Boiler Operation
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Transcript Boiler Operation
INSTRUCTOR:
ROBERT A. MCLAUGHLIN
ZAILI THEO ZHAO
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AUTOMATED CONTROL VALVES &
PRESSURE REGULATORS
WEEK - 6
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POWER
EQUIPMENT
LEARNING OBJECTIVES
Understanding of the fundamentals of
automation, classification, and terms associated
with automatic valves.
Identify the types of actuators and control valves
used in automation systems.
Determine the three categories of control valves.
Define the functions of the control valve actuator,
application and limitations, troubleshooting and
repair of actuators.
Identify the types of pressure reducing valve and
operating principles associated with each type.
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AUTOMATED VALVES
automate?
Advantages:
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Why
Increases process efficiencies
Reduced manpower/maintenance
Increase safety
Quicker more reliable control
Increase mechanical advantage
Disadvantages
Greater initial cost
Complexity, more personal training
Higher qualification of workers
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DEGREES OF AUTOMATION
Fully automated
control
No direct human
intervention
needed
Semi-automated
control
Periodically
requires human
intervention
Part automated – Part
manual
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AUTOMATION CLASSIFICATION
THREE BROAD CATEGORIES
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Position control
Speed control
An elevator
Rotating equipment like turbines, diesels etc.
Process control
Regulating of temperatures, pressures, fluid flow,
tank levels, density control
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AUTOMATION CLASSIFICATION
SIMPLE ‘DISPENSING’ SYSTEM
Process control valves include the
three categories:
Dispensing
Seem like flow control
Dissipating
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Seem like pressure control
Distributing
Seem like direction control
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AUTOMATION CLASSIFICATION
SIMPLE ‘DISPENSING’ SYSTEM
Process control valves - three
categories:
Dispensing
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The control valve dispenses steam or
water for functions like temperature
control.
A lube oil cooler will have a valve
control valve to regulate the flow of
the cooling fluid through the cooler
to maintain a constant temperature
A superheater attemperator control
valve dispenses saturated steam to
the superheated steam to control
the final temperature
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CLASSIFICATION
SIMPLE ‘DISSIPATING’
SYSTEM
Dissipating
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Pressure dissipation
Pressure regulation falls into this category
The automated valve regulates system pressure, high
pressure inlet and a controlled lower pressure outlet
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AUTOMATION CLASSIFICATION
SIMPLE ‘DISTRIBUTING’ VALVE
Distributing
Dividing a process flow into separate lines or tanks
The main slide valve has two coupled shells.
The inner shell is the actual distributing valve.
In the fully left position, it connects the left cylinder end with the
channel B and the right cylinder end with the channel A.
When the main slide valve moves to the right by the action of the
eccentric, it closes both ports.
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TERMS AND DEFINITIONS ASSOCIATED
WITH AUTOMATED VALVES
Valve capacity
Rate of flow through a valve
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Dead Band
The amount of diaphragm pressure change that will
not bring about valve stem movement.
A good way to think of dead band in mechanical
systems is to consider the lost motion in a connecting
linkage due to excessive bushing or pin wear.
Excessive friction can also cause dead band to occur.
This is also known as lost motion.
In some control systems, we do not want small variations in
the feedback signal to move the stem.
Most automated valves will allow the operator to set the
sensitivity of the valve, in other words set the dead band.
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TERMS AND DEFINITIONS ASSOCIATED
WITH AUTOMATED VALVES
Feedback signal
I ’s the measured signal that determines what the valve
has to do.
In a system in which pressure is being regulated, as the
controlled pressure is rising, the feedback tells the valve to
move in the closed direction.
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Diaphragm Pressure Span
The low and high actuating signals that the diaphragm
valve operates.
For example,
a diaphragm pressure control valve may be controlling system
pressure to 25 psi.
To regulate this signal, the diaphragm will receive a pneumatic
air signal from the air pilot controller from 5 psi to 25 psi.
From this situation, the diaphragm pressure span is 20 psi.
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TERMS AND DEFINITIONS ASSOCIATED
WITH AUTOMATED VALVES
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Direct or Indirect
Acting
Refers to is the control air signal to the valve is to the
top or bottom of the diaphragm.
In direct acting valves the signal is on the top of the
diaphragm and increasing air pressure causes the
diaphragm and actuator stem to move downward.
In indirect or reverse acting actuators, the signal is
to the bottom of the diaphragm and increasing air
pressure causes upward movement of the actuator
stem.
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TERMS AND DEFINITIONS ASSOCIATED
WITH AUTOMATED VALVES
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Fail Open
Fail Closed
The valve opens wide if an air pressure failure occurs
The valve closes if air pressure failure occurs
Fail Safe
The valve remains the in a fixed position when air
pressure failure occurs
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TERMS AND DEFINITIONS ASSOCIATED
WITH AUTOMATED VALVES
Closed Loop System
The valve process output is
measured and sends a
feedback signals to a controller.
