ASME Section I Pressure Relief Valves and Devices

Pressure Relief Valves

• The primary function of pressure relief valves is to prevent overpressure in a system.
• There are five types of pressure relief devices defined in ASME PTC-25.

Importance of Accurate Terminology

• Using accurate terms to describe each type of pressure relief valve is crucial to ensure proper understanding and application.

ASME BPVC I Definitions

• According to ASME BPVC I, Part PG-73.1.4, the following definitions apply to pressure relief devices:
  • Accumulation: the pressure increase above the maximum allowable working pressure.
  • Set pressure: the pressure at which a pressure relief valve starts to open.
  • Overpressure: the pressure increase above the maximum allowable working pressure.

Pressure Relief Valve Design and Operation

• Pressure relief valves are designed to actuate by opening in response to an increase in pressure.
• All pressure relief valves have in common the ability to open in response to an increase in pressure.
• PRV is used as an acronym for Pressure Relief Valve in this chapter.

Boiler Pressure Relief Valves

• According to ASME BPVC I, a boiler with more than 500 ft2 of bare tube water-heating surface should have two or more pressure relief valves.
• Cast iron and bronze safety valves are commonly made of bronze or other corrosion-resistant materials.

Safety Valve Operation

• When the steam pressure reaches the set point pressure, the safety valve will lift and relieve the pressure.
• The point at which the safety valve will lift and relieve the pressure can be adjusted using a spring or other adjusting mechanism.
• If the safety valve is opened without pressure in the boiler, it will not seat properly, causing problems.

Huddling Chamber

• A huddling chamber is formed in a safety valve disc to improve the valve's performance and stability.
• The huddling chamber allows the valve to open rapidly and fully, ensuring effective pressure relief.

Superheater and Reheater Pressure Relief Valves

• According to ASME BPVC Section I, Part PG-73, valves with superheaters operating over 230°C must meet specific requirements.
• The set pressure of a superheater safety valve must take into consideration the steam pressure drop from the steam drum to the superheater outlet.
• Superheater and reheater pressure relief valves are required to be made of steel or alloy steel according to ASME BPVC Section I, Part PG-68.
• If a superheater PRV is set to the same pressure as drum PRVs, it can lead to inadequate pressure relief.

Blowdown and Adjustment

• The blowdown of a pressure relief valve is the difference between the set pressure and the reseating pressure.
• If the blowdown adjusting ring completely blocks off the huddling chamber escape ports, the valve will not seat properly.
• The PRV set pressure can be adjusted using a spring or other adjusting mechanism.
• Before adjusting the pressure at which the safety valve opens, the boiler should be shut down and depressurized.

Discharge Pipe

• The purpose of a discharge pipe for a safety valve is to direct the released steam or fluid to a safe location.

Explore the code requirements, construction, and operation of ASME Section I Pressure Relief Valves and Devices. Learn about the different types defined in ASME PTC-25 Pressure Relief Devices and the terminology used to categorize them.