Learn More > Safety Valve Sizing

Safety Valve Sizing

Considerations about sizing a safety valve

In any pressure system, the proper sizing of safety valves is crucial to prevent overpressure incidents. Safety valves must be positioned, set, and sized accurately to ensure that the pressure within the system remains within safe limits. This is essential to avoid potential hazards associated with exceeding the maximum allowable accumulated pressure (MAAP).

Determining the required discharge capacity of a safety valve involves considering the potential flow through all relevant branches upstream of the valve. When multiple flow paths exist, sizing the safety valve becomes more complex. Two main approaches are commonly used:

  • Sizing Based on Maximum Flow: In this method, the safety valve is sized according to the maximum flow experienced in the flow path with the highest flow rate. This ensures that the valve can handle the most significant potential scenario.
  • Combined Flow Sizing: Alternatively, the safety valve can be sized to discharge the combined flow from all flow paths. This approach is necessary when there's a risk of multiple devices failing simultaneously. The valve must accommodate the total flow from all failed devices to prevent overpressure.

The choice between these methods depends on factors such as the level of risk and cost considerations. If the risk of simultaneous device failure is low, sizing the valve based on the highest fault flow may suffice. However, if there's any possibility of multiple failures, the valve must be sized to handle the combined flow.

Ultimately, the responsibility for determining the sizing method lies with the company overseeing the plant, considering safety standards and risk assessments. By ensuring that safety valves are correctly sized and set, the risk of overpressure events can be minimized, enhancing overall system safety and reliability.

Choosing the correct sizing method and data

To ascertain the flow rate during a fault scenario through a Pressure Relief Valve (PRV), or any valve or orifice for that matter, several factors must be taken into account:


There are various scenarios to consider when determining the required discharge capacity of a safety valve. Here are the main events to take into account:

  • Failure of Pressure Controller or Regulator: The fault flow is determined by the manufacturer of this component.
  • Plant Exposed to Fire: Sizing methods outlined in API 521 and ISPESL Collection E can help define the fault flow in case of external fire leading to plant overpressure.
  • Failure of Cooling System or Ambient Temperature Changes: Standards such as EN13136 and ASHRAE 15 assist in this sizing method.
  • Ambient Temperature Changes Leading to Fluid Expansion: API 520, ASME VIII Div.1, and ASME XIII commonly recommend selecting a safety valve of size ¾” x 1”.
  • Increase in Flow Rate Due to Improperly Closed or Opened Isolation Valves: The fault flow should be evaluated as the maximum flow passing through the pipe or component protected by the safety valve.

Safety valve positioning and install

To ensure that the maximum allowable accumulation pressure of any system or apparatus protected by a safety valve is never exceeded, careful consideration of the safety valve’s position in the system is crucial.

When supplying one piece of apparatus with a pressure relief valve (PRV) where the maximum allowable working pressure (MAWP) is less than the PRV supply pressure, it's essential to fit the apparatus with a safety valve.

If a PRV is supplying multiple apparatus and any item has a MAWP lower than the PRV supply pressure, there are two options: either fit the PRV station with a safety valve set at the lowest possible MAWP of the connected apparatus, or equip each affected apparatus with its safety valve.

The safety valve's location must prevent pressure accumulation in the apparatus via other routes, such as separate fluid lines or bypass lines. This ensures that the system remains within the designated pressure limits and operates safely.

Click here for more info about safety valve maintenance and malfunction reason