# CONTROL_VALVES by stariya

VIEWS: 20 PAGES: 23

• pg 1
```									       Nomenclature

Flow Capacity: -        The valve sizing coefficient most commonly used as a measure of
the capacity of body and trim of a control valve is Cv. Cv is defined as one U.S gallon
per minute of 60OF of water that flows through a valve with a one PSI pressure drop. The
general equation is
Specific gravity at flowing temperature
Cv = Flow      -----------------------------------------------
Pressure drop
As a normal practice the calculated Cv is used to determine the valve size and the trim
sizes and the manufacture has to select the Cv in such a way that the minimum and
maximum operation of the process will be able to achieve by opening 20% to 80%.

Pressure profile : - The control valve acts as a restriction in the flow stream. As the
fluid stream approaches this restriction, its velocity increases in order for the full flow to
pass through the restriction. Energy for this increase in velocity comes from a
corresponding decrease in pressure.
Maximum velocity and minimum pressure occur immediately downstream from the
throttling point at the narrowest constriction from the fluid stream, known as vena
contracta. Down stream from the vena contracta, the fluid slows and part of the energy
(in the form of velocity) is converted back to pressure. The slight pressure losses between
inlet and outlet are due to frictional effects.

P1 (inlet pressure)
Dp (valve pressure drop)
P2 (outlet pressure)
PRESSURE
PROFILE of fluid
Passing through a
---------------------------------- Pv (vapor pressure)   Valve
Pvc (Pressure at vena Contracta)

Allowable Pressure Drop
From the definition of Cv it is clear that as the Dp (pressure drop) increases for a given
Cv there should be an increase in flow. This occurs up to a point after which increase in
Dp will not yield an increase in flow. This point is the chocked flow.

Flow rate

Dp       choked pressure drop
In liquids when the pressure at any point in the valve drops below the vapor pressure of
the liquid the vapor bubbles form. These bubbles occupy more volume than the liquid
from which they are formed. As further increase in pressure drop, the proportion of
bubbles to liquid increases until the volume of the flow is so great that the valve can not
pass additional flow. The pressure drop at this point is the chocked flow.

In gases as the down stream pressure decreases (consequently increasing the pressure
drop), the velocity of the gas across vena contracta increases due to the increasing
volume of the gas. When the velocity reaches sonic (Mach=1), any further increase in
pressure drop will not result in additional flow. The pressure drop corresponds to sonic
velocity condition across the vena contracta is the chocked pressure drop.

When sizing a control valve the smaller of the actual pressure drop or the chocked
pressure drop is always used to determine the correct Cv. This pressure drop is the
allowable pressure drop.

Cavitations
In liquids when the pressure at the vena contracta drops below the vapor pressure of the
fluid, vapor bubbles begin to form in the fluid stream. Down stream from the vena
contracta, the fluid decelerates with a resultant increase in pressure. If this pressure is
higher than the vapor pressure, the bubble collapse (or implodes) as the vapor returns to
the liquid phase. This two step mechanism called cavitations produces noise, vibration
and physical damage to the valve and down stream piping.

The onset of cavitations known as the incipient cavitations is the point when the bubbles
first begin to form and collapse. The point at which the chocked pressure cavitations
occurs (severe damage, vibration and noise) is at the chocked pressure drop.

Flashing
If the down stream pressure is equal to or less than the vapor pressure, the vapor bubbles
created at the vena contracta do not collapse, resulting in liquid gas mixture down stream
of the valve, this is commonly called flashing. Velocity of this two phase flow is very
high resulting in the possibility for the erosion of the valve and the piping components.

Velocity
As a general rule the valve outlet velocity should be limited to the following maximum
values.
Liquids                       : - 50 feet per second
Gases                         : - Approaching Mach 1.0
Mixed gases and liquids       : - 500 feet per second.
In general smaller sized valves handle slightly higher velocities and larger valves handle
lower velocities.
Liquid applications where the fluid temperature is close to the saturation point should be
limited to 30feet per second to avoid reducing the fluid pressure below the vapor
pressure.
Valves in cavitating service should be limited to 30 feet per second. To minimize damage
to the downstream piping.
In flashing services the velocities to be below 500 feet per second. Erosion damage can
be limited by using chrome molybdenum body material and stellite overlayed trim.

Sizing Cv for liquids
q            Gf
Cv = -----        ------
Fp Fr        dPa
q       :- flow rate in gallons per minute
Fp      : - Piping geometry factor
Fr      : - Reynolds number factor
Pa      : - Allowable pr.drop across the valve in psi
Gf      : - specific gravity at flowing temperature.

Piping geometry factor (Fp)
Valve sizing coefficients are determined from tests run with the valve mounted in a
straight run of pipe which is the same diameter as the valve body. If the process piping
configurations are different from the standard test manifold, the valve capacity is
changed. The effect of reducers and expanders can be approximated by the use of the
piping geometry factor, Fp.

