FIELD OF THE INVENTION
The present invention relates to a system for automatic temperature control of
pickling bath for efficient heating at acid pickling line in cold rolling mill of steel
plants. More particularly, the present invention is directed to providing a system for
automatic temperature control of pickling bath for improving pickling reactivity, bath
condition and efficiency of pickling line through controlled steam flow rate to the heat
exchanger, involving pneumatically controlled steam flow control valve with I/P
converter, microprocessor based PID single loop temperature controller and RTD,
which helps to maintain and control the temperature of pickling solution within a
range of ± 2°C.
BACKGROUND OF THE INVENTION
In the conventional Pickling Lines(PL) of Cold Rolling Mill(CRM) at the applicants steel
plant, H2SO4 is used as acid pickling media. The required pickling bath temperature
is maintained by indirect closed-loop pickling acid circulation through graphite block
heat exchanger. The indirect heating of the acid is carried out by cross-flow of acid
through the pores of graphite block and steam through the shell of heat exchanger.
The suction of the acid is taken from the drain connections of the tank mounted at
the side wall of tank and pumped through heat exchanger for gaining the
temperature and finally poured back into the tank at other end.
In the conventional pickling line, there was no proper pickling bath temperature
control system to maintain the uniform bath temperature. The heating through the
Graphite block heat exchangers was not effective due to frequent clogging of pores
leading to pressurized leakages and cross contamination of steam and acid at the
joining block layer and resulting to frequent shutdown of the system. Moreover,
when the bath temperature goes beyond 95°C, the generation of mono-ferrous
sulphate(Fe2SO4,H2O) was promoted due to localized heat concentration, acid
concentration and acid salt level. Mono-ferrous sulphate is insoluble in acid solution
and very hard to separate out during regeneration process in. This salt gets
deposited in running equipments like heat exchangers, pipelines tanks etc and lead
to major breakdowns. Absence of automatic temperature control system normally
leads to this kind of breakdown, especially when line is getting frequent stoppages

due to some process abnormalities or if steam valve is not closed. This leads to
pickling process defect and resulting in poor quality of pickled strip.
There has been therefore a continuous need in the pickling line for acid pickling of
cold rolled products to developing a system for automatic temperature control of the
pickling bath with desired accuracy, reliability, easy maintainability, improved pickled
product quality and enhanced plant availability. The present invention attempts to
solve the limitations and disadvantages of the existing pickling line as described
above by providing a controlled steam flow based heating of the bath involving a
newly designed efficient heat exchanger along with flow control valves and
associated instrumentation to meet the desired temperature control of the acid
pickling bath.
OBJECTS OF THE INVENTION
The basic object of the present invention is directed to providing a system for
automatic temperature control of pickling bath at desired level with desired accuracy
by precise control of steam involving efficient heat exchanger alongwith flow control
valves.
A further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath which would avoid clogging of heat
exchanger and cross contamination of steam and acid.
A still further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath which is capable of achieving high
accuracy, reliable performance, easy expandability, easy maintainability and
diagnostic features using standardization of items & and makes for maximizing
interchangeability.
A still further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath which would be able to maintain
temperature of bath at desired level with desired accuracy by involving Control Valve
with pneumatic actuator, pneumatic positioner with separate I/P converter,

Microprocessor based single loop PID temperature controller and other supporting
instrumentation.
A still further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath wherein the control valve manipulates
a flowing fluid, i.e. steam to compensate for the heat load disturbance and keep the
pickling tank temperature as ciose as possible to the desired set point.
A still further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath which is capable of reducing the
steam consumption substantially.
A still further object of the present invention is directed to providing a system for
automatic temperature control of pickling bath adapted to ensure safety of heat
exchanger by avoiding overheating thereby avoiding formation of mono hydrate
ferrous salt in the tank, thus safeguarding the equipment and tank installed in the
line.
SUMMARY OF THE INVENTION
Thus the basic aspect of the present invention is directed to a system for automatic
temperature control of baths involving heat exchangers for improving bath
condition and efficiency comprising
(a) atleast one heat exchanger operatively connected in order to maintain the
desired bath temperature based on through controlled steam flow rate to the heat
exchanger from a steam supply source,
b) Control Valve with pneumatic actuator, pneumatic positioner with separate I/P
converter
said control valve adapted to control the flow of steam based on the actuation of said
I/P converter,

