Please give the formula for pressure setting of steam safety valve of boiler?

Formula for Pressure Setting of Steam Safety Valve in Boilers

Safety valves are one of the most critical components of a boiler. They act as the last line of defense against overpressure, ensuring safe operation and protecting both equipment and personnel. The Indian Boiler Regulations (IBR) mandate strict guidelines for the design, sizing, and pressure setting of steam safety valves.  

In this blog, we’ll explore the formula for calculating the area of safety valves, the importance of valve sizing, and the rules for pressure resetting.  

🔍 Safety Valve Requirements

- Every boiler must be fitted with at least two safety valves.  

- These valves may be mounted on a common manifold with a single connection to the boiler.  

- The minimum valve size must not be less than 38 mm in diameter.  

- The aggregate area of the valves is calculated using a standard formula.  

📐 Formula for Safety Valve Area

The formula for calculating the required safety valve area is:  

C * A * P = 9.81 * H * E

Where:  

- H = Total heating surface (m²)  

- E = Evaporation rate (kg steam per m² of heating surface per hour)  

- P = Working pressure of safety valves (MN/m² absolute)  

- A = Aggregate area through the seating of the valves (mm²)  

- C = Discharge coefficient (depends on valve type)  

⚙️ Discharge Coefficient (C Values)

The discharge coefficient varies depending on the type of valve:  

🌍 Explanation of Parameters

- Heating Surface (H): The total area exposed to heat transfer. Larger heating surfaces generate more steam, requiring bigger safety valves.  

- Evaporation Rate (E): The amount of steam produced per unit heating surface per hour. Higher evaporation rates increase steam flow, demanding larger valve capacity.  

- Working Pressure (P): The absolute pressure at which the boiler operates. Safety valves must be sized to relieve steam at this pressure.  

- Valve Area (A): The effective opening area of the valve seat. This ensures sufficient steam discharge during overpressure conditions.  

- Discharge Coefficient (C): A correction factor based on valve design. Advanced valves have higher coefficients, allowing more steam discharge through smaller openings.  

⚖️ Pressure Resetting of Safety Valves

Safety valves must be reset carefully to ensure reliable operation:  

- The valve should be reset at a pressure at least 2.5% below the set pressure.  

- It must not be reset at more than 5% below the set pressure.  

This ensures that the valve reseats properly after lifting, preventing continuous steam leakage while maintaining safety.  

📌 Components Linked with Safety Valves

1. Tonnage – Refers to the steam-generating capacity of the boiler. Larger boilers require bigger or multiple safety valves.  

2. Pressure – The operating pressure of the boiler determines the valve setting and sizing.  

✅ Conclusion

The formula for steam safety valve sizing and pressure setting ensures that boilers operate safely under varying loads and conditions. By considering heating surface, evaporation rate, working pressure, and valve type, engineers can calculate the correct valve area and discharge capacity.  

Adhering to IBR guidelines not only ensures compliance but also protects equipment, reduces risk, and enhances operational reliability.