Why Are Gate Valves Required to Have a Backseat?

A backseat is a sealing mechanism which can prevent medium leaking into the packing material when the valve is fully open.

The backseat typically realize sealing through the metal-to-metal contact between the valve stem and the bonnet. These sealing surfaces are precision-machined and resistant to wear and corrosion. The backseat only be effective when the valve is fully open, ensuring reliability of sealing .

When a gate valve is in the closed position, the pressure in the body cavity is lower than the working pressure, so the medium pressure on the packing is very small. However, when the valve is open, the packing must withstand the working pressure. Traditional packing seals may degrade over time due to aging, wear, or loosening, leading to leakage. The backseat seal, through direct metal-to-metal contact, provides an additional layer of sealing in case of packing failure, extending the service life of the valve.

Industrial standard API 600, explicitly require that gate valves be designed with a backseat seal to ensure safety and reliability, particularly under high-temperature, high-pressure, or specialized operating conditions.

Another critical function of the backseat seal is to facilitate maintenance and packing replacement. The primary purpose of the backseat seal is to isolate the medium from the packing chamber when the valve is fully open, effectively sealing off the packing chamber. This design allows for the replacement or maintenance of packing without draining the medium from the pipeline, reducing downtime and improving operational efficiency.