Reduced Pressure Zone (RPZ) Valve Sizing Calculator
Determine the appropriate RPZ valve size based on design flow rate, inlet pressure, and system pressure-loss constraints. The calculator uses the valve flow coefficient (Cv) method and standard RPZ pressure-drop characteristics.
Results will appear here.
Formulas Used
1. Required Flow Coefficient (Cv)
Based on the ISA/ANSI S75.01 liquid valve sizing equation:
Cv_required = Q × √(SG / ΔP_dynamic)
Where:
- Q = flow rate (US GPM)
- SG = specific gravity of fluid (water = 1.0)
- ΔP_dynamic = allowable dynamic pressure drop = ΔP_max − ΔP_inherent (psi)
2. RPZ Inherent Pressure Drop
ΔP_inherent ≈ 10 psi
Standard RPZ assemblies (ASSE 1013) have a fixed internal pressure drop of approximately 10 psi regardless of flow, comprising: 1st check valve (~5 psi), differential pressure relief valve (~2 psi), and 2nd check valve (~3 psi).
3. Total Actual Pressure Drop
ΔP_total = ΔP_inherent + (Q / Cv_selected)² × SG
4. Pipe Velocity
V = Q / A [ft/s] A = π × (ID/2)² [ft²] Q converted: GPM ÷ 448.831 = ft³/s
Recommended maximum velocity through an RPZ assembly: 8 ft/s.
Assumptions & References
- RPZ inherent pressure drop of 10 psi is used per ASSE 1013 standard and manufacturer data (Watts Series 009, Febco 825Y, Wilkins 375).
- Cv values are representative assembly Cv values from published manufacturer data; actual values vary by manufacturer and model — always verify with the specific product's submittal sheet.
- The ISA/ANSI S75.01 flow coefficient equation is valid for non-cavitating, turbulent, incompressible liquid flow.
- Minimum inlet pressure for RPZ operation is typically 15–25 psi above the relief valve set point; consult manufacturer specifications.
- Maximum recommended flow velocity through the valve body is 8 ft/s to prevent erosion and noise (per AWWA M14 and manufacturer guidelines).
- Pipe IDs are based on Schedule 40 carbon steel per ASME B36.10M.
- For parallel RPZ installations (flow splitting), divide total flow equally and size each assembly independently.
- References: ASSE 1013 (RPZ Backflow Preventers), AWWA M14 (Backflow Prevention), ISA-75.01.01 (Flow Equations for Sizing Control Valves), USC Foundation for Cross-Connection Control and Hydraulic Research.