wherein the output of the I/P converter is adapted to control the steam movement of
control pneumatically which in turn controls the steam flow rate to heat exchanger
and in the process control bath temperature.
A further aspect of the present invention is directed to a system for automatic
temperature control of pickling bath for improving pickling reactivity, bath condition
and efficiency of pickling line, comprising
(a) a heat exchanger operatively connected to pickling bath in order to maintain the
desired bath temperature based on through controlled steam flow rate to the heat
exchanger from a steam supply source,
b) Control Valve with pneumatic actuator, pneumatic positioner with separate I/P
converter
c) Microprocessor based single loop PID temperature controller;
d) Pressure transmitter;
e) Steam trap unit;
f) Steam valve; and
g) Resistance temperature detectors (RTD) unit installed inside the tank.
A still further aspect of the present invention is directed to a system wherein said
heat exchanger is tantalum shell and tube heat exchanger.
Yet another aspect of the present invention is directed to said system wherein said
control valve is a single seated cage guided globe valve comprising (a) a control
valve body and (b) a control valve actuator.
A further aspect of the present invention is directed to said system, wherein said
microprocessor based single loop PID temperature controller is adapted to read the
sensor signal from said RTD, convert the measurement to engineering units e.g.
Degrees C, subtract the measurement from a desired set-point to determine an error
and minimize the error by adjusting the process control inputs via control element
i.e. the position of a control valve, to control steam flow rate to maintain said set
point.
A still further aspect of the present invention is directed to said system, wherein said
pressure transmitters used is of smart type with variable capacitance type sensor.

built in local indicator, two ways manifold and mounting accessories etc. , for display
outlet steam pressure of the control valve and provide signal required for controller
to control the movement of control valve based on the steam pressure.
A still further aspect of the present invention is directed to said system, wherein said
steam trap is inverted bucket type steam trap resistant to water hammer installed
after the heat exchanger to ensure that only condensate come out from the system
discharged immediately at steam temperature avoiding any leakage of steam.
A still further aspect of the present invention is directed to said system, wherein said
RTD is of type PT100 having its element made from platinum, installed inside the
tank and immersed in the pickling solution to measure the pickling solution
temperate and to give the signal to PID temperature controller.
A still further aspect of the present invention is directed to said system, wherein said
control valve is configured based on the heat load requirement of installed heat
exchanger and input available steam pressure and temperature.
Yet another aspect of the present invention is directed to said system for automatic
temperature control of pickling bath, wherein said system is adapted to maintain
pickling bath at uniform temperature consistently within a range of ± 2°C leading to
better pickling.
A further aspect of the present invention is directed to a method for automatic
temperature control of pickling bath involving the system as described above
comprising steps to maintain the following operating parameters:



Importantly, said method comprises controlling the bath temperature to within +/-
2°C.
The various objects and advantages of the present invention is described hereunder
in greater details with reference to the following accompanying non limiting
illustrative drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows the schematic arrangement of the conventional pickling lines at cold
rolling mill of steel plant.
Figure 2: shows the schematic flow diagram for the automatic temperature control
system for acid pickling bath according to the present invention.
Figure 3: is the graphical plot showing the flow characteristic of the flow control
valve according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWINGS
The present invention relates to a system for automatic temperature control of
pickling bath for efficient heating at acid pickling line in cold rolling mill of steel
plants.
Reference is first invited to the accompanying Figure 1 that schematically illustrate
the schematic arrangement of conventional pickling line uses H2SO4 acid as pickling
media. The required pickling bath temperature is maintained by indirect closed-loop
pickling acid circulation through graphite block heat exchanger. The indirect heating

