Control Valves: Optimizing Flow and Pressure

هوالعليم
CONTROL VALVES

شير كنترل
CONTROL VALVE
• ISA-S75.05: A Control Valve is a power-operated device used to
modify the fluid flow rate in a process system.

شير كنترل
CONTROL VALVE
همانطور كه از اسم آن بر مي آيد اين شير جهت كنترل جريان سيال درمسير 
عبور آن نصب مي گردد. اين شير به طور اتوماتيك ومعمولاً با فشار هوا كار مي
كند (در موقع لزوم مي توان آنرا با دست بازوبسته نمود).
اين شير با توجه به اينكه چه چيزي را بايستي كنترل كند بطور اتوماتيك 
بازوبسته مي شود. به طور مثال اگر قرار باشد فشار را كنترل كند از محل ديگري
كه فشار آن مورد نظر است به طور مداوم با شير ارتباط برقرار است (سيگنال
سسننسسوورر)) وو ببااتتغغييييرر ففششاارر آآنن ممححلل،، ششييرر ببااززتترر وويياا ببسستتهه تترر مميي ششوودد تتاا ففششاارر ثثااببتت
بماند.
معمولاً اين شير براي كنترل فشار، دبي، دما و يا سطح مايع در يك ظرف 
مورد استفاده قرار مي گيرد و نسبت به تغييرات آنها نيز باز وبسته مي گردد.
از اين شير نمونه هاي مختلفي وجود دارد كه از لحاظ عملكرد يكي مي باشند 
و فقط از لحاظ ساختمان با يكديگر تفاوت دارند.
دو نمونه از شيرهاي كنترل عبارتند از:
Air to open/Fail to Close . - شيرهايي كه با قطع جريان هوا بسته مي شود
Air to close/Fail to Open . - شيرهايي كه با قطع جريان هوا باز مي شود

شير كنترل
در گذشته شيرهاي كنترلي با حركت خطي كاربرد بيشتري داشتند اما از سال ۱۹۵۰ تا سال •
از ۱% به ۵۰ % رسيد. Rotary ۱۹۸۰ رشد شيرهاي كنترلي از نوع
شير پروانه اي است كه اولين نوع از اين شيرها بود. Rotary معروفترين شير كنترلي نوع •
شيرهاي ربع گرد از لحاظ هزينه و همچنين عملكرد مزايايي را نسبت به شيرهاي خطي از •
خود نشان دادند.
ااننتتخخاابب ششييررههاايي ككننتتررلل ققببلالا ببرر ممببنناايي پپااررااممتتررههاايي ااووللييهه اايي ننظظييرر ففششاارر ققااببلل تتححمملل،، ددااممننهه •
جريان، افت فشار و ... بود ولي اكنون تاكيد عمده بر هزينه ها مي باشد بنابراين شيرهاي
كنترل بايد از لحاظ هزينه هاي اوليه و هزينه هاي تعميراتي مناسب باشند ضمن اينكه مي
بايست خواص كنترلي خوبي داشته باشند.
پارامترهاي ثانويه در انتخاب شير كنترل شامل نشتي هاي مجاز، خصوصيات جرياني(فلو)، دما، •
لزجت و سايش مي باشد.
مواردي نظير نويز، كاويتاسيون، فلاشينگ و .... نيز كه از فاكتورهاي عمومي و مورد تاكيد •
تمامي شيرها هستند در اينجا نيز مورد توجه قرار مي گيرند.

CONTROL VALVE
controller
Inst Air

Control valve in FLOW CONTROL
controller
Inst Air
dP Flow measurement

Control valve in LEVEL CONTROL
controller
Inst Air

Control valve in PRESSURE CONTROL
controller
Inst Air
Pressurized vessel

Control valve in Temperature Control
controller
Inst Air
Super heated Steam in Hot water out
Steam out
Cold water in

Controller
meter controller valve
INPUT SET POINT (commonly from the operator) OUTPUT

معيارهاي كنترل
پارامتر تحت كنترل
T, P, flow rate, level, etc.
ووااسسططهه ككننتتررلليي
Flow Rate (Opening/Closing Valve)
پارامتر تحت اندازه گيري
T, P, flow rate, level, etc.

قسمتهاي اصلي يك شير كنترلي

Control Valve Classification
Control
Valve
Linear
Motion
Rotary
Motion
Globe
Globe
Single
Seated
Double
Seated
Diaphragm Pinch Ball Butterfly Plug
Angle 3 way

۱- مسير جريان پيچ و خم داراست.
است (از لحاظ بازيافت فشار). Low Recovery ۲- شير از نوع
۳- قابليت و بازه كنترلي در درصدهاي پايين بهتر است.
۴- طرحهاي تريم مي تواند متنوع باشد.
۵- براي فشارها و دماهاي بالا مناسب ميباشد.
۶- معمولا به صورت فلنجي يا پيچي ساخته مي شود.
آن قابل جدا شدن است. Bonnet -۷
ممققااييسسهه
۱- مسير جريان تقريبا مستقيم است.
است (از لحاظ بازيافت فشار). High Recovery ۲- شير از نوع
۳- سايش پكينگ كمتر است.
۴- طرحهاي تريم مي تواند متنوع باشد.
۵- سيالهاي ساينده و دوغابي را نيز مي تواند پوشش دهد.
۶- بازه كنترلي بالايي دارد( از صفر تا صد).

انواع شيرهاي كنترلي خطي
1. Globe
1.1. Single Seated
1.2. Double Seated
1.3 Three way
2. Diaphragm
3. Pinch

Single Seated Globe Valve
• ويژگيها
دقت بالا در كنترل o
نيروي بسيار مورد نياز جهت باز و o
بست شير
بباا ننييرروو وو ققددررتت ززيياادديي ببسستتهه مميي ششوودد o
كاربردهاي سايز پايين o
• كاربردها
بازه كاربري بسيار وسيعي داشته و جزء •
قديميترين شيرهاي كنترلي است.

October 2005
Unbalanced / Balanced

Double Seated Globe Valve
• ويژگيها
دقت بالا در كنترل o
نيروي كمتر مورد نياز جهت باز و o
بست شير
ننششتتيي ننسسببتتاا بباالالا o
كاربردهاي سايز بالا o
مقدار عوامل نويز و كاويتاسيون و ... o
بالاتر است.
فلو و فشار بالا •

Diaphragm Valves
• ويژگيها
آب بند از نوع ديافراگمي o
قابل استفاده در سيالات o
خخووررننددهه
خوردگي فيزيكي كمتر o
در صنايع بهداشتي و خوراكي •

Pinch Valves
• ويژگيها
آب بندي بهتر o
تله كمتر براي ذرات و سيالات o
The flexible sleeve allows the
valve to close drop tight around
solids , solids that would
typically be trapped by the seat
or stuck in crevices in globe,
diaphragm, butterfly, gate or ball
valves.
در صنايع بخار و سيالات با دماي بالا •
سيالات خورنده •

انواع شيرهاي كنترلي دوراني
1. Butterfly
2. Ball
3. Plug

Ball Valves
• ويژگيها
آب بندي بسيار خوب o
مقاومت بسيار كم در برابر o
سسيياالل
قابليت تحمل فشار و دماي بالا o
بسيار متنوع مخصوصا براي افت •
فشار كم

