Home / Steam & combustion

Steam pipe sizing and pressure reducing valve

Enter steam flow, pressure and condition to find the commercial pipe size that respects the allowable velocity and pressure drop. Then size the PRV Cv/Kv and the outlet pipe, which must be larger because the steam expands.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Steam
Upstream line

Maximum velocity changes automatically with the steam condition (100 ft/s saturated, 160 ft/s superheated); adjust to your practice.

Pressure reducing valve

If downstream equipment cannot withstand upstream pressure, install a safety relief valve after the PRV, sized for the PRV’s full-open capacity.

—Recommended steam line size
—

Velocity and pressure drop by size

Bars: velocity at the line inlet. Line: pressure drop over the total length (actual + equivalent). Green: meets both criteria.

Tap the chart to see the values.

Required PRV Kv versus load

Kv per IEC 60534-2-1 from 10% to 120% of the flow entered, compared with the selected commercial Kvs.

Tap the chart to see the values.

Commercial pipe sizes (ASME B36.10M)

Next step

Take this page’s results to another calculator with the fields already filled in.

Steam properties at this condition →Cost of this steam →

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Volume flow: Q = ṁ ÷ ρ; velocity: v = Q ÷ (π · ID² ÷ 4)
Darcy–Weisbach: Δp = f · (L + Leq) ÷ ID · ρm · v² ÷ 2, with ρm at mean pressure (iterated)
Colebrook: 1 ÷ √f = −2 · log10(ε ÷ (3.7 · ID) + 2.51 ÷ (Re · √f)); Re = ρ · v · ID ÷ μ
PRV (IEC 60534-2-1): Kv = W ÷ (3.16 · Y · √(x · p1 · ρ1)), W in kg/h, p1 in kPa(a); x = Δp ÷ p1 ≤ Fγ · xT; Y = 1 − x ÷ (3 · Fγ · xT); Fγ = γ ÷ 1.4; γ = 1.33; Cv = 1.156 · Kv
Cross-check (Spirax Sarco, steam): Kv = ṁ · (1 + 0.00126 · ΔTsup) ÷ (12 · P1 · √(1 − 5.67 · (0.42 − χ)²)), ṁ in kg/h, P1 in bar(a), χ = Δp ÷ P1; for χ ≥ 0.42 the root equals 1
Downstream: isenthalpic expansion h2 = h1 at p2 (IAPWS-IF97) → T2 and ρ2; pipe by velocity criterion

Assumptions and limits

  • Dry saturated or superheated steam; properties from IAPWS-IF97. Viscosity approximated by μ = (8.04 + 0.0407 · T[°C]) × 10⁻⁶ Pa·s, within ±3% of IAPWS 2008 below 290 psia.
  • Carbon steel pipe per ASME B36.10M; velocity taken at the line inlet. The drop is computed as incompressible flow with mean density, valid for drops up to about 10% of absolute pressure.
  • PRV: no reducers next to the valve (Fp = 1); default globe-valve xT; suggested Kvs keeps the valve at up to 80% of rated capacity at maximum flow. Check with the manufacturer’s capacity chart and the relief valve sizing.

Standards and technical documents

  • IEC 60534-2-1:2011: Industrial-process control valves — Flow capacity — Sizing equations for fluid flow under installed conditions
  • ASME B36.10M: Welded and Seamless Wrought Steel Pipe
  • ASME B31.1: Power Piping
  • IAPWS R7-97(2012): Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam; IAPWS R12-08: viscosity of ordinary water substance
  • Spirax Sarco: The Steam and Condensate Loop (pipe sizing and control valve sizing for steam)
  • Crane Co.: Flow of Fluids Through Valves, Fittings and Pipe (Technical Paper 410)

U.S. regulations

  • ASME B31.1 Power Piping, as adopted by state boiler and pressure vessel laws (paragraph on pressure reducing valves requires a downstream relief valve when the low-pressure side cannot withstand inlet pressure)
  • OSHA 29 CFR 1910.147: control of hazardous energy (lockout/tagout) for steam line work
  • ASME B31.3 Process Piping, where the owner specifies it

Frequently asked questions

What is a good steam velocity in pipes?

For saturated steam, 80 to 130 ft/s in mains; above that, noise, droplet erosion and pressure drop rise. For superheated steam, 165 to 230 ft/s. With 6,000 lb/h at 150 psig, a 4 in Schedule 40 line runs at about 52 ft/s.

Why is the pipe after the PRV larger?

Steam expands as pressure falls. From 150 to 60 psig, density drops from about 0.36 to 0.16 lb/ft³, so the same flow takes more than twice the volume and needs a larger pipe to keep velocity in range.

What is choked flow in a PRV?

It is when the drop is so large that flow no longer increases as outlet pressure falls. Per IEC 60534, with xT = 0.72 and steam (γ = 1.33), it happens when the drop exceeds about 68% of absolute inlet pressure.

Cv or Kv?

Cv is water flow in US gpm at 1 psi drop; Kv is water flow in m³/h at 1 bar. Cv = 1.156 × Kv. US catalogs list rated Cv at full travel.