of the acid is carried out by cross-flow of acid through the pores of graphite block
and steam through the shell of heat exchanger. The suction of the acid is taken from
the drain connections of the tank mounted at the side wall of tank and pumped
through heat exchanger for gaining the temperature and finally poured back into the
tank at other end.
An efficient automatic bath temperature control system has thus been designed and
developed by way of the present invention to avoid the disadvantages of the
conventional pickling line and for improving pickling reactivity, bath condition and
efficiency of pickling line. The modified automatic temperature control system has
been introduced at Tank no. 4 of a pickling line in conjunction with a new generation
heat exchanger i.e. "tantalum shell & tube heat exchanger" in order to maintain the
desired bath temperature. The basic principle of automatic temperature control
system is based on through controlled steam flow rate to the heat exchanger, which
helps to maintain and control the temperature of pickling solution within a range of ±
2°C. The system mainly comprises of pneumatically controlled steam flow control
valve with I/P converter, microprocessor based temperature PID single loop
controller and RTD. The RTD installed in the tank for temperature sensing and feed-
back control system. The schematic diagram of automatic temperature control
system according to the present invention has been illustrated in the accompanying
Figure 2. The detail design basis of automatic temperature control system is given
in following Table-1. The desired steam flow rate was calculated based on the
required heat load of Tank no. 4 for nearly constant bath temperature at the desired
line speed.
Design and selection of temperature control for the system according to the present
invention have been decided based on the following criteria:
(i)High accuracy, reliable performance, easy expandability.
(ii)Standardization of items and makes for maximizing interchangeability
and minimizing inventories.
(iii)Easy maintainability and diagnostic features.
(iv)Provision for auto/ manual operation for all automatic control loops.
(v)AII field-mounted instruments shall be of suitable weatherproof
construction, as per details indicated for respective items.
(vi)Continued availability & supply of equipment including their spares parts
& consumables shall be ensured.


Designing of automatic temperature control system
Based on the design criterion and consideration of other parameters mentioned in
the Table 1 given above, the automatic temperature control system has been
developed and commissioned at one of the pickling line of the CRM. The schematic
diagram of the automatic temperature control system is illustrated in the
accompanying Figure 2. The Automatic temperature control system consists of
mainly a) Control Valve with pneumatic actuator, pneumatic positioner with separate
I/P converter, b) Microprocessor based single loop PID temperature controller c)
Pressure transmitter, d) Steam trap unit, e) Steam valve, and f) RTD unit installed
inside the tank.
The details of the features and functions of the components of the temperature
control system are as follows:
(a) Control Valve with pneumatic actuator, pneumatic positioner with
separate I/P converter

The control valve manipulates a flowing fluid, i.e. steam to compensate for the heat
load disturbance and keep the pickling tank temperature as close as possible to the
desired set point. The control valves consist of two parts: the control valve body and
control valve actuator. Control valve body is the housing through which steam flows.
It provides inlet and outlet connections; and a movable restrictor which varies the
steam flow as it opens and closes the port. The other term, an actuator, is part of
control valve which causes the valve stem to move by providing the force it's
needed. The body of a control valve regulates the steam flow as the position of the
valve is changed by the actuator. Therefore, it is very important for the valve body
to be able to permit actuator thrust transmission, resist chemical and physical effects
from the process, and easily flange up with the adjacent piping connections. Here the
control valve selected is single seated cage guided globe valve. The cage guided
control valves has inherent advantages of large flow at relatively tougher conditions,
smooth operation through stroke length and economy of actuator selection due to
reduced unbalanced force. The main features of the selected control valves are:
(i) Good pressure balance;
(ii) Smaller actuator for higher differential pressure;
(iii)Greater resistant to wear & vibration;
(iv) Soft seated trim for tight shut off valves;
The control valve is pneumatically controlled and controls the flow of steam through
it as per actuation of I/P converter. The flow characteristic of the control valve is
illustrated graphically in accompanying Figure 3. The output of I/P converter
controls the stem movement of control valve pneumatically which in turn controls
the steam flow rate to the heat exchanger and helps to heat the pickling solution
passing through the heat exchanger and thereby controlling the tank temperature.
Air filter regulator ensures dry and clean air supply to the control valve diaphragm
actuator. A special cover was also designed to protect control valves and its
accessories from H2SO4 acid corrosive environment.
(b) Microprocessor based single loop PID temperature controller
A PID (Proportional Integral Derivative) controller is a common instrument used in
industrial control applications. A PID controller can be used for regulation of speed,
temperature, flow, pressure and other process variables. In this work, it has been
used to regulate temperature. A PID controller reads the sensor signal from RTD,

