Total_Mechanical_Course_Valves_Actuators_1636284465

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INSTRUMENTATION MAINTENANCE 
 
 
VALVES AND ACTUATORS 
 
 
 
 
 
 
TRAINING MANUAL 
Course EXP-MN-SI040 
Revision 0
 Field Operations Training
Instrumentation Maintenance
 
 
Valves and Actuators
 
Training Manual: EXP-MN-SI040-EN 
Last Revised: 09/04/2008 Page 2 / 129
 
INSTRUMENTATION MAINTENANCE 
 
VALVES AND ACTUATORS 
 
SUMMARY 
 
1. OBJECTIVES ..................................................................................................................6 
2. INTRODUCTION .............................................................................................................7 
2.1. LOCATION IN A REGULATION LOOP.....................................................................7 
2.2. DEFINITION..............................................................................................................8 
2.3. ROLE OF THE VALVE..............................................................................................8 
2.4. CONSTRAINTS.........................................................................................................8 
2.4.1. Due to the fluid ..................................................................................................8 
2.4.2. Due to the effect of the environment on the valve .............................................9 
2.4.3. Due to the effect of the valve on the environment .............................................9 
2.4.4. Due to the assembly conditions.........................................................................9 
2.5. TECHNOLOGY OF A REGULATION VALVE .........................................................10 
2.6. CHARACTERISTICS OF REGULATION VALVES .................................................12 
2.6.1. Inherent flow characteristic..............................................................................12 
2.6.1.1. Definition ....................................................................................................12 
2.6.1.2. The linear characteristic .............................................................................12 
2.6.1.3. The equal percentage characteristic ..........................................................13 
2.6.1.4. The quick opening characteristic................................................................14 
2.6.1.5. Inherent adjustment coefficient or rangeability ...........................................14 
3. VALVE TYPES ..............................................................................................................15 
3.1. LINEAR ACTION VALVE ........................................................................................15 
3.1.1. Plug type valve with single-seat body..............................................................15 
3.1.2. Plug type valve with double-seat body ............................................................17 
3.2. CAGE VALVE .........................................................................................................19 
3.3. 3-WAY VALVE ........................................................................................................21 
3.4. DIAPHRAGM VALVE..............................................................................................23 
3.5. VERTICAL LIFT GATE OR GUILLOTINE VALVE...................................................24 
3.6. MICRO-FLOW CONTROL VALVE WITH ADJUSTABLE Cv ..................................25 
3.7. ROTARY VALVE.....................................................................................................27 
3.7.1. Butterfly valve..................................................................................................27 
3.7.2. Spherical plug ball valve, known simply as a "Ball valve"................................28 
3.7.3. Semi-rotary valve with eccentric shutter ..........................................................30 
4. TYPES OF PLUG ..........................................................................................................33 
4.1. QUICK OPENING LINEAR PLUG...........................................................................34 
4.2. LINEAR PLUG ........................................................................................................34 
4.3. MODIFIED LINEAR PLUG ......................................................................................34 
4.4. EQUAL PERCENTAGE PLUG................................................................................35 
4.5. PARABOLIC PLUG.................................................................................................35 
5. TYPES OF CAGE..........................................................................................................36 
5.1. QUICK OPENING CAGE ........................................................................................36 
5.2. LINEAR CAGE ........................................................................................................36 
5.3. EQUAL PERCENTAGE CAGE ...............................................................................37 
5.4. LOW NOISE CAGE.................................................................................................37 
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6. THE CAP .......................................................................................................................38 
6.1. THE PACKING GLAND...........................................................................................39 
6.2. GLAND PACKING...................................................................................................40 
7. THE SERVOMOTOR.....................................................................................................42 
7.1. PNEUMATIC SERVOMOTOR ................................................................................43 
7.1.1. Standard diaphragm type servomotor .............................................................43 
7.1.1.1. Operation ...................................................................................................44 
7.1.1.2. Description .................................................................................................45 
7.1.2. Diaphragm type servomotor with multiple springs ...........................................46 
7.1.3. Rolling diaphragm type servomotor.................................................................46 
7.1.4. Piston type servomotor....................................................................................47 
7.2. HYDRAULIC SERVOMOTOR.................................................................................49 
7.2.1. Constitution .....................................................................................................49 
7.2.2. Operation.........................................................................................................50 
7.3. ELECTRIC SERVOMOTOR....................................................................................51 
7.3.1. Servomotor with motor and gearbox ...............................................................51 
7.3.2. Solenoid type servomotor................................................................................52 
7.4. DIRECTION OF ACTION ........................................................................................53 
7.4.1. Direction of action of the valve body................................................................53 
7.4.2. Direction of action of the servomotor...............................................................54 
7.4.3. Direction of action of the positioning device ....................................................54 
7.4.4. Special case with the "two-way" servomotor type piston .................................55 
7.5. FAIL-SAFE POSITION............................................................................................55 
7.5.1. Fail-safe aspect of thevalve (body + servomotor)...........................................55 
7.5.2. Fail-safe aspect of the valve with its positioning device ..................................56 
8. VALVE ACCESSORIES ................................................................................................57 
8.1. POSITIONING DEVICE ..........................................................................................57 
8.1.1. Pneumatic positioning device ..........................................................................58 
8.1.1.1. Features .....................................................................................................58 
8.1.1.2. Constitution ................................................................................................58 
8.1.1.3. Principle of operation .................................................................................59 
8.1.1.4. Faults .........................................................................................................61 
8.1.2. Electro-pneumatic positioning device ..............................................................62 
8.1.2.1. Constitution ................................................................................................62 
8.1.2.2. Principle of operation .................................................................................63 
8.1.2.3. Faults .........................................................................................................65 
8.1.3. Intelligent (digital) positioning device...............................................................65 
8.1.3.1. Constitution ................................................................................................65 
8.1.3.2. Principle of operation .................................................................................66 
8.1.3.3. Faults .........................................................................................................68 
8.2. THE ELECTRO-PNEUMATIC CONVERTER (I/P)..................................................69 
8.3. THE LUBRICATOR.................................................................................................70 
8.4. THE POSITION SENSOR.......................................................................................71 
8.4.1. Microswitch......................................................................................................71 
8.4.1.1. Microswitch on a linear valve .....................................................................72 
8.4.1.2. Microswitch on a rotary valve .....................................................................73 
8.4.2. Inductive limit switch........................................................................................73 
8.4.3. Capacitive limit switch .....................................................................................74 
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8.5. THE BOOSTER.......................................................................................................75 
8.6. ELECTROVALVE OR "ELECTRO-DISTRIBUTOR VALVE" ...................................77 
8.6.1. Pneumatic distributor valve .............................................................................77 
8.6.1.1. Purpose......................................................................................................77 
8.6.1.2. Principle of operation .................................................................................78 
8.6.1.3. Diagrams....................................................................................................79 
8.6.1.4. The 3/2 distributor valve.............................................................................79 
8.6.1.5. The 5/2 distributor valve.............................................................................79 
8.6.2. Control of distributor valves .............................................................................80 
8.6.2.1. The monostable distributor valve ...............................................................81 
8.6.2.2. The bistable distributor valve......................................................................81 
8.6.3. Installation of the distributor valve ...................................................................82 
8.6.4. The solenoid....................................................................................................83 
8.7. MANUAL CONTROL...............................................................................................84 
9. MAINTENANCE.............................................................................................................86 
9.1. REPLACEMENT OF SEAL PACKINGS..................................................................86 
9.2. VALVE CALIBRATION............................................................................................88 
9.2.1. Calibration of an I/P converter .........................................................................88 
9.2.2. Calibration of an electro-pneumatic positioning device ...................................90 
9.2.2.1. Zero adjustment .........................................................................................90 
9.2.2.2. Adjustment of the scale..............................................................................91 
9.2.2.3. Replacement of the solenoid......................................................................91 
9.2.2.4. Rocker alignment .......................................................................................92 
9.3. DEFECTIVE OPERATION OF THE I/P POSITIONING DEVICE............................93 
9.3.1. Pneumatic system check.................................................................................93 
9.3.2. Electrical system check ...................................................................................93 
9.3.3. Cleaning of the pneumatic system ..................................................................95 
9.3.3.1. Calibrated orifice ........................................................................................95 
9.3.3.2. Controller....................................................................................................95 
9.4. MAINTENANCE OF SERVOMOTOR FOR ROTARY VALVE ................................97 
10. TROUBLESHOOTING.................................................................................................99 
10.1. CAVITATION AND VAPORISATION ....................................................................99 
10.1.1. Variation of the static pressure in a valve ......................................................99 
10.1.2. Cavitation ......................................................................................................99 
10.1.3. Vaporisation ................................................................................................100 
11. VALVE SIZING ..........................................................................................................101 
11.1. THE Cv AND THE Kv of a VALVE ......................................................................101 
11.1.1. What is the Cv of a valve?...........................................................................101 
11.1.2. What is the Kv of a valve? ...........................................................................102 
11.1.3. Standard formulae for calculation of a valve Cv ..........................................103 
11.1.4. Cv calculation formulae according to the manufacturer Masoneilan ...........103 
11.1.4.1. For liquids in imperial units.....................................................................103 
11.1.4.2. For liquids in metric units........................................................................104 
11.1.4.3. For gases and steam, in imperial units...................................................105 
11.1.4.4.For gases and steam in metric units ......................................................106 
11.1.5. Cv calculation for a valve.............................................................................107 
11.1.5.1. Equivalent Cv with 2 valves in parallel ...................................................107 
11.1.5.2. Equivalent Cv with 2 valves in series .....................................................107 
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11.2. CHOICE OF VALVE............................................................................................108 
12. TAG AND IDENTIFICATION OF VALVES.................................................................109 
12.1. ALL-OR-NOTHING VALVES...............................................................................109 
12.1.1. Blow Down Valve ........................................................................................109 
12.1.2. Emergency Shut-Down Valve......................................................................109 
12.1.3. Remote Operated Valve ..............................................................................109 
12.1.4. Shut-Down Valve.........................................................................................110 
12.1.5. Surface Safety Valve ...................................................................................110 
12.1.6. Surface Controlled Sub-Surface Safety Valve.............................................110 
12.2. REGULATING VALVES ......................................................................................111 
13. APPENDICES............................................................................................................112 
13.1. CRITICAL CONSTANTS OF CERTAIN LIQUID AND GAS BODIES..................115 
14. EXERCISES ..............................................................................................................119 
15. FIGURES...................................................................................................................122 
16. TABLES.....................................................................................................................126 
17. ANSWERS TO THE EXERCISES .............................................................................127 
 
