Control valve related book

1. CHAPTER-4
CONTROL VALVES
1

2. INTRODUCTION
Control Valves also called final control element are a most important
element in the control system of a process plant. It is the last device in
control Loop.
Definition:-
A valve is a device that regulates, directs or controls the flow of a fluid
by opening, closing or partially obstructing various passageways.
Function:-
1) Stopping & Starting Fluid Flow
2) Varying (throttling) the amount of fluid flow
3) Controlling the direction of fluid flow
4) Regulating downstream system or process pressure
5) Reliving component or piping over pressure
2

3. PRINCIPLES OF OPERATION:-
The most common final control element in the process control
industries is the control valve. The control valve manipulates a
flowing fluid, such as gas, steam, water, or chemical compounds, to
compensate for the load disturbance and keep the regulated process
variable as close as possible to the desired set point.
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4. WHY CONTROL VALVES USED?
Process plants consist of hundreds, or even thousands, of control loops
all networked together to produce a product to be offered for sale. Each
of these control loops is designed to keep some important process
variable such as pressure, flow, level, temperature, etc. within a required
operating range to ensure the quality of the end product. Each of these
loops receives and internally creates disturbances that detrimentally
affect the process variable, and interaction from other loops in the
network provides disturbances that influence the process variable.
To reduce the effect of these load disturbances, sensors and
transmitters collect information about the process variable and its
relationship to some desired set point. A controller then processes this
information and decides what must be done to get the process variable
back to where it should be after a load disturbance occurs. When all the
measuring, comparing, and calculating are done, some type of final
control element must implement the strategy selected by the controller.
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5. NUMBER OF PLUGS
single-seated valve:-
only one plug is present in the control valve, so it is single seated
valve. The advantage of this type of valve is that, it can be fully
closed and flow variation from 0 to 100% can be achieved. But
looking at its construction, due to the pressure drop across the
orifice a large upward force is present in the orifice area, and as a
result, the force required to move the valve against this upward
thrust is also large. Thus this type of valves is more suitable for small
flow rates.
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6. 6

7. Double-seated valve:-
there are two plugs in a double-seated valve; flow moves upward in
one orifice area, and downward in the other orifice. The resultant
upward or downward thrust is almost zero. As a result, the force
required to move a double-seated valve is comparatively much less.
But the double-seated valve suffers from one disadvantage. The flow
cannot be shut off completely, because of the differential
temperature expansion of the stem and the valve seat. If one plug is
tightly closed, there is usually a small gap between the other plug
and its seat. Thus, single-seated valves are recommended for when
the valves are required to be shut off completely. But there are many
processes, where the valve used is not expected to operate near
shut off position. For this condition, double-seated valves are
recommended.
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8. 8

9. VALVE TERMINOLOGY
Balanced Valve:-
A body design in which the same pressure acts on both sides of the
valve plug, thus reducing the actuator force required to open the
valve. This characteristic is inherent in the design of a double port
valve but is also available in single port valves.
Bellow Seal:-
a physical barrier or seal which uses a bellows for sealing against
leakage around the valve plug stem.
Bonnet Assembly:-
Are assembly including the part through which a valve plug stem
moves and a means for sealing against leakage along the stem. It
usually provides a means mounting the actuator.
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10. VALVE TERMINOLOGY
Capacity:-
Rate of flow through a valve under stated conditions.
Cavitation:-
If the speed through the valve is high enough, the pressure in the
liquid may drop to a level where the fluid may start bubble or flash.
The pressure recovers sufficiently and the bubbles collapse upon
themselves.
Vena Contracta:-
Velocity=maximum
Static pressure=minimum
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11. CAVITATION
11

