Fatigue Failure Analysis of a Cooling Fan Blade: A Case Study

Triloki Nath Kushwaha*

NTPC Limited,Farakka,742236,india

Email:-tnkushwaha@ntpc.co.in

Abstract Cooling tower (CT) fan blade are long in size and made up of FRP(Fibber reinforced plastic) because of their mass saving advantage, high stiffness-to-density etc. During fan operation, the blade undergoes many forces such as gravitational, centrifugal, forces due to fluid (Drag force) and in addition of it many un-design forces such as non uniform airflow through the fan caused by unevenness of blade pitch angle, difference in height of blades etc. These multiple forces cause loading and unloading of the blade numerous times per revolution and is cyclic in nature and causes fatigue failure of the blade. The failure of one blade also damage other blades, drive shaft and motor frame leg and hence causes huge financial impact as well as fan availability. In this article, the author present a case study of fatigue failure analysis of the fan blade used in a 500MW unit of a thermal power plant. A 3D modelling of the blade has been developed by Pro-Engineer based on dimensions of the actual blade used at site and same FEM analysis is carried out by Ansys software. Concurrence between FEM results and observations of failing pattern at site has been found.

Key words: CT Fan, Blade, Fatigue failure, FRP, FEM

1. Introduction

Thermal power plants use a significant volume of water to dissipate the heat of the hot water received from the condensers. Cooling tower (CT) are used in thermal power plant to cool the circulating water are an important part of power plants as it has direct impact on unit efficiency. Induced draft cooling tower as shown in Fig. 1 which required fan to create the daft across inlet and outlet of the tower

IDCT fan availability is vital for the power plant efficiency as well as to keep unit running. The fan assembly consists of fan blades, hub plate and holding V-block gear box and drive shaft shown in Fig. 3

Fig:-1.A top view of CT showing fan and stack Fig.:-2. Exploded view of The fan assembly

Fig:-3. Schematic representation of the cooling fan assembly, gear box, drive shaft and motor

The blades are made up of FRP (Fibber reinforced plastics) the nature of FRP allows us to make any desired shape and twist of the blade as needed to achieve maximum performance. A properly designed fan blade will have a non-symmetrical air foil with a variable chord and twist to compensate for the peripheral change in speed from the blade tip to the wide chord.

. Fig.:-4. Schematic representation of the cooling fan blade.

2. Problem Definition

The blade of IDCT fan of the 500MW thermal power unit was failing while in operating at its design

point although the fan having its on line protection system for high vibration (velocity above 10mm/s)

of its gear box assembly. As blade having low RPM (105) and made up of FRP as shown in Fig.4 the

pre failure symptom could not got acknowledge many times by accelerometer mounted at its gar box

casing. The damage of any one blade become primary reason of failure for the other components like

drive shaft and motor leg as shown in Fig.-3a-e. The failure is in catastrophic in nature and hence

causes huge financial impact as well as availability of the fan

a b

Fig. 3 .Actual image of damaged Fan components(a)Complete Fan Assembly with Gear box (b) Failed fan blade

(c)failed from its neck side (d)Fan motor Damaged leg (e)Damaged drive shaft

2. Material and method

FRP composites are frequently used in such structural applications, Because of their excellent formability, their mass saving advantage, their high stiffness-to-density and strength-to-density ratios and the greater freedom to use these properties in the desired orientation and position. Furthermore, these lightweight structural materials have some precise objectives, which cannot be reached with some other conventional materials. The necessary raw materials for the fan blade include Fibber Reinforced Epoxy Resin in various forms like chopped strand mat, ravings, woven ravings, cloth surface mat and resin (vinyl ester & polyester), catalysts, accelerator, hardener, pigments, surface treatment agents. The material property of Discussed FRP is given in Table-1.

Fig:-4 Actual image of FRP blade used at site

Table -1: Material Properties of FRP Composite

4. Forces on fan blade During its operation, the fan blade undergoes many forces such as gravitational, centrifugal and air resistance force (Drag force). These forces can cause the failure of the blade .Many factors such as variation of the centrifugal stress, shut up/down of the cooling system and mass imbalance make the fatigue to be the most probable cause of failure in cooling fan systems. The various forces acting during fan operation are shown in Fig.5 the primary component of the blade force on the air is directed axially from inlet to outlet. The blade force necessarily has an additional component in the tangential direction, providing the reaction to the driving torque. Since mounting orientation of cooling fan shaft is horizontal, weight of blades and its moment must be considered in analysis. From input data and detailed results for the fan type, weight, Fw centrifugal, Fc axial, Fa and tangential, Ft forecasting on blades are calculated as follows

c e d

Fw = mg =(64 kg)x ( 9.81m/s

2)=627.84

Fc = mrmw

2 = 64 kg x (1.93 m)x(10.9red/sec)

2

=1357.48N Fa =Axial thrust/6 =9761/6=1626.8 N Ft =Fan shaft power/6 x rA x w =123; 700 W

Fig. 5 Applied forces on the cooling fan assembly.