 
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1. OBJECTIVES 
 
 
The purpose of this course is to provide a future instrument engineer with knowledge of all 
the types of valves and actuators on an industrial site which has a predominantly 
petroleum-related activity. 
 
At the end of the course, the trainee should have the following knowledge concerning 
valves and actuators: 
 
 Knowledge of all existing types of regulating valve, 
 
 Ability to change the direction of action of a valve, 
 
 Knowledge of the accessories of a regulating valve, 
 
 Ability to distinguish between an I/P converter and an electro-pneumatic positioning 
device, 
 
 Ability to adjust a regulating valve, 
 
 Basic knowledge of how to calculate the Cv of a valve. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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2. INTRODUCTION 
 
2.1. LOCATION IN A REGULATION LOOP 
 
In a regulation loop, the final adjustment component is usually an automatic valve which, 
by acting on the flow-rate of a fluid (gas or liquid), enables the measured value to be 
regulated: 
 
 Pressure 
 
 Flow-rate 
 
 Level 
 
 Temperature, etc. 
 
The automatic valve is the final component of a regulation system; it is the component that 
acts directly on the process. 
 
In a regulation loop, it is every bit as important as the "sensor-transmitter" and as the 
"regulator". 
 
 
REGULATOR 
CONTROL 
COMPONENT 
 "Valve" 
PROCESS 
MEASURING 
COMPONENT 
"Sensor Transmitter"
W Y 
X 
GR 
 
 
Figure 1: Location of the "regulating valve" in the regulation loop 
 
 
W : Setpoint 
Y : Control signal from the regulator 
GR : Adjusting value 
X : Measurement from the sensor-transmitter 
 
 
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2.2. DEFINITION 
 
Valves are components with a variable orifice, which enable a fluid flow to be adjusted. 
 
They are the actuators of most regulation systems, and this means that they are 
significantly important components. It is for this reason that the catalogues issued by valve 
manufacturers are extremely well presented and constitute the best possible 
documentation on the subject. 
 
The role of the instrument engineer is often limited to the maintenance and adjustment of 
installed valves. 
 
Sometimes, when observing the operation of regulation systems that are not performing 
properly, it can be observed that the valve is operating in an abnormal manner: this almost 
always occurs very close to the closing point or, conversely, the valves are too often found 
to be fully open. 
 
 
2.3. ROLE OF THE VALVE 
 
A regulating valve modifies a fluid flow-rate (adjustment value), as a function of the signal 
from a regulator (control signal) or a transmitter, and it does this whatever the constraints 
connected with the circulation of the fluid. 
 
 
2.4. CONSTRAINTS 
 
2.4.1. Due to the fluid 
 
The fluid is either a liquid or a gas (or vapour), or it can be a two-phase mixture (liquid-
solid, water-steam), and these states depend on certain service conditions and the 
chemical composition of the fluid. 
 
Examples: 
 
 Corrosive fluid: this may or may not attack the materials, 
 
 Toxic fluid: danger in case of leakage; sealing class, 
 
 Flammable fluid 
 
 Explosive fluid: in the presence of air or a spark, 
 
 Dangerous fluid: in the sense of a molecular transformation or a reaction with 
other products (e.g. Oxygen (O2) with grease), 
 
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 Viscous fluid 
 
 Fluid laden with solid particles: erosion, clogging, etc. 
 
 Change of phase: solidification, vaporisation, cavitation, etc. 
 
 Pressure 
 
 Temperature: High, very high, or very low (cryogenic effect), 
 
 Food product. 
 
 
2.4.2. Due to the effect of the environment on the valve 
 
 Explosive Atmosphere, 
 
 Corrosive Atmosphere, 
 
 Dry or Humid Atmosphere, 
 
 Salty Atmosphere (Sea-front company), 
 
 Vibrations, 
 
 Interference (electric motor, thunderstorm, etc.). 
 
 
2.4.3. Due to the effect of the valve on the environment 
 
 Noise: acoustic decibels (dBA), 
 
 Vibration: screw tightness problem. 
 
 
2.4.4. Due to the assembly conditions 
 
 Nominal diameter of pipe 
 
 Space remaining for shut-off valves and bypass valves. 
 
All these conditions will have a determining effect on the choice and type of valve to be 
used in an operating process. 
 
