Safety relief valves, also known as pressure safety valves, are the last line of defense to protect equipment and personnel from overpressure. They are installed to ensure safety, prevent equipment damage from overpressure, and conserve material. Overpressure can be caused by events like closed outlets, reflux failure, valve malfunctions, or loss of power. A pressure safety valve has a cross sectional view showing how it opens at a set pressure to relieve overpressure. Equations are provided to calculate the minimum flow area required based on the mass flow and properties of liquids or vapors and gases being relieved. Standards like API 520 and 521 provide more information on sizing and rating pressure safety valves.

1. PRESSURE SAFETY
VALVE
FEATURE & SIZING
Pressure Safety Valve

2. • Safety Relief Valves Are Last Line Of Defense To
Protect Personnel & Equipment From The
Consequences Of Energy & Mass Accumulation
Greater Than What Is Allowed By Design Limits
Pressure Safety Valve

3. WHY SAFETY VALVE
INSTALLED ?
• Safety Of Operating Personnel With Respect To
Overpressure
• Prevention Of Destruction Of Capital Investment Due To
Overpressure
• Conservation Of Material Loss During & After An Upset
Which Produces Temporary Overpressure
• Prevention Of Damage To Downstream Equipment Due
To Overpressure Being Transmitted Through Connecting
Equipment & Piping
Pressure Safety Valve

4. NOMENCLATURE
• Relief Valves
• Safety Valves
• Conventional Pressure Relief Valve
• Balanced Pressure Relief Valve
• Pilot Operated Pressure Relief Valve
• Accumulation
• Overpressure
• Set Pressure
• Cold Differential Test Pressure
• Back Pressure
• Blow Down
Pressure Safety Valve

5. CAUSES OF OVERPRESSURE
• Closed Outlet On Vessel
• Reflux Failure
• Inadvertent Valve Opening
• Check Valve Malfunction
• Utility Failure
• Electrical Or Mechanical Failure
• Loss Of Instrument Air Or Electric Power
• Abnormal Heat Input From Reboiler
• Heat Exchanger Tube Failure
• External Fire
• Hydraulic Expansion
Pressure Safety Valve

6. CROSS SECTIONAL VIEW OF
PSV
Pressure Safety Valve

7. RATING OF FLOW AREA
• FOR LIQUIDS , AO=
0.6211 𝑚
𝑎 𝑤 𝜌∆𝑃
• FOR VAPOUR & GASES, AO=
0.1791 𝑚
𝜑 𝑎 𝑤 𝑃 𝑅
𝑇 𝑍
𝑀
• Where,
• AO – Minimum Cross Sectional Flow Area (mm2)
• m – Mass Flow To Be Discharge (kg/hr)
• aw – Outflow Coefficient(Specified By Manufacture)
• ∆𝑃 − Absolute Relieving Pressre – Absolute Backpressure
• 𝜑 – Outflow Function
• PR – Absolute Relieving Pressure (BAR A)
• T- Inlet Temperature(k)
• Z – Compressibility Factor
• M –Molar Mass (Kg/Kmol)
Pressure Safety Valve

8. • FOR MORE INFORMATION ABOUT PRESSURE
SAFETY VALVE REFERRED API 520 & API 521
Pressure Safety Valve

9. • THANK YOU
Pressure Safety Valve