DIB & DBB BALL VALVES

It has been a common misunderstanding in the industry that DBB valve provides double isolation barriers, infact it does not. At times designers specify DBB valves where infact they should be refering to DIB valve. To clarify this confusion, API 6D defines the DBB valve as “Single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces”. However, DIB valve is defined as “Single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces.”
DBB ball valves have SPE seats on the upstream and the downstream side. SPE seats will provide a seal in one direction only. Consider the case when the valve is fully closed, the single piston effect at the upstream side actually pushes the valve seat against the ball to provide the seal. In the case of seat wear or seat leakage, the cavity pressure will increase. Excess body cavity pressure will push the downstream seat ring away from the ball creating a gap, and relieving overpressure from the body cavity. It will make the valve seat on the downstream side to leak prior to the upstream seat leakage. The upstream side is pressurized while the downstream side has no or minimum pressure and there would be no force to keep the contact between the ball and the seat at the downstream side. DBB ball valves are not recommended if a true isolation is required for operational safety during shutdown.
DIB ball valves have two different configurations called as DIB-1 and DIB-2. DIB-1 ball valves have both upstream and downstream seats as DPE. However, in case of DIB-2 ball valve, one seat is DPE and the other one is SPE.
DIB-1 ball valves have DPE seats and both upstream and downstream seats will provide a seal in both directions. The cavity pressure cannot release to the upstream or downstream side and similarly, upstream or downstream pressure cannot cause the cavity pressure to increase. With the ball in the closed position and pressure on the upstream side, the cavity pressure will increase in case of upstream seat failure. The cavity pressure will cause a double piston effect on the downstream seat and thereby creating a second seal on the ball. This kind of configuration should have a body cavity relief valve that should relieve the cavity pressure during over-pressurization due to seat leakage.
The main advantage of DBB valve is that it has self-relieving seats. However, if the upstream seat leak the downstream seat will not provide isolation. On the other hand, the main benefit of the DIB is that even if the upstream seat leak, the second seat will provide the seal and the valve will not leak. It should be noted that DIB valve should have body cavity relief valves. If we want to combine positive features of both DIB-1 and DBB valve then we should use DIB-2 valve that has one SPE seat at the upstream and DPE seat at the downstream. If the upstream seat leak in DIB-2, then the cavity pressure can relieve on the upstream side and the downstream seat will provide isolation due to double piston effect. It should be noted that the installation of DIB-2 valve is critical and SPE seat should be on the upstream/pressurized side.

[@Bruno-qx8nh]

Very well explained sir, thank you!

[@ProductProcess]

Thank You

[@heshammatoug4860]

a question, please
we need to install API 608 ball valve in one of the applications, but the supplier just offered an API 6d, is that acceptable?

[@ProductProcess]

Generally acceptable. But, it generally depends on application and what selection criteria you have. I can help you further if you provide me service and application.

[@ProductProcess]

In general, the API 6D ball valve is primarily used in oil and gas industry pipeline systems, including long-distance oil and gas pipelines including ASME B31.4 and B31.8, with a diameter range of NPS (4 ~ 60) and pressure levels of 150, 300, 400, 600, 900, 1500,2500. Generally fixed ball structure, sealed at the inlet. API 608 ball valves are used in petroleum, petrochemical and industrial applications, mainly for ASME B31.3 process pipeline, diameter range NPS (1/4 ~ 24), small diameter, pressure class 150, 300, 600, 800 pounds, generally floating ball structure, sealed at the outlet.