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2.5. TECHNOLOGY OF A REGULATION VALVE 
 
The valve is broken down into two separate assemblies: 
 
 The body 
 
 The actuator 
 
 
Figure 2: Technology of a regulating valve 
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Figure 3: The two assemblies of a regulating valve 
 
The actuator is a component than enables the passage area to be modulated by 
changing the position of the rod that supports the shutter. 
 
The body comprisesthe body of the valve with its seat, shutter, studs, etc., and the 
packing gland cap. 
 
Note: The flow running through the body is a function of the passage area, but also of the 
pressure upstream of the flange. 
 
It is the element of the valve that is connected to the pipe, and through which the fluid 
flows. 
 
Small valves are connected by means of "unions". 
 
Large valves are connected by means of flanges or by welding. 
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Comment: 
 
The actuator can be: 
 
 A simple hand wheel: this is known as a "manual" valve or a "hand" valve, 
 
 An electromagnet: with two states, energized or not energized; this is an all or 
nothing electric valve, 
 
 A cylinder, 
 
 An electric motor, 
 
 A servomotor: This is the name generally given to the device located above the 
body, and which functions with pneumatic power. 
 
As the cylinder, the electric motor and the servomotor are all "powered", they can be 
remote controlled and still therefore be used for analog and digital regulation. 
 
 
2.6. CHARACTERISTICS OF REGULATION VALVES 
 
2.6.1. Inherent flow characteristic 
 
2.6.1.1. Definition 
 
This is the law that represents the flow-rate as a function of the displacement of the plug 
(or shutter), for a constant ∆P. 
 
There are three fundamental characteristics: 
 
 The linear characteristic, 
 
 The equal percentage (equal %) characteristic, 
 
 The quick opening characteristic. 
 
 
2.6.1.2. The linear characteristic 
 
The flow changes linearly as a function of the signal. The characteristic is a straight line. 
Equal increases in the valve signal cause equal increases in flow-rate. 
 
 
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Figure 4: Linear flow characteristic 
 
 
2.6.1.3. The equal percentage characteristic 
 
The characteristic is an exponential function. Equal increases in the valve signal cause 
equal increases in relative flow-rate. 
 
 
Figure 5: Equal percentage flow characteristic 
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2.6.1.4. The quick opening characteristic 
 
 
Figure 6: Quick opening flow characteristic 
This characteristic consists of a rapid increase in flow at the beginning of the opening 
range, reaching approximately 80 % maximum flow for less than half the command signal. 
It is also known as an "All or Nothing Flow characteristic ". 
 
This characteristic is very often used for safety applications with All or Nothing valves. 
 
 
2.6.1.5. Inherent adjustment coefficient or rangeability 
 
A regulating valve can only provide efficient adjustment within a specified flow range. This 
is defined by a coefficient R. 
 
R = (maximum controllable flow-rate) / (minimum controllable flow-rate) 
 
Rangeability defines the ability of a valve to control low flow-rates. This means that a valve 
with a rangeability of 100 will be capable of controlling a minimum flow-rate that is 100 
times less than the maximum flow-rate. 
 
Another way of saying this is that the adjustment range is from 1 to 100. 
 
 
 
 
 
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3. VALVE TYPES 
 
We will begin by presenting the various types of valve body. 
 
The size of the regulating valve body is proportional to the displacement of the shutter. 
 
There are two body types: 
 
 Longitudinal bodies: Translational displacement of the shutter. Usually known 
as "Linear Valves", 
 
 Angular bodies: Rotational displacement of the shutter. Usually known as 
"Rotary Valves". 
 
 
3.1. LINEAR ACTION VALVE 
 
These valves are also called "standard valves". The shutter is a plug which is displaced by 
the servomotor with a translational movement. 
 
3.1.1. Plug type valve with single-seat body 
 
ADVANTAGES DISADVANTAGES 
 
Good to very good sealing 
 
Relatively high pressure differentials 
 
Relatively simple construction 
 
Requires a large servomotor (high pressure 
on the plug) 
 
Table 1: Advantages and Disadvantages of the single seat 
 
The position of the plug in front of the seat determines the passage area for the fluid. L 
 
Sealing around the stem is achieved using Teflon 
packing (for example). 
 
The shape of the plug determines the static 
characteristic of the valve. Good sealing can be 
obtained when the valve is closed because the plug 
presses against the mating face of the seat. 
 
Figure 7: Fluid displacement in a single-seat body 
 
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The thrust of the fluid against the plug can be very high in cases of high pressure 
differentials, requiring the use of a powerful servomotor. 
 
The figure above clearly shows how the fluid flows as it passes through a single seat body. 
This provides a better view of the operation of the valve when the plug stem rises and lifts 
the plug off its seat. 
 
 
1 Plug stem 11 Body seals 
2 Packing gland flange studs 12 Plug guide 
3 Packing gland flange nut 13 Cage 
4 Packing gland flange 14 Seat 
5 Packing gland bush 15 Seat seal 
6 Packing gland seal 16 Plug 
7 Packing gland spacer 17 Plug pin 
8 Cap 18 Body 
9 Body studs 19 Top nut 
10 Body stud nuts 
 
Figure 8: Single-seat body 
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3.1.2. Plug type valve with double-seat body 
 
 
 
1 Plug stem 11 Body stud nut 
2 Packing gland flange nut 12 Body stud 
3 Packing gland flange 13 Body seal 
4 Packing gland flange stud 14 Plug guide 
5 Top nut 15 Lower seat 
6 Cap 16 Upper seat 
7 Body 17 Packing gland seal 
8 Plug pin 18 Seal spacer 
9 Plug 19 Packing gland bush 
10 Bottom flange 
 
Figure 9: Double-seat body 
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The forces on the shutter system tend to balance themselves out due to the fact that the 
fluid attempts to open one plug and to close the other. 
 
These weak forces improve the stability of the valve, which means that a smaller diameter 
servomotor can be chosen for a valve of the same capacity. 
 
Most shutter systems are also reversible. 
 
They do not provide very good sealing when closed, due to the fact that both plugs can 
never be perfectly seated on their respective seats at the same time. 
 
 
 
Figure 10: Fluid displacement in a double-seat body 
 
The above figure shows the same fluid displacement principle as seen in a double-seat 
body. 
 
 
ADVANTAGES DISADVANTAGES 
 
The forces are almost perfectly balanced 
 
Inferior sealing to that of the single seat 
 
No need for a large servomotor 
 
More complex design 
 
Table 2: Advantages and Disadvantages of the double seat 
 
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3.2. CAGE VALVE 
 
This is a single-seat / plug type valve which also has the advantages of the double seat / 
plug valve. 
 
The shutter system incorporates an excellent guide for 
the plug (piston) and enables the cage (cylinder) to be 
quickly replaced. 
 
The possibility of installing an O-ring around the piston 
reduces the likelihood of leakage. 
 
The piston is balanced, because the downstream 
pressure acts on both sides of these faces. 
 
The preferential flowdirection is from the outside to 
the inside of the piston, to ensure better stability. 
 
The cage openings are machined according to the 
flow characteristic. 
 
