Module 4. Final control element : I/P converter – pneumatic and electric actuators – valve...
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Process Control Instrumentation II Module 4
Module 4. Final control element : I/P converter – pneumatic and electric actuators – valve positioner – control values – characteristics of control valves
Final control element : I/P converter pneumatic and electric
actuators valve positioner control values characteristics of
control valves inherent and installed characteristics valve body
commercial valve bodies control valve sizing cavitation and
flashing selection criteria.
Slide 3
I/P Converter
Slide 4
ACTUATORS If a valve is used to control a flow, some mechanism
must physically open or close the valve. For a temperature control
some mechanism must turn the heater ON or OFF. these are examples
of ACTUATORS. Between Signal conversion unit and Control
Valve.
HYDRAULIC ACTUATORS P H = F 1 /A 1 = F w /A 2 F w = F 1 A 2 /A
1
Slide 17
HYDRAULIC ACTUATORS
Slide 18
ACTUATORS - COMPARISON Actuator typesAdvantagesDisadvantages
Electrical(servomotor or stepping motor) Direct interface with
computer system. Simple design. Low thrust. Slow speed. No
mechanical fail safe hazardous. Electromechanical(Motors combined
with gear boxes) High thrust High stiffness coefficient Flexible
adaptation Complex design No mechanical fail safe Large, heavy
structure Hazardous Hydraulic and Electro hydraulic High thrust
Fast speed High stiffness coefficient Self lubrication Complex
design Large, heavy structure Hazardous Fluid Viscosity Sensitive
Pneumatic and Electro- pneumatic Low cost Mechanical Fail safe
Simple design Small package Suitable for highly hazardous areas
also Good control with control device Slow speed Lack of stiffness
Instability Moderate trust Quality air requirement
Slide 19
POSITIONERS It guarantees that the valve does move to the
position where the controller wants to be. Can correct many
variations, including changes in packing friction due to dirt,
corrosion, or lack of lubrication; variations in the dynamic forces
of the process; sloppy linkages; or non-linearities in the valve
actuator.
Slide 20
ELECTRONIC VALVE POSITIONERS (For Electric Actuators) High Gain
Proportional controller. It is considered as a slave controller of
a cascade loop in which the master controller is the primary
controller itself. The primary controllers output signal is the set
point for the slave controller (positioner)
Slide 21
ELECTRONIC VALVE POSITIONERS
Slide 22
VALVE POSITIONERS for PNEUMATIC Actuators Refer
Krishnaswamy
Slide 23
Pneumatic Motion Balance Positioner
Slide 24
Pneumatic Force Balance Positioner
Slide 25
Electro-pneumatic Force Balance Positioner
Slide 26
CONTROL VALVES Control Flow Rate
Slide 27
CONTROL VALVE CHARACTERISTICS The relationship between control
valve capacity and valve stem travel is known as the Flow
Characteristic of the Control Valve. Trim design of the valve
affects how the control valve capacity changes as the valve moves
through its complete travel. Because of the variation in trim
design, many valves are not linear in nature. Valve trims are
instead designed, or characterized, in order to meet the large
variety of control application needs. Many control loops have
inherent non linearity's, which may be possible to compensate
selecting the control valve trim.
Slide 28
CONTROL VALVE CHARACTERISTICS Inherent Characteristics
Installed/ Effective Characteristics
Slide 29
CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal)
These curves are based on constant pressure drop across the valve
(pressure difference is determined by the valve alone). based on
the design conditions, parameters and experimental results.
Different from actual because of the variations in parameters and
limitations in manufacturing process.
Slide 30
CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal)
ASSUMPTIONS: 1.The actuator is linear. 2.The pressure difference
across the valve is constant. 3.The process fluid is not flashing,
cavitating or approaching sonic velocity (Choked flow).
Slide 31
Inherent Characteristics.
Slide 32
1. Quick Opening CV Used for Full On/ Full OFF applications.
Relatively small motion of valve stem results in maximum possible
flow rate through the valve. Eg: 30% travel of stem may results in
90% of full flow Also called DECREASING SENSITIVITY TYPE VALVE The
valve sensitivity (Q/S) at any flow decreases with increasing
flow.
Slide 33
2. Linear Valve Flow rate varies linearly with stem position.
Q/Q max = S/ S max Sensitivity Constant. Eg: if valve open 25% the
flow through the valve is 25% of full flow. Process tank level
control, De-aerated level control etc.
Slide 34
3. Equal Percentage Valve A given percentage change in stem
position produces an equivalent change in flow. These valves does
not shut off flow completely. RANGEABILITY, R = Q max /Q min
INCREASING SENSITIVITY TYPE The valve sensitivity at any given flow
rate is a constant percentage of the given flow rate, thus equal
percentage.
Slide 35
3. Equal Percentage Valve Eg: consider a valve having a maximum
flow rate of 60 tonnes/hr. Case 1 when valve delivering 10
tonnes/hr, when the valve is permitted to open 10% more, then the
incremental flow is 1 tons/hr. Case 2 when valve delivering 40
tonnes/hr, when the valve is permitted to open 10% more, then the
incremental flow rate is 4 tonnes/hr.
Slide 36
Slide 37
CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real)
When valve is installed as part of a process plant, its flow
characteristics are no longer independent of the rest of the
system. Fluid flow through the valve is subject to frictional
resistances in series with that of the valve. Distortion
Coefficient,
Slide 38
CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real)
1.Deviation in inherent valve characteristics. 2.Actuators without
positioner will introduce nonlinearities. 3.Pump curves will also
introduce nonlinearities.
