Actuator modulation and Plant efficiency

1. Continuous Modulating Actuators Improve
Plant Efficiency
Issue 8 and Volume 116.
8.1.12

By Stephan Schulze, ABB Automation Products GmbH
The use of highly precise and continuously modulating actuators — in power stations
for example — increases the efficiency of plants and reduces power consumption.
The capacity of current new-build coal power stations is typically one gigawatt and a
0.1 percent improvement in efficiency equates to one megawatt. Assuming there are
300 days of block operation in a year, this is equivalent to the annual consumption of
around 2,000 households and a CO2 reduction of 6,500 tonnes per year.
ABB actuator in burner air control reduces actuator lifecycle
cost because it is designed for up to 10 years of maintenance-
free operation. Photo courtesy ABB
In many applications with steam boilers, superheaters create the necessary high
steam temperature. Special injection control valves on the boilers control the volume
of cooling water injected into the hot steam in the superheater and reheater. The
injection volume is decisive for the steam temperature. To operate a superheater
with the lowest possible steam cooling and, simultaneously, the maximum
permissible hot steam temperature, the mass flow rate of the injected cooling water
must be regulated continuously and precisely. To regulate the temperature,
actuators are used.
Owing to certain general conditions, the control algorithms of many process control
systems in certain plants are not optimised for optimal process control, but rather for

2. observing the permissible switching frequency and switch-on time of the drives
technology used. This has negative consequences on the efficiency of the plant.
After a brief introduction to actuator technology, this article describes an opportunity
to improve the efficiency of steam power plants with continuously modulating
actuators from ABB and, thereby, reduce operating costs.
Actuators — the intelligent field devices
Power station operators are keen to use automation technology to generate more
energy and actuators play an important role in this, in systems and supply
engineering and are used in almost all areas of industry. In the automation of
processes, actuators regulate material, mass and energy flows by adjusting final
control elements such as valves, flaps and cocks. The actuator and valve create a
single unit — the control valve. The actuator normally comprises the motor and
transmission as well as an output shaft or push rod and a position sensor.
Actuators perform very different motion sequences, with a distinction made between
actuators for linear motions and actuators for pivoting or rotating motions. The
actuators are powered by pneumatic, hydraulic or electrical auxiliary energy.
The actuator receives a control signal from the automation system, such as a control
unit, operator station or process control system, which it must convert into a motion
so that the control element of the actuating element, which can be the valve cone for
example, assumes a corresponding position. With control valves, this is a stroke
motion, whereas with flaps, ball cocks or rotary plug valves, this is a pivoting motion.

3. The positioner influences the auxiliary energy of the drive and in doing so, sets the
desired setting of a control valve or maintains this despite external influences. In
electrical drives, it is the power electronic circuits that control the run time and
direction of rotation of the drive.
Besides the motion type of electrical actuators, the duty/operation mode is also an
important selection criterion. This raises the question of whether the final control
element is to be used as a shut-off valve in open-close duty or whether it is to be run
in positioning duty at intermediate positions or whether the final control element
position is even to be adjusted at short intervals in a modulating duty in order to
regulate the flow of a pipe, for example. These are decisive conditions for both the
design of the final control element and also the actuator, as the loads in the various
duties vary significantly. Accordingly, there are actuators for open-close duty,
positioning duty, as well as actuators equipped for the high demands of modulating
duty.
In open-close duty, the final control element is activated relatively infrequently. There
could be a few hours or even several months between two setting stages. In
positioning duty, the final control element is moved to a defined intermediate
position, e.g. to achieve a constant flow. Here, there are run-time restrictions, similar
to those in open-close duty. In modulating duty, the actuating element must be
frequently updated owing to ongoing changes in conditions. This is required in
second intervals in sensitive control applications, which places demanding
requirements on the actuator. The motor and mechanics must be designed so that
the high number of switching operations can be sustained over long periods. The
control accuracy must also remain consistent.
An actuator must move a given load within a given time to a set position with a
specific accuracy.
The electric motors used in actuators normally work over a short term rather than in
continuous operation and often have to meet particularly demanding requirements.
These include a high accelerating and braking torque, a high holding torque at
standstill, high speed rigidity, uniform rotation, a large speed control range, high
efficiency, robustness, low maintenance costs, low noise and vibration and a high
protection class. Often, the requirements contradict each other requiring
compromises to be made in most cases. The right selection of actuators is of
decisive significance for the function, investment, operating costs and reliability of
the entire process technology.
Contrac increases plant efficiency and lowers
operating costs
The efficiency of a plant is the relationship between power output and power input.
The typical efficiency of a coal power station lies between 25% and 45%. The
efficiency of a steam power plant increases with the temperature of the steam
created in the power station boiler. However, permissible maximum temperature
limits of the boiler tube material and a turbine to be exposed to the steam must not
be exceeded. The more closely the temperature can be kept to its target value, the