Butterfly Valves
• ويژگيها
(Ball فلوي بالا(كمتر از شير o
نشت بند نسبتا خوب o
• ككااررببررددههاا
بسيار متنوع مخصوصا براي مايعات •
و سيالات با ذرات جامد.
صنايع استيل، شكر و نساجي •

ضريب فلوي شير (ضريب شير)
Flow Factor/Valve Coefficient/Capacity Coefficient
Cv
Cv - gallons per minute (GPM) of 60oF water with a pressure drop of 1 psi (lb/in2).
• Cv = Flow coefficient or flow capacity rating of valve.
• F = Rate of flow (US gallons(3.73liters) per minute).
• SG = Specific gravity of fluid (Water = 1).
• ΔP = Pressure drop across valve (psi) @16oC.
به عبارتي برابر است با مقدار فلوي حجمي عبوري آب از درون يك شير در مدت يك دقيقه به نحوي كه اختلاف •
1 باشد. psi فشار دوسر شير
برابر ۵۰۰۰۰۰ است. ، Grove مقدار آن براي يك شير توپي معمولي مجراكامل سايز “ ۵۶ ساخت شركت •

روش اندازه گيري ضريب فلوي شير

ضريب فلوي شير
Cv Kv
Cv (Gallon/Min) 1 0.268
Kv (Liters/Min) 3.73 1

ممققااييسسهه ضضررييبب ففللوويي ششييررههاا

Too small Cv
Valve undersized
Starving for fluid
Buildup of upstream pressure
Higher backstream pressure damaging equipments
Cavitation & Flashing
Too Large Cv
Large oversized valve is selected
Cost, size & weight increases
Higher pressure drops and faster velocity causing
Cavitation, flashing & corrosion
Bath tub stopper effect

مشخصه فلو - ۱
Flow Characteristic
Quick Opening: Quick open
plugs are used for on-off
applications designed to produce
maximum flow quickly
Linear: produces equal changes
in flow per unit of valve stroke
regardless of plug position, used
where valve pressure drop is a
major portion of the total system
pressure drop
Equal Percentage: change in
flow per unit of valve stroke is
directly proportional to the flow
occurring just before the change is
made, generally used for pressure
control
In each angle, pressure differential should be kept constant; then flow
measurement is done.

مشخصه فلو - ۲
Flow Characteristic

ممششخخصصهه ففللوو ددرر ششييررههاا

ااصصلالاحح ممششخخصصهه ففللوو
يكي از موثرترين روشها براي اصلاح مشخصه فلو، تغيير شكل مجرابند است.
تغيير شكل قفس مجرابند
تغيير شكل مجرابند نشستني

ااصصلالاحح ممششخخصصهه ففللوو

Triangle-Shape BALL VALVE
ببرراايي ككااررببررددههاايي ككننتتررلليي •

DeZurik Operation
V ported valves:
October 2005
Eccentric type:

Common Body Designs; 1
In-Line
In-line style bodies feature smooth, streamlined, constant internal area passages with no
pockets, permitting high capacity with minimum turbulence.

Angle Form
The angle-style body form for many applications such as handling erosive fluids. The
angle valve incorporates a selfdraining design. The design also allows for smaller space
requirements than as globe valve.

Three-Way Form
Three-way bodies are used for either combining or diverting services.
A standard globe valve usually, easily converts to three-way service with the addition of
a three-way adaptor, upper seat ring, two gaskets, a three-way plug, and bonnet flange
bolting.

Offset Form
When inlet and outlet piping can be offset, this design is the simplest, least expensive
barstock style. It works best in self cleaning applications. Other than the body, the offset
design is completely interchangeable with the standard globe valve.

Steam Jacketed
Steam jackets are used to heat
the fluid passing through the
control valve.
Expanded Outlet
The expanded outlet valve(Reduced Bore), permits the
installation of a small valve in a larger line without using
line reducers or expanders. The valve is a standard in-line
globe valve, except for the body which incorporates
expanded outlets. Because line expanders and reducers
are not used, field installation expenses are reduced.

Common Designs
Designed for most gas and liquid
applications. The valve’s unique
construction handles pressures
from vacuum to 15,000 psi (1034
Bar) and temperatures from -423 to
1500F (-253 to 816C).
High positioning accuracy,
repeatability, controlled high speed,
and instant response.

Common Designs
The ShearStream ball valve features
a segmented V-notch ball to reduce
clogging, to improve shearing
action and to exceed 300:1
rangeability. The one-piece body
provides high performance.
A high performance rotary control valve using an eccentric plug
which provides high rangeability, zero breakout torque and
durable trim with a significant increase in valve life.
Applications from petrochecmical to low content slurry and pulp
service to severe service.
The eccentric plug provides rangeability greater than 100:1,
compared to 50:1 for typical globe valves and 20:1 for most
butterfly valves.
The shutoff rating reaches Class IV for metal seats and Class VI for
soft seats.

Special Designs
The Offset design is used for installation in offset
piping configurations. All parts, except the body,
are identical to the conventional globe type.
high pressure classes but low sizes
where fast delivery is required

Special Designs
The pressure-balanced regulator used primarily
by the aerospace industry in high temperature
gas applications. Upstream pressure is delivered
to a small piston within the diaphragm actuator.
(This piston has the same area as the plug.)
Steam Jackets are used to heat the fluid passing
through the control valve. The steam jacketed
valve body uses a standard globe-style body with
oversized, blind flanges for a full jacket or
standard flanges for a partial jacket. The jacket
usually is rated for 150 psi and comes equipped
with 3/4-inch NPT supply and drain connection.

Severe Services
The trims are designed to eliminate cavitation, reduce high noise levels and
handle flashing applications often associated with high pressure drop service.
This cartridge uses specially
designed channels and intersecting
holes (plenums) in series to
prevent single point, large pressure
recovery which can cause
cavitation and hydrodynamic noise.
For less serious cavitation
applications, use of some retainers
to minimizes cavitation damage by
controlling the location of
imploding vapor bubbles are
offered.

Severe Services
Attenuator reduces gaseous noise
levels with staged pressure
reduction through a series of
drilled-hole cylinders.
Above design effectively reduce gaseous and
hydrodynamic noise more and eliminate the
damaging effects of cavitation in liquids.

Downstream Devices
Silencer Plate; upto 15dB
Installed between raised face flanges
immediately downstream from the
valve, the plate incorporates a series of
stages to control line turbulence and
absorb the pressure drop. The number
of stages varies according to the
application.

Downstream Devices
Diffuser; upto 30dB
Diffusers share the high pressure drop
with the valve. The length of the diffuser
and the number of holes vary to
accommodate the flow capacity required.

Downstream Devices
Vent Element; upto 25dB
In a blow off or vent system, tremendous
energy in the form of noise is released at
the open exit. Vent silencers attenuate
this noise energy before it is released to
the outside environment.

ككووييتتااسسييوونن
زمانيكه يك مايع از ميان يك شير نيمه باز عبور مي كند فشار آن در ناحيه اي كه o
سرعت افزايش پيدا كرده كاهش مي يابد و به فشار بخار مايع مي رسد. مايع در ناحيه
كم فشار شروع به بخار شدن مي كند وحفره هارا توسط حبابهاي گاز توليد شده از بخار
مايع پر مي كنند. وقتي مايع مجدداً به فشار استاتيك برسد، حبابها بطور ناگهاني جمع
مي شوند. اين پديده كه باعث ايجاد شكاف در شير مي شود را كويتاسيون مي گويند.
ااگگرر اايينن ععمملل ببططوورر ممررتتبب تتككرراارر ششوودد ددرر ننههااييتت ببااععثث ششككسستتگگيي وواازز ببيينن ررففتتنن ششييرر مميي o
شود كه البته اين پديده در لوله ها نيز ممكن است اتفاق بيافتد.

ضضررييبب ككووييتتااسسييوونن
يكي از مشخصه هايي كه براي مصرف كنندگان شير مدنظر مي باشد ضريب o
كويتاسيون است كه بايستي شدت آن مشخص باشد. اين پارامتر به صورت ذيل بيان
مي شود:
ضريب كويتاسيون : C
فشار بخار مايع :Pv
فشار در لوله اي كه ۵ برابر قطر لوله متصل به جريان پايين دست شير است. :Pd
فشار در لوله اي كه برابر قطر لوله متصل به جريان بالادست شير است. :Pu
را براي شيرهاي گوناگون نشان مي دهد. C نمودارهاي صفحه بعد مقدار

ررووششههاايي ممممااننععتت اازز ككووييتتااسسييوونن
براي جلوگيري از پديده مي توانيم قطر لوله خروجي از شير را بطور ناگهاني افزايش دهيم
كه مقدار آن نيز قابل محاسبه مي باشد.
۵ برابر قطر لوله وطولي معادل ۸ برابر قطر لوله بعد از شير / اگر بتوانيم يك افزايش به قطر ۱
بگذاريم خواهيم توانست از كويتاسيون كه در نهايت باعث از بين رفتن شير مي شود
جلوگيري بعمل آوريم.
افزايش قطر
لوله خروجي
۱
اايينن االلگگووههاا مميي تتوواانننندد سسررععتت وو يياا ااخختتلالافف ففششاارر ننااگگههااننيي رراا ككااههشش ددههنندد..
استفاده از
الگوهاي خاص
در مسير
جريان
۲

ففلالاششييننگگ
نرسد آنگاه فلاشينگ Pv درصورتي كه هنگام افت فشار در شير، فشار سمت دوم به o
رخ مي دهد.
در اين حالت معمولا خوردگي فيزيكي در قسمتهاي پايينتر شير و لوله اتفاق مي افتد. o
فلاشينگ غالبا چاره اي ندارد جز اصلاح ساختار و ابعاد شير. o

چچوو ؛؛ ششددنن سسررععتت
در اين حالت گاز در ناحيه اختلاف فشار به حداكثر سرعت خود (معادل سرعت صوت) o
مي رسد و با شدت تمام به بدنه و قسمتهاي مختلف داخل شير برخورد مي كند و اثري
مشابه كويتاسيون و فلاشينگ برجاي مي گذارد.

ااففززااييشش ننااگگههااننيي سسررععتت
اين پديده در محدوده وسيعتري نسبت به كويتاسيون رخ مي دهد و موجب لرزش o
شير و تاسيسات، جريان سرگردان سيال و نيز نويز صوتي شديد مي شود.

ضضررببهه ققووچچ
ضربهٴ قوچ افزايش فشار يا موجي است كه در سيالهاي در حال حركت پس از توقف o
يا تغيير مسير ناگهاني پيش مي آيد. اين افزايش فشار معمولاً هنگامي رخ مي دهد
كه شير در مسير حركت سيال (آب يا گاز) ناگهان بسته مي شود.
در اين حالت يك ضربه ناگهاني و قوي به سيستم وارد مي شود كه نويز شديدي نيز o
دداارردد..

ااثثررااتت ممححييططيي
اين اثرات ممكن است شامل نشتي گاز به محيط، صداهاي آزاردهنده، فضاي اشغالي و o
... باشد.

Valve Sizing
1- Flow Capacity; Cv
The valve sizing coefficient most commonly used as a measure of the capacity of the
body and trim of a control valve is Cv
One Cv is defined as one U.S. gallon per minute of 60 degree Fahrenheit water that
flows through a valve with a one psi pressure drop.

Valve Sizing
2- Pressure Profile
Maximum velocity and minimum pressure
occur immediately downstream from the
throttling point at the narrowest constriction of
the fluid stream, known as the vena contracta
.(انقباض رگ)
Downstream from the vena contracta, the fluid
slows and part of the energy (in the form of
velocity) is converted back to pressure.

Valve Sizing
3- Allowable Pressure Drop
The curve departs from a linear
relationship at the onset of "choking"
described using the Fi factor.
The flow rate reaches a maximum,
qmax, at the fully choked condition due
to effects of cavitation for liquids or
sonic velocity for compressible fluids.
The transition to choked flow may be
gradual or abrupt, depending on valve
design.

Valve Sizing
3- Allowable Pressure Drop
For liquid ; ANSI/ISA sizing equations use a
pressure recovery factor, FL, to calculate the
ΔΔΔΔP at which choked flow is assumed for sizing
purposes.
For compressible fluids ; a terminal
pressure drop ratio, xT, similarly describes the
choked pressure drop for a specific valve.
ΔPa : When sizing a control valve, the
smaller of the actual pressure drop or the
choked pressure drop is always used to
determine the correct Cv.
This pressure drop is known as the allowable
pressure drop, ΔPa .

Valve Sizing
4- Cavitation
In liquids;
when the pressure anywhere in the liquid
drops below the vapor pressure of the fluid,
5- Flashing
If the downstream pressure is equal to or
less than the vapor pressure, the vapor
bubbles created at the vena contracta do not
vapor bubbles begin to form in the fluid
stream.
As the fluid decelerates there is a resultant
increase in pressure. If this pressure is higher
than the vapor pressure, the bubbles
collapse (or implode) as the vapor returns to
the liquid phase.
This two-step mechanism – called
cavitation – produces noise, vibration, and
causes erosion damage to the valve and
downstream piping.
collapse, resulting in a liquid-gas mixture
downstream of the valve. This is commonly
called flashing.
When flashing of a liquid occurs, the inlet
fluid is 100 percent liquid which experiences
pressures in and downstream of the control
valve which are at or below vapor pressure.
This two phase (vapor and liquid) fluid at the
valve outlet and in the downstream piping; the
velocity of this two phase flow is usually very
high and results in the possibility for erosion of
the valve and piping components.

Valve Sizing
6- Liquid Pressure Recovery Factor, FL
7- Liquid Critical Pressure Ratio Factor, FF
The liquid pressure recovery factor, FL, predicts the
amount of pressure recovery that will occur between the
vena contracta and the valve outlet.
The liquid critical pressure
ratio factor, FF, multiplied by
the vapor pressure, predicts
the theoretical vena contracta
FL is an experimentally determined coefficient that
accounts for the influence of the valve’s internal
geometry on the maximum capacity of the valve. It is
determined from capacity test data.
High recovery valves such as butterfly and ball valves
have significantly lower pressures at the vena contracta
and hence recover much farther for the same pressure
drop than a globe valve. Thus they tend to choke (or
cavitate) at smaller pressure drops than globe valves.
* High recovery valves are valves that lose little energy
due to little flow turbulence.
pressure at the maximum
effective (choked) pressure
drop across the valve.

Valve Sizing
8- Chocked Flow
9- Reynolds Number Factor, FR
in gases and vapors ; Choked flow occurs when the fluid
velocity reaches sonic values at any point in the valve body,
trim, or pipe. As the pressure in the valve or pipe is lowered,
the specific volume increases to the point where sonic
velocity is reached.
The Reynolds Number
Factor, FR, is used to
In liquids, vapor formed as the result of cavitation or
flashing increases the specific volume of the fluid at a faster
rate than the increase in flow due to pressure differential.
Lowering the downstream pressure beyond this point in
either case will not increase the flow rate for a constant
upstream pressure. The velocity at any point in the valve or
downstream piping is limited to sonic (Mach = 1).
As a result, the flow rate will be limited to an amount
which yields a sonic velocity in the valve trim or the pipe
under the specified pressure conditions.
correct the calculated Cv
for non-turbulent flow
conditions due to high
viscosity fluids, very low
velocities, or very small
valve Cv .

Valve Sizing
10- Piping Geometry Factor, FP
11- Ratio of Specific Heats Factor, Fk
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 ratio of specific heats
factor, Fk, adjusts the
equation to account for
the standard test manifold, the apparent valve capacity is
changed. The effect of reducers and increasers can be
approximated by the use of the piping geometry factor, FP.
different behavior of gases
other than air.
12- Terminal Pressure Drop Ratio, xT
The terminal pressure drop ratio for gases, xT, is used to predict the choking point where
additional pressure drop (by lowering the downstream pressure) will not produce additional
flow due to the sonic velocity limitation across the vena contracta.
This factor is a function of the valve geometry and varies similarly to FL, depending on the
valve type.

Valve Sizing
13- Expansion Factor, Y
The expansion factor, Y, accounts for the
14- Compressibility Factor, Z
The compressibility factor, Z, is a
variation of specific weight as the gas passes
from the valve inlet to the vena contracta.
It also accounts for the change in cross-sectional
area of the vena contracta as the
pressure drop is varied.
function of the temperature and the
pressure of a gas.
It is used to determine the density
of a gas in relationship to its actual
temperature and pressure conditions.

Valve Sizing
15- Velocity
As a general rule, valve outlet velocities should
be limited to the following maximum values:
Gas applications where special noise
attenuation trim are used should be
limited to approximately 0.33 Mach.
In addition, pipe velocities
downstream from the valve are critical
The above are guidelines for typical
applications. In general, smaller sized valves
handle slightly higher velocities and large valves
handle lower velocities.
Special applications have particular velocity
requirements; a few of which are provided here:
to the overall noise level.
Experimentation has shown that
velocities around 0.5 Mach can create
substantial noise even in a straight pipe.
The addition of a control valve to the
line will increase the turbulence
downstream, resulting in even higher
noise levels.

Valve Sizing
15- Velocity
Liquid applications – where the fluid
temperature is close to the saturation point –
should be limited to 30 feet per second to
avoid reducing the fluid pressure below the
vapor pressure. This is also an appropriate limit
In flashing services, velocities become
much higher due to the increase in
volume resulting from vapor formation.
For most applications, it is important
to keep velocities below 500 feet per
second. Expanded outlet style valves
for applications designed to pass the full flow
rate with a minimum pressure drop across the
valve.
Valves in cavitating service should also be
limited to 30 feet per second to minimize
damage to the downstream piping. This will
also localize the pressure recovery which
causes cavitation immediately downstream
from the vena contracta.
help to control outlet velocities on such
applications.
Erosion damage can be limited by
using chrome-moly body material and
hardened trim.
On smaller valve applications which
remain closed for most of the time such
as heater drain valves higher velocities
of 800 to 1500 feet per second may be
acceptable with appropriate materials.

Valve Sizing
هدف : بدست آوردن مناسبترين(كوچكترين) سايز و ابعاد شير و مشخصات تريم باتوجه به گروه 
محصولات سازنده، با لحاظ كردن پارامترهاي مهم (اقتصادي) : سايز شير مي تواند كمتر از سايز لوله باشد
اما بزرگتر، نه! انتخاب مشخصه يا ويژگيهاي فيزيكي و ذاتي شيرها در اينجا صورت نمي گيرد.
سسپپسس ششررااييطط سسررععتت سسيياالل.. غغااللبباا Cv پپااررااممتتررههاايي تتححتت ااننددااززهه گگييرريي ججههتت ننييلل ببهه ههددفف:: ااببتتدداا 
سايز بدنه شير و از روي سرعت سيال، تريم انتخاب مي گردد. Cv ازروي
در شرايط خاص ممكن است جهت برآوردن نياز و عدم وجود تريم مناسب در سايزي خاص(باتوجه به
سايز را بالاتر انتخاب كرد. ،(Cv
درصورتيك از انتخابها راضي نيستيم بايستي به سراغ سازنده ديگر يا گروه شير ديگر برويم.
داشته ها : مشخصات رده شير مدنظر (نوع و مشخصه فلو، مشخصات هندسي و جداول مربوطه، جدول 
مشخصات سيال و فرآيند از جمله فشار اوليه و ثانويه مجاز. ،(Cv

Calculating Cv for Liquid
The Equation for the flow coefficient (Cv) in non-laminar liquid flow is:

The following steps should be used to compute the correct Cv, body size and trim number:
Step 1: Calculate Actual Pressure Drop
The allowable pressure drop, ΔPa, across the valve for calculating Cv, is the smaller of the
actual ΔP from Equation 3.2 and choked ΔPch from Equation 3.3. (P1 and P2 Should be given.
فشار مينيمم درخروجی شير است.) p فشار ماکزيمم در ورودی است و 2 p1
Step 2: Check for Choked Flow
Use Equation 3.3 to check for choked flow:
If ΔPch (Equation 3.3) is less than the actual ΔP
(Equation 3.2) , use ΔPch for ΔPa in Equation 3.1.

It may also be useful to determine the point at which substantial cavitation begins. The
following Equation defines the pressure drop at which substantial cavitation begins:
In high pressure applications, alternate analysis may be required; verify analysis with
factory if ΔP > ΔP (cavitation)> 300 psi (globe valves) or 100 psi (rotary valves).
The required Cv for flashing applications is determined by using the appropriate ΔP
allowable [ΔPch calculated from Equation 3.3].

Step 3: Determine Specific Gravity
Specific gravity is generally available for the flowing fluid at the operating temperature.
The appendix provides fluid property data for 268 chemical compounds, from which the
specific gravity, Gf can be calculated.
Step 4: Calculate Approximate Cv
Generally the effects of nonturbulent flow can be ignored, provided the valve is not
operating in a laminar or transitional flow region due to high viscosity, very low
velocity, or small Cv. In the event there is some question, calculate the Cv, from Equation
3.1, assuming Fp=1, and then proceed to steps 5-7. If the Reynolds number calculated in
Equation 3.6a is greater than 40,000, FR can be ignored (proceed to step 8 after step 5.

Sizing Valve using Cv for Liquid
Step 5: Select Approximate Body Size Based on Cv
From the Cv tables in section 4, select the smallest body size that will handle the
calculated Cv.
Step 6: Calculate Valve Reynolds Number Rev and
Reynolds Number Factor FR
Use Equation 3.6a to calculate valve Reynolds Number Factor:
Use Equation 3.6b to calculate valve Reynolds Number Factor FR if Rev < 40,000,
otherwise FR = 1.0:

Step 7: Calculate the Final Cv
If the calculated value of FR is less than 0.48, the flow is considered laminar; and the Cv
is equal to Cvs calculated from Equation 3.6c. If FR is greater than 0.98, turbulent flow
can be assumed (FR = 1.0); and Cv is calculated from Equation 3.1. Do not use the
piping geometry factor Fp if FR is less than 0.98. For values of FR between 0.48 and
0.98, the flow is considered transitional; and the Cv is calculated from Equation 3.6e:
For laminar and transitional flow, note the ΔP is always taken as P1 - P2 .

Step 8: Calculate Piping Geometry Factor
If the pipe size is not given, use the approximate body size (from step 5) to choose the
corresponding pipe size. The pipe diameter is used to calculate the piping geometry factor,
Fp, which can be determined by Tables 3-III and 3-IV. If the pipe diameter is the same as
the valve size, FP is 1 and does not affect Cv.
Using the value of FP, calculate the required Cv from Equation 3.1.
Step 10: Calculate Valve Exit Velocity
The following Equation is used to calculate entrance or exit velocities for liquids:

After calculating the exit velocity, compare that number to the acceptable velocity for
that application. It may be necessary to go to a larger valve size.
Step 11: Recalculate the Cv if Body Size Changed
Recalculate Cv if the FP has been changed due to selection of a larger body size.

Step 11: Select Trim Number
First identify if the valve will be used for on/off or throttling service. Using the Cv tables
in Section 4, select the appropriate trim number for the calculated Cv and body size
selected. The trim number and flow characteristic (Section 9) may be affected by how
the valve will be throttled. When cavitation is indicated, refer to Section 14 to evaluate
special trims for cavitation protection.

Example1; p1

Example1; p2

Example1; p3

Example1; p4

Example1; p5

Example2; p1

Example2; p2

Example2; p3

Example2; p4

Flashing Liquids Velocity Calculations
When the valve outlet pressure is lower than or equal to the saturation pressure for the
fluid temperature, part of the fluid flashes into vapor. When flashing exists, the following
calculations must be used to determine velocity. Flashing requires special trim designs
and/or hardened materials.
Flashing velocity greater than 500 ft/sec requires special body designs.
If flow rate is in lb/hr:
if the flow rate is given in gpm, the following
Equation can be used:

Calculating Percentage Flash
The % flash (x) can be calculated as follows:
For water, the enthalpies (hf1, hf2 and hfg2) and specific volumes (vf2 and vg2) can be found
in the saturation temperature and pressure tables of any set of steam tables.

Example1; p1
Assume the same conditions exist as in Example1, except that the temperature is 350°F rather
than 250° F. By referring to the saturated steam temperature tables, you find that the
saturation pressure of water at 350°F is 134.5 psia, which is greater than the outlet pressure of
105 psia (90 psia). Therefore, the fluid is flashing. Since a portion of the liquid is flashing,
Equations 3.9 and 3.10 must be used. x (% flashed) can be determined by using Equation3.10:

Example1; p2
Therefore, the velocity in a 3-inch valve can be determined by using Equation 3.9:
Flashing velocity is less than 500 ft/sec, which is acceptable for Mark One bodies.
Hardened trim and CavControl should also be considered.

Calculating Cv for Gas
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. The form of the
equation used is one of the following, depending on the process variables available:

The following steps should be used to compute the correct Cv, body size and trim number:
Step 1: Select the Appropriate Equation
Based on the information available, ”just” select one of the four equations to calculate Cv
from flow rate: 3.11, 3.12, 3.13 or 3.14. the calculation will be done later(step 5).
Step 2: Check for Choked Flow
Determine the terminal pressure drop ratio, xT, for that particular valve by referring to Table
3-V. Next, determine the ratio of specific heats factor, Fk, by using the Equation below:

Calculate the ratio of actual pressure drop to absolute inlet pressure, x, by using Equation 3.16:
Choked flow occurs when x reaches the value of FkxT. Therefore, if x is less than FkxT, the
flow is not choked. If x is greater, the flow is choked. If flow is choked, then FkxT should be
used in place of x (whenever it applies)

Step 3: Calculate the Expansion Factor
The expansion factor, Y, may be expressed as:
* (If the flow is choked, use FkxT for x)
Step 4: Determine the Compressibility Factor
To obtain the compressibility factor, Z, first calculate the reduced pressure, Pr, and the
reduced temperature, Tr, Using the factors Pr and T, find Z in Figures 3-4 or 3-5.

Sizing Valve using Cv for Gas
Step 5: Calculate Cv
Using the above calculations, use one of the four gas sizing Equations to determine Cv :
(assuming Fp is 1)
Step 6: Select Approximate Body Size Based on Cv
From the Cv tables of valves, select the smallest body size that will handle the calculated Cv.
Step 7: Calculate Piping Geometry Factor
If the pipe size is not given, use the approximate body size (from step 6) to choose the
corresponding pipe size. The pipe size is used to calculate the piping geometry factor, Fp,
which can be determined by Tables 3-III or 3-IV. If the pipe diameter is the same as the
valve size, Fp is 1 and is not a factor.

With the calculation of the Fp, figure out the final Cv.
Step 9: Calculate Valve Exit Mach Number
Equations 3.20, 3.21, 3.22 or 3.23 are used to calculate entrance or exit velocities (in
terms of the approximate Mach number).
Use Equations 3.20 or 3.21 for gases, Equation 3.22 for air and Equation 3.23 for steam.
Use downstream temperature if it is known, otherwise use upstream temperature as an
approximation.

After calculating the exit velocity, compare that number to the acceptable velocity for that
application. Select a larger size valve if necessary. Refer to section 13 to predict noise level.
Caution: Noise levels in excess of 110 dBA may cause vibration in valves/piping resulting in
equipment damage.
Step 10: Recalculate Cv if Body Size Changed
Recalculate Cv if Fp has changed due to the selection of a larger body size.
Step 11: Select Trim Number
Identify if the valve is for on/off or throttling service. Using the Cv tables in Section 4, select
the appropriate trim number for the calculated Cv and body size selected. The trim number
and flow characteristic (Section 9) may be affected by how the valve is throttled.
After selecting trim number, calculate the mach number for trim(bore) size for not being
larger than 1.0. if mach number is greater then select larger trim; if the trim size is
maximum, you should choose larger body size.

* Step 12: Calculate Valve Trim Mach Number
After selecting trim number, calculate the mach number for trim(bore) size for not being
larger than 1.0. if mach number is greater then select larger trim; if the trim size is
maximum, you should choose larger body size.
Note: it is possible that if you select small trim size in large body size while mach number is
high, the velocity of gas reaches over the sonic velocity which it is not possible! So you
should consider the mach number in trim not to be mare than 1.0 .

Example1; p1

Example1; p2

Example1; p3

Example1; p4

Example1; p5

Example1; p6

Step 12: by considering the mach number during pass of flow from trim it is clear that if
the exit velocity is close to 0.5 mach, the square of body size to bore size should not be
more than 0.5 (i.e. the trim size should be more than 4.6”) . Hence the selected trim no. is
not suitable and we have to select trim size 5.0 .
By selecting this trim size, Cv will be large no. which results in lower controllibility.