Reynolds number factor (Fr)
Fr is used to correct the calculated Cv for non turbulent flow conditions due to high
viscosity fluids, very low velocities, or very small valve Cv’s.

Sizing Cv for Gases
Because of compressibility, gases and vapors expand as the pressure drops at the vena
contracta, decreasing their specific weight. To account for the change in specific weight,
an expansion factor, Y, is introduced into the valve sizing formula.
x
Q = 7320 Fp Cv P1 Y        -------
MT1Z
Fp = piping geometry factor
P1 = Inlet pressure in absolute
Y =Expansion factor
x =ratio of actual pressure drop to absolute inlet pressure (Dp / P1)
M = molecular weight
T1 = Absolute upstream pressure in degrees R (degree F + 460)
Z = Compressibility factor
Q = Gas flow in SCFH (standard cubic feet per hour)

Body materials
The control valve user normally specifies the body material. Which is often the same
material as the pipe. The most common choices of body material are carbon steel, chrome
molybdenum steel and stainless steel.
Carbon steel is the most commonly used for bodies. It handles most non corrosive liquids
and gases up to 800oF for continuous service or to 1000oF for occasional service. Carbon
steel can be used for most condensate and steam services.

Chrome – moly steel is used for higher temperatures and pressures than carbon steel,
including such services as high pressure steam or flashing condensate which requires
corrosion and erosion resistance. This is stronger than carbon steel and in some cases is
as strong as stainless steel. It costs less than SS but not as corrosion resistance.

SS is specific for higher temperature services (1000oC and above) or in corrosive
application.

Special alloys such as HastalloyB or C, monel, nickel and titanium are also available.

The valve body can be cast, forged, wrought or fabricated. Castings are the first choices
in standard sizes and ratings. Forgings are used for smaller sized bodies (generally these
are high pressure ratings). Barstock are recommended when the delivery is critical and a
casting or forging is not available. Fabricated type is a convenient way to manufacture
large angle valves.

Bonnets are manufactured from barstock of the same material as the body. Exception is
the smaller low pressure chrome moly valves where SS bonnet is the standard.

Gaskets are used to prevent leakage around the seat ring, bonnet or pressure balanced
sleeve.

This is the first economical choice as far as possible with the pressure and temperature
limits.

6000

5000

4000
Pressure (psi)                           Reinforced TFE
3000
Virgin TFE
2000

1000

0
-200           0            200              400
Temperature (oF)
Flat Kel-F gaskets are used primarily for cryogenic services. The temperature range for
Kel-F is –423 to 350oF.

This consists of alternate layers of metal and non metallic materials wound together.
AFG is a non asbestos filler material for standard spiral wound gaskets and may be
directly substituted for asbestos material in most applications. This can be used in steam
services up to 1000deg F and in air at 1500degF.

limit          limit
304SS/Asbestos         -20 to 750oF   ANSI     class Valves through 8” in carbon
2500           steel and chrome moly.
316SS/ Asbestos        -20         to ANSI     class SS, carbon and chrome moly
1000oF         2500           valves sizes 10” and above
316SS/ Grafoil         -320        to ANSI     class High        pressure,       high
1000oF         2500           temperature severe service
applications up to 1000oF-
especially    severe     service
valves
Inconel/ grafoil       -20         to At a full pr. Used       for    high      temp.
1500oF         rating                            o
Applications- 1000 F above, or
where inconel is preferred over
316SS for that particular fluid.

Unbalanced and pressure balanced trim

Valtek offers 3 unbalanced trim designs 1) standard full area trim which provides
maximum Cv with a removable seat ring 2) reduced trim which provides a lower Cv in a
wide variety of sizes or when larger bodies are required. 3) Integral seat trim which
utilizes the seat machined into the body and an oversized plug to provide additional Cv
beyond the capabilities of standard full area and reduced area trim designs.

Unbalanced trim design generally requires that the direction of flow should assist the
motion of failure, for ex. Flow over or fail closed and flow under for fail open. The force
required to fail open or closed is a function of the off balanced area. This area is equal to
the seat area in fail open applications and the seat area minus the stem area in fail closed
applications.

Pressure balanced trim
For high pressure drop applications, pressure balanced trim may be required to reduce the
thrust necessary to stroke the plug by reducing the trims off balance area. However with
high thrust cylinder actuators, pressure balanced trim may not be required. Often when
tight shut off is required an oversized cylinder actuator may be the most economical
choice...
Because the pressure balanced plug fits closely to the sleeve the trim should be used in
relatively clean services.

As a standard, pressure balanced trim is designed to classII shut off. Flow direction is
under the plug for fail closed applications and over the plug for fail open applications.
