One of the most critical automatic safety devices in a steam system is the safety valve. It protects lives, equipment and property from potentially dangerous levels of temperature and forces caused by excessive steam system pressure.
Codes and insurers require safety valves be part of the steam system (Figure 1). Therefore, it’s important to have well documented up-to-date records for the plant’s steam system safety valves. Develop a database containing the facility’s relevant safety device information using readily available technology. Update the database periodically, depending on plant standards, insurance company recommendations, and the local, state, or federal government requirements.
The code that establishes the requirements for safety valves is governed by the American Society of Mechanical Engineers (ASME). Through its committees, ASME has published and continues to update the Boiler and Pressure Vessel codes for safety valves. It’s the responsibility of plant personnel, primarily the steam team, to know which codes apply to the different parts of the steam system. In the United States, major considerations for safety valves are proper sizing and correct installation.
Sizing guidelines for safety valves
A partial listing of sizing and installation highlights is listed below:
- The safety valve set point should provide a differential between operating and set steam pressures of at least 20%.
- The total steam capacity of a safety valve downstream of a steam pressure control valve at the set point must exceed the steam control valve’s maximum steam flow capacity (largest orifice available from the manufacturer) if the steam valve fails open.
- Do not oversize a safety valve. A larger safety valve is not better in this case because a larger-than-required valve could experience chatter, leakage and premature failure.
- Many times, a single safety valve isn’t possible because of high steam capacity requirements, physical limitations or economic considerations. An acceptable alternative is to use multiple safety valves on the same steam system. The safety valves should use the same set point and the steam flow capacities must be equal or exceed the steam flow into the equipment or steam system piping. Additionally, size the vent pipe to account for the steam venting capacity of all the safety valves fully opening at the same time.
- The safety valve set pressure should be at or below the maximum allowable working pressure (MAWP) of the steam component with the lowest set point in the system. This includes, but isn’t limited to, steam boilers, pressure vessels, equipment and piping systems. In other words, if two steam components on the same system are rated at different pressures, set the safety device protecting both of these devices at the lower of the two ratings.
This is the second key element of successful safety valve operation. Some guidelines to consider include:
The steam system must be clean and free of any dirt or sediment before commissioning the steam system with a safety valve.
Figure 2. A proper safety valve installation with a vertical orientation, drip pan elbow and drain lines off the safety valve and drip pan.
Figure 3. This safety valve is not installed correctly because it’s not oriented vertically. Safety valves are tested and set at the manufacturer or assembler; if mounted off of the vertical position, this could or will change the set point.
- Mount the safety valve vertically with the valve’s spindle in the vertical position above the pipe centerline (Figures 2 and 3). A safety valve not mounted in the vertical position can influence the set pressure of the valve.
- The inlet steam piping to the safety valve must be equal to or larger than the safety valve inlet connection.
- Never install intervening shut-off valves between the safety valve inlet and the protected steam component or steam system because this configuration could isolate the safety valve from the system.
- Never plug or cap safety valve drains and vent openings. The openings provide a function on the safety valve.
- Safety valves are set, sealed and certified to prevent tampering. If the wire seal is broken, the valve is unsafe and shouldn’t be used. Contact the supplier immediately.
- If you have multiple safety valves using a single connection, make sure the internal cross-sectional area of the inlet is equal to the combined inlet areas of all the safety valves.
- Install a drip pan elbow on the outlet from all safety valves. The drip pan elbow changes the outlet of the safety device from horizontal to vertical. Drip pan elbow installation has its own guidelines.
- Never attach the vent discharge piping directly to the safety valve. This could place undue stress and weight on the valve body and change the set pressure of the safety valve.
- Don’t let the safety valve vent pipe touch the drip pan elbow.
- The drains on the drip pan elbows direct condensed vapor and rain safely away to the drain. Don’t plug these openings.
- Steam won’t escape from the drip pan elbow if the vent line is sized correctly.
Figure 4. The vent pipe size must be at least as large as the valve outlet to prevent excessive backpressure from affecting the valve performance.
The guidelines that apply to vents are: