1. CHARACTERISTICS OF
CONTROL VALVES
Prem Baboo
Sr. Manager (Prod)
National Fertilizers Ltd. India
F.I.E., Institution of Engineers (India)
Technical Advisor & an Expert for
www.ureaknowhow.com
2. CHARACTERISTICS
• CONTROL VALVES ARE CLASSIFIED
ACCORDING TO THEIR
CHARATERISTICS ALSO
– LINEAR VALVES
– EQUAL PERCENTAGE VALVES
– QUICK OPENING VALVES
– MODIFIED CHARACTERISTICS
3. Control Valve
Characteristics
• The relationship between
flow through valve to the
corresponding stem
position is plotted to get
valve characteristics
– Inherent flow
characteristics
– Installed Flow
Characteristics
4. –Inherent flow
characteristics
•It is derived from testing a
valve with water as the fluid
and with a constant pressure
drop across the valve.
–Installed Flow
Characteristics
•The flow characteristic when
the pressure drop across the
valve varies with flow and
6. QUICK OPENING
• A FLOW CHARACTERISTIC that
provides maximum change in flow
rate at low travels. The curve is
basically linear through the first 50-60
% of travel. It then flattens out
indicating little increase in flow rate
as travel approaches the wide open
position.
• Q = ky2
K = CONSTANT
y = Valve Opening
Q = Flow
7. LINEAR
• A characteristic where flow or (Cv)
increases linearly with valve travel.
Flow is directly proportional to
valve travel.
• Q = ky
– k = constant
– y = valve opening
– Q = Flow
8. EQUAL PERCENTAGE
• It is described as the type of valve
flow characteristic where for equal
increments of valve plug travel the
change in flow rate with respect to
travel may be expressed as a
constant percent of the flow rate at
the time of the change.
• Q = b eay
ALTERNATELY
• Q = Q0 ef(ky)
Q = FLOW
a & b ARE CONSTANTS
y IS VALVE STROKE
9. MODIFIED PARABOLIC
• A FLOW CHARACTERISTIC that
lies somewhere between LINEAR
and EQUAL PERCENTAGE.
13. • ACTUATOR:
A fluid -powered or electrically
powered device that supplies force and
motion to a valve closure member.
– DIAPHRAGM:
A flexible pressure responsive
element that transmits force to the
diaphragm plate and actuator stem.
– FORCE PLATE
The support plate which gives
support to the diaphragm and exerts
force uniformly
14. • SPRING
– This is required for single acting
actuators to return to normal
position when air supply is failed.
• YOKE
– Yoke is the fixed connection
(mounting) between body and
actuator.
15. • BODY
– The body of the valve is the main
pressure boundary. It provides the pipe
connecting ends and the fluid flow
passageway. It can also support the
seating surface and the valve CLOSURE
MEMBER.
• BONNET
– The bonnet is that portion of the valve
pressure retaining boundary which may
guide the stem and contains the PACKING
BOX and STEM SEAL.
16. • TRIM
– Includes all the parts that are in flowing
contact with the process fluid except the
body, BONNET, and body flanges and
gaskets
• STUFFING BOX
– The chamber located in the BONNET
which surrounds the stem and contains
the PACKING and other stem-sealing
components.
20. Cv & Kv FOR CONTROL VALVES
• flow coefficent Cv
– the flow of water at 60 oF in US
gallon/minute at a pressure drop of 1
psi across the valve.
• flow factor Kv
– the number of cubic meters per hour
of water at 20 oC which will flow
through the valve with a pressure
drop of 1 kg/cm2
21. • FOR LIQUIDS
– Cv = Q(G/Dp)1/2
• FOR GASES
– Cv = Q/1360 * {TfG / [dP*(P2)]}1/2
• FOR VAPOURS
– Cv = W / 63.3 * {v / Dp} 1/2
– G = sp.gravity Tf =Flow temp. in Rank
– dP = P1-P2 v = Down stream sp.vol
22. Valve Leakage
Classifications
• Class I. Identical to Class II, III, and IV in
construction and design intent, but no
actual shop test is made.
• Class II. Intended for double-port or
balanced singe-port valves with a metal
piston ring seal and metal-to-metal seats.
Air or water at 45 to 60 psig is the test fluid.
Allowable leakage is 0.5% of the rated full
open capacity.
• Class III. Intended for the same types of
valves as in Class II. Allowable leakage is
limited to 0.1% of rated valve capacity.
23. • Class IV. Intended for single-port and
balanced single-port valves with extra-tight
piston seals and metal-to-metal seats.
Leakage rate is limited to 0.01% of rated
valve capacity.
• Class V. Intended for the same types of
valves as Class IV. The test fluid is water at
100 psig or operating pressure. Leakage
allowed is limited to 5 X 10 ml per minute
per inch of orifice diameter per psi
differential
• Class VI. Intended for resilient-seating
valves. The test fluid is air or nitrogen.
Pressure is the lesser of 50 psig or
operating pressure. The leakage limit
depends on valve size and ranges from 0.15
to 6.75 ml per minute for valve sizes 1
through 8 inches.
24. ACTUATORS
• HOW DOES ACTUATOR WORKS
AGAINST HIGH PRESSURE TO
GET SHUT OFF
• TYPES OF ACTUATOR
30. Multiplication of Force
F / A = P
242
x .7854 = 452 sq. in.
220 lb. / 452 sq. in. =
.48 psi
1442
x .7854 = 16286 sq. in.
.48 psi x 16286 sq. in. =
7817 lb. of force
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