0 0 17 17 17 17 17 17 17 17 -

Turbine D FY 747 427 427 427 427 747 427 213 608 -

Seismic FX FZ -39 -39 -39 -39 -39 -39 -39 -39 -16.08 -16.08 -7.32 -7.32 -23.52 -23.52 -7.32 -7.32 -16.08 -16.08 -23.52 -23.52 -8.46 -8.46 -32.52 -32.52 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -1.8 -3.06 -3.06 -3.06 -3.06 -3.06 -3.06 -3.06 -3.06

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DESCRIPTION AND MODELING OF STRUCTURE

The geometry is considered as per foundation outline drawing. The columns are assumed to be fixed on top of base raft at FL ( –)4.05m. The top deck level is considered as FL (+) 12.0m & FL(+) 11.2m for Turbine & Generator respectively. It can be seen from the geometry that the TG pedestal is built-up of large sections. Hence, the solid brick finite elements are used to represent the geometry for dynamic analysis. The solid model is built in ANSYS software based on this geometry and then the finite element is created by mapped mesh using brick elements. The mapped volume mesh contains only hexahedron elements. Basic geometric dimensions are: Top deck thickness at E.L.11.2 = 1700mm Sizes of columns = 1600X1600, 2540X1600, 2500X1600 mm Thickness of deck at E.L.+12.0 = 2500mm .

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SOLID MODEL-ANSYS

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MESHED SOLID MODEL-ANSYS

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SUPPORT CONDITIONS

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MODAL ANALYSIS  – NATURAL FREQUENCIES

The Mode-Frequency analysis for natural frequency and mode shape determination is carried out in ANSYS. The assumptions made in this analysis are •The structure has no time varying forces, displacements, pressures, or temperatures applied, which means that this is free vibration analysis. •There is no damping in the structural system. •The structure has constant stiffness and mass effects. 3D MASS 21 element (from ANSYS element library) is used to represent machine mass application points on top of deck. The natural frequencies are obtained for first seventy five modes of vibration.

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Estimation according to DIN 4024 Part 1, Clause 5.3.2: 1.

First order natural frequency, f 1 1.25*f m or f 1 0.8*f m , f m = Machine operating frequency, 50 Hz f 1 = 2.8586 Hz

0.8*50 = 40 Hz

Hence condition 1 is o.k.

2) Higher order natural frequencies Higher order natural frequencies that approach the service frequency: f n

0.9*f m

f n+1

1.1*f m

and

This condition is not met If condition 2a) is not met, it shall suffice that f n is less than f m where n is equal to 10 or 6. f 10 = 27.3487

50 Hz

Hence clause 2b) is satisfied. From the above frequency table, it can be seen that the fundamental structural frequencies are within 30 Hz where the predominant portion of applied mass is participated.. 25

TUNING OF MASS AND STIFFNESS

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FREQUENCY SEPARATION CRITERIA Estimation according to IS 2974 Part 3: From the above Table it is clear that the Frequecy saparation in any mode is atleast 20% which meets the criterion specified in IS 2974 Part 3.

MODE

MODE

NATURAL FREQ. (Hz)

MACHINE FREQ. (Hz)

FREQ. SAPARATION (%)

X-TRANS

1

2.85863

50

94.28274

Y-TRANS

4

17.6915

50

64.617

Z-TRANS

2

3.58554

50

92.82892

ROT-X

4

17.6915

50

64.617

ROT-Y

1

2.85863

50

94.28274

ROT-Z

4

17.6915

50

64.617

27

MODE 1

28

MODE 2

29

MODE 3

30

MODE 4

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HARMONIC ANALYSIS  – VIBRATION AMPLITUDES

The harmonic response analysis for obtaining forced vibration amplitudes. This analysis solves the time-dependent equations of motion for TG foundation undergoing steady-state vibration. The assumptions made in this analysis are The entire structure has constant stiffness, damping, and mass effects. The structure damping of 2% is considered in the harmonic analysis for normal operating condition in accordance with Cl. 9.1.1 f) of IS 2974 Part-3.  All loads and displacements vary sinusoidal at the same known frequency (50 Hz in present analysis case). The harmonic load is specified in ANSYS with three pieces of information the amplitude, the phase angle, and the forcing frequency range . The amplitude is the maximum value of the load. The phase angle is a measure of the time by which the load lags (or leads) a frame of reference. The phase angle is required only if multiple loads are present that are out of  phase with each other. The bearing locations are shown indicatively below. 32