12. VALVE TERMINOLOGY
Characteristic:-
characteristic that defines the relationship between 'valve opening'
and flowrate under constant pressure conditions.
Corrosion:-
The reaction between materials of the valve and the fluids handled
which caused valve deterioration.
Cv (flow coefficient) :-
Define the capacity of valve. It is defined as “the number of gallons
of water per minute at room temperature which will pass through
a given flow restriction with a pressure drop of one psi.
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13. VALVE TERMINOLOGY
Dead Band:-
The amount the diaphragm pressure can be varied without initiating
valve plug motion.
Diaphragm actuator:-
A fluid pressure operated spring or fluid pressure opposed
diaphragm assembly for positioning the actuator stem in relation to
the operating fluid pressure or pressures.
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14. VALVE TERMINOLOGY
Equal percentage flow characteristic:-
An inherent flow characteristic which for equal increment of rated
travel, will ideally give equal percentage of the existing flow.
Erosion:-
A wearing action on valve trim and body, resulting from high velocity
fluids and fluids containing solids particles. It is prevalent in steam
service, in flashing service, in some chemical services and where
high pressure drops occur.
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15. VALVE TERMINOLOGY
Extension bonnet:-
A bonnet with an extension between the packing box assembly and
bonnet flange.
Galling:-
A high friction condition between the valve stem and stem guides,
bushing or packing resulting from lack of lubricity between the
moving parts.
Guide bushing:-
A bushing in bonnet, bottom flange or body to align the movement
of a valve plug with a seat ring.
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16. VALVE TERMINOLOGY
High recovery characteristic:-
A term applied to the design characteristic of a valve which allows a
high pressure ratio of downstream pressure to upstream pressure.
Upstream Pressure:-
the pressure against the flow is called upstream pressure
downstream pressure:-
the pressure toward the flow is called downstream pressure
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17. VALVE TERMINOLOGY
Inherent flow characteristic:-
Flow characteristic when constant pressure drop is maintained
across valve.
Inherent rangeability:-
Ratio of maximum to minimum flow within which the deviation from
the specified inherent flow characteristic does not exceed some
stated limits.
Installed flow characteristic:-
An inherent flow characteristic which can be represented ideally by a
straight line on a rectangular plot of flow versus percent rated travel.
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18. VALVE TERMINOLOGY
Leakage:-
Quantity of fluid passing through an assembled valve when valve is
in fully closed position under stated closure forces, with pressure
differential and temperature as specified.
linear flow characteristic:-
An inherent flow characteristic which can be represented ideally by a
straight line on a rectangular plot of flow versus percent rated travel.
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19. VALVE TERMINOLOGY
Low recovery characteristic:-
A term applied to the design characteristic of a valve which causes a
low percentage ratio of downstream to upstream pressure.
Modulate:-
The function of a controller which causes a valve to respond to an
infinite number of positions between the closed and full open
positions.
Normally closed:-
Applying to normally closed control valve assembly, one which closes
when the actuator pressure is reduced to atmospheric.
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20. VALVE TERMINOLOGY
Normally open:-
Applying to normally open control valve assembly, one which opens
when the actuator pressure is reduced to atmospheric.
∆P:-
The pressure drop across a valve.
Packing box assembly:-
The part of the bonnet assembly used to seal against leakage around
the valve-plug stem.
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21. VALVE TERMINOLOGY
Plug:-
A movable part which provides a variable restriction in a port.
Rangeability:-
The ratio of maximum to minimum usable sizing coefficient.
Rated Cv :-
The value of Cv at the rated full open position.
Rated travel:-
Linear movement of the valve plug from the closed position to the
rated full open position.
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22. VALVE TERMINOLOGY
Seat:-
The portion of a seat ring or valve body which a valve plug contacts
for closure.
Seat ring:-
A separate piece inserted in a valve body to form a valve body port.
Stem:-
A rod extending through the bonnet assembly to permit positioning
the valve plug.
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23. VALVE TERMINOLOGY
Trim:-
The parts (except body) of a valve which come in to contact with the
flowing fluid.
Turndown:-
The ratio of maximum to minimum flow requirements.
Valve body:-
A housing for internal valve parts having inlet and outlet flow
connections.
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24. VALVE TERMINOLOGY
Valve plug guide:-
That portion of a valve plug which aligns its movement in either a
seat ring. Bonnet, bottom flange or any two of these.
Yoke:-
A structure by which the diaphragm case assembly is supported
rigidly on the bonnet assembly.
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25. VALVE CAPACITY
The valve coefficient Cv is defined as
25

26. VALVE CAPACITY
Q=volumetric flow rate through the valve in gallons per minute.
∆P=pressure drop across the valve in psi
G=specific gravity of the following fluid
It is defined as “the number of gallons of water per minute at room
temperature which will pass through a given flow restriction with a
pressure drop of one psi.
The flow is similarly determined at various increments of valve lift
and Cv at each increment is obtained.
A plot of these values reveals the characteristic curve of the valve.
The curve is made by plotting the percentage of maximum travel
against the percent of maximum flow.
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27. VALVE CAPACITY
27

