Wednesday, October 17, 2012
Quick Facts About Control Valves
Pressure Drop for
Good Control
·
DP = 30
to 35% of total dynamic drop for equal percentage characteristic valves
·
but > 15% adequate for most applications
·
increase % DP
for wide flow ranges
·
decrease % DP
for small flow variations
·
best to optimize pumping costs with respect to
valve type and DP valve
Capacity Requirements
·
Design flowrate (normal flow)
·
valve to control from 25 to 60% opening should
be approxim. 65% of Cv rating to give control margin
·
Maximum flowrates
·
if not known then use 1.3-1.5 times design rate
(generally 1.3)
·
Minimum flowrate
·
may be 3 to 4 % of rated capacity of valve
·
lower flowrates clash with leakage rates of
valves especially double ported types
Rangeability (Turndown)
·
Assume rangeability of 15:1 to 20:1 for globe
valves and 30:1 to 50:1 for ball valves
Valve Characteristics
Depending on Function
·
Flow control
·
for wide flow range use linear characteristic
·
for small flow range (but high DPV) use equal percentage characteristic
·
Pressure control
·
for liquids use equal percentage
·
compressible fluids use equal percentage
·
if < 3m of downstream pipe
·
use linear characteristic if downstream system
has a receiver or line > 30m length
·
if pressure drop > 5:1 use equal percentage
·
Liquid level control
·
for constant DP
use linear characteristic
·
when DP
decreases with load increase use linear characteristic
·
if "full load DP" < 20% of no-load DP
use equal percentage
·
if DP
increases with load increase use linear characteristic. If increase > 2:1
use quick opening type.
Flow Characteristics
of Types
·
Equal percentage
·
for fast processes
·
for high rangeability
·
when system dynamics not well known
·
at heat exchanger where product rate change
demands greater change in heating or cooling
to be used:
·
when major portion of system DP is not available through the valve
·
valve DP
is high at low flows, low at high flows
·
when oversizing is desirable due to limited data
·
for past pressure control loops
·
for most flow control loops
·
Quick opening
·
for on-off control
·
when maximum valve capacity required quickly
·
for directly connected valves such as pump
governors, back pressure regulators and high capacity reducing regulators >
25 mm.
·
Linear
·
for slow processes
·
when more than 40% of system DP occurs across
valve
·
when major process changes are a result of load
changes
·
when a variable head flowmeter is used
·
for most level control loops
·
for slow pressure control loops
·
loops where measurement is linear and DP across
valve is small
·
3-way valve systems
·
2-way valves in 3-way service
·
Other characteristics for special applications
are:
·
hyperbolic
·
square root
Body Design
(Characteristics of Types)
·
Sliding gate
·
reduced turbulence
·
fast response
·
self cleaning
·
tight shutoff
·
high rangeability (100:1)
·
linear characteristic
·
direct actuators available
·
fluid must be clean
·
Double port globe
·
high flow capacity compared to single port
·
high rangeability
·
requires smaller actuator than single port
(balanced design)
·
reversible port
·
used for > 50.8 mm size
·
high leakage rates > 1%
·
low pressure recovery
·
erosion can occur
·
no good for high flow, low DP
·
Single port globe
·
high rangeability
·
good shut-of
·
reversible plugs
·
used for < 50.8 mm sizes
·
unbalanced design means larger actuator
·
low pressure recovery
·
3-way globe
·
good for blending and diverting
·
often used for temperature control of heat
exchangers
·
cannot control total flow
·
need to know flow conditions precisely
·
Split
body
·
good control
·
tight shutoff
·
simple, economic construction
·
easy maintenance
·
free of pockets where sediments/solids collect
·
limited pressure drop due to top guiding
·
Venturi angle
·
good control
·
high capacity, good rangeability
·
tight shutoff
·
minimizes erosion
·
conserves space
·
can handle sludges and slurries
·
good for flashing
·
only available > 50.8 mm
·
Diaphragm
·
high capacity
·
low cost
·
self-cleaning
·
diaphragm protects working parts from fluid
·
tight shutoff if pressure is low
·
good for corrosive fluids
·
poor control characteristics
·
low rangeability
·
short diaphragm life
·
slow response
·
not for high DP
applications
·
temperature limited by diaphragm material
·
Butterfly
·
high capacity
·
economical in large sizes
·
high recovery characteristic
·
good for slurry services
·
minimum space for installation
·
available in large sizes
·
operating torques are high
·
tight shutoff depends on resilient seals
·
throttling control limited
·
Vee-ball
·
high capacity
·
good control, rangeability
·
reasonable cost
·
good for slurry services
·
limited operating pressure
·
no good for high DP
applications
Pressure-Temperature
Rating, Valve Type and CV
·
Need to establish required Cv (65 to 75% rated
CV), pressure and temperature rating eg. ANSI 150 class, 300 class etc.
