Boiler explosions have been responsible for widespread damage to companies throughout the years, and that’s why today’s boilers are equipped with safety valves and/or relief valves. Boiler safety valves are designed to prevent excess pressure, which is usually responsible for those devastating explosions. That said, to ensure that boiler safety valves are working properly and providing adequate protection, they must meet regulatory specifications and require ongoing maintenance and periodic testing. Without these precautions, malfunctioning safety valves may fail, resulting in potentially disastrous consequences.
How Boiler Safety Valves and Relief Valves Work
Boiler safety valves are activated by upstream pressure. If the pressure exceeds a defined threshold, the valve activates and automatically releases pressure. Typically used for gas or vapor service, boiler safety valves pop fully open once a pressure threshold is reached and remain open until the boiler pressure reaches a pre-defined, safe lower pressure. 
Boiler relief valves serve the same purpose – automatically lowering boiler pressure – but they function a bit differently than safety valves. A relief valve doesn’t open fully when pressure exceeds a defined threshold; instead, it opens gradually when the pressure threshold is exceeded and closes gradually until the lower, safe threshold is reached. Boiler relief valves are typically used for liquid service.
There are also devices known as “safety relief valves” which have the characteristics of both types discussed above. Safety relief valves can be used for either liquid or gas or vapor service.
Boiler Safety Valve Nameplate Requirements
Safety valves require nameplates that contain important information about the device, including:
- Capacity
- Set PSI pressure
- Manufacturer
- Design
- Date assembled
- Date(s) tested
- ASME symbol
Nameplates must be fastened securely and permanently to the safety valve and remain readable throughout the lifespan of the valve, so durability is key.
Boiler Safety Valve Guidance: The National Board of Boiler and Pressure Vessel Inspectors
The National Board of Boiler and Pressure Vessel Inspectors offers guidance and recommendations on boiler and pressure vessel safety rules and regulations. However, most individual states set forth their own rules and regulations, and while they may be similar across states, it’s important to ensure that your boiler safety valves meet all state and local regulatory requirements.
The National Board published NB-131, Recommended Boiler and Pressure Vessel Safety Legislation, and NB-132, Recommended Administrative Boiler and Pressure Vessel Safety Rules and Regulations in order to provide guidance and encourage the development of crucial safety laws in jurisdictions that currently have no laws in place for the “proper construction, installation, inspection, operation, maintenance, alterations, and repairs” necessary to protect workers and the public from dangerous boiler and pressure vessel explosions that may occur without these safeguards in place.
The documents are meant to be used as a guide for developing local laws and regulations and also may be used to update a jurisdiction’s existing requirements. As such, they’re intended to be modifiable to meet any jurisdiction’s local conditions.
Boiler Safety Valve Regulations from ASME
The American Society of Mechanical Engineers (ASME) governs the code that establishes guidelines and requirements for safety valves. Note that it’s up to plant personnel to familiarize themselves with the requirements and understand which parts of the code apply to specific parts of the plant’s steam systems.
Proper sizing and correct installation are the primary concerns for boiler safety valves. Some general guidelines to ensure proper sizing include:
- The differential between operating and set steam pressure should be at least 20%.
- Safety valves should never be oversized; oversized valves can lead to issues such as leaks, chattering, and premature failure.
- Total steam capacity of a safety valve located downstream from a pressure control valve must exceed the control valve’s maximum steam flow capacity.
- High steam capacity requirements, physical or economic constraints may make the use of a single safety valve impossible. In these cases, using multiple safety valves on the same system is considered an acceptable practice, provided that proper sizing and installation requirements are met – including an appropriately sized vent pipe that accounts for the total steam venting capacity of all valves when open at the same time.
- The lowest rating (MAWP or maximum allowable working pressure) should always be used among all safety devices within a system, including boilers, pressure vessels, and equipment piping systems, to determine the safety valve set pressure.
General guidance on proper installation may seem like common sense to experienced installers and inspectors. A few of the most important guidelines and best practices include:
- Always ensure that the total steam system is clean and free of debris before installing any safety valves.
- Safety valves should always be mounted in a vertical position to avoid influencing the valve’s set pressure.
- Avoid isolating safety valves from the system, such as by installing intervening shut-off valves located between the steam component or system and the inlet.
- Don’t plug or cap drains or vent openings, as these features serve a purpose in the effective operation of a safety valve.
- Contact the valve supplier immediately for any safety valve with a broken wire seal, as this indicates that the valve is unsafe for use. Safety valves are sealed and certified in order to prevent tampering that can prevent proper function.
- Avoid attaching vent discharge piping directly to a safety valve, which may place unnecessary weight and additional stress on the valve, altering the set pressure.
Further Reading on Safety Valves, Regulations, and More
For more information on various types of safety valves, boiler components, regulations, and related topics, visit the following resources:
- How Does a Hydraulic Flow Control Valve Work? Function of Hydraulic Flow Control Valves, Types, Design Considerations, and More
- What to Look for When Buying Equipment Nameplates: Environmental Considerations, Durability, and More
- How Does a Boiler Deaerator Work? What’s the Function of a Boiler Deaerator, and How do They Operate?
- How to Select a Valve Actuator: Types of Valve Actuators, Appropriate Sizing, Safety Criteria, and More