 
Figure 11: Cage valve 
 
 
Comment: 
 
There are different types of cage: 
 
 balanced or non-balanced cage, 
 
 single or double seat cage, 
 
 low-noise cage, 
 
 anti-cavitation cage.
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ADVANTAGES DISADVANTAGES 
Excellent ability to withstand large pressure 
differentials 
 
More complex design 
 
Excellent sealing 
 
Non-reversible straight body 
 
Low noise 
 
Possible jamming of the shutter in the cage 
when using fluids laden with solid particles 
 
 
Balancing by holes in the shutter 
 
 
 
Anti-cavitation 
 
 
 
Easy to replace the cage 
 
 
 
Anti-flash 
 
 
 
Can be used under extreme conditions: 
 
 velocity up to 130 m/s 
 operating temperature -200 to +600 °C 
 pressure up to 2,500 bars 
 
 
 
Easy and quick to maintain 
 
 
 
Table 3: Advantages and Disadvantages of the cage valve 
 
 
 
 
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3.3. 3-WAY VALVE 
 
3-way valves are designed to regulate either a fluid mixing process or a fluid bypass. It 
should be particularly noted that this type of valve has a high flow capability and a low 
recovery. 
 
The flow capability is among the best of all currently-available 3-way valves. Pressure 
recovery is small. 
 
These valves are also designed to be installed with the fluid tending to open the double 
plug (mixing valve) or each of the plugs (bypass valve). This configuration has the 
advantage of ensuring stable operation of the valve. 
 
 
 
Figure 12: 3-way mixing valve 
 
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Figure 13: 3-way bypass valve 
 
 
 
 
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3.4. DIAPHRAGM VALVE 
 
The diaphragm valve is an alternative to the spherical plug ball valve. It is used as an All-
or-Nothing valve in small applications (e.g. hot water injection to clean a level sensor 
flange separator, etc.). 
 
It is controlled by an "electrovalve" (also called a “solenoid 
valve” refer to the valve accessory chapter). When the opening 
command is given, the solenoid of the electrovalve is energised 
and therefore sends the air from the distributor into the valve 
head, and this distorts the diaphragm which in turn allows the 
fluid to flow through the body of the valve. 
 
It is used in applications where the fluids are heavily laden with 
solid particles or are very corrosive. The passage area is 
obtained between a deformable diaphragm, usually made of 
synthetic rubber, and the bottom part of the valve body. 
 
 
Figure 14: Diaphragm valve 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 15: Functional diagram of the diaphragm valve 
 
 
The force "Fs" developed by the servomotor must overcome the force "Fp" created by the 
static pressure on the diaphragm. 
FP 
Fs 
Flexible diaphragm 
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ADVANTAGES DISADVANTAGES 
 
Usable with any type of product 
 
Valve which changes with use (due to the 
elasticity of the diaphragm) 
 
Low pressure losses 
 
Temperature less than 120 °C 
 
Inexpensive solution 
 
Unspecified characteristic 
No need for packing glands and their 
possible leakage 
 
Very imprecise adjustment 
 
 
 
Low maximum pressure rating 
 
 
 
Badly defined static characteristic 
 
 
Table 4: Advantages and Disadvantages of the diaphragm valve 
 
 
3.5. VERTICAL LIFT GATE OR GUILLOTINE VALVE 
 
 
 
Figure 16: Guillotine valve 
 
 
 
 
 
 
 
 
 
 
 
 
 
Table 5: Advantages and Disadvantages of the guillotine valve 
ADVANTAGES 
 
Low pressure losses (direct flow) 
 
DISADVANTAGES 
 
Sharp fluid passage cut-off 
 
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3.6. MICRO-FLOW CONTROL VALVE WITH ADJUSTABLE Cv 
 
 
1 Body 9 Packing gland flange 
2 Seat 10 Packing gland flange studs 
3 Plug 11 Packing gland stud nuts 
4 Seat seal 12 Safety plug 
5 Seat clamp ring 13 Valve coefficient adjustment 
6 Seals 14 Cap 
7 Packing gland bush 15 Manual control 
8 Packing gland spacer 
 
Figure 17: Micro-flow control valve with adjustable valve coefficient (Varipak) 
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The possibility of adjusting the Cv on this 
needle valve eliminates valve sizing 
uncertainties; such uncertainties often 
lead to the choice of a valve that turns 
out to be too large and works with an 
excessively small opening. 
 
The Cv flow coefficient of the Varipak is 
adjustable without changing the 
pneumatic control signal. This very easy 
manual operation can be carried out 
before installing the valve, but it can also 
be performed when the valve is 
operating. 
 
The plug on this type of valve is a needle. 
 
Figure 18: Example of a micro-flow valve 
 
 
 
 
Figure 19: Adjustment of the Cv 
 
To find out what the Cv of a valve is, refer to the "Valve Sizing" chapter of this course. 
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3.7. ROTARY VALVE 
 
3.7.1. Butterfly valve 
 
The shutter is a disk whose diameter is equal to the inside diameter of the duct. When 
closed, the surface of this disk is perpendicular to the fluid flow direction. The variation of 
the passage area is achieved by tilting this disk away from the vertical. 
 
The stem of the shutter rotates, which is much better for the packing gland (better sealing). 
This rotation is often limited to an opening angle of 60°, due to the extent of the torque 
applied by the fluid. 
 
 
Figure 20: Butterfly valve 
 
 
ADVANTAGES DISADVANTAGES 
Direct flow valve (the fluid path is relatively 
undisturbed when the butterfly is fully open)
 
Tendency to cavitate 
 
 
The butterfly rotates with or without a stop 
(the stop provides better sealing) 
 
The poor balancing limits the acceptable 
pressure differential, even when the 
butterfly is inherently balanced due to its 
shape. 
 
Simple and robust design 
 
 
 
Valve mostly used for gases and large 
diameter pipes 
 
 
 
Table 6: Advantages and Disadvantages of the butterfly valve 
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This type of valve can only be manufactured for large diameters ND > 4". Considering the 
surface area and shape of the shutter, it cannot be used for very high pressures. Due to 
the long length of the mating surface of the butterfly on the body (which also constitutes 
the seat), sealing in the closed position is difficult to obtain, and is therefore usually poor. 
 
Also note that there is friction due to the thrust of the liquid, which presses the shutter stem 
against the seal (transverse effort). 
 
 
3.7.2. Spherical plug ball valve, known simply as a "Ball valve" 
 
 
 
Figure 21: Ball valve 
 
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Contains a sphere or ball with a nominal diameter which is generally equal to that of the 
pipe. 
 
The ball can pivot through 90° thanks to a stem coupled 
to a servomotor. 
 
The ball is in continuous contact with an O-ring, which 
provides excellent sealing. 
 
Standard ball valves are used in safety systems (All or 
Nothing), or as regulating valves. 
 
Modified ball valves with a "V"-shaped opening have an 
equal percentage characteristic, and are suitable for fluids 
which are viscous or laden with solid particles or fibres. 
 
The fluid tends to close the shutter system, and the 
servomotor must counter this effect. 
 
 
Figure 22: Example of a ball valve 
 
 
ADVANTAGES DISADVANTAGES 
 
Shutter consisting of a hollow sphere with a 
cut-out that depends on the required 
inherent characteristic 
 
Tendency to cavitate 
 
Direct flow valve, for viscous or fibre-laden 
fluids 
 
 
 
Good sealing 
 
 
 
Accepts high pressure differentials 
 
 
 
Table 7: Advantages and Disadvantages of the ball valve 
 
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3.7.3. Semi-rotary valve with eccentric shutter 
 
 
 
Figure 23: Valve with eccentric spherical shutter 
 
The principle of operation is based on a spherical shutter with an eccentric rotary 
movement, inside a direct-flow body. The spherical part of the shutter is connected by one 
or two flexible arms pressed onto the shaft. 
 
The actuator pushes the lever to a varying extent based on the pneumatic signal it 
receives, and this causes the shaft to rotate and therefore the shutter to rotate as well. 
 
A slight lateral play of the hub on the shaft enables the shutter to self-centre. 
 
The extremely efficient sealing between the seat and the shutter is obtained by elastic 
distortion of the shutter arms. 
 
The slightly chamfered seat is secured inside the body by means of a threaded clamp ring. 
 
This type of valve is regularly used in industrial applications, and is universal. 
 
 
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Figure 24: Cross-sectional view of the eccentric spherical shutter 
 
 
 
 
Figure 25: Functional diagram of the eccentric spherical shutter valve 
 
 
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ADVANTAGES DISADVANTAGES 
 
Direct flow valve with a small footprint, 
usually installed between flanges 
 
Greater tendency to cavitate than straight 
valves, but not as much as butterfly valves 
 
Suitable for viscous and particle-laden 
fluids 
 
 
 
Good sealing with a Teflon or plastic coated 
contact surface 
 
 
 
Table 8: Advantages and Disadvantages of the eccentric shutter valve 
 
 
 
 
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4. TYPES OF PLUG 
 
To obtain the 3 fundamental characteristics described above, we need to change the type 
of plug on a valve. This plug will modify the flow of fluid passing through the valve body. 
 
 
 
Figure 26: Different plugs and their flow characteristics 
 
 
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4.1. QUICK OPENING LINEAR PLUG 
 
A maximum increase in flow is obtained for a displacement of approximately 30 %, and 
this increase then diminishes as the displacement approaches 100 %. 
 
These plugs are mainly used in All-or-Nothing regulation loops and in safety systems. 
 
 
 
Figure 27: Quick opening plug 
 
 
4.2. LINEAR PLUG 
 
The flow is directly proportional to the 
opening of the valve over the entire length 
of its displacement. 
 
These plugs are used in level regulation 
loops, and more generally in processes 
with a constant gain. 
 
 
Figure 28: Linear plug 
 
 
 
4.3. MODIFIED LINEAR PLUG 
 
These plugs are a compromise between the linear and quick opening characteristics. 
 
In extreme areas with high flow-rates, and more 
particularly with low flow-rates, a long displacement 
produces a small variation in flow. 
 
 
Figure 29: Modified linear plug 
 
 
 
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4.4. EQUAL PERCENTAGE PLUG 
 
Equal increments in plug displacement 
produce equal increases in flow. 
Near the closing point, variations in flow 
are small; between 0 and 30 % displace-
ment, the flow varies from 0 to 
approximately 9 %. 
 
Figure 30: Equal percentage plug 
 
Near the full open point, 
variations in flow are relatively 
high; between 80 and 100 % 
displacement, the flow varies 
by approximately 50 %. 
 
These plugs are used in pressure regulation loops, and more generally in 
processes where only a small proportion of the total pressure differential can 
be absorbed by the valve. 
 
 
Figure 31: Equal percentage plug turned with a Vee-shaped aperture 
 
 
 
 
 
4.5. PARABOLIC PLUG 
 
These plugs are a compromise between the linear and equal percentage characteristics. 
They have a linear characteristic with high flow and displacement characteristics. 
 
These plugs are used in pressure 
regulation loops, and more 
generally in processes where a 
high proportion of the total 
pressure differential can be 
absorbed by the valve. 
 
 
 
Figure 32: Parabolic plug 
 
 
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5. TYPES OF CAGE 
 
In the same way as for plugs, cage valves have different types of cage which modify the 
flow characteristic. 
 
The quick opening, linear, and equal percentage characteristics are determined by the 
shape of the openings in the cage. 
 
 
5.1. QUICK OPENING CAGE 
 
 
 
Figure 33: Quick opening cage 
 
5.2. LINEAR CAGE 
 
 
 
Figure 34: Linear cage 
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5.3. EQUAL PERCENTAGE CAGE 
 
 
 
Figure 35: Equal percentage cage 
 
 
5.4. LOW NOISE CAGE 
 
 
 
Figure 36: Low noise cage 
 
 
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6. THE CAP 
 
This part is installed on the top of the valve body, and its purpose is to provide sealing 
around the plug stem. 
 
It acts as a guide for the plug stem(s), contains the packing gland and supports the 
servomotor. 
 
For temperatures >= 200 °C, cooling fins are provided. 
 
For temperatures <= 20 °C, an extension type cap is used 
 
 
 
 
Figure 37: Diagram of valve cap 
 
 
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6.1. THE PACKING GLAND 
 
The packing gland is a very widely used sealing system. Its principle of operation consists 
in providing sealing by compression of several braids (packings) around the stem of the 
plug by means of the packing gland rammer. 
 
The washers of the packing gland, or the 
braids, must seal the body with a minimum 
amount of friction on the stem of the plug. 
 
They are usually made of a flexible and 
compressible material such as Teflon 
(t < = 230 °C) or graphite (t > 230 °C). 
 
The material used for the packing gland 
depends on the fluid to be sealed. 
 
The pressure on the washers must be properly 
adjustedon a periodic basis, in order to 
minimise leakage; this is done by means of the 
flange and rammer. 
 
Figure 38: Packing gland of a valve 
 
To obtain absolute sealing, a secondary boot can be joined to the plug stem. This is known 
as an "extension" or an "extension boot"). 
 
The sealing boot ensures total sealing between the valve stem and the 
cap. This technology is typically proposed for applications involving toxic, 
flammable or explosive fluids for which any leakage could have serious 
consequences for personnel and and/or the environment. 
 
 
 
 
 
 
Figure 39: Sealing boot 
 
 
Boot 
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6.2. GLAND PACKING 
 
In the world of maintenance, the term "Braids" is often used when referring to gland 
packings. 
 
There are two types of seal packing: 
 
 Packing rings: These are ready-made seals, to the diameter you need. 
 
 
 
Figure 40: Graphite and PTFE packing rings 
 
 Braids: These are in the form of coils, and it is up to the instrument engineer to 
cut them to the correct length so that it corresponds to the diameter of the 
rammer. 
 
 
 
Figure 41: Examples of graphite and PTFE braids 
 
Both types of braid shown in the figure above are the most commonly used. Graphite 
braids are often used on valves in heating systems, operating at very high temperatures 
and at high pressures. 
 
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For all other applications, we use PTFE (Teflon) braid. 
 
 
 
During maintenance, it is important to check the packings because we are so used 
to simply saying to ourselves "Oh, I had to tighten the packing gland of that valve 
because it was leaking a bit". 
 
But it is important to know that by constantly retightening valve packing glands, the 
braids end up becoming crushed and no longer provide proper sealing. 
 
 
 
Figure 42: Poor sealing, leakage from the packing gland 
 
The figure above show what happens when a valve is not properly maintained. 
 
 
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7. THE SERVOMOTOR 
 
The servomotor is the component that enables the plug stem of the valve to be actuated. 
The force developed by the servomotor serves two purposes: 
 
 It counters the pressure acting on the plug; 
 
 It ensures the sealing of the valve. 
 
These two criteria determine the sizing of the servomotors. The driving fluid can be air, 
water, oil or gas. 
 
The supply fluid (at 1.4 bars or 2.1 bars) is usually air, and the command pressure varies 
from 0.2 bars to 1 bar. The following types of servomotor are available: 
 
 Standard diaphragm type servomotor (direct or reverse action). 
 
 Rolling diaphragm type servomotor, principally used for rotary valves (example: 
Masoneilan CAMFLEX valve), 
 
 Piston type servomotor, used in applications where very high forces are required. 
The command pressure can be very high. The driving fluid can be air, water or 
oil, 
 
 Electric servomotor, used for rotary valves. An electric motor is associated with a 
gearbox, thus enabling very high torque values to be obtained, 
 
 Hydraulic servomotor, used on shutdown valves. 
 
 
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7.1. PNEUMATIC SERVOMOTOR 
 
7.1.1. Standard diaphragm type servomotor 
 
 
 
Figure 43: Diaphragm type servomotor 
 
 
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7.1.1.1. Operation 
 
The diaphragm of the servomotor is subjected to 2 forces: 
 
 On one side, the force due to the pressure in the servomotor (modulated 
pressure from the regulator). It is proportional to the air pressure and to the 
surface of the diaphragm (F = P x S). 
 
 On the other side, the force due to the compression of the spring which 
increases as the spring is compressed. 
 
For a given air pressure in the servomotor, the spring contracts by a length such that the 
resulting force (proportional to the shortening of the spring) is equal to the corresponding 
motive force. 
 
For each pressure value, the displacement of the diaphragm is transmitted by the stem to 
the plug, the displacement of which is proportional to the air pressure exerted on the 
diaphragm. 
 
For Split-Range valves (refer to the course on "the regulator and its functions"), calibration 
of the valve consists in adjusting: 
 
 The tension of the spring, and 
 
 The length of the plug stem. 
 
An adjustment system enables the spring tension to be adjusted. If there is no other 
resistance on the stem or plug, the valve stem moves through its full range when the air 
pressure varies from 200 to 1,000 mbars. 
 
This results in the following correspondence: 
 
 
 
 
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7.1.1.2. Description 
 
The rolling diaphragm type servomotor is the most commonly used. 
 
 
 
 
Figure 44: Simplified diagram of a diaphragm type servomotor 
 
This servomotor consists of: 
 
 A rubber diaphragm, 
 
 A return spring, 
 
 A stem range adjustment. 
 
 
 
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7.1.2. Diaphragm type servomotor with multiple springs 
 
This servomotor design with multiple return springs 
reduces the forces on the plug stem. 
 
Its rolling diaphragm is subjected to less strain, 
which means that it wears less. 
 
 
 
 
Figure 45: Cross-sectional view of a servomotor 
with multiple return springs 
 
 
 
 
 
7.1.3. Rolling diaphragm type servomotor 
 
 
Figure 46: Rolling diaphragm type servomotor 
 
 
The rolling diaphragm type servomotor consists of a cylinder clamped in a flange by 4 BTR 
screws. The rolling diaphragm is secured both to the cylinder and to the piston which is 
connected to the position of the spring. 
 
Spring 
Diaphragm 
plate 
Coupling with 
the valve lever 
Spring 
stem 
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Unlike the standard servomotor in which the displacement of the diaphragm plate 
assembly is small, this type of Servomotor has a large displacement. 
 
The coupling between the servomotor and the lever of the valve consists of a small 
cylinder fitted with a circlip. 
 
This improves the precision of the positioning for the shutter. 
 
The rolling diaphragm is used in more and more applications and can be found on 
spherical plug ball valves and on rotary valves. 
 
Its cost is relatively low and it is extremely easy and quick to maintain. 
 
 
7.1.4. Piston type servomotor 
 
 
Figure 47: Piston type servomotor 
 
Piston type servomotors operate at much higher pressures than diaphragm type 
servomotors. 
 
These pneumatic or hydraulic pressures can be up to several tens of bars. 
 
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They are capable of producing much higher forces and of operating over much longer 
ranges, and can therefore overcome very high pressure differentials through the valve 
body. 
 
Safety valves use pistons: 
 
 one-way with return spring to return the 
valve to its fail-safe position if there is a lack 
of air.Figure 48: Simplified diagram of a one-way piston 
type servomotor for a linear valve 
 
 two-way with hydraulic accumulator or 
pneumatic tank, used to return the valve to its fail-safe position if there is a lack of 
hydraulic pressure. 
 
 
Figure 49: Two-way piston type servomotor for a rotary valve 
 
Without a return spring, there is no possible fail-safe position. 
 
The two-way servomotor has the particularity of having two air inlets, because we 
introduce air on one side of the piston to open the valve, and on the other side of the 
piston to close the valve, which is normal because there is no return spring. 
 
Piston type servomotors are very often associated with butterfly valves. They can be used 
as All-or-Nothing valves, but they can also be used as regulating valves with an electro-
pneumatic positioning device. 
 
Generally speaking, when a piston type servomotor is used as a regulating valve, this is 
because the operating conditions involve very high pressures in large diameter pipes. 
 
They are similar to diaphragm type servomotors in that they can be installed either on a 
linear displacement valve or on an angular displacement (rotary) valve. 
Air to close the 
valve 
Air to open the 
valve 
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7.2. HYDRAULIC SERVOMOTOR 
 
7.2.1. Constitution 
 
 
 
Figure 50: Hydraulic servomotor 
 
A hydraulic servomotor consists of: 
 
 A hydraulic pump, 
 
 A hydraulic reservoir, 
 
 A hydraulic distributor valve, 
 
 A valve control, 
 
 The actuator, consisting of one or two cylinders. 
 
This type of servomotor is used in very high pressure processes, in which the operating 
pressure can be up to about a hundred bars. It can also be used in low-pressure systems, 
but the valve opening takes longer because the hydraulic system has a lot of torque. 
 
In the same way as for the piston type servomotor, the hydraulic servomotor can be a one-
way or a two-way device. 
 
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7.2.2. Operation 
 
In this simple hydraulic actuator, a 
stream of hydraulic fluid can be 
directed into either of two chambers. 
 
If hydraulic fluid is added to 
chamber 1, the piston moves right, 
closing the valve. 
 
When hydraulic fluid is added to 
chamber 2, the action is reversed. 
Hydraulic pressure pushes the piston 
to the left and the valve opens. 
 
For more details concerning the 
hydraulic actuator, please refer to the 
"Hydraulics and Pneumatics" course. 
 
 
 
 
Figure 51: Functional diagram of the 
control of a hydraulic servomotor 
 
 
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7.3. ELECTRIC SERVOMOTOR 
 
7.3.1. Servomotor with motor and gearbox 
 
 
 
Figure 52: Electric servomotor with motor and gearbox 
 
The advantage of this actuator is that we only need an electrical signal to "rotate" the 
motor, so the associated valve can either open or close. 
 
This type of motor is reversible because it rotates in both directions. It is equipped with a 
gearbox which enables the torque to be reduced, in order to prevent the electric motor 
from applying too much force on its rack. 
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This type of actuator is often used on All-or-
Nothing safety valves. 
 
 
Figure 53: Example of an electric servomotor 
 
 
 
7.3.2. Solenoid type servomotor 
 
A solenoid is a coil of electric wire wound around a cylinder in with all the turns in 
contact with each other. The induction of the magnetic field generated by a solenoid 
is proportional to the number of turns and to the intensity of the current passing 
through it, and is inversely proportional to its length. 
 
The solenoid is mounted directly on the valve body. 
The valve body has a diaphragm equipped with a 
return spring. 
 
This diaphragm is installed on a core, over which the 
solenoid is fitted. 
 
Figure 54: Valve with solenoid type servomotor 
 
This type of valve can be classified as an 
"electromagnetic valve". 
 
When an electrical signal is injected into the solenoid, this generates a magnetic field 
which will act on the spring and cause the diaphragm to distort. 
 
Depending on whether or not the diaphragm is distorted, the plug of the valve will be either 
open or closed. 
 
This type of valve is used in low-pressure applications, and 
has the advantage of rapidly opening or closing the plug of the 
valve. 
 
The valve is installed in line for small applications (e.g. boiler 
pilot gas supply, vacuum pump buffer tank water 
replenishment, etc.). 
 
The solenoid can be energised at 230 VAC, 110 VAC or 
48 VAC (refer to the manufacturer's document). 
 
Figure 55: Example of a solenoid type servomotor 
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7.4. DIRECTION OF ACTION 
 
7.4.1. Direction of action of the valve body 
 
The direction of action of the valve body depends on the shutter system (plug + seat). 
 
 Direct action valve body: extension of the plug stem closes the shutter system. 
 
 
 
Figure 56: Direct action valve body 
 
You will have noticed that the figure on the left shows a single-seat valve body, 
and that the figure on the right shows a double-seat body. 
 
 Reverse action valve body: extension of the plug stem opens the shutter 
system. 
 
 
 
Figure 57: Reverse action valve body 
 
Comment: 
 
Some valve bodies are reversible, in other words the action of the plug can easily be 
reversed by simple disassembly. 
 
 
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7.4.2. Direction of action of the servomotor 
 
If an air failure occurs, the counter-spring causes the servomotor to move to an extreme 
position so that the shutter can move to a completely open or a completely closed position. 
 
These types of servomotor therefore do not pose any particular problem when it comes to 
complying with the specification, for direct or reverse action servomotors. 
 
 Direct action servomotor: the action is direct, so 
when the modulated air pressure increases, the 
stem of the plug descends. 
 
 
Figure 58: Direct action servomotor 
 
 
 Reverse action servomotor: the action is 
reversed, so when the modulated air pressure 
increases, the stem of the plug rises. 
 
 
 
 
 
 
 
 
Figure 59: Reverse action servomotor 
 
 
 
 
 
 
 
 
 
 
 
7.4.3. Direction of action of the positioning device 
 
 Direct positioning device: When the input signal increases, the output signal 
increases. 
 
 Reverse positioning device: When the input signal increases, the output signal 
decreases. 
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7.4.4. Special case with the "two-way" servomotor type piston 
 
If an air failure occurs, the piston takes any position depending on the force exerted by the 
fluid on the shutter of the valve. 
 
In order to force the position of the shutter, it is therefore necessary to provide a device 
comprising both a reserve of compressed air and switching components which enable the 
valve to be moved to the selected position in case of failure of the distribution system air 
supply. 
 
 
7.5. FAIL-SAFE POSITION 
 
7.5.1. Fail-safe aspect of the valve (body + servomotor) 
 
When there is no more pressure on theservomotor, the spring returns the valve to its open 
or closed position, as defined by the construction of the device. 
 
 A "Fail Closed" valve, i.e. one which is closed when there is a lack of air, closes 
when there is no longer any pressure on the servomotor. 
 
 A "Fail Open" valve, i.e. one which is open when there is a lack of air, opens 
when there is no longer any pressure on the servomotor. 
 
The choice depends essentially on the safety conditions for the process. 
 
 
 
Figure 60: Valve fail-safe position 
 
 
Comment: When the positioning device is electro-pneumatic, the chances are it will be of 
the direct type. 
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Figure 61: Different possibilities for the fail-safe position of a valve 
 
 
7.5.2. Fail-safe aspect of the valve with its positioning device 
 
 
 
Table 9: Combinations of valve fail-safe and positioning device positions 
Action direction 
of positioning 
device 
Fail-safe 
aspect of 
the valve 
Timing sequence of particular states 
P S No supply 4 mA 200 mbars 
20 mA 
1,000 mbars
Direct Direct Open Open Closed 
Direct Reverse Closed Closed Open 
Reverse Direct Open Closed Open 
Reverse Reverse Closed Open Closed 
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8. VALVE ACCESSORIES 
 
8.1. POSITIONING DEVICE 
 
What is the purpose of a positioning device? 
 
The positioning device is a device used to slave the displacement of the plug to the control 
signal from the regulator. 
 
For the regulation system to operate correctly, it is essential for the displacement of the 
plug to remain precisely proportional to the value of the regulator output signal. 
 
However, certain interference forces can hinder the movement of the plug: 
 
 Thrust exerted by the fluid (particularly in the case of single seat plugs) 
 
 Friction of the transmission stem in the Packing gland 
 
 Spring exerting a force which is not precisely proportional to the displacement it 
undergoes (hysteresis) 
 
 Variation of surface area due to the distortion of the diaphragm, etc. 
 
 
These forces depend on the operating conditions: severe conditions → High forces 
 
 Viscous or solid-laden fluid 
 
 High pressure differential, etc. 
 
It is therefore necessary, in order to obtain a plug position that corresponds to the value of 
the control signal, to complete the regulation system by a positioning device. 
 
 
There are 3 types of positioning device that can be adapted to a regulating valve: 
 
 Pneumatic positioning device 
 
 Electro-pneumatic positioning device 
 
 "Intelligent" (digital) positioning device 
 
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8.1.1. Pneumatic positioning device 
 
8.1.1.1. Features 
 
The function of the pneumatic positioning device is to ensure linear or other slaving 
between the displacement of the valve and a pneumatic signal output by a regulator. 
 
It also has another function, which is to modify the natural characteristic of a valve by 
means of a cam whose profile depends on the required characteristic. 
 
It can also be configured for the "cascaded" (Split-Range) control of several valves, and 
can be used with an augmented pneumatic supply enabling it to operate the valves under 
higher differential service pressures. 
 
It is also possible to reverse the direction of action of the valves by means of the 
positioning device. 
 
 
8.1.1.2. Constitution 
 
The pneumatic positioning device consists of the following components: 
 
 The profiled cam: this is the intermediate component between the reaction 
device, the valve actuator and the spring of the positioning device. Its profile 
determines the relationship between the position of the shutter of the valve and 
the signal output by the regulator. "Linear" or "equal percentage" characteristics 
are available by selecting the appropriate sector of the cam. 
 
 The controller: this is a small 3-way distributor valve. The valve adjusts the 
compressed air flow-rates from the supply to the outlet on the actuator, and from 
the outlet to the exhaust orifice. The position of this valve is controlled by the 
diaphragm, and determines the discharge pressure of the actuator. The action of 
the pneumatic positioning device can be reversed by reversing the supply and 
exhaust connections and by changing the cam sector and the orientation of the 
lever. 
 
 The return spring: this enables the slide valve of the controller to slide in the 
distributor valve. 
 
 The reaction spring: this enables the cam to be rotated by varying the rotation 
of the lever. This variation is due to the pressure exerted on the diaphragm. 
 
 
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8.1.1.3. Principle of operation 
 
The pneumatic positioning device is based on the principle of a force equilibrium device: 
the pressure of a pneumatic signal applied to diaphragm opposes the force of a reaction 
spring. 
 
In the state of equilibrium, if the pneumatic signal varies, the diaphragm assembly moves. 
The movement drives the slide valve of the controller, which is pressed by the return 
spring. 
 
The displacement of the slide valve alternately sets the outlet system into 
communication with the supply system or with the exhaust system, thus modifying the 
pressure applied on the actuator. 
 
The cam transmits the displacement of the valve shutter to the reaction spring. 
 
The valve shutter continues its movement until the force of the spring precisely balances 
that developed by the pressure of the pneumatic signal on the diaphragm. In this state of 
equilibrium, the position of the valve shutter in front of the seat corresponds to that ordered 
by the signal from the regulator. 
 