Example2; p1

Example2; p2

Example2; p3

Example2; p4

Example2; p5

Noise Prediction
Control valve noise is generated by turbulence created in the valve and radiated to the
surroundings by the downstream piping system. Major sources of control valve noise are
mechanical vibration of the valve components, and hydrodynamic and aerodynamic fluid
noise.
Noise produced by mechanical vibration is usually well below 100 dBA and is described
as a mechanical rattling.
Aerodynamic noise levels can be above 100 dBA and reach as high as 150dBA in certain
services.
Noise Sources :

Noise Prediction
The predicted noise value can be considered accurate to + 5 dBA when the outlet velocity
is less than sonic and is correct for single throttling point trims only.
NOTE: any level below 70 dBA usually does not require low noise trim or other accessories.
For calculating noise level, these data should be available:
o Required(not installed) valve sizing coefficient, Cv .
o Upstream pressure, psia, P1 .
o Downstream pressure, psia, P2 .
o Flowing temperature of fluid, T .
o Flowing fluid, Q .
o Pipe size and schedule D, sch,
At the next Slides, the method of calculating(predicting) noise level will be presented. After
Calculating Noise Level, add below quantities if the corresponding condition is ready:
1. For a valve installed near a reflective surface (a hard floor or wall), add 3 dBA.
2. For a valve installed near two reflective surfaces (a hard floor and wall), add 6 dBA.
3. If the valve is near three reflective surfaces (two hard walls and hard floor), add 9 dBA.
4. A valve installed in a small room with all reflective walls, floor, and ceiling can easily
produce noise levels 30 or 40 dBA.

Hydrodynamic Noise Prediction
1- Hydrodynamic Noise Prediction
The total sound pressure level is easily found by finding DPs, Cs, Rs, Ks, and Ds from Figures 13-2
through 13-5 and Tables 13-I and 13-II and then substituting them into the noise equation.
To obtain DPs and Rs, it is first
necessary to calculate the pressure
drop ratio, DPf, where:

If DPf calculated is greater than 1, flashing is occurring in the valve and these formulas do
not apply to flashing service.

Example1; p1

Example1; p2

Aerodynamic Noise Prediction
2- Aerodynamic Noise Prediction
The total sound pressure level is easily found by finding Vs, Ps, Es, Ts, Gs, As and Ds from
Figures 13-6 through 13-9 and Tables 13-I and 13-III and 13-IV then substituting them into
the noise equation.

Example1; p1

Example1; p2

Noise Attenuation
1. CHANGING THE PROCESS FLOW CONDITIONS
2. CHANGING THE LOCATION OF THE VALVE
3. CHANGING THE STYLE OF THE VALVE BODY
4. CHANGING (Reversing) THE INSTALLED FLOW DIRECTION OF THE VALVE BODY
5. USE MULTIPLE VALVES OR DOWNSTREAM RESTRICTION DEVICES
6. USE OF HARDENED TRIM
7. USE OF THICKER PIPING WALL SCHEDULE
8. USE OF DOWNSTREAM NOISE SUPPRESSION(plate, diffuser, …)
9. INCREASE FRICTIONAL LOSSES
10. USE OF EAR PROTECTION OR INSULATION
11. Use Of Special Designs or Equipment

Flow Characteristics
Inherent flow characteristic
Vs.
Installed flow characteristics
When a constant pressure drop is maintained
across the valve, the characteristic of the valve
alone controls the flow; this characteristic is
referred to as the “inherent flow
characteristic.” “Installed characteristics”
include both the valve and pipeline effects.
The difference can best be understood by
examining an entire system.

Quick Review
Equal Percentage
Equal percentage is the characteristic 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 characteristic of the valve itself may be equal
percentage, most control loops will produce an installed characteristic approaching linear
when the overall system pressure drop is large relative to that across the valve.
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.
Quick Open
Quick open plugs are used for on-off applications designed to produce maximum flow
quickly.

Example1; p1
Effect of other equipments
A centrifugal pump supplies water to a system in which a control valve is used to maintain the
downstream pressure at 80 psig. The pump characteristics are shown below:

Example1; p2
The maximum flow required is 200 gpm. at which the pump discharge pressure
(P1) is 100 psig. Piping losses are negligible. Using the ISA liquid sizing formula,
the flow coefficient, or Cv, can be determined :
To determine the plug characteristic which should be specified, let us analyze the
installed flow characteristic of “equal percentage” and “linear” trim in this valve.

Example1; p3

Example1; p4

Example2; p1
Effect of other equipments
The previous example was idealized in that the downstream pressure was held constant
and the pressure drop variation was due to the pump characteristic alone. Now
consider a more realistic installation where the delivered pressure must be held
constant after passing Through the valve and with some line restriction, R, in series with
the valve.

Example2; p2
To find the installed characteristics of equal percent and linear trim in a suitably sized
valve, a pressure drop distribution must be chosen. A suitable choice would be 4 psi
across the valve at a flow of 200 gpm. The control valve can then be sized for the
maximum required Cv :
Since the pressure drop across the restriction will vary with flow in accordance
with the square root law ( Q = R sqrt(ΔP) ) the available pressure drop across the
valve at various flowing quantities can be determined, keeping in mind the pump
characteristic. This is shown in Table 9-II.

Example2; p3

Example2; p4

ننششتتيي ممججاازز ششييررههاايي ككننتتررلل -- ۱۱

ننششتتيي ممججاازز ششييررههاايي ككننتتررلل -- ۲۲

Actuator Sizing
Linear Actuator Sizing, Pressure-balanced Trim
In throttling services three questions must be answered:
1. Will the actuator handle the throttling differential pressure?
2. Will the actuator provide sufficient thrust to overcome application pressures to open or
close the valve, and generate enough seat loading for tight shutoff with the given air
supply pressure?
3. Will the spring fail the valve in the proper direction?
With on/off services, only questions 2 and 3 must be answered.

Actuator Sizing
The equations in this section use the following variables:

Actuator Sizing
When an actuator’s stroke length exceeds the longest stroke length shown for that size
actuator in Table 16-VI, the actuator will not have a spring. For actuators without a spring,
SE = SR = 0. AR is not used when sizing actuators for valves with standard trim or when
sizing actuators for Class 150 through 600 MegaStream valves.

Actuator Sizing

Actuator Sizing
• Step 1: Determine Actuator’s Maximum Allowable
Throttling Pressure Drop
Determine the maximum allowable throttling pressure drop (ΔPa) that the selected
actuator can handle by using equations (16.7) and (16.8):

Actuator Sizing
Compare the maximum allowable throttling pressure drop to the actual pressure drop.
If the actual throttling drop is less than ΔPa, the selected actuator is sufficient.
However, if the actual throttling drop is greater than ΔPa, the next larger actuator size
should be chosen and the above calculation should be repeated.

Actuator Sizing
Step 2: Determine Actuator’s Size For Actuation Thrust
Calculate the actuator cylinder areas required by using the applicable group of equations
in the following tables. Compare the calculated areas to the corresponding areas for the
actuator size selected in Step 1. Actuator areas are listed in Table 16-III. If the calculated
areas are less than or equal to the corresponding areas for the selected actuator, the
actuator size is adequate. If the calculated areas are larger, an actuator with cylinder
areas larger than the calculated areas must be selected.
When determining the required actuator size, various service conditions should be
considered. For each sizing equation, the conditions to be considered for that equation
are listed with the equation. Each equation should be evaluated for each listed condition
that will actually occur. The condition numbers refer to the following list.

Actuator Sizing
Service Conditions to be considered:
1. P1 and P2 for flow conditions. If more than one flow condition will occur, each should
be evaluated.
2. P1 and P2 at shutoff. If more than one set of pressures will occur during shutoff, each
set of pressures should be evaluated. The possibility of P2 dropping to atmospheric
pressure (0 psig) should be considered. Pressures used to bench test the
valve should also be considered.
3. P1 and P2 equal to the maximum value of P. This condition may occur if the pipeline is
pressurized and the pipe downstream from the valve is blocked. For this condition, set
RSL = 0 in the sizing equations.
4. P1 and P2 equal to 0. This condition will occur if the pipeline is depressurized. This
condition will also occur during bench testing of the valve. For this condition, set RSL= 0
in the sizing equations.

