2. Agitator Blade Bolt Failure Problem - History
• Agitator blade bolt failure problem is being observed in leaching plant
reactors since commissioning .
• The blade bolt frequency is seen to be very high ( every 3-4 months ) in JP
& HL reactors in Hydro-1 plant and CN reactors in Hydro-2 plant. The
frequency is very low ( almost zero) in neutral leaching , WAL and
purification section reactors.
• Initially it was thought that this to be the problem of normal corrosion
and bolts used may not be of SS 904 L material. Hence SS 904 L bolts were
procured and used but even these bolts are found cracked. To avoid the
possibility of bolts breaking after loosening , the bolt heads and nuts were
tack welded. However no improvement was observed
3. Probable reasons
• As the bolt cracking frequency was high in JP and HL section, it was
thought that it may be related to high acidity / high temperature
environment.
• Upon study, it came out that phenomenon could be of stress
corrosion cracking (SCC). Stress corrosion cracking is a special
corrosion process whereas Austenite steel components starts
cracking in presence of following
1. Stress
2. Acidity
3. Chloride ion
4. Probable reason – cont.
• Rate of SCC crack propagation increases with increase in amount of stress
level, acidity and concentration of chloride ion.
• Below a certain limit of stress, SCC crack propagation may not start at all.
• Based on above fact , a simplified load analysis was made for agitator
blade.
5. Agitator Blade – Free Body Diagram
Hydraulic
thrust F
r
R
Agitator shaft
Agitator
Blade
Blade mounting bolts
X
X
Weight of
blade W
6. Agitator Blade Force analysis
• As these agitators have axial flow impellers, the blade movement causes downward force on
liquid and as reaction to it, liquid exerts a hydraulic thrust on blade. For simplicity of analysis
it is assumed that resultant hydraulic thrust F acts on centre of blade as shown in previous
slide.
• This hydraulic force on blade tends to lift blade along the mounting plate edge X-X.
• Assumed all the bolts are located at an average distance of r from edge X-X and are
subjected to stress level of σ.
• Balance of force moments w.r.t. to edge X-X gives
(F-W)R= nσ(π/4*d2)r where n = no. of bolts and d = bolt diameter
σ = 1.27 (F-W) R / d2 r n
Conclusion : Stress level in bolts could be decreased by increasing
1. Distance r between blade mounting plate and bolt holes
2. Increasing bolt size
3. Increasing no. of bolts
7. Action taken / proposed
• In past, the distance “r“ has been increased by extending the blade
mounting plate by insitu welding of additional plate. This has increased
the time between failure from earlier 3-4 months to 20 / 24 months.
• In new reactors JP-8 (Hydro-1) and CN-11 (Hydro-2) no. of bolts were
increased from 6 to 9 and also the distance “r” was increased. These
agitators are working without any blade bolt failure from last 11/12
months.
• It is proposed to increase the no. of bolts from 6 to 9 with increase in
length of bolt mounting plate ( distance r) for at least all the JP/CN
reactors. This would require taking out the agitator shaft and sending it to
workshop .