and converts the measurement to engineering units e.g. Degrees C. It then subtracts
the measurement from a desired set-point to determine an error. The controller
attempts to minimize the error by adjusting the process control inputs. The PID
controller calculation (algorithm) involves three separate constant parameters, and is
accordingly sometimes called three-term control: the proportional,
the integral and derivative values, denoted P, I, and D. These values can be
interpreted in terms of time: P depends on the present error, I on the accumulation
of past errors, and D is a prediction of future errors, based on current rate of
change. The weighted sum of these three actions is used to adjust the process via
control element i.e. the position of a control valve, to control steam flow rate to
maintain set point. In order for control loops to work properly, the PID loop must be
properly tuned.
The feedback tank temperature and its signal is taken as input to the single loop PID
controller. Based on the set point for maximum and minimum operating
temperature, PID controller controls send the signal in 4 - 20 mA to I/P converter
mounted on the control valve. I/P converter convert the current signal in terms of
pressure. PID parameter optimizes and controls the temperature of the pickling tank
within ±2°C.
(c) Pressure transmitter
The pressure transmitters used is of smart type with variable capacitance type
sensor. Transmitter has built in local indicator, two ways manifold and mounting
accessories etc. Pressure transmitter has been used for display outlet steam pressure
of the control valve and provision has been kept for any signal required for controller
to control the movement of control valve based of the steam pressure.
(d) Steam trap unit
The temperature control system is incomplete without that crucial component called
steam trap. A steam trap quite literally 'purges' condensate, out of the system. It is
imperative that the correct trap is selected to carry out a given function under given
conditions. They may involve variations in operating pressure, heat load or
condensate pressure. Steam traps may be subjected to extremes of temperature or

even water-hammer. Whatever the conditions, correct steam trap selection is
important to system efficiency. In this application, steam trap was used after the
installed heat exchanger to ensure only condensate as a output from heat
exchanger. Leakage of steam at this point is inefficient and uneconomical. The steam
trap has to allow condensate to pass whilst trapping the steam in the process. If
good heat transfer is critical to the process, then condensate must be discharged
immediately and at steam temperature. Inverted bucket type steam trap selected to
ensure that only condensate come out from the system. Inverted bucket traps are
the most robust type of the mechanical traps. This type of steam trap also helps in
resistant water-hammer. Size of steam trap selected same as size of outlet of heat
exchanger.
(e) Resistance thermometers
Resistance thermometers also called resistance temperature detectors (RTD),
are sensors used to measure temperature by correlating the resistance of the RTD
element with temperature. Most RTD elements consist of a length of fine coiled wire
wrapped around a ceramic or glass core. The element is usually quite fragile, so it is
placed inside a sheathed probe to protect it. The RTD element is made from a pure
material, platinum, nickel or copper. The material has a predictable change in
resistance as the temperature changes; it is this predictable change that is used to
determine temperature. RTDs are relatively immune to electrical noise and therefore
well suited for temperature measurement in industrial environments. RTD installed
inside the tank and immersed in the pickling solution to measure the pickling solution
temperate and to give the signal to PID temperature controller. The type of RTD
used is PT100 and its element is made from, platinum.
The detail Technical Specification of major items installed under Automatic
Temperature Control System are given below:
a) Control Valve with pneumatic actuator, pneumatic positioner with separate I/P
converter:

The main advantages of this Automatic temperature control system according
to the present invention comprises:
(i) Maintain pickling bath at uniform temperature consistently within a range of
± 2°C leading to better pickling;

(ii) Ensures safety of heat exchanger by avoiding overheating thereby avoiding
formation of mono hydrate ferrous salt in the tank, thus safeguarding the
equipment and tank installed in the line;
(iii) Reduction in steam consumption by 30%;
(iv) Design of control valve is implemented based on the heat load
requirement of installed heat exchanger and input available steam
pressure and temperature;
(v) Design of total temperature control system implemented to control tank
temperature within ± 2°C;
(vi) Saving in energy consumption is achieved by optimizing the steam flow
to heat exchanger and eliminating any wastage of steam.
It is thus possible by way of the present invention to providing a system for
automatic temperature control of pickling bath at desired level with desired accuracy
by precise control of steam involving efficient heat exchanger alongwith flow control
valves and associated instrumentation. The system according to the present
invention is capable of maintain pickling bath at uniform temperature consistently
within a range of ± 2°C leading to better pickling by adopting microprocessor based
control on steam flow through the heat exchanger through control valve based on
the heat load requirement of installed heat exchanger and input available steam
pressure and temperature. The present system of temperature control of pickling
bath is also suitable for application in other chemical process or process industry
where bath temperature is maintained through some kind of heat exchanger.

