robert a. mclaughlin zaili theo zhao automated control valves ...

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5 Νοε 2013 (πριν από 4 χρόνια και 4 μέρες)

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POWER
EQUIPMENT



INSTRUCTOR:


ROBERT A. MCLAUGHLIN


ZAILI THEO ZHAO


AUTOMATED CONTROL VALVES &
PRESSURE REGULATORS

W
EEK

-

6

01:06

1

L
EARNING

O
BJECTIVES


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.

01:06

2

A
UTOMATED

V
ALVES


Why automate?


Advantages:


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


01:06

3

D
EGREES

OF

AUTOMATION


Fully automated
control


No direct huma
n

intervention
needed


Semi
-
automated
control


Periodically
requires huma
n

intervention


Part automated


Part
manual


01:06

4

A
UTOMATION

C
LASSIFICATION

T
HREE

B
ROAD

C
ATEGORIES




Position control


An elevator



Speed control


Rotating equipment like turbines, diesels etc.



Process control


Regulating of temperatures, pressures, fluid flow,
tank levels, density control

01:06

5

A
UTOMATION

C
LASSIFICATION

S
IMPLE

‘D
ISPENSING
’ S
YSTEM


Process control valves include the
three categories:


Dispensing


Seem like flow control


Dissipating



Seem like pressure co
ntrol


Distributing


Seem like direction

co
ntrol


01:06

6

A
UTOMATION

C
LASSIFICATION

S
IMPLE

‘D
ISPENSING
’ S
YSTEM


Process control valves
-

three
categories:


Dispensing


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


01:06

7

AUTOMATION
CLASSIFICATION


SIMPLE ‘DISSIPATING’
SYSTEM


Dissipating



Pressure dissipation


Pressure regulation falls into this category


The automated valve regulates system pressu
re, high
pressure inlet and a controlled lower pressure outlet

01:06

8

A
UTOMATION

C
LASSIFICATION

S
IMPLE

‘D
ISTRIBUTING
’ V
ALVE


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.

01:06

9

T
ERMS

AND

DEFINITIONS

ASSOCIATED

WITH

AUTOMATED

VALVES


Valve capacity


Rate of flow through a valve


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.

01:06

10

T
ERMS

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.


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.

01:06

11

T
ERMS

AND

DEFINITIONS

ASSOCIATED

WITH

AUTOMATED

VALVES




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.

01:06

12

T
ERMS

AND

DEFINITIONS

ASSOCIATED

WITH

AUTOMATED

VALVES







Fail Open


The
valve opens
wide
if an air pressure failure
occurs


Fail Closed


The valve closes if air pressure failure occurs


Fail Safe


The valve remains the in a fixed position when air
pressure failure occurs

01:06

13

T
ERMS

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.


Open loop system
has no feed back signal


Transducer



A device that changes a pressure or temperature
signal into an electrical signal.


01:06

14

V
ALVE

A
CTUATORS

T
YPES


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




01:06

15

V
ALVE

A
CTUATORS

T
YPES


Diaphragm actuators


very common in
steam/condensate systems


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



01:06

16

V
ALVE

A
CTUATORS

T
YPES


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


01:06

17

V
ALVE

A
CTUATORS

T
YPES


Electric motor actuators


they are usually used
when high speed valve movement is necessary or
long strokes are required in large valves.


Relatively low torque outputs.


Very expensive


01:06

18

V
ALVE

A
CTUATORS

T
YPES



Solenoid Actuators


used for on/off applications
only and are quite small.


01:06

19

V
ALVE

A
CTUATORS

T
YPES


Hydraulic actuators


use hydraulic fluid to
position the valve.


Requires a complete hydraulic system


Very expensive


Very powerful for their small size.


01:06

20

A
CTUATOR

PROBLEMS


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


Packing gland to tight


Stem worn or damaged


Corrosion inside the valve


01:06

21

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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.


01:06

22

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


There are
two ways to
automate pressure reduction.


Multi component
automated control loops


Self contained pressure
regulating valves


01:06

23

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


Multi component control
loops


Minimum requirements


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.

01:06

24

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


Multi component control loops


Two types of valves are


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.



01:06

25

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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




01:06

26

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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.


01:06

27

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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.

01:06

28

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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.

01:06

29

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES



01:06

30

P
RESSURE

R
EDUCING

AND

R
EGULATING

V
ALVES


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.



01:06

31


THANK YOU




01:06

32