Allowing the fluid pressure to act on both sides of the plug results in a net force equal to
the pressure times the off balance area. This balancing force is made possible by transfer
holes in the plug. Leakage past the plug is prevented by a seal around the top of the plug

Flow characteristics

Flow characteristics are the relationship between flow coefficient and the valve stroke.
Many valve types such as butterfly, eccentric disk and ball valves have inherent
characteristics which can not be changed (except with characterizable positioner cams).
Flow characteristics of globe valves can be determined by the shape of the plug head.

The 3 most common types of flow characteristics are quick opening, equal percentage
and linear. These characteristics can be approximated by contouring the plug. However in
as much as there is body effects and other uncontrollable factors, plus the need for
maximizing the flow capacity for a particular valve, the real curves often deviate
considerably from these ideals.

When a constant pressure drop is maintained across the valve, the characteristics of the
valve alone controls the flow, this characteristic is the inherent flow characteristics. But
in stalled characteristics include both the valve and pipeline effects.

100

80
Quick open
60
Flow, %                linear
40

20
Equal %
0       20      40     60     80       100
Valve lift, %

Equal Percentage
This is the characteristics most commonly used in process control. The change in flow
per unit of valve stroke is directly proportional to the flow occurring just before the
change is made. While the flow characteristics may be equal%, most control loops will
produce an installed characteristic approaching linear when the overall system pressure
drop is large relative to that across the valve. (Such is the case in the majority of
applications)

Linear
An inherently linear characteristic produces equal changes in flow per unit of valve
stroke regardless of plug position. Linear plugs are used on those systems where the
valve pressure drop is a major portion of the total system pressure drop and the inlet
pressure is constant.

Quick open
Quick open plugs are used for on/off applications designed to produce maximum flow
quickly.

Trim design
The design concepts relative to valve trim that affect not only the characteristic curve but
also how the valve responds to erosion, cavitations, vibration, high pressure drop and
other similar problems.

Guides
Proper operation of control valves depends on a positive relation between the valve plug
and seat. Accurate guiding is accomplished by

Top guiding
The plug is aligned by a guide bushing in the bonnet or body.

Top and bottom guiding
Aligned by the guide bushing in the bonnet and the bottom flange.

Port guiding
Plug is aligned in the body port or ports only.

Top and port guiding
Aligned in the bonnet and the port.

Stem guiding
Plug is aligned by guide bushing acting on the valve plug stem.

Cage
To help problems associated with high pressure drops and their attendant and noise
problems, various designs of cage trim have been developed. Cage trim valves provide
higher capacity for the same size valve body than other globe valves making them
economically attractive. This acts as an excellent guide for the plug. The cage results in
longer trim life under high pressure drops.

Bonnet assembly
This is the part through which the valve plug stem moves and is the means for sealing
against leakage along the stem.
Bolted flange bonnet with a standard packing box is used for temperature from 0o to 450o
F.

Extension bonnet
For cryogenic or low temperature services, the packing box is kept at reasonable
temperature by extension bonnets. The bonnet may be cast or fabricated; they may be
insulated if necessary.

Finned Bonnet
In services with higher temperature requirements high temperature packings or finned
bonnets are used to provide a large heat radiating area.

Bellows seal
This type of bonnet assembly is used on applications where no leakage along the stem
can be tolerated. These are used on installations where the process fluid is pyrophoric,
toxic, explosive, highly expensive or difficult to contain such as hydrogen gas. Two types
of pack less sealing are made, one in which process fluid is sealed inside the bellows and
one in which fluid is sealed outside the bellows.

Trim Materials
Hardened trim is considered for all chocked flow conditions or for temperatures above
600oF. Generally SS 316 is used as standard.
Hardened trim is used to protect against erosion and/or corrosion. The hardened surface
may include the seat surface of the plug and seat ring, the full seat ring bore, the full
contour of the plug or the lower guide area of the plug stem. Stellite is commonly used
which has good relative hardness and corrosion resistance. For corrosion resistance
special alloys such as Alloy 20, Hastalloy C and monel are also available.

Aside from corrosion, the main factors that cause wear in valve trim are the conditions of
the process fluid.
 Gas versus liquid
 Velocity and pressure differential
 Temperature
 Flashing
 Presence of abrasive solids

Gas versus Liquids
Clean gases even at high velocities are not usually a source of trim erosion. However
entrained solids or liquid droplets in high velocity gas can wear the trim rapidly.

Depending on the fluid’s composition, liquids at high velocity can produce accelerated
erosion. For ex. At high velocities water can cause more damage than lubricating oil.
With liquids another harmful effect is cavitations which can erode most trim material
even hardened trim. Liquid application valves require hardened trim more often than
gas application valves.

Velocity and pressure differential
Erosion caused by flowing fluid is a function of the velocity of the fluid. Velocity is
depending on flow rate and area. In order to have a large flow rate through a relatively
small area, large differential pressures are required. Therefore hardened trim selection
becomes a function of differential pressure.
