Machine Foundation Design

Aug 2013

MACHINE FOUNDATION ANALYSIS-ONLY PRACTICAL VIEW A.PAVAN KUMAR

AGENDA :-

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Objective of machine foundation analysis

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Types of machine foundation

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

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Machine foundation analysis

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Modelling options –Solid element,Shell Element

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Softwares Available –ANSYS,SAP 2000, etc

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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

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Possible options of foundations

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Possible options of foundations

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STRUCTURAL DRAWING OF TG BUILDING

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Schematic diagram of machine foundation system

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MODELLING OPTIONS FOR FOUNDATION-SOLID SHELL,PLATE

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MODELLING OPTIONS FOR SOIL-SPRINGS,CONTINUM

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ISOLATION PRINCIPLE and TRANSMISSIBILTY

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REAL PROBLEM-TABLE TOP FOUNDATION-TG FOUNDATIONNAGAI PROJECT

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SCOPE 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 Works

Machine Manufacturer’s Drawings 2 3 4 5

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 CODES FOR DESIGN OF BASE RAFT

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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 Foundations – Foundations For Rotary Type Machines (Medium and High Frequency)

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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

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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 MODELANSYS

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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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  1.

First order natural frequency, f1  1.25*fm or f1  0.8*fm , fm = Machine operating frequency, 50 Hz   f1 = 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:   fn  0.9*fm and   fn+1  1.1*fm   This condition is not met   If condition 2a) is not met, it shall suffice that fn is less than fm where n is equal to 10 or 6.   f10 = 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 25 of applied mass is participated..

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)

MACHIN E FREQ. (Hz)

FREQ. SAPARATION (%)

XTRANS YTRANS ZTRANS ROT-X ROT-Y ROT-Z

1

2.85863

50

94.28274

4

17.6915

50

64.617

2

3.58554

50

92.82892

4 1 4

17.6915 2.85863 17.6915

50 50 50

64.617 94.28274 64.617

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MODE 1

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MODE 2

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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   #1 #2 #3 #4

UNBALANCED FORCE AT RATED SPEED (50 Hz) (Kips) (KN) 8.2 8.2 8.2 8.2

36.3 36.3 36.6 36.6

LOCATION

TURBINE TURBINE GENERATOR 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 applying unbalance forces at all bearing points in both horizontal and vertical directions. 90o phase difference 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 o rotor. The harmonic analysis is carried out with different relative phase angles and it is noted that 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 are34 calculated and tabulated as below.

VIBRATION AMPLITUDES The maximum displacement amplitudes obtained from the harmonic analysis for 2% damping are tabulated below.The same results are presented graphically.The vibration amplitudes are listed on top of deck at corresponding bearing locations.

Vibration Amplitude Table for 2% Damping – Normal Operating BEARIN 2% DAMPING Condition G LOCATI ON

NODE

UX (µm)

UY (µm)

UZ (µm)

1

1750

2.2283 59 2.1000 78 1.4559 65 2.1000 78 0.4765 84 0.9830 78 1.0681

2.0949 875 1.8002 342 0.6442 127 1.8002 342 0.7991 964 2.4001 657 1.4327 693 0.5894 506 -

0.7163 144 0.8717 515 1.5667 955 0.8717 515 0.7032 227 0.5480 38 0.7475 793 1.0218 484 -

2.4001

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1793 2

1560 1524

3

4459 4468

4

4607 4760

UX, MAX UY, MAX

1750 4468

1.2016 05 2.2283 59 -

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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 Sec 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

Vertical

Static Spring 4Gv Rv Kv  Constants 1 

Dynamic Impedance

Horizontal

Rocking

8Gh Rh Kh  2

8Gr Rr Kr  31   

K v  k v  ia 0 cv  K h  k h  ia 0 ch 

3

K r  k r  ia 0 c r 

Torsion 16Gt Rt Kt  3

3

K t  k t  ia 0 ct 

DYNAMIC PROPERTIES • The classic single lumped mass machinefoundation-soil system with circular foundation on elastic half-space summarized by Richart, Woods, HallMotion (1970): Spring Constant Reference Vertical

K

Horizontal Rocking Torsion

Ky 

4G R 1 

32(1   )GR 7  8 8GR 3 K rz  31    16 K ry  G R3 3 Kx 

Timoshenko & Goodier (1951) Bycroft (1956) Borowicka (1943) Reissner & Sagoci (1944)

A Frequency Independent Model, Applied for 0 < a