Safety Valve and PRV (ASME/API 526)

A safety valve protects people, the environment, and other plant assets from overpressure that may cause explosions or other damaging events. A safety valve opens automatically anytime a set pressure is exceeded. Safety valves are last-resource devices that intervene when other protection devices have failed. Safety valves may be spring or pilot operated.

WHAT IS A SAFETY VALVE?

A safety valve opens automatically and at once anytime the pressure of the fluid passing through the valve goes above a set-pressure (in bars or psi).

When an overpressure is detected, the disc of the valve opens so it can discharge the fluid restore normal pressure conditions in the piping system.

As the fluid pressure decreases below a set value, the valve closes back again. These movements are mechanical, i.e. there are no actuators or external devices to the valve the execute the open/close operations.

Safety valves may be installed on pressure vessels, boilers, gas storage tanks and in piping systems.

The standards for safety valves are API 526 or ASME.

SAFETY VALVE VS. PRESSURE RELIEF VALVE

Sometimes, the terms safety valve and pressure relief valve are used as synonyms.

However, differences exist between safety vs pressure relief valves:

• A safety valve opens fully, and suddenly, as the set overpressure limit is reached by the equipment (vessel, boiler). Safety valves relieve the pressure without the assistance of a controller and an operator.

• A pressure relief valve (called also, “PRV”) is a pressure release device mounted on a vessel filled with a fluid. The opening of the disc of a PRV valve is proportional to the increase in the vessel pressure (i.e., it’s gradual) and not sudden (like in a safety valve). Pressure relief valves generally operate through the combination of a pressure controller and an actuating mechanism (which opens the valve disc proportionally to the overpressure signal received by the controller).

CAUSES OF VESSEL OVERPRESSURE

The pressure in a vessel or a piping system can exceed a threshold limit due to multiple reasons. The API 521 specification illustrates the typical root causes:

• Blocked discharge
• Chemical reaction
• Tubes and other devices rupture
• Fire case (the equipment is exposed to external fire)
• Thermal expansion
• Cooling system failure

Each of these adverse events may occur separately or simultaneously and can create different mass or volume of flow to be discharged, e.g. small mass flow for thermal expansion and large mass flow in case of a chemical reaction.

The engineering team shall determine the worst-case scenario for the sizing and selection of a suitable pressure relief device, considering the above factors.

SAFETY VALVE TYPES

There are three main types of relief valves:

SPRING LOADED SAFETY VALVE

This is the most common type: the load of the spring is designed to press the “Disc” against the inlet pressure. As the pressure exceeds a set value, the spring opens releasing the disc and the fluid flowing through the valve.

Depending on the fluid type, such as steam, gas or liquid, bellows can be added to the spring to clear the back pressure effect.

 

The image shows the action of the spring on keeping the disc on the seat and have a close or open position for the valve.

For more information, consult the Leser’s guide to spring operated safety valves.

DEAD-WEIGHT SAFETY VALVE

Dead weight valves do not have a spring and are used for low-pressure vessels.

This type of safety valve adjusts the pressure by the action of the disc weight (when the pressure increases, the disc is lifted and the valve discharges the overpressure).

PILOT OPERATED

Pilot-operated valves are composed of a “pilot valve” and a “main valve”.

Differently from spring operated valves, the pressure is monitored and adjusted by the pilot valve and not by the main valve.

This type is used for large size valve and high-pressure applications, that require stronger torque and reliability.

Pressure relief valves are available in most material grades from carbon steel to stainless steel and can manage overpressure of any fluid (hydrocarbons, gas, steam or water).

For each type of valve, hundreds of possible configurations are actually available (based on the combinations of parameters as valve type, sub-type, bore size, pressure rating, manufacturing norm, body and trim materials, valve operation, etc).

For more information, consult the Leser’s guide to pilot operated safety valves.