Instruments - Part 2

Control System Training
Instruments – Part 2

Classification: Restricted
Agenda:
• Instruments
• Transmitters
• Control valves
• Valve actuators
• Valve positioner

INSTRUMENTS
Transmitters used in Process Instrumentation
In the process industries, there four common process variables of interest:
(a) Pressure
(b) Temperature
(c) Flow Level
(d) Level
In addition to these variables, there is also the need to analyze various
chemical compositions and components.
The following transmitters are commonly used in the process industries.
• Pressure Transmitters
• Temperature Transmitters
• Flow Transmitters
• Level Transmitters
• Analytic Transmitters – O2(Oxygen), CO (Carbon Monoxide), PH etc.

Pressure Transmitters
Pressure transmitters are essentially used in measuring various types of
process pressures.
Absolute Pressure Transmitter ‐ This transmitter measures the pressure
relative to perfect vacuum pressure.
Gauge Pressure Transmitter ‐ This transmitter measures the pressure
relative to atmospheric pressure at a given location. When the pressure
gauge reads 0 PSI, it is means pressure is atmospheric.
Differential Pressure Transmitter ‐ This transmitter measures the difference
between two or more pressures introduced as inputs to the sensing unit.
They are used to measure the pressure drop across an oil filter for example.
They are also popularly used to measure flow or level in pressurized vessels.
Various technologies can be used to create pressure transmitters – vibrating
wire sensor, capacitance pressure sensor, strain gauge sensor, LVDT (Linear
Variable Differential Transformer) etc

Level Transmitters
Level transmitters are used to measure the level of a liquid or solid material
within a vessel or space. These transmitters can measure level continuously or
at determined points:
• Point Level Transmitters ‐ Provides an output when a specific level
measurement is reached. This output is generally in the form of an audible
alarm or an electrical signal to turn on a switch.
• Continuous Level Transmitters – Measures level within a specified range and
provide an output as a continuous reading in proportion to the changing
level.

INSTRUMENTS
• There are various types of level transmitters in use in the process industries.
Some of them include:
• Ultrasonic Level Transmitters – Used for non‐contact level sensing of
highly viscous liquids, as well as bulk solids.
• Conductive Level Transmitters – Used for point level detection of a wide
range of conductive liquids such as water, and is especially well suited for
highly corrosive liquids such as caustic soda, hydrochloric acid, and similar
liquids.
• Pneumatic Level Transmitters – Used in hazardous environments and
where there is no electric power. They are also ideal in applications
involving heavy sludge or slurry.
• Capacitance Level Transmitters – They are used in liquids that are
non‐conductive and have a high dielectric constant and can be used for
continuous level monitoring.
• Hydrostatic based Level Transmitters – These transmitters use the
hydrostatic pressure at a point in a liquid to determine level.

Temperature Transmitters
• A temperature transmitter comprises a temperature sensor and transmitter.
The transmitter receives a signal from temperature sensors such as a
thermocouple or RTD, computes the temperature based on this signal and
then converts it to a 4 – 20mA output signal meant for a receiving device such
as a controller.
• There are different types of temperature transmitters used in the process
industries utilizing various temperature measurement technologies. The most
common types include:
• Thermocouple type Temperature Transmitter – With a thermocouple, the
electromotive force generated by changes in the process temperature is used
to calculate temperature.
• RTD type Temperature Transmitter – When an RTD is used, changes in
process temperature results in change in the electrical resistance of the RTD
sensor. This relationship between process temperature and electrical
resistance is then used to calculate temperature by the transmitter.

Flow Transmitters
• A flow transmitter is used to measure and indicate flow. It combines a flow
sensor and transmitter in one piece.
• The flow signal from the flow sensor is used by the transmitter to generate a
4 – 20m A output that represents changes in flow in the actual process.
• Various technologies are used in flow transmitters to measure flow.
• They range from velocity based flow sensors, Ultrasonic flow sensors to
Differential pressure flow sensors.

Final Control Elements
• Final control elements are devices that complete the control loop. They link
the output of the controlling elements with their processes. Some final
control elements are designed for specific applications. The final control
element is the last element of the closed control loop that implements the
control action. It receives the output signal (control or actuating signal) from
a process controller and adjusts accordingly the value of the manipulated
variable by changing the amount of matter or energy entering the process in
a way to bring the controlled variable (process variable) to its set point. The
final control element is probably the most important because it exerts a
direct influence on the process.
• Final control elements are general application devices such as valves,
dampers, pumps, and electric heaters. Valves and dampers have similar
functions. Valves regulate flow rate of a liquid while dampers regulate flow of
air and gases. Pumps, like valves, can be used to control flow of a fluid.
Heaters are used to control temperature.

Control Valves
• A control valve is a valve with a pneumatic, hydraulic, electric or other
externally powered actuator that automatically, fully or partially opens or
closes the valve to a position dictated by signals transmitted from controlling
instruments.
• The control valve is the most widely used type of final control element and it
must perform satisfactorily with a minimum amount of attention, even in
severe conditions of temperature, pressure, corrosion and contamination.
• A control valve functions as variable resistance in a pipeline. It provides a
pressure drop, called throttling, which limits the flow through a pipeline.
• There are many different kinds of control valves in common use like globe
valves, butterfly valves, ball valves, eccentric disc valves and diaphragm
valves.

GLOBE VALVE
Globe valves are the most frequently encountered control valves in process
plants. Figure shows the side view of a typical globe valve, which is named after
its globular shaped valve body.
The globe valve consists of two main components:
• The valve body: which contains and regulates the fluid flow
• The valve actuator: which converts electrical or pressure energy into stem
movement which changes the regulating effect of the valve body.

