A MODIFIED ROLL COOLING SYSTEM AT REVERSING HOT ROLLING MILL
FIELD OF INVENTION
The present invention relates to a modified roll cooling system at reversing rolling mill.
More particularly the present invention discloses improved design of intense cooling of
Horizontal rolls using double row of spray concept and mild cooling for vertical rolls with
single row of spray concept since efficient roll cooling becomes critical for controlling
the thermal condition of roll surface.
PRIOR ART
Roll is used as vital tool to transfer the required deformation energy to the work-piece
for producing steel in the desired shapes and sizes. All the key performance
parameters of any mill, viz. productivity, product-quality and production-cost are highly
dependent on the performance of rolls being used in the mill. Rolls are required to
retain their original profile for longer campaign size of rolling to obtain higher mill
productivity and good dimensional accuracy of the rolled products. The wear rate of
rolls is highly dependent on its temperature. Rolls experience thermal gradient in all its
orthogonal directions, i.e. along the circumferential, axial and radial directions. The
stress generated due to cyclic heat load promotes generation and propagation of
micro-cracks in the rolls and may cause roll peeling, banding or even their spalling. The
heat picked up by the roll body during hot rolling need to be extracted by cooling.
Therefore, the need for efficient roll cooling becomes critical for controlling the thermal
condition of roll surface. The previous roll cooling system at hot reversible mill was with
primitive design of roll cooling having numerous drill holes in the spray headers and
improper location of coolant application resulted in poor impinging coolant pressure,
non-uniform spray pattern on the roll surface, higher roll coolant surface temperature,
generation of fire-cracks on roll surface and premature roll thermal failure.
The high productive hot reversible Mill of BSL commissioned in 1970's with a capacity
of 4.0 MT of producing different grades of slabs, catering to the hot strip mill. No
technological up-gradation on equipments had taken place in the mrl zone.
The previous roll coolant application system was designed with unscientific coolant
spray with numerous drill holes in the spray headers. The previous spray header for top
horizontal roll was having two rows and the spray was from top of the roll. The spray
headers for bottom horizontal were located at entry, middle & exit side. But all these
sprays were not directed towards the bite. Similarly for vertical rolls the sprays were not
perpendicular to the roll surface. This leads to non-uniform distribution of flow onto roll
surface, over-flooding of rolls, low impinging coolant spray on rolls which resulted in
higher roll thermal crown, excessive generation of fire-cracks in roll surface, higher
grinding off-take and premature roll thermal failure.
SUMMARY OF INVENTION
Rolls in hot rolling mills are required to sustain the arduous rolling conditions and
therefore their performance does not only depend on the roll material but also on the
mill service conditions. The excessive heat generated during the rolling process is
mainly due to deformation energy and conduction of heat. This generated heat being
picked up by the roll body during rolling need to be extracted judiciously by effective
cooling with adequate amount of water distributed over the roll body at desired
pressure. The continuous heat absorption and cooling of roll surface lead to cyclic
thermal stress on the roll. Improper or insufficient cooling of rolls can lead to excessive
differential expansions and sometimes it results to premature roll thermal failure. The
differential expansion also affects the shape or crown of roll and result in non-uniform
shape of the rolled product. Therefore, the need for efficient roll cooling becomes
critical for controlling the surface detonation caused by friction as well as the thermal
condition of the roll surface and over-all mill performance.
The previous roll coolant application system (Fig.1) in Reversing Slabbing mill was
designed with improper roll cooling system on the basis of spray arrangement as well
as spray header configuration. Improper location of spray headers with numerous drill
holes in the headers led to inadequate heat transfer rate from the roll body. The spray
header for top horizontal roll was having two rows of numerous small slit holes and the
spray was from the top of vertical roll centre. The spray headers for bottom horizontal
were located at the entry & exit side and located towards the bottom of the vertical
centre of roll. But all these sprays were not directed towards the roll-bite for effective
cooling. Similarly for the vertical Squeezer rolls, the spray headers were also having
numerous drill holes (6mm f) and the spray were not perpendicular and not having full
coverage of spray along the entire of the roll barrel length. These inadequate coolant
application system leading to non-uniform distribution of coolant flow onto roll surface,
over-flooding of rolls, low impinging coolant spray on rolls which resulted in higher roll
thermal crown, excessive generation of fire-cracks in roll surface, higher grinding off-
take and premature roll thermal failure.
