3.6　Seamless Steel Tube Reheating Furnace

3.6.1　The design of seamless steel tube heating refurnace shall conform to the following provisions.

1　The fuel of seamless steel tube heating furnace shall be gas or fuel oil.Except for application of regenerative burner，the calorific value of gas shall be greater than 7500KJ/m3.Mandrel preheating furnace can use afterheat of the fume exhausted from other furnaces in the same workshop.

2　Rotary hearth furnace should be selected for tube heating before rolling mill，and walking beam furnace for tube preheating before diameter reducing and sizing mill.

3　Automation and thermal measurements must be set up for mandrel preheating furnace，and for furnace which will be used to heat tube before rolling mill，diameter reducing and sizing mill.

3.6.2　The body design of rotary hearth furnace shall conform to the following provisions.

1　Nominal size of rotary hearth furnace should be marked as"furnace center diameter×furnace inside width".

2　Heating rate of tube blanks in rotary hearth furnace can refer to the Table 3.6.2.Heating time of tubes is relevant to placement of tubes in the furnace，distribution of the furnace heat load as well as fuel type.Select upper heating rate limit in case of lower calorific value of the fuel.The following table is not applicable to alloy tube.

3　The placement of tube blanks inside the rotary hearth furnace shall conform to the following provisions.

1）For small scale tube rolling mill with little tube diameter change，the placement angle of tube blanks in the furnace shall be fixed，even when tube blank diameter is changed.

2）For tube rolling mill with multible tube blank diameter and production change，the placement angle of tube blanks in the furnace can be variable.

4　Number of tube lines along the furnace width can be one，two and three.The space between two tubes is in the range of 100-300mm.

5　Heating load of rotary hearth furnaces should be set according to heat balance calculation，and nominal specific consumption should be designed according to the Table 3.1.4 of the current code.Combustion equipments with greater capacity than the rated amount should be equipped for them.

6　Necessary mechanical interlock shall be provided for rotary hearth furnace.

7　There should be several partition walls inside the furnace，one or two partition walls may ben provided between charging door and discharging door.Provision of two partition walls is convenient for clearing furnace hearth.There should be one partition wall between charging hole and smoke exhaust hole，and one partition wall between discharging hole and soaking section.For small-diameter rotary hearth furnace，there can be only one partition wall.

8　There should be annual gap set between inner and outer walls and rotary hearth.Walls shall project properly compared to the rotary hearth at the annular gap joint.The cross section of the gap is a little tortuous.

9　Water seal trough and water seal plates must be provided at the lower part of the annular gap.Scale removing devices shall be provided for water seal trough to allow for no downtime scale removal.The parallelism tolerance of water seal plates is±5mm along circumferential face.

10　Steel structure of rotary hearth furnace should bear furnace body（excluding rotary furnace hearth），accessories as well as expansion force of furnace inner and outer wall.Ring beams shall be provided for furnace steel structure，and be set up at where temperature is relatively low.

3.6.3　The design of hearth rotary mechanism shall conform to the following provisions.

1　The design of hearth rotary mechanism shall meet the stress requirement of furnace hearth，including the load of furnace hearth，expansion force of hearth refractory layers along width and length，expansion force and temperature stress of furnace bottom steel structure due to temperature difference，as well as power for rotary movement，brake and centring of furnace rotary hearth.

2　Bottom structure of rotary hearth furnace with relatively big diameter shall be set in layers.Expansion joint shall be provided for upper structure so that expansion of the upper structure will not impact structure diameter.

3　Centring devices should be set up for hearth rotary mechanism to regulate its rotary center and ovality.

4　There can be three or four drive points set up for larger-scale rotary hearth furnace distributed uniformly along circumference，and one for small-scale rotary hearth furnace.

5　The rotation of the rotary hearth shall meet the requirements of charging and discharging rhythm.

6　Stop position of furnace rotary hearth shall meet the requirements of maximum opening angle of clamps of furnace tongs charger，with tolerance less than 10mm.