The controller compares this
signal to the set point signal,
and then tells the valve what to do (open more, …… ).
This is a closed loop system.
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Open loop system has no feed back signal
Transducer
A device that changes a pressure or temperature
signal into an electrical signal.
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VALVE ACTUATORS TYPES
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A valve actuator is what
causes the position of the
seat to actually move.
Some common actuators are:
Diaphragm actuators
Piston actuators
Electric motor actuators
Solenoid actuators
Hydraulic actuators
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VALVE ACTUATORS TYPES
Diaphragm actuators – very common in
steam/condensate systems
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By using large diaphragms, a very small air signal to
the diaphragm can have huge mechanical advantage
to move the valve disc.
Direct acting – signal is on top of diaphragm
Indirect acting – signal is to the bottom of
the diaphragm
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VALVE ACTUATORS TYPES
Piston (cylinder) actuators – uses a piston to
position the valve disc.
Most common application is ball and butterfly
valves
More expensive than diaphragm valves
Greater internal friction
Very small and quite
powerful for their size
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VALVE ACTUATORS TYPES
Electric motor actuators – they are usually used
when high speed valve movement is necessary or
long strokes are required in large valves.
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Relatively low torque outputs.
Very expensive
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VALVE ACTUATORS TYPES
Solenoid Actuators – used for on/off applications
only and are quite small.
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VALVE ACTUATORS TYPES
Hydraulic actuators – use hydraulic fluid to
position the valve.
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Requires a complete hydraulic system
Very expensive
Very powerful for their small size.
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ACTUATOR PROBLEMS
Packing gland to tight
Stem worn or damaged
Corrosion inside the valve
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Leaking diaphragms – causes
Poor reaction to signal change
Excessive air consumption
Control valve will not stay in position
Note – a leaking air line from a pilot controller
to the valve diaphragm can cause the same
symptoms.
Valve stem sticking – causes
Poor valve reaction
Stem sticking can be cause by
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PRESSURE REDUCING AND
REGULATING VALVES
Pressure reduction is achieved
by placing a restriction in the
line
A simple pressure reduction
can be accomplished by
placing an orifice in a line.
If either the inlet pressure
varied or the demand changed
on the low pressure side, the
pressure would vary.
To keep the regulated pressure
constant, you need to vary the
orifice size.
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PRESSURE REDUCING AND
REGULATING VALVES
There are two ways to
automate pressure reduction.
Multi component
automated control loops
Self contained pressure
regulating valves
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PRESSURE REDUCING AND
REGULATING VALVES
Multi component control
loops
Minimum requirements
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Pressure sensing device
Pilot pressure controller
Actuated valve
The control pilot controller
receives a signal from the
reduced pressure side of the
system and sends a operating
signal to the diaphragm control
valve.
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PRESSURE REDUCING AND
REGULATING VALVES
Multi component control loops
Two types of valves are
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An upward seating
single valve disc
A downward seating
single valve disc.
Both of these valves are single
seated unbalanced valves
They are the least expensive valve seating
arrangement.
The valve can have a single seat
which is balanced
Balance seats are arranged so high pressure
acts on both sides of the seat which allows the
valve to be operated with less effort.
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PRESSURE REDUCING AND
REGULATING VALVES
Multi component control loops
Valve can also have
two discs on one valve
stem, which are
also
balances.
It is called a double
seat pressure balanced plugs.
The high pressure medium enters between the
two seats and exerts opposing forces on each of
the seats
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PRESSURE REDUCING AND
REGULATING VALVES
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Self contained pressure
regulator has all of the
control components
needed in one valve.
Automatically reduce
supply pressure to a
preselected pressure
As long as the supply
pressure is at least as
high as the selected
pressure.
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PRESSURE REDUCING AND
REGULATING VALVES
The principal parts:
An upward-seating main
valve that has a piston on
top of its valve stem,
An upward-seating
auxiliary valve,
A controlling diaphragm,
An adjusting spring and
screw.
The pressure entering
the main valve assists
the main valve spring in
keeping the reducing
valve closed by pushing
upward on the main
valve disk.
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PRESSURE REDUCING AND
REGULATING VALVES
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Some of the high pressure is
bled to an auxiliary valve.
The auxiliary valve controls
the admission of high
pressure to the piston.
The piston has a larger
surface area than the
main valve disk,
Open the main valve.
The auxiliary valve
is controlled by a
diaphragm.
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PRESSURE REDUCING AND
REGULATING VALVES
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PRESSURE REDUCING AND
REGULATING VALVES
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The control system below
shows a supply service
arrangement.
The variable pressure
sensing line is actually
measuring the pressure
exerted by the column of
water above the sensing
point of the variable pressure
line to the water level.
As the water level changes,
the pressure will vary
in the variable line.
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THANK YOU
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