Steam Control Valve Sizing

Calculation for Saturated and Superheated Steam ($C_v/K_v$) per ISA-75.01-2012 / IEC 60534-2-1 Standards.

US CUSTOMARY METRIC / SI

1. Project Data

Project Name

Tag Number

Service Description

Engineer

Nominal Line Size (ID, in)

2. Process Data

Steam State:

Dry Saturated Steam

Superheated Steam

Case Description	Flow (lb/h)	Inlet P₁ (psig)	Outlet P₂ (psig)	Temp T₁ (°F)	Req. $C_v$ / $K_v$
Maximum					/
Normal					/
Minimum					/

3. Steam Properties

Density is calculated automatically based on P1 (and T1 if superheated).

Ratio of Heats ($k$)

Dry Sat: ~1.31, Wet/Equil: ~1.13

Compressibility ($Z$)

Auto-calculated

Normal Case Density (lb/ft³)

4. Valve Details

Trim Logic

Pressure Drop Ratio ($x_T$)

Piping Factor ($F_p$)

5. Specification Datasheet

Selected Valve -

Rated Capacity 0.0 / 0.0

Installed $x_T$ -

Piping $F_p$ -

Point	Flow ()	P₁ ()	T₁ ()	DP ()	Req $C_v$	Opening %	Regime	Inlet Mach	Outlet Mach	Exp. $Y$

Notes / 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. Flow Equation & Density

Steam sizing follows the compressible fluid equations of ISA-75.01. The key factor is accurate steam density ($\rho_1$) or specific volume ($\nu_1$) at inlet conditions.

C_v = \frac{W}{N_6 F_p Y \sqrt{x P_1 \rho_1}}

$W$: Mass flow rate ($lb/h$ or $kg/h$).
$\rho_1$: Inlet steam density. For Saturated steam, this is a function of $P_1$ only. For Superheated, it depends on $P_1$ and $T_1$.
$Y$: Expansion factor, accounting for density change across the valve orifice.
Pressure Basis: Inputs are Gauge ($psig/barg$), calculated as Absolute ($psia/bar(a)$).

2. Choked Flow (Critical Pressure)

Just like gas, steam flow chokes when velocity reaches sonic conditions (Mach 1). This limits the maximum flow regardless of downstream pressure drop.

x_{choked} = F_k \cdot x_T, \quad F_k = \frac{k}{1.40}

The specific heat ratio ($k$) varies:

Dry Saturated Steam: $k \approx 1.31$ (Standard for dry/rapid expansion).
Wet/Equilibrium Steam: $k \approx 1.13$.

3. Superheated vs Saturated

Saturated Steam: Use this mode for boilers, headers, and sterilizers where steam is at equilibrium. Temperature is fixed by pressure.

Superheated Steam: Common in turbines and power generation. Density is lower than saturated steam at the same pressure, requiring larger valve coefficients ($C_v$) for the same mass flow.

4. Velocity Limits (Erosion & Noise)

High velocity steam can cause severe erosion (wire-drawing) and noise.

Location	Service Type	Max Mach Number
Valve Inlet	Continuous	≤ 0.20 Mach
Valve Outlet	Continuous	≤ 0.50 Mach
Valve Outlet	Intermittent / Relief	≤ 0.70 Mach

5. Valve Type Selection Guide (Steam Service)

Globe Valves (High Recovery)

Best For: High-pressure throttling, severe service, and tight shutoff.
Steam Performance: High $x_T$ (~0.75) allows for larger pressure drops before choking occurs. Excellent resistance to wire-drawing erosion on seats.
Trade-off: Lower capacity ($C_v$) and higher cost/weight than rotary valves.

Segmented Ball

Best For: Low-to-medium pressure steam, high flow requirements, high rangeability (100:1).
Steam Performance: High capacity allows smaller valve sizes. However, lower $x_T$ (~0.35) means flow chokes earlier; susceptible to noise/vibration at high pressure drops.
Trade-off: Seat materials must be selected carefully for high-temp steam.

High Performance Butterfly

Best For: Large utility steam lines (> 6") with low pressure drop.
Steam Performance: Economic choice for large pipes. Very low recovery ($x_T \approx 0.25$) severely limits pressure drop capability before choking/noise becomes excessive.
Trade-off: Not suitable for precise control at low opening angles.