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

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:

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

Maximum Leakage Rate

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

Valve End Connections

Also Read: Control Valves Terminology