Steam Control Valve Sizing Calculator

STEAM CONTROL VALVE SIZING · Cv / Kv
Steam Control Valve Sizing Calculator

Size a control valve for saturated steam. Enter the steam flow and your inlet and outlet pressures to get the required Kv and Cv, automatic choked-flow detection, and a suggested ARI-STEVI® valve size.

SOLVE FOR
Steam Mass Flow
Inlet Pressure P1
Outlet Pressure P2
Required Kv
--m3/h.bar
metric coefficient
Required Cv
--US
= 1.1561 x Kv
Pressure Drop dP
--bar
P1 - P2 (abs)
Method — Saturated Steam (Spirax Sarco simplified)
Subcritical: m = 12 x Kv x sqrt(dP x P2) — m in kg/h, pressures in bar abs

Guidance only. Uses simplified saturated-steam sizing equations (critical pressure ratio 0.58). For critical processes confirm with full IEC 60534 or ARI-myValve. Valve selected when Kvs exceeds required Kv; ideal operation 20–80% of Kvs.

How to Size a Steam Control Valve

Sizing a steam control valve means finding the flow coefficient — Kv (metric) or Cv (imperial) — that will pass your required steam flow at the available pressure drop, then choosing a valve whose rated Kvs comfortably exceeds it.

What you need

Three figures size a steam valve: the steam mass flow (kg/h or lb/h), the inlet pressure and the outlet pressure (both absolute, or enter gauge and tick the box).

Choked (critical) flow

Steam flow through a valve increases with pressure drop only until the downstream pressure falls to about 58% of the absolute inlet pressure. Below that, the flow chokes and the calculator applies the correct critical-flow equation.

Steam Valve Sizing FAQs

How do you size a steam control valve?
Determine the required flow coefficient Kv (or Cv) from the steam mass flow and pressure drop, then select a valve whose rated Kvs exceeds the required Kv. For saturated steam: subcritical flow uses m = 12 x Kv x sqrt(dP x P2) and critical flow uses m = 12 x Kv x P1 — pressures in bar absolute, flow in kg/h.
What is choked or critical flow in a steam valve?
Choked flow occurs when the downstream pressure drops below about 58% of the absolute inlet pressure. At this point the steam velocity at the valve throat reaches sonic speed and further reducing outlet pressure cannot increase flow. The calculator detects this automatically.
What is the difference between Cv and Kv?
Kv is the metric flow coefficient (m3/h of water at 1 bar pressure drop) and Cv is the imperial equivalent (US gallons per minute at 1 psi drop). The conversion is Cv = 1.1561 x Kv.
What is Kvs?
Kvs is the fully-open rated flow coefficient of a valve as given by the manufacturer. Your required Kv must be below the Kvs, and ideally the valve should operate between 20–80% of Kvs for stable control.
Why must steam pressures be absolute?
The sizing equations use absolute pressures because they relate to the physical state of the steam. Gauge pressure is relative to atmosphere; tick the gauge checkbox and the calculator adds 1.013 bar (1 atm) automatically.