4. closer the target value can be set to the permissible temperature limit. Thus, higher
efficiency is achieved when operating the power plant.
In many applications with steam boilers, superheaters create the necessary high
steam temperature. The steam temperature must be controlled so that the
permissible material temperatures of the steam turbine and boiler are not exceeded.
The temperature is controlled via a steam cooler, which injects a specific volume of
cooling water into the overheated steam flow. In a power station, there are two
locations of use for steam coolers; the superheater and the reheater. The
superheater cooling occurs before the steam is introduced into the high-pressure
turbine. The reheater cooling is applied to the steam after the high-pressure turbine.
This steam is reheated in the boiler before being fed to the medium-pressure or low-
pressure turbine. The volume of the injection water is controlled via an external
control valve. To achieve precise temperature control, the valve must respond
quickly to downstream temperature changes and in order to cope with low flows and
different load states it must also have a good control ratio. Decisive performance
criteria include: precise control at a low flow rate and a high control ratio in order to
keep the outlet temperature of the steam superheater constant; and a sufficient
seating contact force to avoid leakage when starting up the plant and a rapid
response to changes in steam temperature.
Special injection valves are required for temperature control in the final stages of the
superheater to dose the required cooling water. To operate a superheater with the
lowest possible steam cooling and, simultaneously, the maximum permissible hot
steam temperature, the mass flow rate of the injected cooling water must be
regulated continuously and precisely. If too much water is injected, excessive steam
cooling has adverse consequences for efficiency. Too little water results in excessive
steam temperatures and pressures and creates a risk of damaging the superheater
as well as the turbine and downstream components. The smallest volumes of water
must be introduced. To achieve this requires highly precise positioning in a valve’s
disproportionate zone. The actuator used therefore influences the efficiency of the
plant. This must fulfil all requirements for the control of the injection valve.
The permissible switching frequency of the installed electrical actuators is decisive
for control accuracy and thus the turbine inlet temperature at the superheater. For
safety reasons, a clearance is maintained between the steam temperature and the
turbine limit temperature. If the differential temperature increases by 1°C, the overall
plant efficiency reduces by up to 0.05%. Through precise control of the steam
temperature, the plant can be operated very close to the turbine’s temperature limit,
thus increasing efficiency. In a fossil-fuel 700-MW power plant, for example, a 1%
efficiency increase translates into a profit of EUR 0.5 million per year. Typical current
new-build coal power stations deliver an output of approx. 1100 MW and an
improvement in efficiency of 0.1% is equivalent to an increased output of 1.1 MW.
Electrical actuators must be serviced in accordance with load and switching
frequency. An average switching frequency of fewer than 700 cycles per year
produces a service interval of seven months. If the plant operator aims for longer
service intervals, e.g. two years, this target will reduce the average permissible
switching frequency to approx. 200 cycles per hour or approx. three cycles per
minute.

5. ABB actuator in feedwater control application. Photo courtesy
ABB
Owing to these general conditions, the control algorithms of many process control
systems are not optimised for optimal process control, but rather for observing the
permissible switching frequency and switch-on time of the actuator technology used.
This has negative consequences for plant efficiency. In contrast, an actuator that
operates continuously in one process without restrictions on switching cycles and
switching times, in the same way as an injection control valve for example, increases
plant efficiency.
Another important factor for power station operators is maintenance costs. These are
even more important than the depreciable investment costs. The plant operator
would like to use plant components with a life cycle that is as long as possible, in
order avoid maintenance costs or potential downtime costs. However, extending
service life normally entails comparably higher investment costs.
The most important stage of a control valve from the plant operator’s perspective is
its actual service life. Besides the operating costs incurred for energy and operating
resources, it is servicing and maintenance costs that are the principal driving factors
when it comes to life cycle costs. The cost behaviour means that, at a certain point in
the utilisation phase of a device, the actuator with the higher acquisition costs
amortises itself. This point of amortisation should be as close to the beginning of the
utilisation phase as possible so that the plant operator can achieve a large saving
potential and as a result, the logical decision is usually made to favour a more
expensive actuator in terms of investment costs.
Owing to the sliding motion, design factors dictate that worm gears wear more
quickly than the spur gears of Contrac actuators. Servicing these actuators is
extremely easy and economical as no transmission components have to be
replaced. The transmission oil simply needs changing and the seal rings and gaskets
replacing.