Actuator Sizing
Step 2: Determine Actuator’s Size For Actuation Thrust
NOTES:
1. On valves larger than 24-inch, the weight of the plug may need to be accounted for;
contact factory.
2. A negative number calculated for AL or AU indicates that the smallest available
actuator will work for the condition being evaluated.
3. For valves with a trim number smaller than the plug stem diameter, AS-Astem will be a
negative number. In this case, the negative sign must be retained during the sizing
calculations.

Actuator Sizing
Step 3: Determine Spring Size
If it will be necessary for the valve to stroke open or closed upon loss of air supply
pressure, the fail-safe spring must be sized. The required spring force is calculated by
using the applicable equations in the following tables. Each sizing equation should be
evaluated for the listed conditions that will actually occur. The condition numbers refer to
the service conditions listed in step 2.
After the required spring force is calculated, it must be compared to the standard spring
force for the actuator selected in steps 1 and 2. This spring force is listed in Table 16-VI.
If the required spring force is less than the standard spring force of the selected actuator,
a standard spring will be sufficient. If the required spring force is greater than that of a
standard spring force, compare the required spring with the dual (or heavy-duty) spring
force for the same size actuator (see Table 16-VI). If the dual spring force is larger than
the required spring force, the dual spring should be used. If the dual spring force is not
large enough, a volume tank or larger actuator will be required. Section 19 contains
volume tank sizing information. If the spring or actuator size selected to provide sufficient
spring force is different from that used during step2, the calculations of step 2 must be
verified using the New spring or actuator information.

Emergency Conditions
• HandWheel:
To bypass Control Valve
• By pass Valves

Positioners
Positioning is based on a balance of two forces:
one proportional to the instrument signal and the other proportional to the stem position.

Positioners
Linearity – Independent
Performance Data
Hysteresis - Maximum position error for the same value of input when
approached from opposite directions.
Dead Band - Maximum change in input required to cause a reversal in valve stem
movement.
Response Level - Maximum change in input required to cause a change in valve
stem position in one direction.
Resolution - Smallest possible change in valve stem position.
Repeatability - Maximum variation in position for the same value of input when
approached from same direction.

Positioners
Steady State Air Consumption
Performance Data
Supply Pressure Effect - Position change for 10 psi supply pressure change.
“Open-loop” Gain - Ratio of output pressure unbalance to instrument pressure
change with locked stem @60 psi.
Maximum Flow Capacity
Frequency Response
Stroking Speed : Closed to open ; Open to closed

Positioner
Positioners are instruments that help improve control by accurately
positioning a control valve actuator in response to a control signal
Positioners receive an input signal either pneumatically or
electronically and provide output power to an actuator
183

Reasons To use Positioners
• Increase control system resolution i.e)fine resolution.
• Allow use of characteristic cams.
• Minimize packing friction effects.
• Allow Split Ranging.
• Overcome seating friction in rotary valves.
• Facilitate operation when the higher number in the bench-set range is greater than 1ksc
• Permit use of piston actuators.
• Allow distance between controller and control valve using the advantage of
4-20ma signal.

POSITIONERS
A device attached to an actuator that receives An electronic or pneumatic signals from
the controller And compares this signal to the actuator’s position.
3-way positioners
Send and exhaust air to only one side of a Single acting actuator that is opposed by a
range Spring.
4-way positioners
Send and exhaust air to both sides of an actuator which is required for double acting
actuators.

Valve Positioners
• Pneumatic valve positioners are the most commonly used valve accessories.
• A valve positioner is a device which will accurately position a control valve in
accordance with the pneumatic control signal.
• The control signal is routed to the positioner where comparison of the valve
position (actual) to the control signal (desired) is used to develop an output
pneumatic signal which operates the valve actuator.
• The positioner compares the control signal (the requested valve position)
with the actual valve position through the mechanical feedback linkage.
• If the valve position is incorrect, the positioner will either load or exhaust air
from the valve actuator until the correct valve position is obtained.
• A positioner requires both a control signal and an instrument supply air for
normal operation.
• Most positioners come equipped with three gauges to indicate supply air
pressure, control signal pressure and actuator diaphragm signal (output) air
pressure.
186
ERT 212/3 Process Engineering
Skills

Valve Positioners
188
Skills

Valve Positioners
• Advantages of the valve positioner include:
1) Minimizing the effect of friction, hysteresis and
deadband on the valve stem. With a high pressure
system, tighter valve stem packing is needed to
prevent leakage and a high frictional force is
generated. With a positioner valve stem movements
of as little as 25 µm are possible.
2) Enables signal range change. A positioner can
amplify the incoming control signal when a greater
actuating force is needed. A 20-100 kPa control
signal can be amplified to 40-200 kPa before being
applied to the actuator.
189
Skills

Valve Positioners
3) Allows signal reversal. A positioner can operate in
either direct or reverse acting mode. For example, in
reverse acting mode, an increase in control signal
pressure causes a decrease in positioner output air
pressure. For example, in reverse mode, a 100 - 20
kPa actuator signal would correspond to a 20 - 100
kPa control signal from the I/P transducer.
4) Increases the speed of response of the actuator.
The speed of response of the valve actuator depends
on:
(a) the actuator volume, and
(b) the flow rate of the control signal air.
190
Skills

Valve Positioners
Allows valve flow characteristic to be changed.
Most valve positioners employ a rotating cam in the
feedback system. This cam can be changed to
simulate different valve flow characteristics. A linear
globe valve can be used to respond in an equal
percentage manner.
Allows split range operation. In a split range control
loop, one controller is used to drive two control
valves.
191
Skills

Valve Positioners
• A positioner ensures that there is a linear relationship
between the signal input pressure from the control
system and the position of the control valve.
• This means that for a given input signal, the valve will
always attempt to maintain the same position regardless
of changes in valve differential pressure, stem friction,
diaphragm hysteresis and so on.
192
Skills

Valve Positioners
• A positioner may be used as a signal amplifier or booster.
• It accepts a low pressure air control signal and, by using
its own higher pressure input, multiplies this to provide a
higher pressure output air signal to the actuator
diaphragm, if required, to ensure that the valve reaches
the desired position.
• Some positioners incorporate an electropneumatic converter
so that an electrical input (typically 4 - 20 mA) can be used to
control a pneumatic valve.
• Some positioners can also act as basic controllers,
accepting input from sensors.
193
Skills

Positioner Working
Some of the mechanisms
i. Force Balance Positioners
ii. Motion balance Positioners
iii. Electronic Positioners
194

Force Balance Positioners
195

Motion balance Positioners; 1
196

Motion balance Positioners; 2
197

Electronic Positioners
Sensor to detect valve position
Micro-Processor Error = Position - Signal
198

DeZurik Operation
Rotary Control Valves
October 2005

پارامترهای موثر در انتخاب شير کنترلی
هزينه •

Actuator
The purpose of the valve actuator is to accurately locate the
valve plug in a position dictated by the control signal
202

Types of Actuators
Pneumatic Valve Actuators: Adjust the position of the valve
by converting air pressure into rotary motion or linear motion.
Piston and Diaphragm Actuators are examples of Pneumatic
Actuators
Electric Valve Actuators: An electric actuator is powered by
motor that converts electrical energy to mechanical torque
Hydraulic actuator: Consists of a cylinder or fluid motor that
uses hydraulic power to facilitate mechanical operation. The
mechanical motion gives an output in terms of linear, rotary or
oscillatory motion
203

Types of Actuators Operation
Multi-turn actuator
gives torque for at least one full
Part-turn actuator
gives torque for less than one full
Linear actuator
opens and closes valves that can be
operated via linear force
204

Electric Actuators
• Electric actuators use an electric motor
with voltage requirements in the
following range: 230 Vac, 110 Vac, 24 Vac
and 24 Vdc.
• There are two types of electrical actuator
1. VMD (Valve Motor Drive)
2. Modulating.