28. VALVE CAPACITY
Figure reveals a typical flow curve for percentage valve plug.
The characteristic curve thus obtained reflects the control
characteristic curve of that particular plug and determines whether
it or another type plug is best suited for a particular application.
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29. VALVE RANGEABILITY
Rangeability of control valve may be defined as the ratio of
maximum controllable flow to the minimum controllable flow.
It is the ratio of maximum to minimum usable sizing coefficient.
It is obtained by dividing the minimum usable sizing coefficient in
percent in to maximum usable sizing coefficient in percent.
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30. VALVE RANGEABILITY
30

31. VALVE RANGEABILITY
Figure shows how rangeability is obtained.
This term should be called inherent rangeability.
Types of control valve Rangeability
Globe valve 30:1 to 50:1
Throttling ball valve 100:1
Butterfly valve 10:1 to 20:1
Pinch & diaphragm valve 5:1(low)
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32. VALVE RANGEABILITY
In selection a valve for control application, the installed rangeability or
operating rangeability is just as inherent rangeability.
Operating rangeability may be defined as relationship between reangeability
and pressure drop. it can be expressed by
32

33. VALVE RANGEABILITY
Where q1=initial flow
q2=final flow
∆P1= initial pressure drop across the valve
∆P2= final pressure drop across the valve
33

34. BODY DESIGNS
Globe Bodies:-
Globe valves may be divided into several categories including
1) single port
2) double port
3) three way
4) split body
5) angle valve
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35. SINGLE PORT
They are simple in construction.
Frequently used in 2 inches and below.
Provide tight shutoff for metal-metal or composition seating.
Have wide rangeabilities.
But may have high unbalanced forces on the plug requiring large
actuators.
They can be constructed to have the valve plug move into or out of
the port with increasing actuator loading pressure.
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36. SINGLE PORT
36

37. SINGLE PORT
The need for low leakage, balanced valves has brought about some
notable design for balancing single seated valves.
Figure shows are typical examples of how balanced has been
achieved.
It is accomplished by porting through the body or plug so that the
same pressure (inlet or outlet) acts on both sides of the valve plug.
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38. SINGLE PORT
38

39. SINGLE PORT
39

40. DOUBLE PORT
They were developed to balance the forces normally acting on single
port valves.
They have higher flow capacities and require smaller stem forces
compared to the same size single port valve.
They are frequently specified for sizes larger than 2 inches.
Not be used when leakage is objectionable.
Special construction may provide composition seating for tight
shutoff when required.
Reversible plug design is available to open or close the valve with
increasing loading pressure.
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41. DOUBLE PORT
41

42. THREE WAY
They are designed to blend(mix) or to divert (split) flowing streams.
Can replace two straight-though valves in many applications.
In blending service, there are two inlet and one outlet, whereas in
diverting service there are one inlet and two outlet ports.
Total flow is proportioned only, not controlled, in either service.
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43. THREE WAY
43

44. THREE WAY
Have characteristic of unbalanced forces on the valve plug and
require large operators.
They are usually installed with the flow tending to open the valve
plug disc to prevent “slamming” of the valve plug.
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45. THREE WAY
45

46. THREE WAY
Balancing a three way valve is accomplished by using four seats.
It is done by using dual double seats in one body.
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47. THREE WAY
47

48. SPLIT BODY
A special type globe body, consisting of two body halves with a seat
ring clamped between them, is called a split body valve.
Its design is applicable only to single seated valves.
Its construction minimizes erosion effects, allows parts to be
replaced easily and is relatively inexpensive.
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49. SPLIT BODY
49

50. ANGLE VALVE
Angle valves, nearly always single ported, are often used where
space is at a premium (may eliminate a 90o ell in the piping).
They are applicable to services requiring high pressure drops or
where the effects of turbulence.
Cavitation or impingement present problems.
They have good control characteristic, high rangeabilities and
high pressure and temperature ratings.
They are easily removed from the line and can handle sludges
and erosive materials.
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51. ANGLE VALVE
51

52. VENTURI FLOW ANGLE VALVE
It is especially good for flashing services, high pressure drops and
erosive appliactions.
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53. VENTURI FLOW ANGLE VALVE
53

54. LONG SWEEP ANGLE VALVE
It has a slightly higher capacity than other angle patterns, uses a
long-radius bend at the valve inlet while the outlet simulates a
venturi.
Its construction is ideal for slurries and highly viscous fluids.
However this design is seldom used.
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