6. Contrac variable-speed actuators are designed for ten years of service-free
continuous operation. The so-called service computer takes account of the load on
the actuator, for example, evaluating temperatures, number of motor reversals and
load peaks, and using these to calculate the remaining time until the next service is
required. This load-dependent servicing facilitates optimal plant management when
compared with time-based servicing.
Despite the supposedly high investment costs, the linear and rotary actuators in the
Contrac series represent the more economic solution when viewed over the entire
service life. If you compare the life cycle costs of a Contrac that has been under
constant load, with competitor products that have been subjected to relatively low
loads, it can be seen that the putatively high initial investment amortises itself after
just a few months and, at the longest, after four years.
The Contrac series continuous electric variable-speed actuators are the ideal
solution for highly precise, continuous position regulation of injection control valves
and reduce operating costs.
Actuators that regulate continuously and precisely
The intelligent field devices of Contrac actuator systems are based upon ABB’s
family of conventional rotary and linear actuators. The name Contrac is an
amalgamation of the words ‘control’ and ‘actuator’. The actuator system sets itself
apart with continuous positioning, precise control, long service intervals, overload
protection in end positions without torque-dependent cut-off as well as its high
protection class. The series comprises tried and tested mechanical components
combined with microprocessor electronics and is compatible with fieldbuses as well
as conventional control methods. The devices offer diagnostic options and
parameter settings are performed via a graphic user interface. The systems are self-
monitoring and offer fail-safe back-up of technical data.
In the most frequently used duty, the Contrac actuator follows an analogue setpoint
signal in continuous operation. As the torque or force increases or decreases
smoothly, the mechanical components are not subjected to load peaks. This
facilitates long service intervals and a long service life of the actuator and actuating
element. In this material-friendly duty, Contrac systems can also be operated when
control commands are received as pulses from a step controller. In this way, the user
can benefit from these unique operating features even in older plants, which
frequently still use step control or simple open-closed commands. Torques and
forces can be set independently of each other as well as the direction of motion. This
can either be set via a constant value or a torque/force characteristic curve. The
speed settings are made in a similar manner.
In the “Drive to end position” duty, individual settings are available for the respective
end position. Depending on the settings, the motor either remains on or is switched
off as soon as the actuator reaches its defined position and the brake is applied to
stop the motor. With the help of the breakaway function, the Contrac actuator can
make up to 200% of its rated torque or rated force available in the end position
areas. This allows jammed actuating elements to be safely moved out of their end

7. position. For most control loops, minimal valve movements near the end position
make little sense from a technical perspective. If, however, process variables change
at this actuating element position, the actuator will follow the resulting control
commands and there is a danger that the valve final control element will sustain
permanent damage if it is approached too often. There is also a danger that valve
positions very close to the end positions will cause cavitation. ABB actuators avoid
this effect by defining a small area in front of the end position. As soon as the
actuator reaches this area, it behaves as if it were set to ‘Drive to end position’.
The three-phase asynchronous motor with cage rotor guarantees safe and reliable
operation. The use of a frequency converter allows the torque and stroke time of the
intelligent actuator to be varied. This means that both parameters can be adapted to
the actuating element or process independently of each other. The motor is
constantly under voltage, and increases or reduces the torque gently and in
proportion to the control deviation. The actuator is always switched on, meaning that
no restrictions are placed on the control loop, even at the maximum permissible
ambient temperature. Where a state of balance exists, the drive force and process
force are equivalent and the actuator will keep the actuating element in the required
position. No temperature or torque monitoring switches are required.
The devices have a very high overall efficiency level of 80 to 85 percent. They do not
require any cooling period or reduction in switch-on time at high ambient
temperatures. Owing to their 100% switch-on time, they are suited to highly-dynamic
control loops, as required for highly-precise control loops for injection control valves
or pressure-reducing stations.
In rotary actuators, the motor drives a low-friction, oil-lubricated spur transmission. At
the end of this spur transmission, a lever mounted on the output shaft transmits
torque to the actuating element via a rod. As the position sensor is mounted directly
on the rear end of the output shaft, position feedback can be provided without any
backlash. This means Contrac is able to offer highly precise positioning.

