Facts About Control Valves

Study facts about control valves include pressure drop of control valves, capacity requirements of control valves, valve rangeability, characteristics of control valves, valve body design etc.

Contents

Facts of Control Valves

Pressure Drop for Good Control

DP = 30 to 35% of total dynamic drop for equal percentage characteristic valves
DP = up to 50% for linear characteristics
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 C_vrating 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

Also Read: Control Valves Terminology

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 C_V

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 DP_max 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:

Valve Operating Temperature Range

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

Inherent Flow Characteristics