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.
Facts About 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 Cvrating 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
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:
Very good article. plz could you provide reference for information(i.e: standards, best practices ... etc) , especially for those listed in the tables? Thank you.
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Very good article. plz could you provide reference for information(i.e: standards, best practices ... etc) , especially for those listed in the tables? Thank you.