Temperature
As temperature increases, many trim materials susceptible to erosion because of the
general deterioration of their mechanical properties. Therefore the selection of the
hardened trim must be compatible with high temperature conditions. For ex., hardened
440C would not be recommended for services above 800oF, where as stellite can be used
up to 1500oF. At the same time at the other end of the temperature scale- such as
cryogenic most hardened materials become excessively brittle and 316SS becomes
relatively hard. When ever the temperature exceeds 550oF, seating surfaces should be
hardened. The plug stem and bushings should be hardened above 600 oF regardless of the
pressure differentials.

Corrosion
The erosion and abrasion of valve and trim is aggravated by the corrosive effect of the
process fluid.

Types of hardened Trim
This covers
 Stainless steel hard faced with stellite.
 Flame sprayed with tungsten carbide or aluminium oxides.
 Hard materials such as wrought stellite 6B or the various sintered metal carbides
and oxides
 Materials hardened by heat treatment such as 416, 17-4pH, 440C, or 329SS or K
Monel K500.

Soft and metal seats
A soft seat is used in applications requiring ANSI class VI bubble tight shut off. Its
design consists of an elastomer insert sandwiched between a metal seat ring and retainer.

Class IV is the industry standard for metal seated valves.

Class V versus Class VI
Due to the common belief that Class VI shut off is more stringent than Class V under all
circumstances, the following should be noted.

Class V allowable leakage is defined as 0.0005cc per minute per inch of orifice diameter
per psi differential. Therefore allowable leakage increases with orifice diameter and
differential pressure.
Class VI allowable leakage is independent of pressure differential and is only a function
of orifice diameter. For large orifice diameter or small pressure drops, Class VI shut off
may be less stringent than Class V.

This shows that Class VI shut off can be obtained with metal seats. It is not true that class
VI shut off can not be obtained in high temperature services simply because a soft seat
can not be used. Keep in mind that seat loading must be increased to 250 to 400lbs. per
linear inch of seating force to obtain class VI shut off.

ANSI B16.104 Seat Leakage Classifications
ANSI B16.104

Maximum
Leakage                        Test                              Testing Procedures Required for
Leakage                         Test Pressures
Class                        Medium                                   Establishing Rating
Allowable
No test required provided user and
I
supplier agree.
Pressure applied to valve inlet,
Air or      45-60psig or max.       with outlet open to atmosphere or
0.5% of rated      water at    Operating               connected to a low head loss
II
capacity           50-125°F    differential,           measuring device, full normal
(10-52°C)   whichever is lower.     closing thrust provided by
actuator.
0.1% rate of
III                        As above    As above                As above
capacity
0.01% of rated
IV                         As above    As above                As above
capacity
Pressure applied to valve inlet
Max. Service
after filling entire body cavity and
0.0005 ml per                  pressure drop across
connected piping with water and
minute of water Water at       valve plug, not to
stroking valve plug closed. Use net
V       per inch of port 50-125°F      exceed ANSI body
specified max. Actuator thrust, but
diameter per psi (10-52°C)     rating. (100 psi
no more, even if available during
differential                   pressure drop
test. Allow time for leakage flow
minimum.)
to stabilize.
Not to exceed                  50 psig or maximum
Air or                              operating conditions specified with
amounts shown                  rated differential
nitrogen at                         full normal closing thrust applied
VI      in following                   pressure across valve
50-125°F                            to valve plug seat. Allow time for
table based on                 plug, whichever is
(10-52°C)                           leakage flow to stabilize and use
port diameter.                 lower.
suitable measuring device.

ANSI B16.104 Class VI Seat Leakage Allowable
Nominal Port Diameter                                      Leak Rate
Inches              Millimeters                ml Per Minute              Bubbles per minute*
1                     25                           0.15                          1
1 1/2                   38                           0.30                          2
2                     51                           0.45                          3
2 1/2                   64                           0.60                          4
3                     76                           0.90                          6
4                    102                           1.70                          11
6                    152                           4.00                          27
8                    203                           6.75                          45
*Bubbles per minute are an easily measured alternative based on a suitable measuring device such
as 1/4" O.D. x 0.032" wall tube submerged in water to a depth of 1/8" to 1/4". The tube end shall
be cut square and smooth with no chamfers or burrs and the tube axis shall be perpendicular to the
surface of the water. Other apparatus may be constructed and the number of bubbles per minute
may vary from these shown, as long as the correctly indicated the flow in ml per minute.

Resilient Seated Butterfly Valve Bubble-Tight Shut Off Class B

Maximum
Test
Leakage            Test Medium                                   Testing Procedures Required
Pressures
Allowable
Valve disc to be closed shut. Pressure
Air over water on
110% of         applied to valve inlet after filling cavity of
underside of disc and
valve rated     underside of disc with water or air over
None           water on disc topside or
shut-off        water. Hold pressure at test parameter for
water on underside of
pressure.       one minute and observe for any water
disc; room temperature.
leakage or air bubbles if air or water test.