Therefore, to enhance the efficiency of roll coolant application system, a new
improvised roll cooling system had been designed and commissioned for Horizontal
rolls & Vertical Edgers at Slabbing Mill of Bokaro Steel Limited (BSL), which is a
Reversing mill. The design criterion of improvised system was based on maximum heat
transfer effect with optimum impact density of the coolant onto the roll surface. The
dove-tail type flat jet spray nozzles were introduced in the modified system for ensuring
perfectly aligned spray band onto the roll surface with proper over-lap of spray to take
care of dry-spotting. Double row cooling concept introduced for intense cooling of
Horizontal rolls (HR) with relocating spray header for spray towards roll-bite and to
achieve effective heat transfer co-efficient (HTC). Filters were introduced in the inlet
coolant supply line for top & bottom spray headers. The spray headers were re-
designed to clog-free operation of the nozzles. A piping arrangement is provided for
continuous uninterrupted supply of the coolant. The total coolant flow had been
designed and redistributed to extract the heat generated due to deformation energy
effectively with higher cooling efficiency, and also to take care of thermal fatigue as well
reduction in specific roll consumption.
Salient Features of the invention:
• Design of new roll cooling application system with improved cooling efficiency
• Flat jet dove-tail type spray nozzle introduced to ensure aligned spray pattern
• Double row spray concept for intense cooling of horizontal rolls & single row for
vertical edgers
• Re-location of spray headers for higher heat removal rate
• Double row & overlap of spray for horizontal rolls to avoid dry spotting of rolls,
• Introduction of filter & re-routing of in-let water line to avoid nozzle choking
• Retrofitting to existing mill logistics
• Smooth operation and ease of maintenance with low investment
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates Roll cooling system at stabbing mill as prior art;
Fig 2 shows a table illustrating the Design / specification of the improved cooling
system at stabbing mill in accordance with the present invention;
Fig 3 illustrates the Roll cooling system at stabbing mill in accordance with the present
invention;
Fig 4 illustrates the modified roll cooling header for top and bottom at entry and exit
side at stand in accordance with the present invention;
Fig 5 illustrates the modified roll cooling header for vertical squeezer roll in accordance
with the present invention;
DETAILED DESCRIPTION
On the basis of objective and pre-set conditions of previous coolant application system,
the modified roll coolant application system (Fig.2) was designed with the following
criterion:
• Design of variable coolant flow distribution along barrel for horizontal rolls.
• Design of intense cooling of Horizontal rolls using double row of spray concept and
mild cooling for vertical rolls with single row of spray concept.
• Reducing number of openings in spray headers for increased coolant pressure and
higher impinging force.
• Use of dove-tail type flat jet nozzles for aligned coolant spray pattern
• Overlap of sprays to avoid dry spotting of roll.
• Design of filters & re-design of in-let water line for horizontal rolls to avoid nozzle
choking.
• Location of spray header closer towards bite for effective heat transfer rate
• Design of headers in relation to coolant application and versatility, ease of
maintenance and inter-changeability.
New Roll cooling System:
As stated, the sprays must impinge as precisely and accurately as possible onto the
roll-bite for maximized effectiveness, which directly depends on the positioning of the
headers within the mill fixtures. This optimal location has to take into account existing
mill furniture, access of services coolant piping to mill stand, rubber hoses and ease of
accessibility for maintenance. Piping must be sized and routed so as to eliminate
pressure losses. Spray headers must be properly designed to accommodate these
flow rates without restrictions. The spray headers are made up of 80NB seamless MS
pipe for HR and 50NB for VR. The number of nozzles at the central zone is higher than
the edge zones. The supply connections were provided in the headers to maintain
balanced and uniform flow rate through each nozzles. The modified system shown in
Fig.3.
The vertical roll spray headers are fitted with lesser number of nozzles for mild cooling
and are located perpendicular to the roll surface. All the spray headers were equipped
with manual flushing arrangement for clog-free operation.
