| name | control-valve |
| description | Control valve sizing — Cv/Kv calculation (ISA/IEC), valve types (globe, ball, butterfly, cage), trim characterization (linear, equal-%, quick-open), flashing, cavitation, noise, ANSI/ISA standards. |
| metadata | {"priority":7,"promptSignals":{"phrases":["control valve","valve sizing","Cv valve","Kv valve","valve flow coefficient","valve cavitation","valve trim"],"minScore":3}} |
Control Valve Sizing — Complete Skill
Flow Coefficient (Cv / Kv)
Cv definition (US/ISA):
Cv = flow [US gpm] through valve at 1 psi pressure drop with water at 60°F
Kv (metric, IEC):
Kv = flow [m³/hr] through valve at 1 bar pressure drop with water at 15°C
Conversion: Kv = 0.865 × Cv
Liquid Sizing (ISA S75.01 / IEC 60534-2-1)
Non-choked flow:
Q = N₁ × Cv × F_p × √(ΔP / (G_f))
Q = volumetric flow [gpm US]; ΔP = differential pressure [psi]; G_f = specific gravity
N₁ = 1.0 (gpm, psi, sp.gr. = 1.0 unit set)
Choked flow condition:
ΔP_choked = F_L² × (P₁ - F_f × P_v)
F_L = liquid pressure recovery factor (0.65–0.98; depends on valve type)
F_f = liquid critical pressure ratio factor = 0.96 - 0.28 √(P_v/P_c)
P_v = vapor pressure of liquid; P_c = critical pressure
Cavitation index:
σ_cv = (P₁ - P_v) / ΔP
σ_cv > σ_incipient → no cavitation; σ_cv < F_L² → fully choked cavitation
Cavitation prevention:
Use anti-cavitation trim (staged pressure reduction; multiple orifices)
Or select valve with higher F_L (globe > butterfly)
Flashing: downstream P < P_v → liquid partially vaporizes permanently
Cannot prevent flashing; select erosion-resistant trim (Stellite, hard carbide)
Gas/Vapor Sizing (ISA)
Non-choked (x < F_γ × x_T):
Q = N₇ × Cv × F_p × P₁ × Y × √(x / (G_g × T₁ × Z₁))
x = ΔP/P₁ = pressure drop ratio
Y = expansion factor = 1 - x / (3 F_γ x_T)
F_γ = specific heat ratio factor = γ/1.4 (γ = c_p/c_v)
x_T = pressure drop ratio at choked flow (0.28–0.72 by valve type)
Choked flow (x ≥ F_γ × x_T):
Use x = F_γ × x_T (flow does not increase further)
Y = 0.667 (choked limit)
Mass flow:
w = N₆ × Cv × F_p × P₁ × Y × √(x × M / (T₁ Z₁)) [N₆ = 63.3 for lbm/hr, psia, °R, lb/lbmol]
Valve Types and F_L Factors
| Type | Inherent F_L | Equal-% range | Notes |
|---|
| Globe (standard) | 0.90 | Up to 100:1 | Best control; expensive |
| Globe (anti-cavitation) | 0.80–0.85 | 50:1 | Multi-stage; staged ΔP |
| Butterfly (concentric) | 0.55–0.70 | 30:1 | Low cost; high Cv/size |
| Butterfly (eccentric) | 0.60–0.75 | 50:1 | Better seating |
| Ball valve (V-port) | 0.55–0.65 | 30:1 | Segmented ball |
| Cage globe | 0.85–0.92 | 50:1 | Good for dirty service |
Valve Trim Characteristics
Equal percentage (most common for process):
m(x) = R^(x-1) where x = valve position [0-1]; R = rangeability (typically 50:1)
Linear portion of response → control loop sees constant gain
Linear trim:
m(x) = x (flow proportional to position)
Better for processes with linear gain (e.g., temperature control)
Quick-opening:
m(x) = √x (most flow at low travel)
Used for on-off service (block valves)
Installed vs. inherent: installed characteristic depends on system curve; equal-% installed often appears linear
Actuator Sizing
Required actuator force:
F_act = P_stem × A_seat + F_packing + F_spring (for spring-return)
P_stem = stem friction; A_seat = seating area; spring provides fail-safe
Typical bench set range (spring range): 0.2–1.0 bar pneumatic
Pneumatic supply: 3–15 psi or 6–30 psi signal; 20–80 psi air supply (5.5 bar typical plant air)
Positioner: electro-pneumatic; converts 4–20 mA input → pneumatic signal → accurate valve position
Noise Calculation
Hydrodynamic noise (liquid):
L_p = calculated from turbulence intensity and flow velocity (IEC 60534-8-2)
Main parameter: inlet pipe velocity V_1; if V_1 > 3 m/s → noise concern
Aerodynamic noise (gas valve):
L_p = function of power dissipation W = Q × ΔP and type of trim
Reduction: multi-hole cage trim reduces noise 10–20 dB; expanding outlet pipe
OSHA 29 CFR 1910.95 limit: 85 dBA (8-hr TWA)
Selection Procedure (ISA S75 Process)
- Determine process conditions: Q, P₁, P₂, T, fluid properties
- Check for cavitation, flashing, or choked flow
- Calculate required Cv or Kv
- Apply piping correction factor F_p if valve size ≠ pipe size
- Select valve type and trim for the service
- Choose next larger commercial Cv (from manufacturer catalog)
- Check rangeability (min Cv at minimum flow > Cv,min of selected valve)
- Specify actuator and positioner
Output
Provide: required Cv (or Kv), valve type selection, trim characteristic, F_L, cavitation check (σ_cv vs. σ_incipient), flashing check, noise estimate [dBA] at pipe exterior, actuator type (spring-return or double-acting), fail position (open/closed/last), positioner recommendation.