We Claim:
1. A system for automatic temperature control of baths involving heat exchangers
for improving bath condition and efficiency comprising
(a) atleast one heat exchanger operatively connected in order to maintain the
desired bath temperature based on through controlled steam flow rate to the heat
exchanger from a steam supply source,
b) Control Valve with pneumatic actuator, pneumatic positioner with separate I/P
converter,
said control valve adapted to control the flow of steam based on the actuation of said
I/P converter,
wherein the output of the I/P converter is adapted to control the steam movement of
control pneumatically which in turn controls the steam flow rate to heat exchanger
and in the process control bath temperature.
2. A system as claimed in claim 1 for automatic temperature control of pickling bath
for improving pickling reactivity, bath condition and efficiency of pickling line,
comprising
(a) a heat exchanger operatively connected to pickling bath in order to maintain the
desired bath temperature based on through controlled steam flow rate to the heat
exchanger from a steam supply source,
b) Control Valve with pneumatic actuator, pneumatic positioner with separate I/P
converter
c) Microprocessor based single loop PID temperature controller;
d) Pressure transmitter;
e) Steam trap unit;
f) Steam valve; and
g) Resistance temperature detectors (RTD) unit installed inside the tank.
3. A system as claimed in claim 2 wherein said heat exchanger is tantalum shell &
tube heat exchanger.

4. A system as claimed in anyone of claims 1 or 2 wherein said control valve is a
single seated cage guided globe valve comprising (i) a control valve body and (b) a
control valve actuator.
5. A system as claimed in claim 1 or 2 , wherein said microprocessor based single
loop PID temperature controller is adapted to read the sensor signal from said RTD,
convert the measurement to engineering units e.g. Degrees C, subtract the
measurement from a desired set-point to determine an error and minimize the error
by adjusting the process control inputs via control element i.e. the position of
a control valve, to control steam flow rate to maintain said set point.

6. A system as claimed in anyone of claims 2 to 5, wherein said pressure
transmitters used is of smart type with variable capacitance type sensor, built in
local indicator, two ways manifold and mounting accessories etc. , for display outlet
steam pressure of the control valve and provide signal required for controller to
control the movement of control valve based on the steam pressure.
7. A system as claimed in anyone of claims 2 to 6, wherein said steam trap is
inverted bucket type steam trap resistant to water hammer installed after the heat
exchanger to ensure that only condensate come out from the system discharged
immediately at steam temperature avoiding any leakage of steam.
8. A system as claimed in anyone of claims 2 to 7, wherein said RTD is of type PT100
having its element made from platinum, installed inside the tank and immersed in
the pickling solution to measure the pickling solution temperate and to give the
signal to PID temperature controller.
9. A system as claimed in anyone of claims 2 to 8, wherein said control valve is
configured based on the heat load requirement of installed heat exchanger and
input available steam pressure and temperature.
10. A system for automatic temperature control of pickling bath as claimed in anyone
of claims 1 to 9, wherein said system is adapted to maintain pickling bath at uniform
temperature consistently within a range of ± 2°C leading to better pickling.

11. A method for automatic temperature control of pickling bath involving the
system as claimed in anyone of claims 1 to 10 comprising steps to maintain the
following operating parameters:

12. A method as claimed in claim 11 comprising controlling the bath temperature to
within +/- 2°C.

ABSTRACT

A system for automatic temperature control of pickling bath for efficient heating at
acid pickling line in cold rolling mill of steel plants is disclosed. More particularly, the
present invention is directed to providing a system for automatic temperature control
of pickling bath for improving pickling reactivity, bath condition and efficiency of
pickling line through controlled steam flow rate to the heat exchanger, involving
pneumatically controlled steam flow control valve with I/P converter, microprocessor
based PID single loop temperature controller and RTD, which helps to maintain and
control the desired temperature of pickling solution within a range of ± 2°C.