Balanced Bellows Safety Valves Under Backpressure, Reliability and Certified Performance

Balanced bellows safety valves are widely used in industrial applications where backpressure can affect the stability and performance of a conventional spring loaded safety valve. Their main function is to minimize the influence of superimposed backpressure on the valve set pressure, ensuring reliable operation in critical services such as oil & gas, power generation, petrochemical and process industries.

The reliability of these valves is strongly connected to the integrity of the bellows element, which is continuously exposed to pressure, temperature variations and mechanical stress. While balanced bellows technology improves pressure stability, it also introduces additional technical considerations compared to conventional safety valves.

An important aspect highlighted by the latest edition of EN ISO 4126-1 is that current product standards no longer clearly define the valve performance requirements in case of bellows failure. Previous editions required the safety valve to maintain certified relieving capacity without exceeding allowable pressure limits even after bellows rupture, but this requirement is no longer explicitly included in the current standard.

This means that users and EPC contractors should carefully evaluate the real performance of a balanced bellows safety valve under backpressure conditions, especially verifying whether the declared capacities have been validated by actual testing and third-party certification. In many cases, valve capacities are certified only under atmospheric backpressure conditions, while real plant operating conditions may be significantly different.

For this reason, the certified backpressure correction factor becomes a key parameter in valve selection and sizing. Using safety valves with tested and certified backpressure performance helps guarantee accurate relieving capacity, stable set pressure and compliance with PED, API 520 and ASME Section VIII requirements. TAI MILANO, in particular, has certified backpressure correction factors for all the conditions.

Additional attention should also be given to installation conditions. The bonnet vent of a bellows valve must remain at atmospheric pressure to maintain proper balancing action. In cold environments, moisture condensation and freezing inside the bonnet may create serious operational risks, potentially locking the valve in the closed position, a condition already addressed by API 520 recommendations.

To further improve reliability, advanced solutions such as balanced bellows safety valves with auxiliary piston systems can maintain balancing capability even in case of bellows damage, increasing operational safety in severe service applications.

TAI MILANO manufactures high-performance safety valves, pressure relief valves and balanced bellows safety valves engineered for critical process conditions, with focus on certified performance, reliability under backpressure and full compliance with international standards.