Module 4
Module 4
 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.
I/P Converter
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.
ACTUATORS TYPES
 ELECTRICAL ACTUATORS


Solenoid
Motors
 PNEUMATIC ACTUATORS

Spring and Diaphragm Actuators

Piston Actuators

Rotary Valve Actuators
 ELECTRO-PNEUMATIC ACTUATORS
 HYDRAULIC ACTUATORS
ELECTRICAL ACTUATORS
SOLENOID
 ON- OFF Control
ELECTRICAL ACTUATORS
MOTORS
 Motor as Direct Actuators
 Motor along with Gear boxes
Common Varieties
 DC Motors
 AC Motors
 Stepping Motors
PNEUMATIC ACTUATORS
SPRING & DIAPHRAGM ACTUATORS
PNEUMATIC ACTUATORS
SPRING & DIAPHRAGM ACTUATORS
PNEUMATIC ACTUATORS
SPRING & DIAPHRAGM ACTUATORS
• DIRECT ACTING
• REVERSE ACTING
PNEUMATIC ACTUATORS
SPRING & DIAPHRAGM ACTUATORS
FAIL SAFE OPERATIONS
FAIL OPEN
(Air to Close)
FAIL CLOSED
(Air to Open)
PNEUMATIC ACTUATORS
SPRINGLESS DIAPHRAGM ACTUATORS
PNEUMATIC ACTUATORS
PISTON ACTUATORS
PNEUMATIC ACTUATORS
PISTON ACTUATORS
ELECTRO-PNEUMATIC ACTUATORS
HYDRAULIC ACTUATORS
PH = F1/A1 = Fw/A2
Fw = F1A2/A1
HYDRAULIC ACTUATORS
ACTUATORS - COMPARISON
Actuator types
Advantages
Disadvantages
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 Electropneumatic
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
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.
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 controller’s output signal is the set point
for the slave controller (positioner)
ELECTRONIC VALVE POSITIONERS
VALVE POSITIONERS for PNEUMATIC
Actuators

Refer
Krishnaswamy
Pneumatic Motion Balance Positioner
Pneumatic Force Balance Positioner
Electro-pneumatic Force Balance Positioner
CONTROL VALVES
 Control Flow Rate
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.
CONTROL VALVE CHARACTERISTICS
Inherent
Characteristics
Installed/ Effective
Characteristics
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.
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).
Inherent Characteristics….
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.
2. Linear Valve

Flow rate varies linearly with stem position.
Q/Qmax = S/ Smax

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.
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 = Qmax /Qmin

‘INCREASING SENSITIVITY TYPE’

The valve sensitivity at any given flow rate is a constant
percentage of the given flow rate, thus equal
percentage.
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.
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,
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.
Valve Body & Commercial Valve Bodies
Refer Krishnaswamy
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.
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
FLOW COEFFICIENT, CV
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
FLOW COEFFICIENT, KV

Whenever the flow coefficient is mentioned in
metric units.
The flow rate of water in m3/hr at about
300C flowing through the fully opened
control valve at a pressure drop of 1
kg/cm2 across the valve.
CV = 1.17 KV
KV = 0.86 CV
FLOW COEFFICIENT, KV
Q = flow rate
SG = Specific Gravity
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.
Guidelines for Sizing of Control Valves…
4.
The selection must be done such that the calculated CV
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 CV.
CAVITATION

Related to Bernoulli’s 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.
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.
PRESSURE PROFILE
CAVITATION
CAVITATION…

Cavitation is usually coupled with vibration and a
sound like rock fragments or gravel flowing through
the valve.
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.
Cavitation Elimination Methods
1.
Revised Process Conditions
2.
Revised Valve
3.
Gas Injection
4.
Revised Installation
Cavitation Elimination Methods
Cavitation Elimination Methods…
FLASHING

Cavitation occurs when P2˃PV , while flashing takes
place when P2˂PV.

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.
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
SELECTION OF CONTROL VALVES…
5.
Control Valve Sizing
6.
Valve Actuator Selection
7.
a.
Whether electrical, pneumatic or hydraulic
b.
Actuator Speed of Response
c.
Actuator power or torque
d.
Valve failure position (Fail safe operation)
Valve Positioner
a.
When not to use positioner
b.
To eliminate dead band
c.
Split range operation
SELECTION OF CONTROL VALVES…
8.
Process Application Considerations
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
High Pressure service
High differential pressure usage
Vacuum service
High temperature service
Low temperature service
Cavitation and erosion
Flashing and erosion
Viscous and slurry service
Leakage
Small flow values
Control valve noise
Piping and installation considerations
Climate and atmospheric corrosion
SELECTION OF CONTROL VALVES…
9. Control Valve Specification form
10. Test report and test specification