STUDY OF BI-DIRECTIONAL FLOW IN METAL SEATED BUTTERFLY VALVE

BI-DIRECTIONAL FLOW IN METAL SEATED BUTTERFLY 

VALVE

The triple offset metal seated valves have an innovative arrangement of providing offsets in the axes of the shaft, the seating and the disc rotation. Because of the unique proprietary arrangement, the valve is able to give perfect sealing in metallic configuration. These types of valves are governed by API 609 as Category B valves because of the offset configuration.

Due to the offsets, the fluid media and the acting pressure differential generated dynamic forces on the valve disc, which are opposite to each other in preferred and non-preferred direction.

Identifying this aspect, the testing standard API 598 also identifies these valves suitably.

Clause no.6.4.3 paragraph 4 of API 598 states - “For Butterfly Valves with a preferred flow direction, the closure test in the non preferred direction shall be based on the reduced differential pressure rating in that direction.”

This is also applicable since these valves are used in uni-directional service most often, and the operating pressures are always much lower than the design pressure – which is complied to in the forward direction.

Accordingly, these valves are provided full rated in the preferred direction, and in the non-preferred direction at 40% of valve rating. In smaller sizes it may be higher due to less dynamic forces on the valve disc.

Given below is self explanatory.

Metal Seated butterfly valve Dics in ON-OFF position.