·
check valve CV limitations to establish valve
types available which meet CV requirements
Shut-off Differential
Pressure, Valve Type and Size
·
establish maximum DP
for valve, plus valve size
·
check valve types available which exceed DPmax and size specifications
Throttling
Differential Pressure, Erosion and Stability
·
establish normal throttling DP, fluid properties (gas, liquid, abrasive,
non-abrasive)
·
check valve type and internals to handle DP and fluid conditions
Valve Operating Temperature Range
·
establish maximum and minimum process
temperature
·
check valve types suitable for temperature range
(ball, globe and material - stainless steel, carbon steel, etc.)
·
typical ranges are:
cryogenic service
|
- 273°C to -18°C
|
nominal service
|
-18°C to 204°C
|
hot service
|
204°C to 482°C
|
high temperature service
|
482°C to 1093°C
|
·
body material ranges are:
Stainless steels
|
- 267°C to 871°C
|
Carbon steels
|
-18°C to 426°C
|
Chrome molybolenum
|
- 28.8°C to 593°C (flashing services)
|
Inconel
|
- 267°C to 1093°C
|
High nickel steels
|
- 267°C to 871°C
|
Iron
|
-18°C to 204°C
|
Ductile iron
|
-180°C to 371°C
|
Aluminium
|
- 267°C to 204°C
|
Bronze
|
- 267°C to 204°C
|
·
bolting materials
Studs
|
|
Stainless A193 Gr B8
|
- 267°C to 871°C
|
Alloy steel A193 Gr B7
A193 Gr B16
|
-18°C to 426°C
-18°C to 621°C
|
Nuts
|
|
stainless A194 Gr8
|
- 267°C to 871°C
|
alloy steel A194
Gr2H
|
-18°C to 621°C
|
·
packing material
TFE
|
-28.8°C to 204°C
|
graphite asbestos
|
-28.8°C to 287°C
|
graphite
|
-240°C to 1093°C
|
Valve types:
·
globe:
extended bonnets
|
- 267°C to -18°C
|
standard bonnets
|
-18°C to 204°C
|
extended bonnet + PTFE packing
|
232°C to 454°C
|
graphite packing
|
> 454°C
|
Seats
|
|
TFE
|
- 267°C to 204°C
|
elastomers
|
-18°C to 204°C
|
metal
|
- 267°C to 454°C
|
integral
|
- 267°C to 1093°C
|
·
butterfly
unlined
|
- 267°C to 648°C
|
elastomer lined (tight shut off)
|
-18°C to 204°C
|
metal and refractory lined
|
- 267°C to 1093°C
|
piston ring disc seal
|
- 128°C to 204°C
|
·
ball
TFE seats
|
- 267°C to 204°C
|
TFE and elastomer seats
|
-18°C to 204°C
|
carbon-graphite
|
-18°C to 537°C
|
stellite seats
|
-18°C to 815°C
|
·
plug
TFE lined
|
- 128.8°C to 204°C
|
metal to metal
|
- 128.8°C to 315°C
|
·
diaphragm
elastomer diaphragms
|
- 34°C to 177°C
|
Inherent Flow
Characteristics
·
establish flow characteristic required (linear,
equal %, quick opening etc) and required rangeability
·
check valve type which provides suitable
rangeability for given characteristic
Valve type
|
Characteristic
|
Rangeability
|
Low flow control
|
Globe
|
linear
equal %
quickopening
|
30:1 to 300:1
30:1 to 60:1
100:1
|
good
good
-
|
Sliding gate
|
linear
|
100:1
|
-
|
Butterfly
|
equal %
|
100:1 (< 305mm)
33:1 to 100:1 (> 305mm)
|
-
-
|
linear
|
100:1
|
-
|
|
Full ball
|
modified linear
equal %
|
50:1
|
good
|
V-ball
|
mod. equal %
|
350:1 (seat)
100:1 (no seat)
|
good
good
|
Diaphragm
|
weir type
pinch
|
5:1
-
|
-
good
|
Maximum Leakage Rate
·
establish acceptable leakage rate at max. DP as % of valve capacity (% valve CV).
·
select valve to lie within following limits
ISA RP 39.6
Class
|
Leakage
Rate
|
Valve Type which conforms
|
2
|
0.5%
|
Single seated globe
Double seated globe
Butterfly (metal lined)
|
3
|
0.1%
|
High quality double seated globe
Single seat globe (balanced with
metal pistone seals)
|
4
|
0.01%
|
Globe - single seatBall valve - metal seat
|
5
|
5 x 10-4 cm;/min
per inch orifice diameter per psi differential
|
Globe valves with heavy duty actuators
|
6
|
bubbles/min (see RP 39.6)
|
globe (resilient seat)
butterfly (elastomer lined)
ball (resilient seat)
diaphragm (weir type)
plug valves
|
Valve End Connections
flangeless
|
up to ANSI 600 class
|
screwed
|
gas to 20670 kPa(g), tem= 204°C
liquid to 34450 kPa(g), temp = 204°C
|
flanged
|
flat face ANSI 185, 150 (204°C)
raised face ANSI 150 to 2500
tongue and groove ANSI 150 to 2500
ring joint: high pressure (815°C)
lengs ring: to 344500 kPa(g)
lap joint: 861 to 1722 kPa(g) (93°C)
|
welded
|
piping temperature and pressure ratings refer ANSI B16.5
|
Also Check-
Labels: control valve, epc-school, instrumentation, valves
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