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Control Valves

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 valveFacts of Control Valves

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

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:

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

Maximum Leakage Rate

  • establish acceptable leakage rate at max. DP as % of valve capacity (% valve CV).
  • select valve to lie within following limits

Valve Selection

Valve End Connections

Valve End Connections

Also Read: Control Valves Terminology

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1 comment

azetou April 30, 2017 at 3:24 pm

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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