The top HR cooling headers (Fig.4) made of 80NB pipe are fitted on the top roll choke
to maintain fixed spray location with varying roll gap. The bottom HR coolant headers
were made of 80NB pipe of fixed type and fitted on the mill housing at the entry and
exit side of stand with similar flow distribution. Each spray headers for HR were having
double row of nozzle, where as the spray headers for VR were having single row of
nozzle. For top and bottom HR spray header one row with 29 numbers of nozzles (9
nozzles at central zone, 12 nozzles at intermediate zone and 8 nozzles at edge zone)
and other row with 21 nos. of nozzles with equal spacing. The row of higher number of
nozzle in the spray header was oriented towards the roll-bite to arrest heat wave
penetration at earliest. The variable flow distribution was carried out with different
nozzle spacing at each zone to control the roll thermal camber. All the nozzles are fitted
to the header with an off-set of 20° and minimum spray overlap of 40% to 60% in order
to avoid dry running of rolls due to nozzle choking. All these roll coolant headers were
provided with flushing arrangement for ease of periodic maintenance and made
retrofitting to pervious coolant supply line. The Vertical roll cooling headers (Fig.5) are
provided with 21 nos. of nozzles with less space interval at the bottom & more towards
the upper portion. A specially designed filter has been installed in the in-let water line
of the horizontal spray header to avoid choking of nozzles. In-let water line for spray
header has been re-designed for clog free operation.
Improvement in techno-economics indices:
The new roll coolant system had exhibited improved heat transfer effect with the help of
perfectly aligned spray band onto the rolls and improved HTC effect.
• Modified coolant application system reduced roll surface temperature by
10°C(avg) and roll thermal crown from 12°C to 6°C.
• The modified system has helped to reduce the specific roll consumption from
0.168kg/t to 0.117kg/t
• Campaign length increased by 10%
• Cost to benefit ratio of 1: 20
• Annual saving is Rs 300 Lakhs
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it has
been presented for purposes of illustration and description and is not intended to be
exhaustive or to limit the invention to the particular embodiments and applications
disclosed. It will be apparent to those having ordinary skill in the art that a number of
changes, modifications, variations, or alterations to the invention as described herein
may be made, none of which depart from the spirit or scope of the present invention.
The particular embodiments and applications were chosen and described to provide
the best illustration of the principles of the invention and its practical application to
thereby enable one of ordinary skill in the art to utilize the invention in various
embodiments and with various modifications as are suited to the particular use
contemplated. All such changes, modifications, variations, and alterations should
therefore be seen as being within the scope of the present invention as determined by
the appended claims when interpreted in accordance with the breadth to which they are
fairly, legally, and equitably entitled.
We Claim:
1. A modified roll cooling system at reversing hot rolling mill comprising of:
at least one dove-tail type flat jet spray nozzle;
a double row cooling unit with relocated spray header for spray towards roll-bite;
at least one.filter unit introduced in the inlet coolant supply line for top & bottom
spray headers; and
a piping arrangement for continuous uninterrupted supply of the coolant.
2. A modified roll cooling system as claimed in claim 1, wherein the jet spray
nozzle is configured for ensuring perfectly aligned spray band onto the roll
surface with proper over-lap of spray to take care of dry-spotting.
3. A modified roll cooling system as claimed in claim 1, wherein the double row
cooling is configured for intense cooling of Horizontal rolls (HR) with relocating
spray header for spray towards roll-bite and to achieve effective heat transfer co-
efficient.
4. A modified roll cooling system as claimed in claim 1, wherein the spray headers
are equipped with manual flushing and are configured for clog-free operation of
the nozzles.
5. A modified roll cooling system as claimed in claim 1, wherein the piping is sized
and routed so as to eliminate pressure losses and the spray headers is
configured to accommodate these flow rates without restrictions.
6. A modified roll cooling system as claimed in claim 1, wherein the spray headers
are made up of 80NB seamless MS pipe for HR and 50NB for VR.
7. A modified roll cooling system as claimed in claim 1, wherein the number of
nozzles at the central zone is higher than the edge zones and the supply
connections were provided in the headers to maintain balanced and uniform flow
rate through each nozzles.
8. A modified roll cooling system as claimed in claim 1, wherein the vertical roll
spray headers are fitted with lesser number of nozzles for mild cooling and are
located perpendicular to the roll surface.
9. A modified roll cooling system as claimed in claim 1, wherein all the nozzles are
fitted to the header with an off-set of 20° and minimum spray overlap of 40% to
60% in order to avoid dry running of rolls due to nozzle choking.
10. A modified roll cooling system as claimed in claim 1, wherein each spray
headers for horizontal row (HR) have double row of nozzle, whereas the spray
headers for vertical row (VR) have single row of nozzle

ABSTRACT

Present invention relates to a modified roll cooling system at reversing hot rolling mill
comprising of at least one dove-tail type flat jet spray nozzle; a double row cooling unit
with relocated spray header for spray towards roll-bite; at least one filter unit introduced
in the inlet coolant supply line for top & bottom spray headers; and a piping
arrangement for continuous uninterrupted supply of the coolant.