VMD ( Valve Motor Drive )
• This basic version of the
electric actuator has three
states:
I. Driving the valve open.
II. Driving the valve closed.
III. No movement
• The controller positions the
valve by driving the valve
open or closed for a certain
time, to ensure that it
reaches the desired
position, Valve position
feedback may be used with
some controllers.

Modulating
• In order to position the control valve in
response to the system requirements a
modulating actuator can be used. These
units may have higher rated motors (Step
Motor) and may have built-in electronics.
• A positioning circuit may be included in
the modulating actuator which accepts an
analogue control signal (typically 0-10 V or
4-20 mA). The actuator then interprets
this control signal, as the valve position
between the limit switches.
• To achieve this, the actuator has a position
sensor (usually a potentiometer), which
feeds the actual valve position back to the
positioning circuit. In this way the actuator
can be positioned along its stroke in
proportion to the control signal.

Pneumatic Actuators
They are basically of two types
1. Piston actuator
2. Diaphragm actuator

Piston Actuator
• They can withstand higher input pressures.
• Can offer small cylinder volumes.
• They are generally used where the stroke of a diaphragm
actuator would be too short or the thrust is too small.

Diaphragm Actuators
• They have
compressed air applied
to a flexible membrane
called the diaphragm
• They are single acting
i.e. air is supplied from
single side of the
diaphragm

Classification of control valve on the basis of actuator action

Classification of control valve on the basis of valve action

Failure mode
Actuator
action
Valve body
action
Control valve
action
Failure mode Valve Color
Direct Direct Air to close FAIL OPEN Green
Direct Reverse Air to open FAIL CLOSE Red
Reverse Direct Air to open FAIL CLOSE Red
Reverse Reverse Air to close FAIL OPEN Green

Air To Close Valve
Δ P FLOW =P1-P2
Δ P SHUTOFF =P1-PLOAD
1. USUALLY Δ P SHUTOFF > Δ P
FLOW
2. Methods to increase Δ P SHUTOFF
1. Increase PLOAD
2. Increase ADiaphragm
3. Reduce FSpring
4. Reduce ζ Packing
ΣFup = ΣFdown
P1 * APort+ FSpring+
ζPacking=PLOAD* ADiaphragm+
P2 * APlug
Note: Inlet Pressure tends to push
open plug

Air To Open Valve
Δ P FLOW =P1-P2
Δ P SHUTOFF =P1-PLOAD
1. USUALLY Δ P SHUTOFF > Δ P
FLOW
2. Methods to increase Δ P SHUTOFF
1. Decrease PLOAD
2. Increase ADiaphragm
3. Increase FSpring
4. Reduce ζζ Packing
ΣFup = ΣFdown
P1 * APort+PLOAD* ADiaphragm +
ζPacking= FSpring +P2 * APlug
Note: Inlet Pressure tends to push
open plug

Accessories
• Positioners ; pneumatic input 3-15 psi
Electro-pneumatic 4-20 ma (HART,
Fieldbus, Profibus)
• Limit switches
• Position feedback
• a standard, pneumatic positioners use 3-15, 3-9, 9-15,
• 3-7, 7-11 and 11-15 psi input signals; electro-pneumatic
• positioners use 4-20, 4-12, 12-20 and 10-50 mA input
• signals. Other non-standard signals, such as 6-30 psi,
• are also available.

Valve Sizing
What is Valve Sizing?
Flasing; Cavitation; Noise
October 2005

Generic Control Valve Types
Features Comparison
Double Single Bal. Split Eccentric
Key Features Seat Seat Trim Angle Y Pattern 3-Way Body Ball Butterfly Plug
Capacity 1 1 1.2 1 to 2 1.5 0.7 1 3 3.2 1.3
Shut Off % Rated Cv 0.5 <0.01 0.01 0.01 0.01 N/A 0.01 0 0 or1 0.01
Cv Ratio 50 50 50 50 50 50 50 100 25 100
Cavitation G S G S P S S P P S
Noise G S G S P S S P P S
High Pressure/High DP G S V.G. S S P S P P S
High Temp/Low Temp S S V.G. S G S S P P G
Erosion/Slurry S S P G S S G S P G
Corrosion S G P S S S G S S G
Maintenance S G S S S S V.G. G G G
Cost 1.25 1 1.12 1.2 1.5 1.8 0.97 0.73 0.4-0.7 0.83

ساخت. در نوع اول double act يا single عمل كننده شيرهاي كنترلي را مي توان بصورت
د شير كنترلي مشخص است اما در ساختار دوم، (تغذيه) fail حالت پيش فرض يا وضعيت
كند در همان حال باقي مي ماند. بدليل حساسيت شيرهاي كنترلي fail شير در هر وضعيتي
ساخته single act در آنها مشخص باشد غالبا بصورت fail و اينكه معمولا بايستي حالت
مي شود.
با استفاده از چهار گيج مي توان فشار double با استفاده از ۳ گيج و در نوع single در نوع
ننققااطط ممخختتللفف ععمملل ككننننددهه رراا تتححتت ننظظرر ددااششتت..
معمولا سايز شيرهاي كنترلي يك رده كوچكتر از سايز خطي است كه برروي آن نصب مي شوند.
عمل كننده نوع ديافراگمي را اگر بصورت عمودي قرار دهيم ميتواند براي شيرهاي ربع گرد
استفاده گردد.

ACTUATORS
A device mounted on a valve that in response to a Signal, automatically moves the valve to
the required position using an outside power source. The addition Of an actuator to a
throttling valve is called a
ACTUATORS
Pneumatic Electronic motor Electro hydraulic
Diaphragm Piston

PNEUMATIC ACTUATORS
Air is relatively inexpensive, 90% of the Industries employ these actuators.
HYDRAULIC ACTUATORS
Exceptional stiffness & high thrust’ are required, Fast stroking speeds. Hydraulic fluid
above and below a piston to position the valve.
ELECTROHYDRAULIC ACTUATOR
Self contained hydraulic system, electrical Signal feeds to an internal pumps, which
uses hydraulic fluid from a reservoir to feed hydraulic fluid above or below the
piston.
Pros – Exceptionally stiff because of the incompressibility of liquids.
Cons – Expensive and Bulky

Valve Actuators
• The diameter of the diaphragm plate determines the
force that will be applied to the actuator stem.
• For example, if the maximum input signal pressure is 100
kPa and the plate diameter is 30 cm, then:
Force applied to stem = Pressure x Plate Area
= 100 kPa x 3.14 x (0. 15)2 m2
= 7.07 KN (1590 lb)
226
Skills

CONVAL
Emerson
ISA
Forms( (سفارش
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Control Valves: Optimizing Flow and Pressure

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