Dip tube failure in

5 th stage cyclone5 th stage cyclone

Abrasion and wear Wrong selection

of metallurgy

Causes for failure

Thermal stressesThermo chemical stresses

CO,Alkali and sulfur attack

Phase transformation

Like sigma formation

Abstract:Steels are now used for a wide variety of conditions entailing heat and corrosion under both static and dynamic stresses, such as aero engine valves, furnace conveyers, retorts, oil cracking units and gas turbines. Three important properties are necessary in material used at elevated temperatures:1. Resistance to oxidation and to scaling.2. Retention of strength at the working temperature. 3. Structural stability as regards carbide precipitation,

spheroidisation, sigma formation and temper embrittlement.

Prerequisites for selection of steel grade

HRCS steel grade casting has good wear

resistance and failure due to purely on abrasion

is ruled out. But certain phase transformation and corrosion

can cause embrittlement and reduce the wear resistant.

At tempertures around 700 to 850 deg C. Alpha Fe2O3

may be converted into Zigma form which causes

embrittlement and causes wear.

spheroidisation, sigma formation and temper embrittlement.

CrNi-Mo steelsCrNi steels containing Mo are prone to sigma phase formation between 500 and 850deg C , with a maximum rate of formation at around 750 deg C.

25-12 CrNi and 25-20 CrNi steels

In most austenitic stainless steels containing about 25%Cr and 12 to 20% Nisigma phase is formed between 650 and 900 deg C, with a maximum rate of

formation at 850 C.

18-8 CrNi steels stabilized with niobium or titanium are prone to

18-8 CrNi steels

Sigma formation

Austenitic stainless steel

18-8 CrNi steels stabilized with niobium or titanium are prone to sigma formation between 650 and 800 degC, with a maximum rate of formation at 750 deg C. Because of their good resistance against creep and oxidation these steels are still used at the temperatures mentioned; by choosing a somewhat lower Cr content and somewhat higher Ni content, the sigma phase formation is kept to a minimum.

Sigma PhaseSigma phase is an intermetallic compound which can precipitate on grain boundaries as a secondary phase.The general composition of sigma phase is (Fe,Ni)3(Cr,Mo)2.

Carburization of Steel by Carbon and Carbon Monoxide

Carbon present in the steel can react with carbon dioxide to form carbon monoxide; C(Fe) + CO2 <==> 2 COThe equilibrium situation of this reaction and the subsequent carburizing cq. decarburizing effect of carbon oxides depends on the respective partial pressures of carbon monoxide and carbon dioxide and on temperature, and is represented by the Bouduard curve.The change of the carbon content of a steel can be minimized by selecting an

BouduardThe change of the carbon content of a steel can be minimized by selecting an alloy that has a carbon content close to the equilibrium amount for the gas in question. Additions of chromium, nickel or silicon to iron make the metal more resistant against carburization.phases formed when iron is brought in contact with carbon oxides depend on the temperature and the relative levels of carbon monoxide and carbon dioxide. The relation between these levels also depends on the temperature and can be represented in the equilibrium diagram as a curve, the so-called Bouduard curve, representing the equilibrium condition of the reaction between oxygen and carbon oxides: C(Fe) + CO2 <==> 2 CO

Bouduard curve

Pictures of Sigma formation

Raw material chemistry and fuel chemistry must be considered

before selecting steel alloys as Sulfur attacks Nickel and Alkalis attack

chromium. Chloride s attack any metals under thermal conditions

Yellow color shows alkali attack on chromiumYellow color shows alkali attack on chromium

Normally dip tubes in the lower most cyclones ( 5 th or 6 th stage)are made

of High heat resistant steel casting , grade - 7

Conclusion

The main reasons to cause Dip tube failure in Cyclone-5

are

• Heavy CO formation can corrode iron. It activates volatile cycle of sulfur and

alkali . Sulfur attacks Nickel and Alkali attacks chromium

• Alkali in the raw mix can attack Chromium as the alkali content in clinker is • Alkali in the raw mix can attack Chromium as the alkali content in clinker is

0.5 % and may rise to 0.9 to 1.0 % in hot raw meal to kiln due to volatile

recycle.

• Frequent stops and HFO firing can cause sulfur cycle which can attack Nickel.

• Embrittlement and wear is due to Sigma formation which needs modified

Metallurgy in the alloy being used .Nickel and chromium ratio needs a

modification to improve corrosion resistance

Solution

• Avoid CO formation

• Avoid frequent shut downs

• Ni-Cr ratio needs modification as per the • Ni-Cr ratio needs modification as per the

rawmeal and fuel chemical composition

• The temperature at which the sigma formation

is to be identified and metallurgical modification

Thanks

K.P.PRADEEP KUMAR

Sr. G M Operation

Chettinad Cements LtdChettinad Cements Ltd

Ariyalur