LD VESSEL OXYGEN LANCE


OXYGEN LANCE

LD Process oxygen lance is fed to the furnace through a water cooled lance. The lance is made of three concentric steel tubes to circulate water around the central tube and concentric Steel Tube and passes oxygen through the most inner tube. Therefore the tip of the Lance is exposed water cooling is made of more effective over there by using a property which is weird to the Steel tubes.

● The oxygen lance is nearly 8-10m long and it's diameter varies with furnace capacity in the range of 20 to 25cm. Water requirements are around 50 to 70 m3/ hr at a pressure of 5 to 7 kg/cm2. The oxygen lance is suspended by a wire rope can be inserted in or withdrawn from the furnace by the helps of an electrically operated lance gear. Jigs are used to hold the oxygen lance in a fixed blowing position. Safety devices are employed to automatically withdrawn the oxygen lance, should the cooling water temperature rise above a maximum specified safe value ~40℃.

● Extra holding devices are provided for emergency, when the usual arrangements fail. An additional oxygen lance is provided as a stand by immediately replace a faulty one to another to continued the blow. It takes a few minutes to replace a faulty lance so that the heat can proceed without any harm done to it.


OXYGEN LANCE DESIGN

● The LD process was developed using oxygen lance with a cylindrical nozzle. The successful development and commercial adoption of the LD Process later on led to the study of physics of the supersonic jets and thereby develop a proper oxygen lance design.

● It is now known that supersonic jet issuing from the nozzle of a oxygen lance in LD Process should penetrate the bath adequately and that the area of its impact on the bath should be maximum. These conditions are essential for efficient refining I.e. for decarburization as well as dephosphorisation.

LD VESSEL OXYGEN LANCE
Oxygen lance

● The static pressure in a jet form a cylindrical nozzle, as it emerges in to the ambient atmosphere, is more than the atmospheric pressure. It interact with the atmosphere generating shock waves and the velocity of the jet decrease with damped fluctuations.

● Oxygen is generally blown at 8 to 10 atmosphere pressure through a leval shaped nozzle so that the jet issuing at the nozzle exit is supersonic and generally has a velocity between 1.5 to 2.5 time the velocity of sound. The characteristics of a supersonic jet, as emerging from a laval nozzle and impinging on the liquid metal bath. The jet has characteristically a potential core, a supersonic core and a supersonic region. As the jet travels, its velocity is retarded due to the ambient atmospher, the supersonic surrounding zone expand rapidly. The potential core may normally extend of about 15 times of diameter of the nozzle from the nozzle tip.

● The velocity of the supersonic core grudually decrease until at a distance of about 30 times the nozzle diameter from nozzle tip, the jet becomes wholly subsonic. This point marks the end of supersonic core and the development of a fully expanded jet. During blow the jet should be expanded to obtain maximum impact area at the surface. At the same time, it should also penetrate the bath surface to maximum extent. The depth of penetration of a jet metal bath varies inversely with impact area at the bath surface.


LD Vessel oxygen lance blowing
Oxygen blowing 



● In the blowing position the lance height from the steel bath level has to be more than the length over which the supersonic core extend in the jet, since the jet is not fully expanded until that point.

● In actual practice the proper height should be around 40 to 50 times the daimeter of the nozzle. The depth of penetration of a jet in a bath can be : 

JFN = gas pressure × Nozzle throat dia / height of nozzle.

Condition : better decarburization is faster for greater JFN value and dephosphorisation is faster for its reverse.
● The gas flow rate from a nozzle can be calculated by assuming a frictionless and adiabatic flow through nozzle. In the supersonic oxygen jet the exit temperature may be lowered by much as 100 to 120℃ because of the joule Thomas effect. Circulating water cools the lance up to nozzel throat and the nozzle itself.

● There are three and six hole lance carried out for blowing. The nozzle are at 17° angle inclination to vertical axis and comparation between both are : 
1. Increase in total through put oxygen without any adverse effects at the same pressure.
2. Improvement in jet spread on metal bath. These two lead to 
a. Less of slopping and spitting and thus less of mechanical losses in turn better yield.
b. Improve mixing of slag and metal and thereby better mass transport and hence better rate of refining.
c. Less of danger of burning vessel bottom in spite of increase oxygen blowing rate.
d. Better gas recovery and improve lining life.
e. Better thermal balance and hence more of coolant scrap or ore is required.
f. Improve slag basicity from around 3 to 3.5
g. Much improved turndown %P from early 0.060 to 0.017
h. High residual Mn in the bath so that less of FeMn is subsequently required for deoxidation.








References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty


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