Machine Foundation Design

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A

A u u

g g 2 2 0 0 1 1 3 3

A.PAVAN KUMAR

MACHINE FOUNDATION ANALYSIS-ONLY

PRACTICAL VIEW

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•

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

AGENDA

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•

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

AGENDA

:-2 2

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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)

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Performance Criteria

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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

FOUNDATION-NAGAI PROJECT

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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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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 3

2165-T-1-VVG-C-502 Turbogenerator Acoustic Enclosure Foundation Loads 4

2165-T-1-UMP-C-501 Turbogenerator Foundation Drawing Plan View & Sections 5

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

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DIN ISO 1940-1 Balance Quality Requirements for Rotors in a constant (rigid) State 8

IS 2974 (Part 3) Design and Construction of Machine F oundations – Foundations For Rotary Type Machines (Medium and High Frequency)

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Material Property Value Units Remarks

Concrete, C40

Density 25 kN/ cum

Characteristic

Strength 40 N/ Sq mm IS-456 (2000)

Modulus of Elasticity 32500 (Dynamic) N/ sq mm IS-2974 (Part 3)

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Normal operation Vacuum Load Generator F Turbine D

LOAD POINT FY MX MZ FY FY FX FZ FY FY FX FZ G1 -650 - - -89 - 135 135 -1790 - -39 -39 G2 -650 - - 89 - 135 135 1790 - -39 -39 G3 -650 - - -89 - 135 135 -1790 - -39 -39 G4 -650 - - 89 - 135 135 1790 - -39 -39 G5 - - - 49 - - - -G6 - - - 88 49 - - - -T1 -268 - - 81 -343 213 213 - 747 -16.08 -16.08 T2 -122 - - - -196 111 111 - 427 -7.32 -7.32 T3 -392 - - - -196 205 205 - 427 -23.52 -23.52 T4 -122 - - - -196 111 111 - 427 -7.32 -7.32 T5 -268 - - -81 -343 213 213 - 427 -16.08 -16.08 T6 -392 - - - -196 205 205 - 747 -23.52 -23.52 T7 -141 - - - -98 49 49 - 427 -8.46 -8.46 T8 -542 - - - - 189 189 - 213 -32.52 -32.52 T9 - - - 300 75 - 608 - -T10 - - - 300 75 - - - -T11 - - - 162 648 - - - -I1 -30 - - - - 0 0 - - -1.8 -1.8 I2 -30 - - - - 0 0 - - -1.8 -1.8 I3 -30 - - - -1.8 -1.8 I4 -30 - - - - 0 0 - - -1.8 -1.8 R1 -30 - - - - 0 0 - - -1.8 -1.8 R2 -30 - - - - 0 0 - - -1.8 -1.8 P1 -51 -22 75 - - 17 17 - - -3.06 -3.06 P2 -51 29 86 - - 17 17 - - -3.06 -3.06 P3 -51 -22 75 - - 17 17 - - -3.06 -3.06 P4 -51 29 86 - - 17 17 - - -3.06 -3.06 Seismic

Non return valve Throttle and Regulation valve Interceptor Valve Generator

Turbine

DEAD LOAD Friction load due to expansion

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20 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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MODAL ANALYSIS

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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 and

f _n+1 1.1*f _m

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..

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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

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.

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HARMONIC ANALYSIS

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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.

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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. 90o _{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.

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

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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 Condition BEARING LOCATION 2% DAMPING NODE UX (µm) UY (µm) UZ (µm) 1 1750 2.22835 9 2.09498 75 0.71631 44 1793 2.10007 8 1.80023 42 0.87175 15 2 1560 1.45596 5 0.64421 27 1.56679 55 1524 2.10007 8 1.80023 42 0.87175 15 3 4459 0.47658 4 0.79919 64 0.70322 27 4468 0.98307 8 2.40016 57 0.54803 8 4 4607 1.0681 1.43276 93 0.74757 93 4760 1.20160 5 0.58945 06 1.02184 84 UX, MAX 1750 2.22835 9 - -UY, MAX 4468 - 2.40016 57 -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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DYNAMIC PROPERTIES

In Veletsos Model, the Dynamic Impedance Expressed as:



k

(

a₀

)

ia₀c

(

a₀

)



K

I _ _s _ _d _ _d

Dynamic Equilibrium Equation:

 

M

 

X



_

 

C

 

X



_

 

K

   

X

_

F

(

t

)

Mode Vertical Horizontal Rocking Torsion Static Spring Constants Dynamic Impedance    1 4 _v _v v R G K    2 8 h h h R G K     1 3 8 r r 3 r R G K 3 16 _t _t3 t R G K _



v v



v k ia c K _ ₀





h h h k ia c K 0  K r



k r ia0cr



K t



k t ia₀ct



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DYNAMIC PROPERTIES

• The classic single lumped mass machine-foundation-soil system with

circular foundation on elastic half-space summarized by Richart, Woods, Hall (1970):

A Frequency Independent Model, Applied for

0 < a

0

<1.0

Motion Spring Constant Reference

Vertical Timoshenko & Goodier (1951)

Horizontal Bycroft (1956)

Rocking Borowicka (1943)

Torsion Reissner & Sagoci (1944)

   1 4GR K _y   8 7 ) 1 ( 32    R G K _x      1 3 8G R3 K _rz 3 3 16 R G K _ry 

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DYNAMIC PROPERTIES

s V R a

_





0 Dimensionless frequency, a₀ Where:

ω: machine speed – equipment; R: foundation radius – foundation; V_s: shear wave speed – soil.

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DYNAMIC PROPERTIES

b₁to b₄in expression above are dimensionless functions of μ. Given by Veletsos for different type of

soils.

Veletsos’ Model – Dynamic Stiffness and Damping

Coefficients:

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DYNAMIC PROPERTIES

•

Veletsos Model, k

_x

& c

_x

to

Frequency Relation in

Horizontal Mode:

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c

_x

is independent of a

0

, or the

frequency.

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k

_x

in sandy soil is kind of

sensitive to a

0