Where m is the mass of each blade and w is the angular velocity of blades. rm is the distance between the origin of the coordinate in the centre of the hub plate and mass centre rA is the distance between the origin of the coordinate in the centre of the hub plate and area centre of blades. After the blade is modelled, rm and rA are calculated using modelling software Pro-E. The directions of applied forces on blades of the cooling fan are shown in Fig. 5. It is noted that the condition of loading is multi axial. Among all forces, the only alternating force is weight force. Except the weight, all forces acting on the blades have fixed direction in relative coordinate system whose axes are rotating with the blade. To obtain the stresses on the blades the forces are exerted to the blade, which contact with hub plate and V-block.. Geometry modelling of the cooling fan including fan blades, hub plate and V-block is shown in Fig. 5.

4.3 Blade failure analysis A failure analysis of twisted FRP blade has been carried out at site by its failure pattern, following

points were observed:

(a)Fan blade is failing due to excessive deflection caused by crack initiation at its root

(b)The excessively deflected blade hits other fan components such as drive shaft got beaked.

(c)As every time blade failed has some similarity about weakest failing cross section.

(d)Failed blade indicates that its weakest cross-section is neck and at distance ¾ of its total length

(L) from tip end.

(e) Failure of any one blade becomes primary reason of failure of other Fan components.

3. Finite element analysis of FRP blade Pro-Engineer software is used for geometry modelling of CT fan blade individually and as complete fan assembly with hub and V-block as shown in Fig. 5. Force applied to the finite element model is according to the operating condition the fan model MPI1000. An axial fan blade can be considered as cantilever beam on which air pressure and centrifugal force are applied Blade failure due to different forces during its operation is analysed by fixing its one end at hub by V-block as shown in Fig. 0 and setting the blade with 16 degree angle with horizontal.FRP composite blade can be considered as a stepped, taper, continuous cantilever beam. The circular (hob) section of FRP blade has been fixed. Ansys simulation has been done by doing modal analysis using ANSYS Workbench. A three dimensional model of blades has been generated in Pro-Engineer. The modal analysis in Ansys is carried out. In modal analysis, first refresh the material and geometry then do meshing, further in an analysis system select number of modes required, Apply fixed support at circular section of the blade then, Find the solution. The Solution gave maximum principle stress, maximum shear stress and Total deformation.

Fig. 6. (a) A 3D Blade modelled in Pro-E (b) Mesh generated in Ansys (c) Blade (FEM) showing maximum principle stress (d) shear stress distribution

5. Results and discussion

On analysis of failed blade by Visual inspection revealed that the blade is failing repeatedly at mainly

two point one at its shank/neck portion shown in fig.8a. and second near 1/3 of the blade length

Fig.8b. From its tip side.

a b

c

d

Fig. 7 FEM result showing distribution of the stress (a) At shank portion (b) Near 1/3 of the blade length from its

tip end

5.1 Results of FEM Simulation

In line with observation from on site failing pattern and data analysis a FEM analysis of CT fan blade

also carried out by use of Ansys software. It involves study of different modal parameters of system.

Ansys simulation has been used to analyze the modal various parameters of FRP blades. Thus

avoiding the need for carrying out several experiments which is time consuming and costly. The

results presented ahead are the most stressed section of the blade from which probability of blade

failing is highest .The section of blade with higher stress is also shown in Fig.8 (a) and Fig.8 (b) .

Fig. 8. Image showing actual crack location of fan blade (a)Crack at the neck/shank of the blade (b) Crack from

L ¾ of blade from tip side

6. CONCLUSIONS

Analysis of loading condition indicates that the blade is under multi axial fatigue. The finite element analysis shows that the nodes having the high stress values are near the neck/shank of the blade and L 1/3 length of the blade from its tip. As compared ANSYS results, with site failure pattern, it is being observed that Good agreement

between ANSYS results and experimental results has been found in the failure for a the FRP

Composite blade. FEM results are close to the site results. Therefore the above obtained FEM failure

analysis can be considered to increase the reliability of the blade while designing it.

b a

a b

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