Valve Selection
Fundamentally there are only 2 known ways of controlling the flow of liquids and gases,
and all valves are in essence are based on one or the other types illustrated below...

Obturating

Rotating
Sliding                                      squeezing
This illustrates a simple tank with liquid flowing from an outlet near the bottom. To stop
or control the flow one can either place a finger against the pipe end (Dutch boy) or
alternatively if the pipe is flexible, it can be squeezed. These are the basic principles on
which valves are constructed.

The first principle is developed in 3 ways.
 Moving the stopper by direct thrust on to the orifice seating. This obturating
movement is the basis of globe type valves.
 Rotating the stopper. This is the basis of plug type valves.
 Sliding the stopper across the face of the orifice seating. This is the basis of gate
type valves.

The second type, squeezing action is the basis of all diaphragm type valves.

Each of the 4 valve motivations- obturating, rotating, sliding and squeezing has its own
advantages and disadvantages. An appraisal of the 4 basic types is given below.

Best shut off                High head loss
Obturating Globe             Good              regulating
characteristics
Quick acting                 Temperature limitation on
Rotating     Conical         Straight through flow        PTFE sleeved valves
(plug)                                       Need for attention to
lubricate in lubricated valve
Quick acting                 Temperature limited by
Rotating     Ball            Straight through flow        seating material
(plug)          Easy operation
Quick acting                 Metal to metal seated type
Good              regulating does not give tight shut off
Rotating     Butterfly       characteristics              Temperature limited by
(plug)          Compact                      seating material on resilient
seated type.
Straight through flow        Slow acting
Sliding      Gate                                         Bulky

Glandless                  Pressure and          temperature
Positive shut off on dirty limited   by           diaphragm
Squeezing    Diaphragm       fluids                     material.

Globe valve
Here the direct thrust of the disk on to the seating provides the best form of shut off and
regulating characteristics closure is positive and it is possible to feel when the valve is
shut. The regulating characteristic is the relationship between movement of the valve
hand wheel and the effect that this movement has on the amount of medium flowing
through the valve.

The globe valve is the most suitable type for throttling, i.e. Fine regulation, because the
wear and tear through erosion around the seating is more evenly distributed than in any
other type of valve. A study of the other type shows that they all wear more at one point
than another. Globe valves have reasonably short up and down movement of the disk,
theoretically equal to a quarter of the seating diameter, and are not therefore big time
wasters when being operated.
The disadvantage is the internal shape of the body, in providing the under and over flow,
the diversion from the straight line crates a loss of pressure. Due to this streamlined globe
valves with its reduced pressure loss is introduced. The more the head loss through
valves, bends, length of the pipe, the bigger the pump and the larger the power
consumption.

Plug valve
Advantages of the plug valve are the straight through flow, hence minimum head loss and
the quick action requiring only 90o (quarter turn) movement.

The disadvantages are of 2 fold. One, its quick action can lead to water hammer in
hydraulic installations when it is closed too rapidly. Secondly, it is difficult to combine
tight shut off with ease of operation.

The essential difference between the plug cock and the plug valve is that the latter
incorporates the features to reduce the friction between the plug and the body during
operation and to seal them against leakage. Probably the best known method, which has
been used for many years, is to introduce a lubricant, specially formulated for the
purpose, between the plug and the body. Such pressure lubricated valves are used for
high pressure high temperature applications.

Another method is to introduce a sleeve, usually of PTFE, between the plug and the body
to avoid metal to metal contact. In these sleeved plug valves it is usual for the sleeve to
be retained in the body and for the plug to rotate with in it. This method provides
excellent shut off and is particularly suited to applications where alloy materials are
needed. The temperature limitations to be considered.

A derivative of the plug cock is the ball valve. The advantages and disadvantages are the
same as sleeved valves, as it is also employed seatings of PTFE. It tends to require less
torque than the sleeved plug valve, but its sealing security in severe applications tends to
be less over a period of service.

An even further development of the plug valve is the butterfly valve. A compact, quick
acting and easy to operate which is well suited to flow regulation. Also for tight shut off
resilient seating which is temperature limited to be used. Also as the disk is in the flow
path the disk to be more substantial for the higher pressures. The head loss may be 3
times that of a gate valve.
Gate Valve
The main advantage is the straight through flow when it is fully opened and hence
minimum resistance to flow.

The disadvantage is that it is the slowest acting of all because the gate has to be moved
greater than the bore of the valve also this valve is bulk in height. However it is relatively
free from mechanical problems and hence the number one choice for the hydraulic
service.
Diaphragm valves
This valve has 2 advantages. One no separate stem gland is necessary as the diaphragm
shuts of the flow as well as acts as a gland. Also the flexibility of the diaphragm provides
a positive shut off even on dirty fluids.