 
 
 
 
 
 
Figure 62: Functional diagram of pneumatic positioning device 
 
 
Output signal 
to actuator 
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Figure 63: Masoneilan pneumatic positioning device 
 
 
 
 
Figure 64: View of the cam with and its reaction spring 
 
 
We have already seen, in the features of the positioning device, that the cam can change 
the characteristic of the valve - the following table gives the cam positions and cam lever 
orientations of a MASONEILAN CAMFLEX II or VARIMAX: 
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Figure 65: MASONEILAN lever orientation and cam position 
 
 
8.1.1.4. Faults 
 
The main faults you are liable to encounter on this type of positioning device are: 
 
 The exhaust orifice is blocked, so the valve will no longer regulate and stays fixed 
in a certain position, 
 
 The cam has worked loose; this is often due to vibrations, 
 
 The slide valve of the controller is blocked; the air in the positioning device 
sometimes condenses and produces a little humidity in the slide valve, causing 
the slide valve to seize up. 
 
Despite these various minor failures that can occur, the mechanism of this pneumatic 
positioning device makes it a very robust device that requires little maintenance. 
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8.1.2. Electro-pneumatic positioning device 
 
8.1.2.1. Constitution 
 
The electro-pneumatic positioning device is made up ofthe following components: 
 
 A rocker system (nozzle-flapper): the imbalance caused by the variation of the 
electrical signal in the electromagnet causes the output signal from the nozzle to 
the actuator to vary. 
 
 
 
Figure 66: Nozzle-flapper system with electromagnet on I/P positioning device 
 
 
 A cam, 
 
 A return spring, 
 
 A controller: this is an amplifier relay which will amplify the output signal from 
the nozzle to the actuator. 
 
 A balancing spring: this enables equilibrium to be achieved between the rocker 
system and the cam lever. 
 
 
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8.1.2.2. Principle of operation 
 
The principle of operation is almost identical to that of the pneumatic positioning device. 
 
In actual fact, we have retained the pneumatic valve positioning system with the cam and 
its lever, but a nozzle and flapper with an electromagnet have been added to enable the 
pneumatic signal of the regulator to be replaced by an electrical signal (4-20 mA). 
 
The purpose is to be able to remotely control the regulating valves. 
 
Here, the controller is not a distributor with its slide valve and push-rod, but an amplifier 
relay with a diaphragm. 
 
In this case, the signal from the regulator is no longer a pneumatic signal (0.2 to 1 bar), but 
an electrical signal (4-20 mA). 
 
The electrical signal (4-20 mA) will pass through the solenoid, and this will move the 
flapper. This results in a change in the nozzle output pressure until the reaction of the ball 
located at the end of the nozzle balances out the new force applied on the lever. 
 
The more the electrical signal increases, the closer the flapper will move to the nozzle: the 
nozzle output signal to the actuator will also increase and therefore tend to open the valve. 
 
The reverse effect will apply if the signal decreases. 
 
 
Figure 67: Functional diagram of electro-pneumatic positioning device 
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Figure 68: MASONEILAN type 8013 electro-pneumatic positioning device 
 
We can also change the direction of action of the positioning device, for which it is 
necessary to: 
 
 Reverse the balancing spring 
with respect to the rocker 
 
Figure 69: Direct action: beneath the 
rocker 
 
 
Figure 70: Reverse action: above the 
rocker 
 
 
 
 Reverse the wires of the solenoid on the terminal strip of the positioning device. 
 
 
 
Figure 71: Solenoid wire reversal to change the direction of action of the positioning device 
 
 
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A great deal of care must be taken to avoid blocking the rocker with the 
wires of the solenoid, otherwise the valve will operate in 
"ALL-OR-NOTHING" mode, in other words either wide open or 
completely closed. The reason I am telling you this is that it happened 
to me once, by mistake. 
 
 
8.1.2.3. Faults 
 
The main faults on the electro-pneumatic positioning device are: 
 
 Balancing spring out of its recess, 
 
 Unserviceable solenoid or integrated circuit card, 
 
 Solenoid jammed in the core. This is due to humidity in the positioning device and 
distortion of the solenoid, 
 
 Flapper jammed on nozzle → it is necessary to check the balance of the rocker 
with the screw of the small counterweight on the flapper, 
 
 Clogged nozzle → The nozzle must imperatively be cleaned with compressed air, 
 
 A disconnected solenoid wire. 
 
 
8.1.3. Intelligent (digital) positioning device 
 
The intelligent positioning device is the latest version of the positioning device, which is 
becoming more and more widely used. 
 
 
8.1.3.1. Constitution 
 
The intelligent positioning device consists of the following components: 
 
 A microprocessor: this comprises an eeprom which is used to store the data, 
and an FSK module for digital communication between a PC and the positioning 
device, 
 
 A nozzle-flapper system, 
 
 A 3-way distributor valve: this is the controller, 
 
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 An electromagnet: this is the solenoid with its core, which enables the flapper to 
be tilted to varying degrees with respect to the nozzle thanks to the magnetic field 
created by the electric current passing through it, 
 
 A digital display: it will enable the positioning device to be configured and will 
display either the fault diagnosis or all the measurement information of the 
positioning device. 
 
This type of positioning device is always supplied at a pressure of 1.4 bars, through a 
pressure reducing filter. 
 
The electrical signal from the regulator is always a current signal of 4-20mA. 
 
 
8.1.3.2. Principle of operation 
 
 
 
Figure 72: Functional diagram of intelligent positioning device 
 
The central processing unit, or CPU, is the functional centre of the positioning device. The 
mechanical and pneumatic components provide only secondary functions. 
 
The input signal (4-20 mA) and the position measurement signal are cyclically checked by 
the CPU and sent to an analog-to-digital converter, thus enabling rapid and precise data 
processing. 
 
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The general program also comprises a self-adjustment routine to automatically adjust the 
device on the valve actuator, and also an adaptive self-regulation mode which ensures 
optimum control and monitoring of the position irrespective of the operating conditions 
(supply pressure variation, for example). 
 
The servomotor is controlled by an I/P converter and a 3-way valve (distributor valve). The 
electrical signal from the CPU is proportionally converted into a pneumatic signal which 
adjusts the 3-way valve. The passage area is constantly modified to inflate or empty the 
servomotor proportionally with respect to the signal. 
 
When the valve position is reached, the 3-way valve is in its neutral position (the air flow is 
practically zero). 
 
To make it easier for you to understand the operation, bear in mind that the positioning 
device operates as a regulator (refer to the course on "the regulator and its functions"). 
 
The 4-20 mA input signal is the setpoint, and the position measurement performed by a 
position sensor is the measurement. The CPU compares these two values and sends a 
proportional electrical signal to the electromagnet, which will provide the means of acting 
on the control component (3-way valve) in order to establish measurement = setpoint. 
 
The standard positioning device is equipped with a local keypad (4 keys) and a 2-line 
display. This keypad is used for local configuration and for monitoring of the parameters in 
operation. Configuration, commissioning and observation can also be carried out remotely, 
via the communication port (FSK module), using a computer. 
 
This communication is based on the HART or FieldBus or PROFIBUS protocols. 
 
You can connect to the device either locally or anywhere on the 4-20 mA link. 
 
The basic model of the positioning device can be modified at 
any time (this will depend on the manufacturer) to receive 
new functions. It is possible to add cards for analog copying: 
this will enable a precise indication of the valve position on a 
DCS. Limit switches can also be added. 
 
Figure 73: ABB model TZID-C intelligent positioning device 
installed on a linear valve 
 
We can also change the direction of action of the positioning 
device (direct or reverse), and also the inherent