Slide 39
Valve Body & Commercial Valve Bodies Refer
Krishnaswamy
Slide 40
CONTROL VALVE SIZING Proper sizing of control valve is
important. If control valve is oversize, The valve must operate at
low lift and the minimum controllable flow is too large. Lower part
of the flow-lift characteristic is most likely to be non uniform in
shape. If control valve is undersize, the maximum flow desired for
operation of a process may not be provided.
Slide 41
Control valve sizing FACTORS AFFECTING SIZING Pressure Drop
across the control valve Flow Rate through the control valve
Specific Gravity Other factors such as type of fluid, gas or
liquid, critical flow conditions for gases and vapours and
viscosity of liquids influence valve size
Slide 42
FLOW COEFFICIENT, C V The flow coefficient indicates the amount
of flow the control valve can handle under a given pressure drop
across the control valve. The number of US Gallons of cold water at
60 F flowing through a fully open control valve for 1 minute at a
pressure difference of 1 psi. US Gallon = 3.785 ltr Imperial Gallon
= 4.53 ltr
Slide 43
FLOW COEFFICIENT, K V Whenever the flow coefficient is
mentioned in metric units. The flow rate of water in m 3 /hr at
about 30 0 C flowing through the fully opened control valve at a
pressure drop of 1 kg/cm 2 across the valve. C V = 1.17 K V K V =
0.86 C V
Slide 44
FLOW COEFFICIENT, K V Q = flow rate S G = Specific Gravity
Slide 45
Guidelines for Sizing of Control Valves 1. The valve shall be
sized for the actual flow condition and not for the ultimate design
capacity of the system. Normal maximum flow rate is normally about
70% of the ultimate design capacity. 2. Most of the pressure drop
of the system should be across the control valve. (about 70%) 3.
When the pipe line is dimensioned with normal allowable velocities
the control valve will be a few sizes smaller than the pipe line.
Only in extreme cases where very high velocities have been used in
the pipe line, the size of the control valve will be the same as
that of the pipe line.
Slide 46
Guidelines for Sizing of Control Valves 4. The selection must
be done such that the calculated C V is attained at about 75 to 80%
of the full wave travel. 5. Regardless of the application such as
flow control or pressure control the valve sizing is done on the
basis of flow coefficient C V.
Slide 47
CAVITATION Related to Bernoullis theorem which describes the
pressure profile as the fluid passes through a narrower passage,
restriction, orifice or control valve. As the fluid accelerates
pressure head is converted into velocity head. Fluid accelerates to
maximum velocity, minimum pressure known as Vena Contracta. Fluid
then slows down as it again expands to full pipe area. Static
pressure recovers but a part is lost due to friction.
Slide 48
CAVITATION If the pressure head drops below the liquid vapour
pressure at that temperature, then vapour bubbles will form
downstream of the restriction. As the static pressure recovers to a
point greater than vapour pressure, the vapour bubbles collapse
back into their liquid phase. This collapse of bubbles produces
high-energy implosions which is called CAVITATION. These implosions
generate noise, fluid shock cells, and gets that impinge upon the
trim metal parts. It generates a tremendous and concentrated impact
force that destroys the metal as it fractures out tiny metal
particles.
Slide 49
PRESSURE PROFILE
Slide 50
CAVITATION
Slide 51
CAVITATION Cavitation is usually coupled with vibration and a
sound like rock fragments or gravel flowing through the valve.
Slide 52
Elimination of CAVITATION No known material can withstand
continuous cavitation without damage and eventual failure. The
length of time it will take is a function of the fluid, metal type
and severity of the cavitation. The greatest damage is caused by a
dense pure liquid with high surface tension. Density governs the
mass of the micro jet stream, and surface tension governs the jet
velocity.
Slide 53
Cavitation Elimination Methods 1. Revised Process Conditions 2.
Revised Valve 3. Gas Injection 4. Revised Installation
Slide 54
Cavitation Elimination Methods
Slide 55
Cavitation Elimination Methods
Slide 56
FLASHING Cavitation occurs when P 2 P V, while flashing takes
place when P 2 P V. Liquid flashes into vapour and stays in the
vapour phase. The specific volume increases as liquid changes to
vapour which in turn causes an increase in the fluid velocity. If
enough vapour is formed, the resulting high velocities can erode
metals. So the piping downstream of a valve needs to be much larger
than the inlet-piping in order to keep the velocity keep the
velocity of the stream low enough to prevent erosion.
Slide 57
SELECTION OF CONTROL VALVES 1. Need for a Control Valve 2.
Collection of Process Data 3. Assigning Valve Pressure Drop 4.
Control Valve Performance a. Valve characteristics b. Gain of
control loop components like process, sensor, controller and valve
gain. c. Process Nonlinearity d. Valve Rangeability e. Control
Valve Sequencing f. Split-ranging or floating
Slide 58
SELECTION OF CONTROL VALVES 5. Control Valve Sizing 6. Valve
Actuator Selection a. Whether electrical, pneumatic or hydraulic b.
Actuator Speed of Response c. Actuator power or torque d. Valve
failure position (Fail safe operation) 7. Valve Positioner a. When
not to use positioner b. To eliminate dead band c. Split range
operation
Slide 59
SELECTION OF CONTROL VALVES 8. Process Application
Considerations a. High Pressure service b. High differential
pressure usage c. Vacuum service d. High temperature service e. Low
temperature service f. Cavitation and erosion g. Flashing and
erosion h. Viscous and slurry service i. Leakage j. Small flow
values k. Control valve noise l. Piping and installation
considerations m. Climate and atmospheric corrosion
Slide 60
SELECTION OF CONTROL VALVES 9.Control Valve Specification form
10.Test report and test specification
