We would almost always like to have control
over different parameters of the process like temperature,
pressure, level, and so on To achieve this, obviously,
we need a controller like Programmable Logic
Controller or PLC for short We also need some sensors and transmitters
to send data over to PLC Finally, we need a piece of
equipment to carry out the PLC’s commands which usually
called as “Final Control Element” A Final Control element can be a pump,
a heater, a control valve, and so on If you would like to figure out how
control valves exactly work via an interesting practical example,
stay with us to the end of this video If you like this style of
video, subscribe and click the bell to receive notifications
of new videos by Upmation Very often by “Control
Valves” we mean the type of valves that are used not only to
fully start and fully stop the flow but also to control or adjust or in other
words throttle the flow of the liquid By throttling the flow,
we achieve our final purpose that is for example, controlling
the temperature of a furnace or the level
of a liquid inside a tank You may say how it is possible! Don’t worry!
We are going to talk about it in detail. Well, this is a “Globe valve”; One of the
most common types of control valves Let’s take a quick look
at its different parts This is the “Body” of the valve
that the fluid will pass through it and this is the Bonnet that is connected
to the body and covers its inner parts We call this part Plug The plug will control,
Stop or Start the flow by exposing the liquid
flowing inside the pipe. This part here is the
“Actuator” that transfers the mechanical power to
the plug using the “Stem” The Actuator will receive the commands
from the Positioner The main role of the “Positioner” is
to be an interface between the PLC and the Actuator to precisely adjust
the plug for being open or closed There are a variety of Actuator
and Positioner types and we are going to check them
all at the end of this video So, let’s start with an example
to understand the operation of the Positioner and its
relation to the PLC and Actuator In this example, we aim to control the
temperature of a liquid within a tank by regulating the flow of an additive This additive will generate
heat by having a chemical reaction with the liquid
already inside the tank. Say the PLC decides to send a
50% open command to the control valve This command is based on two items First, the PLC logic and second, the feedback it has
received by the temperature sensor installed on the tank This command signal often is in the form
of a 4-20mA DC electrical current and is sent from the PLC The device that receives this signal
on the field is the “Positioner” of the control valve As we are using a specific type of
pneumatic actuator in this example which needs compressed air
to become activated therefore we chose
an electro-pneumatic positioner Don’t worry about the names at the moment! In this case, the positioner
plays the role of a “translator” and converts the PLC language
to the actuator language! It means that the Positioner
will convert the 4-20mA signal to an air pressure signal How it happens? Well, inside of this
Positioner we have a converter unit which is
called “I to P Transducer” In fact, this transducer converts
the 4mA DC current to a 3PSI air pressure and the 20mA DC current
to a 15PSI air pressure and of course, they are
proportional in the middle range We call 3-15PSI air pressure
as the “Pneumatic Signal” So with a simple calculation,
we understand that in order for the PLC to open
the valve for 50% of its full range it should send a 12mA signal
to the Positioner Then the Positioner will convert it
to a 9PSI signal accordingly and will send it directly to the actuator But wait! It sounds like the 9PSI pressure
is not enough to move the actuator Therefore, we need another input
to the Positioner as the “Air supply” The Air supply input will provide us
with a clean, filtered, regulated air with sufficient pressure, thanks to
this filter/regulator device After all, using the Air supply input
and a built-in pressure amplifier the positioner will be able to convert
the 9PSI pressure signal to sufficient air pressure for moving
the actuator to the right amount Now, we see that the actuator
is able to overcome its spring force and so far so good! But same as the PLC that requires
feedback by the sensor to decide about the command
it is going to send the Positioner also needs
to receive feedback to precisely position the valve stem
and open the valve to 50%. This feedback is sent by
the “control valve” to the Positioner using a mechanical mechanism In this way, the Positioner
will decide how much pressure the actuator needs to move the valve stem In general, Positioners come
into three different categories; First, the “Electro-Pneumatic”
Positioners or I/P Positioners that we already
discussed in the example Second, the “Pneumatic” Positioners,
in which their control signal is a pneumatic signal and they do not need
any I/P Transducer modules integrated The last is “Digital” Positioner or
“Digital Valve Controller” In addition to an “I to P Transducer” these positioners take advantage
of a “Microprocessor” to fill in the place of
the mechanical position feedback. The input signal or setpoint from the PLC
will directly send to the Microprocessor The valve position feedback
which is measured electronically will also enter the Microprocessor Comparing these two electronic signals the Microprocessor is able
to adjust the valve position quite accurately in comparison
with the other types of positioners. Using Digital Positioners, we are able
to communicate with the valve by different types of protocols such as
HART or Fieldbus protocols like Profibus By such digital communications,
we can calibrate control valves way more easily using
and-held communicators without tackling time-consuming and
sometimes difficult mechanical adjustments Besides, we can send some feedbacks
from the control valve to the PLC or DCS via these communication protocols The last point is that the standalone
I/P transducers can control the valve independently in case that the
accuracy is of less importance Meaning that, they are not integrated
into any kind of positioners and therefore there is
no feedback in this case Now that we’ve got introduced
to the positioners let’s get into the Actuators
and their different types Generally, we can classify the Actuators
into four different categories Pneumatic, Hydraulic, Electric, and Manual The Pneumatic actuators are
the most used kinds of actuators due to their simple design fairly low price
and of course being Intrinsically Safe Electric Actuators have
an electric motor inside In the first place, they had been designed
for on/off applications but nowadays some of them are upgraded
for continuous control applications The primary application of them is in the locations that we don’t
access the compressed air In this video, we learned how a
Spring-and-Diaphragm actuator along with an Electro-Pneumatic Positioner adjusts the valve in order for the valve to control the flow of the liquid
according to the PLC commands Well, it’s time to ask us your question or tell us about your experiences dealing
with the control valves in the comments Again if you like these kinds
of videos please subscribe and tell us which subjects
you are most interested in Thanks for watching!