8. Mounting the actuator on the final control element via a lever linkage has the
advantage over direct drive that there is no direct heat transfer. This reduces the
load on the actuator and facilitates longer service intervals. The design with
lever/linkage bar assembly reduces servicing costs owing to the lower temperature
loading. A specific lever length ratio between the actuator lever and final control
element lever and optimisation of the angle in the lever linkage both increases the
resulting torque at the final control element and allows the torque profile to be
adapted to the final control element.
The manufacturer offers the rotary actuators for rated torques from 50 to 16,000 Nm.
The breakaway torque ranges from 200 to 32,000 Nm. The stroke time can be set
between 10 and 900 seconds (at 90 degrees).
Rotary actuators are used, for example, to activate multi-leaf dampers or regulate
the mill air in cement works. In power plant technology, they are required to regulate
the fuel volume, to control the flaps for the burner air or to optimise the fresh air or
induced draught.
ABB linear actuators feature a highly efficient ball roller spindle. The motor drives the
spindle nut located on the thrust rod via a spur transmission, which then moves the
thrust rod out or in, depending on the motor’s direction of rotation. The special
feature of this rotary/linear conversion is a recirculating ball unit with extremely low
friction. Integrated springs absorb any peak loads that may occur owing to kinetic
energy when approaching the end positions of valves and also compensate for
temperature-related alterations to the length of the thrust rod or valve stem that may
occur when using the actuator on a superheated steam pipe. The linear actuators
can be supplied with a rated actuating force of 2 to 100 kN and a breakaway force of
8 to 200 kN. The actuating speed can be set within a range of 0.1 to 10 mm per

9. second. These actuators are used in the most diverse applications, including
injection control valves, drum level control valves, pressure reducing stations, feed
water pre-heating, feed water control valves, bowl level control valve , minimum
volume control valves or start-up control valves.
Power control units are a central component of actuators. These can be installed in
the field near the actuator or remotely in a mounting rack. An integrated version is
available for the smallest rotary and linear actuators. In addition to the connection
terminals, a microprocessor, a frequency converter for motor control, analogue and
binary inputs and outputs, communication interfaces and a female connector for
connection to a PC are also included. In order to be able to accurately position the
actuating element, the power control adjusts the motor torque smoothly until an
equilibrium of forces is achieved between the control actuator and final control
elementfinal control element. High response sensitivity and positioning accuracy with
short stroke times produce excellent control quality with long service life. The control
concept sets the actuators apart from other manufacturers. A continuous positioner
keeps the dead zone at +/- 0.05%. This precision facilitates highly precise positioning
in the entire work area of each valve.
The Contrac generation of actuators can be controlled by conventional signals.
Users can retain their current system concept. However, they also have the facility to
operate their plants with modern communication options at some point in the future
in accordance with future concepts. The actuators communicate either through an
RS232 interface, an FSK connection via the HART protocol or a profibus connection.
A PC and the graphic user interface with the Contrac Device Type Manager (DTM)
can be used to configure all actuator functions as well as to access diagnostics and
service information. The software comprises two elements: the device software and
the optional configuration software. The device software is loaded to the actuator
electronic unit and contains the firmware, motor characteristics and software objects
with the parameters relevant to the actuator, e. g. force/torque limits and start
behaviour with and without breakaway. The DTM configuration software enables
actuator data to be parameterised. The user also receives comprehensive
diagnostics, service and maintenance information. The DTM can be loaded to a
frame application, such as Smart Vision, which supports FDT/DTM technology. This
enables it to be used either locally with the frame application or within a control
system.

10. The components of a control valve are characterised by different service lives.
Depending on the application, a component, and thus the entire system, can fail
prematurely. A holistic asset management approach is essential to avoid such
failure. The diagnostics function of Contrac variable-speed actuators enables
predictive maintenance by monitoring the device itself.
Some error patterns require detailed information to ascertain the precise cause.
Contrac saves the parameters relating to service life (e.g. number of motor reversals,
maximum temperature values for the transmission and electronics, peak loads and
the dynamic loads to which the actuator has been exposed). The graphic user
interface displays the saved data or the user can use this for evaluation at a later
time. In order to establish the cause of non-reproducible errors, parameters relevant
to the positioning loop (e.g. setpoint, actual position value, temperature and motor
frequency) can be recorded during operation. Recordings can also be made over
longer periods via the sample rate setting options.
Conclusion
In energy and steam creation, the regulation of certain plant components is
influenced by the general conditions discussed, including defined service intervals or
switch-on times of actuators. Highly precise and continuous regulation of plants can
contribute to an increase in plant efficiency. In order to achieve this, actuators are
used that are continuously regulated and have long service intervals. The higher
investment costs for special variable-speed actuators are amortised within just a few
months.

11. ABB’s Contrac actuators are the only variable-speed actuators on the market
offering continuous regulation and high precision. The actuators set themselves
apart with low life cycle costs and can also be used in areas with a high risk of
explosion.