The main disadvantages are, the diaphragm has to be made from an elastomer, usually
either natural or synthetic rubber, and this limits its application in respect to both
temperature and pressure. Also because of the stresses induced in the diaphragm the
valve has a shorter working life than other valve types.

Approaching the problem of valve selection we need first to know about the flow
resistance of the various types of valves. Normally a liquid or gas offers some resistance
to flow. With a length of straight pipe, the resistance is caused by boundary friction
between the internal surface of the pipe and the medium passing through it. When a valve
is inserted into a pipe line it adds its own resistance to flow. With a plug or gate valve as
the flow is straight through the resistance is less, but in a globe valve the flow can be
deflected through as many as 3 right angles, the resistance is high.

Resistances to flow are found by experiment and are usually expressed in equivalent
lengths of straight pipe. Within quite narrow limits a particular basic type of valve can be
described as having a constant number of meters of straight pipe equivalent per
millimeter of valve size. For a conventional globe valve this is 0.4, means 25mm globe
valve is equal to 25*0.4=10meters of 25mm pipe. While the resistance of a 50mm globe
valve is 50*0.4=20meters of 50mm pipe and so on.

The resistance constants of the various valve types are shown below.
0.4- Globe (Conventional)
0.27 - Globe (stream lined)
0.16- Y Valve

0.16 - Angle valve
0.025- 0.04 - Butterfly
0.025 - Plug Valve

0.01 - Gate and Ball valve.
It can be seen that the rotating and sliding valve offer almost no resistance. The
obturating type varies quite appreciable from Y-Valve, which is a globe valve with the
head work inclined at an angle to the pipe, at 0.16m per mm, to the conventional globe
type at 0.4m per mm size.

With the basic theory, the 4 valve types, their individual advantages and disadvantages
and resistance to flow, the first requirement is to establish precisely duty of the particular
valve to perform. This can be done by providing answers to the 3 questions.

 Is the valve for use on a gas, including steam and air, or a liquid?
When conveying liquids, head losses are far more important than when conveying gases.
Hence for a liquid, a straight through type with minimum head loss should be used. With
gas this is not so important.

 Is the valve to be used often?
If the valve to be used frequently, the time taken in opening and closing could be
considered. For frequent use, quick acting plug type should be chosen, with the
obturating type as the second choice and the sliding type avoided.

 Is the valve to be used as on/off or regulating?
Obturating type is the most suitable for throttling or regulation. All valves should off
course, unless otherwise qualified be capable of isolation.

Obviously the 3 questions can cause conflicting answers in choosing the ideal valve and
hence to be compromised. The following diagram is an attempt to give the best
compromise solution.
Regulate               Isolate
OBTURATING
OBTURA
Gas                                                  Frequently

ROTATING

SLIDING
Liquid                                                       Infrequently

APPLICATION DIAGRAM
Order of determination
1.    gas or liquid
2.    operated frequently or infrequently
3.    regulate or isolate

The chart is used in the order of answering the questions connecting gas or liquid too
frequently or infrequently and then dropping a perpendicular from either regulate or
isolate. The point of intersection gives the best overall choice; the one nearer to the point
of intersection would be the second choice.

In this example, a valve for liquid (used frequently for isolation), a rotating type of valve
to be chosen as this gives straight through flow and the quick action needed for frequent
use. If however a rotating type to meet either pressure or temperature conditions can not
be obtained then the second choice would be the obturating type, because for a valve to
be fully opened and closed frequently, more emphasis would be given to the time
required to operate the valve than the straight through flow characteristics of the slow
acting sliding through valves.

It is interesting to note that there are 8 possible answers to 3 questions and hence 8
different types of valving conditions. Obturating and sliding provides 2 of this each,
while rotating provides other 4 sets of conditions. This probably explains the increasing
popularity of sleeved plug valves, ball valves and butterfly valves.

After selecting from the diagram it is necessary to qualify for this choice according to
working pressure, temperature and in case of obturating type the allowable pressure loss.

Obtuarating (Globe) valves
These are available in all materials and all types of seating designs to fill every possible
role. The only factor to consider is the resistance to flow. Decide between conventional
globe, streamlined globe, and Y valve on the basis of flow resistance.

Rotating (Plug) valves
The ordinary pattern is suitable for infrequent use on gas and water, while the lubricated
type is preferable for larger sizes and higher pressures. The sleeved plug valves and ball
valves are more suitable for frequent use.

Sliding (gate) valves
This is the number one choice for hydraulic applications where used infrequently, but
should not be too infrequent. Gate valves benefit from being used occasionally, as this
helps to prevent build up of sediment on the seat faces which, when the valve is open, are
exposed to the flowing medium.

Many seating variations are available. Ex. PTFE inserts for tight shut off on gas and low
surface tension fluids.