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BEARING

UNBALANCED FORCE AT RATED SPEED (50 Hz)

LOCATION

(Kips)

(KN)

#1

8.2

36.3

TURBINE

#2

8.2

36.3

TURBINE

#3

8.2

36.6

GENERATOR

#4

8.2

36.6

GENERATOR

Unbalanced forces at bearings Bg-1 to Bg-4 are distributed on the foundation top as per the given Drawing. The excitation forces applied in the analysis are listed in below table. . The unbalanced force can be acting at all the bearings simultaneously, with random distribution of  the relative phase angles. The peak vibration amplitudes are calculated by performing harmonic response analysis by applyin unbalance forces at all bearing points in both horizontal and vertical directions. 90 o phase differen is considered between horizontal and vertical directions. The unbalanced force at each bearing point is applied at two points on top of foundation symmetrical to centerline of rotor. The lever arm effect due to horizontal force acting at bearing point at higher elevation is considered in form of push and pull on top of foundation on either side of rotor. The harmonic analysis is carried out with different relative phase angles and it is noted th the maximum displacement amplitude is occurring for the case of same phase angle for unbalance forces applied at all bearing points. The unbalanced forces at each bearing point are calculated and tabulated as below. 34

VIBRATION AMPLITUDES The maximum displacement amplitudes obtained from the harmoni analysis for 2% damping are tabulated below.The same results ar  presented graphically.The vibration amplitudes are listed on top of dec at corresponding bearing locations. Vibration Amplitude Table for 2% Damping  – Normal Operatin BEARING 2% DAMPING Condition LOCATION

NODE

UX (µm)

UY (µm)

UZ (µm)

1

1750

2.22835 9 2.10007 8 1.45596 5 2.10007 8 0.47658 4 0.98307 8 1.0681

2.09498 75 1.80023 42 0.64421 27 1.80023 42 0.79919 64 2.40016 57 1.43276 93 0.58945 06 -

0.71631 44 0.87175 15 1.56679 55 0.87175 15 0.70322 27 0.54803 8 0.74757 93 1.02184 84 -

2.40016 57 -

-

1793 2

1560 1524

3

4459 4468

4

4607 4760

UX, MAX

1750

UY, MAX

4468

1.20160 5 2.22835 9 -

UZ, MAX

1560

-

1.56679

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From the above table it can be seen that the vibration amplitudes in both directions are very less and well within the manufacturer‟s specified limits and also VDI guideline. This is also obvious from the natural frequency table in Se 3.0 that the contribution of vibration modes to amplitude response in concentrated around lower modes only and its effect is tapered off towards higher modes. Rating according to VDI-guideline 2056, Machine group „T‟ (Refer to chart in next page)  At 50 Hz: Amplitudes < 12.5 µm ≡ Rating: “Good” (2% Damping) Hence, the foundation system adopted is classified as Good for normal operating conditions.

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Vibration Amplitude in Y direction for node 4468

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DYNAMIC PROPERTIES

Dynamic Equilibrium Equation:

 M  X   C  X    K  X    F (t ) In Veletsos Model, the Dynamic Impedance Expressed as:





 I    K  s  k d  (a0 )  ia0 cd  ( a0 )

Mode Static Spring Constants Dynamic Impedance

Vertical  K v

Horizontal

4Gv Rv

 K h



1



 K v k v







 





ia0 cv  K h k h

Rocking 3

8Gh Rh 2





 K r 



 

ia 0 ch

Torsion





8Gr  Rr 

 K t 

31   

 K r  k r 



ia 0 c r 







16Gt  Rt 

 K t  k t 

3

3

 ia 0 ct  

DYNAMIC PROPERTIES

•

The classic single lumped mass machine-foundation-soil system with circular foundation on elastic half-space summarized by Richart, Woods, Hall (1970): Motion Vertical Horizontal Rocking Torsion

Spring Constant  K  y

4G R 

 K  x  K rz 

 K ry

1



Timoshenko & Goodier (1951)

 

32(1 

7





8G R 



31



16 

Reference

3

 )G R

8 

Bycroft (1956)

3



Borowicka (1943)

 

G R 3

Reissner & Sagoci (1944)

A Frequency Independent Model, Applied for 0 < a0