7　The slope of the rotary hearth should be adjustable to ensure that the outer periphery of its steel structure top is at the same level when the hearth is in continuous rotation under cold conditions.Tolerance of top surface vertical fluctuation range is±5mm.

3.6.4　The body of tube reheating furnace shall be designed as per the follows.

1　Charging and discharging mode and cycle shall meet the requirements of mill output.

2　To prevent bend of tubes after their entry into the furnace，great difference between furnace temperature at charging end and tube charging temperature is not permissible.

3　The heating rate of tubes in the preheating furnace should be designed according to the Table 3.6.4.

4　Heat supply for the reheating furnaces should be carried out by low NOX burners.

5　Thermal load of reheating furnace should be based on heat balance calculation or historic experience data.

6　Several heat supply sections shall be set along reheating furnace width.Detection and control item shall conform to the 3rd paragraph of 3.6.1 of the current code.

7　Walking beam and fixed beam made of refractory steel should be set up in the furnace.There shall be saw teeth on the top surface of beams，one tube one notch.The clearance between two tubes can be in the range of 60-150mm and should be more than tube bending.Tooth space of walking beam and fixed beam as well as walking distance should be designed to guarantee tube turnover while walking forward.

8　The space of walking beams inside furnace can be 1800-3000mm，and of fixed beams can be 1800-2200mm.

9　Positioning stop plate or length-measuring & positioning devices can be set up at the entry end of side charging furnace.

3.6.5　Design of reheating furnace walking machinery shall conform to the following provisions.

1　Downward stroke of walking mechanism lifting and lowering should be more than the sum of lower tube slope-deflection on fixed beam and tooth height.Upward stroke can be smaller than downward stroke.Translational walking distance shouldn't be equal to tooth space.Difference between translational walking distance and tooth space should be the displacement of steel tube which can walk forward or backward in design.

2　The sum of walking cycle of walking mechanism and running time of other linkwork should be less than the minimum tube discharging cycle.

3　Walking mechanism can take the form of crank with connection rod，and can be driven by hydraulic cylinder.Along furnace width，one lifting crank should be set every 5-6m，and one translation crank every 8-12m，number of lifting crank and translation mechanism along width shouldn't be less than two.It can take other forms also.

4　Action of charging and discharging rollers，tube positioning device，walking beam as well as charging and discharging doors etc.shall be controlled by program；interlock shall be provided in case of manual operation.

5　The gap between column of walking beam and furnace body can be sealed by steel plate or other seal piece.

3.6.6　The design of reheating furnace charging and discharging roller shall conform to the following provisions.

1　Number of notches on surface of charging rollers shall be the same as number of tubes on condition that several tubes enter the furnace at the same time from the side.

2　Adjustable running speed of charging roller is 1.5-2.0m/s，and of discharging roller is 1.5-5.0 m/s，both should be regulated by DC motor or variable-frequency motor.

3　Cantilever rollers inside the furnace should be made of heat-resistant steel，and the middle of the roller shall be equipped with middle water-cooled axes.

4　Steel tube positioning device should be equipped for charging rollers with multible notches on their surface.

5　Structure of discharging roller shall be the same as changing roller except that there is only one notch on surface of discharging roller.

3.6.7　Mandrel reheating furnace shall be designed as per the follows.

1　Requirements of mandrel preheating，dehydrogenation and annealing of shall be met.

2　Mandrels should be processed in groups intermittently，with 6 nos.in each group.Charging and discharging equipment of mandrel reheating furnace should meet requirements of nimble，gentle and reliable mandrel charging as well as discharging.

3　Burner equipment and its control system should meet the requirements of uniform furnace temperature，flexible and reliable adjustment，adaptability with low heating load，as well as low-temperature heating.

3.6.8　There are several furnace types like bogie-type furnace，roller-type furnace and cap cover furnace，etc.to be selected.

3.6.9　For direct firing mandrel reheating furnace，sub-high speed burners or high speed burners should be selected；moreover，it should be equipped with pulse combustion control system.

3.6.10　Light heat insulator should be selected for reheating furnace lining.

3.6.11　Heat control instruments should be equipped for mandrel reheating furnace according to different kind of thermal source provided.