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LicenceLicense : Non spécifiée / IncluseUnspecified / Included
Bachelor Degree in Chemical Engineering
Process Instrumentation and Control
(Strumentazione e Controllo dei Processi Chimici)
CONTROL VALVE SIZING
AND INHERENT CHARACTERISTIC
Rev. 2.3 of March 25, 2021
ACTUATOR Magnani, Ferretti e Rocco (2007)
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NOMINAL PIPE SIZE (NPS): it is the identification value of the interface dimension of a valve and of the
other piping elements. Elements having the same NPS and NP (nominal pressure) can be coupled.
BODY: The body of the valve is the main pressure boundary. It provides the pipe connecting ends and the
fluid flow passageway. It can also support the seating surface and the valve closure member.
PORT: fixed opening or orifice in the body, which determines fluid passing (for globe valves)
TRIM: The assembly of the inside components of a valve, which are in contact directly with the fluid. The
seat, plug, stem and ball are all trim components. With their shapes they determine the flow characteristic
of the valve.
CLOSURE MEMBER: it is a properly shaped movable element. Its position determines the section area
through which the flow crosses the valve. The closure member can assume every position between two limit
positions: closed position, at which corresponds ZERO or the minimum flowrate, and the full-open
position, at which corresponds the maximum flowrate.
FULL CLOSE: a valve is defined full close when the flowrate is zero as the closure member is in closure.
TRAVEL or STROKE or LIFT or OPENING: it is the displacement of the closure member from the closed
position. Usually it is from ½” to 1½“ in a globe valve.
NOMINAL TRAVEL or RATED TRAVEL: it is the travel corresponding to the full-open position.
RELATIVE TRAVEL: it is the ratio between the travel and the nominal travel. It is indicated with h. In closure
position is h=0; in fully open position is h=1.
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Example of a globe valve
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The flow rate of the fluid crossing the valve can be expressed as either
volumetric or mass. It depends on valve characteristics and on the state and
properties of the fluid, such as:
Inlet pressure, P1
Inlet temperature, T1
Outlet pressure, P2
Pressure drop, ΔP = P1 - P2
Vapor pressure of the liquid, Pv, calculated at the inlet temperature, when
considering negligible the temperature change inside the valve;
Specific gravity of the fluid, Gf = ρ/ρ0, where ρ is the density of the fluid at the
inlet conditions and ρ0= 1000 kg/m3 (T=4°C) is the density of water at ambient
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Flow coefficient Cv (for liquids)
• For the flow characteristics of a valve, it is necessary to put the flow
rate passing through the valve to the physical reference state of the
• The parameter, of British/American origin, which is used to define the
flow characteristics of a valve is, traditionally, the flow coefficient Cv
• In particular,
Cv of a valve is the volumetric flow rate of water in US gal min-1
(one United States gallon is equal to 3.785 liters) crossing the
valve at a temperature between 5÷40°C (Gf=1) for a specified
(partially or full open) valve opening with a static pressure drop
ΔP = 1 psi [=] 6895 Pa
Cv [=] US gal (H2O) min -1 psi-1/2
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Cv assumes values between the smallest flow coefficient Cvmin and the largest flow coefficient Cvmax when
the closure member moves from the close position (h=0) to the full opening position (h=1). To understand
the relationship between Cv and h, the following definitions hold:
Nominal (or Rated) value of Cv at the nominal travel (Cvn=Cvmax)
flow coefficient Cvn
Relative flow a non-dimensional ratio [0,1]
Opening value of Φ expressed as a percentage [0,100]
Rangeability r = ratio between the largest and the smallest flow coefficient;
Cvmax/Cvmin r for Cvmin = 0
The higher the number, the better. The rangeability is 5:1 on
typical small globe valves (1/2"), 75:1 on a large globe valve (2"),
100:1 on rotating valves, 6:1 practically on butterfly valves.
Inherent characteristic the relationship between the relative flow coefficient Φ and the closure
Φ(h) member travel h with constant pressure drop ΔP across the valve.
The inherent characteristic depends on the conformation of the trim,
of which the shaping of the closure member is also part (in the cage
15/09/2021 valves, from the shape
Process Instrumentation and and dimensions
Control of the holes of the cage) 7
- Prof. M. Miccio
• The inherent flow characteristics of a valve are defined and provided by the
• a reference fluid (i.e., water)
• a reference temperature
• a reference pressure drop
• They are defined from the flow coefficient CV (or AV or KV)
• They allow to characterize the dependence of the flow rate on the valve's own
characteristics and to compare the ability to pass a flow rate in different valves.
• The flow coefficient Cv (h) results as a monotonically increasing function of the
relative travel h of the closure member.
• When the control valve is a rotary one, the relative travel h is replaced by the
• Following the IEC 534-2-4 norm, the manufacturer must provide the inherent
characteristic of a control valve as table or diagram. In this latter case, the Cv
at 5%, 10% up to 100% of the relative travel at 10% intervals has to be provided.
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THE MOST COMMON
Hypothesis: ΔP = const.
b. equal percentage or exponential
c. quadratic or parabolic
d. quick opening
e. butterfly valve or V-Port globe (green)
ℹ️ The ON/OFF valves generally allow, by varying the degree of opening, an intermediate
position of the closure member and, hence, a partial flow rate.
However, there is no unique correspondence between closure member position and
flowrate. Therefore, they cannot be used for control purposes.
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THE MOST COMMON