The butterfly can be considered as an alternate to gate valve, especially as size increases,
since it requires much less head room for installation. The disk of a butterfly simply
rotates with in its own bore and is quicker in operation and more suited to regulation than
the gate valve.

The parallel slide valve is best suited where stresses are caused by pipe expansion and
contraction. For example on steam and high pressure hot water heating circuits.
The essential difference between the parallel slide valve and the wedge gate valve is that
as the names imply the gate in the parallel slide valve has faces which are parallel and in
opening slide across the parallel seating in the body of the valve. The wedge gate valve or
sluice valve has a tapered wedge which mates by a wedging action with similarly tapered
seats in the valve body.

FINAL SELECTION
Obturating globe Decide between conventional globe, streamlined globe and Y
valves           valve on the basis of flow resistance.
Rotating    plug Ordinary type for infrequent use on gas and water with lubricated
valves           type for larger sizes and higher pressures. Sleeved plug and ball
valves for frequent use.
Sliding     gate No.1 choice for hydraulic service with infrequent use.
valves           Butterfly can be considered as alternative as size increases
requiring less head room and giving better regulation.

Valve rangeability
Maximum controllable flow/ minimum controllable flow.

Type                           Range ability
Globe                          30:1 to 50:1
Throttling Ball valves              100:1
Std. Butterfly valve           10:1 to 20:1
Pinch and Diaphragm valve      as low as 5:1

Operating range ability
Ro = (q1/ q2) dP2 / dP1
‘q1 initial Flow      q2  Final Flow
dP1 & dP2 Initial and final pr.drop across the valve.

Body Design
Globe Valves
Can be divided as single port, double port, while 3 way split body and angle valves are
classified as special type globe valves.

Single port valves
 Simple in construction
 Usually used in sizes 2” and below
 Tight shut off
 Wide range ability
 High unbalanced forces on the plug requiring large actuators.
Double port valves
 Generally 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”
 Not to be used when leakage is objectionable

3 way valves
Most of this type require the characteristics of unbalanced forces on the valve plug and
require large actuators. They are usually installed with the flow tending to open the valve
plug discs to prevent slamming of the valve plug.
Balancing can be done with 4 seats i.e. Dual double seats in one body.

Split Body
This is a special type globe body with a seat ring clamped between them.
 This is applicable to single seated valves only.
 Its construction minimizes erosion effects
 Allows parts to be replaced easily
 Relatively inexpensive

Angle valves
 Mainly single ported type
 Applicable to services requiring high pressure drops or where effects of
turbulence, cavitations or impingement present problems.
 Have good control characteristics, high range ability, high pressure and
temperature ratings
 Can be easily removed from line and can handle sludges and erosive materials.
 Venturi type flow angle body is good for flashing services, high pressure drop and
erosive applications.

Needle valves
Barstock body design and forged bodies are for high pressure applications requiring small
flows and high range abilities.

Ball valves
One type of ball valve employs a cage to carry a solid ball into the mouth of the body
opening. This is used in paper and pulp industry. Self cleaning, tight shut off, wide range
ability and accurate flow control are the characteristics.
Hard to handle fluids such as paper sludge, polymer slurries and other fluids with
entrained solids can be controlled by ball valves.
These have high recovery (low pressure loss), good control characteristics and high range
abilities.

Butterfly valves
High pressure drop, high static pressure and tight shut off types are available.
These are economical especially in larger sizes. The main disadvantage is the
requirement of large operating torque. These are used between 10o and 60o range because
torque conditions cause instability beyond this range.
At 0o and 90o forces are balanced and in others it is unbalanced.

Diaphragm type valves (Saunders type valves)
This consists of a body, bonnet and flexible diaphragm. Closure is made by forcing a
flexible dome like diaphragm against a weir.
Well suited for slurries and viscous fluids, have high capacity. This shows poor control
characteristics and low turn down ratio.

Pinch Valves
For heavy slurries such as metallic ores, fibers, sand, coal, pulp & paper stock chemicals.
They are made of a sleeve molded of rubber or synthetic material, with flanged or
clamped ends for pipe connections and for a pinching mechanism for control. Air or
hydraulic pressure is applied to the sleeve for closure. These are inexpensive, high
capacities but poor control characteristics and low range abilities.

Drag Valves
This utilizes a patented multiple disc technique which divides the incoming flow into a
series of smaller streams with tortuous flow paths. The paths are engineered to maintain
the fluid velocity through the valve at or near line velocity. Control is obtained by
positioning the plug inside the stack of discs to change the flow area.
The valve was developed for difficult control applications, such as high pr. Drops, high
temperature and pressure, flashing service and erosive applications. This reduces noise
inherent in its design. It is more expensive than a standard valve.

PRESSURE CONTROL- REDUCING AND RETAINING VALVE
Pressure reducing valve
This is installed where it is required to reduce from one level of pressure to another and
to maintain the reduced pressure on the down stream side with in limits, irrespective of
fluctuations in the inlet pressure or change in flow demand. The valve is automatic in
operation.