GLOBE VALVE BODY
The bonnet assembly is the part of the valve body through which the valve plug
stem moves. The bonnet provides a means of sealing against leakage along the
stem by using
packing in the packing box. Force is exerted
by the stud and nuts on the packing material
to squeeze it against the walls of the packing
box and the valve stem. This acts as a
effective seal. The valve stem extends
through the bonnet to permit positioning
of the valve plug, and therefore provide a
variable restriction to the fluid flow.

GLOBE VALVE
• Globe valve bodies can be classified as either direct or reverse acting.
• In a direct acting valve body, a downward movement of the valve plug stem
results in the valve closing.
• In a reverse acting valve body, a downward movement of the valve plug stem
results in the valve opening.

VALVE ACTUATORS
A control valve actuator is a device which is used to drive the valve plug stem
and therefore sets the position of the plug with respect to the valve seat.
The most common valve actuator is the pneumatic diaphragm actuator. It is
simple in construction and very reliable.
It operates by the injection of a
single, low pressure air signal
into the diaphragm housing.
The diaphragm housing is made
up of two sections. Between the
two sections is a flexible
diaphragm.

VALVE ACTUATORS
• The air pressure applied on the diaphragm develops a working force. This
force is transmitted to the actuator stem via the diaphragm plate, which is a
supportive metal disk attached to the diaphragm.
• The actuator spring provides a restoring force which positions and returns the
actuator stem.
• The travel indicator (a pointer attached near the stem connector) indicates
the valve travel on the indicator scale.
• The actuator is supported rigidly on the valve bonnet assembly by the yoke.
• The actuator stem is connected by the stem connector to the valve plug
stem.

PNEUMATIC ACTUATOR
• The pneumatic valve is an air-operated device which controls the flow
through an orifice by positioning appropriately a plug.
• An air to close (A/C) valve, can be obtained with the combination of a reverse
acting actuator and a reverse acting valve body or a direct acting actuator
and a direct acting valve body. In the air to close valve the pressure on the
diaphragm causes the plug to move downwards and close the opening thus
restricting the flow.
• Similarly, an air to open (A/O)/fail close valve, can be obtained with a
combination of direct actuator and reverse body or reverse actuator and
direct body. In the air to open valve the pressure on the diaphragm causes
the stem to move upwards thus, causing the plug to move away from the
opening causing the fluid to flow.

PNEUMATIC ACTUATOR
Air to Close (A/C) Valve Air to Open (A/O) Valve

HYDRAULIC ACTUATORS
• Pneumatic actuators are normally used to control processes requiring quick
and accurate response, as they do not require a large amount of motive
force. However, when a large amount of force is required to operate a valve
(for example, the main steam-stop valves), hydraulic actuators are normally
used.
• Hydraulic actuators use fluid displacement to move a piston in a
cylinder positioning the valve as needed for 0-100% fluid flow. Although
hydraulic actuators come in many designs, piston types are most common.

HYDRAULIC ACTUATORS
• It consists of a cylinder, piston, spring, hydraulic supply and returns line, and
stem. The piston slides vertically inside the cylinder and separates the
cylinder into two chambers. The upper chamber contains the spring and the
lower chamber contains hydraulic oil. The hydraulic supply and return line are
connected to the lower chamber and allows hydraulic fluid to flow to
and from the lower chamber of the actuator. The stem transmits the
motion of the piston to a valve.
• When the hydraulic force is greater than the spring force, the piston
begins to move upward, the spring compresses, and the valve begins to
open. As the hydraulic pressure increases, the valve continues to open.
Conversely, as hydraulic oil is drained from the cylinder, the hydraulic force
becomes less than the spring force, the piston moves downward, and
the valve closes. By regulating amount of oil supplied or drained from
the actuator, the valve can be positioned between fully open and fully
closed.

ELECTRIC SOLENOID ACTUATORS
• Solenoid actuators are used on small valves and employ an
electromagnet to move the stem which allows the valve to either be fully
open or fully closed.
• It consists of a coil, armature, spring, and stem. The coil is connected to an
external current supply. The spring rests on the armature to force it
downward. The armature moves vertically inside the coil and transmits its
motion through the stem to the valve.

INSTRUMENTS
When current flows through the coil, a magnetic field forms around the coil.
The magnetic field attracts the armature toward the center of the coil. As the
armature moves upward, the spring collapses and the valve opens. When the
circuit is opened and current stops flowing to the coil, the magnetic field
collapses. This allows the spring to expand and shut the valve
• A major advantage of solenoid actuators is their quick operation. Also, they
are much easier to install than pneumatic or hydraulic actuators.
• However, solenoid actuators have two disadvantages.
• First, they have only two positions: fully open and fully closed.
• Second, they don’t produce much force, so they usually only operate
relatively small valves.

VALVE POSITIONER
• Pneumatic valve positioners are the most commonly used valve accessories.
A valve positioner is a device which will accurately position a control valve in
accordance with the pneumatic control signal.
• With a positioner, the control signal goes to the positioner instead of directly
to the valve actuator. The positioner outputs another pneumatic signal which
operates the valve actuator.

VALVE POSITIONER
The positioner compares the control signal (the requested valve position) with
the actual valve position through the mechanical feedback linkage.
• If the valve position is incorrect, the positioner will either load or exhaust
air from the valve actuator until the correct valve position is obtained.
• A positioner requires both a control signal and an instrument supply air
for normal operation.
Most positioners come equipped with three gauges to indicate supply air
pressure, control signal pressure and actuator diaphragm (output) air pressure.

Topics to be covered in the next session:
• Programmable Logic Controller(PLC)
• Types of PLC’s
• PLC architecture
• Scan cycles
• Scan patterns
• PLC programming
• Ladder diagram programming
• Latch and Unlatch
• DCS architecture

Thank you!

Instruments - Part 2

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Instruments - Part 2