4BE3.5 U3 C1 Obj2 Medium

ASME Code Requirements for Safety Valves and Relief Valves

• ASME BPVC, Section IV Rules for Construction of Heating Boilers contain rules for safety valves and safety relief valves installed on heating boilers.

Safety Valve Requirements

• Each steam boiler shall have one or more officially rated safety valves of the spring pop type adjusted and sealed to discharge at a pressure not to exceed 15 psi (100 kPa).
• No safety valve for a steam boiler shall be smaller than NPS ½ (DN 15) or larger than NPS 4 (DN 100).
• The safety valve capacity for each steam boiler shall be such that with the fuel burning equipment installed, and operated at maximum capacity, the pressure cannot rise more than 5 psi (35 kPa) above the maximum allowable working pressure.
• A body drain below seat level shall be provided by the Manufacturer for all safety valves and safety relief valves to prevent the collection of condensate around the valve and seat.

Mounting Safety and Safety Relief Valves on Heating Boilers

• Safety valves and safety relief valves must be installed with their spindles vertical.
• Safety valves must be connected directly to one of the following:
  • A tapped or flanged opening in the boiler
  • A fitting connected to the boiler by a short nipple
  • A y‐base
  • A valveless header connecting steam or water outlets on the same boiler
• Safety valves and safety relief valves must be located in the highest practicable part of the boiler proper.
• The opening or connection between the boiler and any safety valve or safety relief valve must have at least the same cross-sectional flow area of the valve inlet.
• No shut-off valve of any description shall be placed between the safety or safety relief valve and the boiler, or on discharge pipes between these valves and the atmosphere.
• Discharge pipes must be used and must be:
  • As short and straight as possible, and arranged so as to avoid undue stress on the valve
  • Properly drained to prevent collection of water
  • Arranged so that there will be no danger of scalding the operator
• The internal cross-sectional area of the discharge pipe must not be less than the full area of the valve outlet

Specific to Safety Valves (Steam Service)

• The safety valve must never be located below the normal operating water level, or below the lowest permissible water level.
• Low-pressure steam heating boilers are designed to operate at no higher than 100 kPa.
• A safety valve especially designed for low-pressure steam heating boilers is described in the text, with a valve body connected to the boiler and a bonnet with an outlet opening to atmosphere.

Safety Relief Valves for Hot Water Heating Boilers

• Each hot water heating or supply boiler shall have at least one officially rated safety relief valve, of the automatic reseating type, and set to relieve at or below the maximum allowable working pressure of the boiler.
• Safety relief valves shall have pop action when tested by steam.
• The design of the safety relief valve is similar to that of the safety valve used for steam boilers, but with some differences.
• Safety relief valves should not open frequently, or leak water, during normal plant operation. Common causes of frequent safety relief valve operation include:
  • Waterlogged heating system
  • Scale, corrosion products, or chemical residue accumulations on the valve seat
  • Installed safety relief valve designed for the wrong pressure range
  • Damaged seating surfaces or weakened compression springs from erosion, corrosion, and lengthy service duration