Liquid Control Valve Sizing

Calculation for Incompressible Flow Coefficients ($C_v/K_v$) as per ISA-75.01-2012 Standards.

US CUSTOMARY METRIC / SI

1. Project Data

Project Name

Tag Number

Service Description

Engineer

Nominal Line Size (ID, in)

2. Process Data

Case Description	Flow ()	Inlet P₁ ()	Outlet P₂ ()	Req. $C_v$ / $K_v$
Maximum Case				0.00 / 0.00
Normal Case				0.00 / 0.00
Minimum Case				0.00 / 0.00

3. Fluid Properties

Liquid Medium

Temp ()

Spec. Gravity

Vapor Press. (Pᵥ)

Critical Press. (P꜀)

4. Valve Details

Trim Logic

Recovery Factor (F_L)

Piping Correction (F_P)

5. Specification Datasheet

Selected Valve -

Rated Capacity 0.0 / 0.0

Installed Fₚ -

Recovery Fₗ -

Case	Flow ()	P₁ ()	ΔP ()	Req $C_v$	Opening %	Regime	Vel ()

Official Remarks

Suggested Valves

Fill process points for results.

Rating Check

Opening for a known valve capacity.

Rated C_v

Rated K_v

Engineering Reference & Technical Basis

1. Primary Flow Equation

The sizing flow coefficient relates volumetric flow and pressure drop per ISA-75.01 standards:

C_v = \frac{q}{N_1 F_p} \sqrt{\frac{SG}{\Delta P_{sizing}}}

Where $ \Delta P_{sizing} $ is the smaller of actual $ \Delta P $ or terminal $ \Delta P_{max} $. $ N_1 $ is 1.0 (US) or 0.865 (Metric).

2. Terminal Pressure Drop

Flow chokes when orifice pressure drops below Vapor Pressure $ (P_v) $:

\Delta P_{max} = F_L^2 (P_1 - F_F P_v)

Valve Trim	Typical $ F_L $
Globe	0.85 - 0.92
Segmented Ball	0.60 - 0.70
Butterfly	0.55 - 0.65

3. Trim Selection Guidance

Selecting between Linear and Equal Percentage (EQP) trim depends on the system pressure drop profile:

Linear Trim: Use when $ \Delta P_{valve} / \Delta P_{system} > 0.70 $. Proportional gain is constant. Best for level control and constant pressure systems.
Equal Percentage Trim: Use when $ \Delta P_{valve} / \Delta P_{system} < 0.30 $. Logarithmic gain compensates for decreasing head at high flows. Best for pressure and flow loops in long pipelines.

EQP Equation: \[ Op\% = 100 \cdot [1 + \frac{\ln(C_v/C_{v,rated})}{\ln(R)}] \]

4. Piping Geometry (Fₚ)

Accounts for turbulence and head losses caused by reducers and expanders when valve size $ (d) $ is smaller than pipe size $ (D) $:

F_p = \frac{1}{\sqrt{1 + \frac{\sum K}{890} \left( \frac{C_{v,rated}}{d^2} \right)^2}}

Rule of thumb for short-radius reducers: $ F_p \approx 0.95 $ (1 size reduction), $ F_p \approx 0.90 $ (2 size reduction).

5. Vena Contracta & Cavitation Theory

The Vena Contracta is the point of minimum pressure within the valve orifice. If pressure at this point drops below Vapor Pressure $ (P_v) $, vapor bubbles form:

Cavitation: Vapor bubbles implode downstream as pressure recovers above $ P_v $. This results in noise, vibration, and mechanical damage.

Flashing: Bubbles remain in the liquid if outlet pressure $ P_2 \leq P_v $. This creates a two-phase flow and limits capacity.

Recommendation: Liquid velocity should stay below 15-20 ft/s (4.5-6 m/s) to prevent erosion.

6. Valve Style Selection Guidance

Globe Valves: The primary choice for high-precision control. Excellent for services with high pressure drops and severe conditions where cavitation or noise-attenuation trims are required. They offer high rangeability ($F_L \approx 0.85-0.92$) but have lower capacity-to-size ratios.

Segmented Ball Valves: High-capacity valves ideal for slurries, fibrous liquids, or high-viscosity applications. They offer a self-cleaning action and very high $C_v$ per size. However, they are high-recovery valves ($F_L \approx 0.60$), making them more susceptible to cavitation at higher pressure differentials.

Butterfly Valves: Most economical choice for large pipe sizes (typically 6" and above). They offer a very high $C_v$ and are lightweight. Best for low pressure drop services and cleaner fluids, usually providing optimal control within the 20% to 70% opening range.