Pressure retaining valves
Also known as surplus valve and used to maintain a level of pressure in the line upstream
of the valve, the valve opening with rising upstream pressure. It is a reverse acting
version of the pressure reducing valve.

Types
Self operated pressure reducing or retaining valves fall into two main categories, direct
acting and pilot operated.
Direct acting valve
The controlled pressure acts directly through a diaphragm, piston or bellows, on an
imposing force from a compressed helical spring, weight or weighted lever or from
compressed air.

Although the pressure controlled is not so accurate with pilot operated valves they are
less costly.

Pilot operated valves
The main is assisted or completely controlled by the operation of a pilot valve which may
be itself a direct acting reducing valve. The pilot valve acts so as to regulate the amount
of opening of the main valve in a way that will maintain the flow at the desired level of
pressure.

These will provide very close accuracy of pressure control, compact in design and
smaller than direct acting valves for the same service. Because of the complex design
pilot operated valves require regular maintenance and clean working conditions, later
being ensured by fitting a strainer.
Valves may be of single seat design if tight shut off is required. Double seated can be
preferred if leakage is acceptable, because this improves the maximum flow range and
accuracy of pressure control.

Safety, Relief and Safety relief valve
Safety valve
A valve which automatically discharges steam, gases or vapors so as to prevent a
predetermined safe pressure being exceeded, such valves have a rapid opening action
(pop action) and obtained their rated discharge capacity with a rise in pressure of 10% or
less.

Relief valves
A valve which automatically discharges liquid so as to prevent a predetermined safe
pressure being exceeded. The term is commonly used for pressure relieving valves in
which the lift is proportional to the increase in pressure above the set pressure.

Safety relief valves
A valve, which depending upon its application automatically discharges gases, vapors or
liquids so as to prevent a predetermined safe pressure being exceeded.

Safety or relief valves should be used on any closed vessel or system in which the
pressure can be other than atmospheric and where any circum stances the design pressure
of the system can be exceeded. In any process, imbalance in rates of fluid flow or energy
transfer into or out of process equipment may result in the pressure exceeding the
operating pressure. If for these or other reasons, pressure exceeds prescribed limits it
must be relieved by a safety or relief valve.
Basic classes
There are mainly 3 basic classes of safety or relief valves.
 Direct
 Pilot operated

Direct acting
The direct acting valve is the simplest and most commonly used class because it is
suitable for most applications. The load is usually applied to such valves by means of
helical coil compression springs, although other means of loading are sometimes used
such as weights.

Pilot Operated Valve
The most common type comprises a main valve and a pilot valve through which the
system pressure loads the main valve. When a pre determined pressure (set pressure) is
reached the pilot valve, which is itself a safety or relief valve, relieves the pressure
loading on the main valve and allows it to open. These are used when the increase in
pressure to open the valve, and the fall in pressure to allow the valve to close, must be
less than can be attained by the use of directing acting valves.
This is similar to direct acting valve, except that an additional load is applied from an
These are used where it is necessary for the system pressure to operate closer, to the
relieving pressure of the valve than can be obtained with a direct acting valve, while
retaining the fail safe feature of the latter.

Capacity tanks
Spring less pneumatic actuators may stay in the last position on air failure but will more
likely to drift slowly to a closed or open position, depending on valve plug forces. When
it is necessary to open or close a valve against line pressure a capacity tank is used.

Stored air pressure on the cushion loading side of the piston provides positive valve
opening or closing, regardless of the magnitude or direction of the forces involved when
air supply failure occurs.
Control valve characteristic guidelines

Liquid Level Control System

Control valve pressure drop                best inherent characteristic

Constant delta pressure                              Linear

Decreasing delta pressure with increasing                Linear
20% of minimum load delta pressure.

Decreasing delta pressure with increasing
20% of minimum load delta pressure.                    Equal percentage

Increasing delta pressure with increasing
200% of minimum load delta pressure.                   Linear

Increasing delta pressure with increasing              Quick Opening
200% of minimum load delta pressure.

Source: Control engineering with data from Fisher Controls

Flow Control Processes

Best inherent characteristic

Flow measurement Location of control wide range of small range of flow but
Signal to controller valve in relation to flow set point large delta pressure change
Is:       measuring element is                at valve with increasing

Proportional to flow     in series              linear       equal percentage

In bypass *              Linear      equal percentage

Proportional to flow     in series              linear       equal percentage
Squared
In bypass *            Equal         equal percentage
Percentage
* When control valve closes, flow rate increases in measuring element.
Source:

Pressure Control System

Application                             Best inherent characteristic

Liquid process                                Equal percentage

Gas process, small volume, less than 10 ft
(3 m) of pipe between control valve and

Gas process, large volume [process has a