The American Petroleum Institute API 6D Standard – Double-Block-and-Bleed Ball Valves
May 21, 2024
API 6D details three double block categories: DBB (double-block-and-bleed), DIB-1 (Double-isolation-and-bleed type 1), and DIB-2 (Double-isolation-and-bleed type 2).
Trunnion Mounted Ball Valve Seat Designs
Trunnion mounted API 6D ball valves have 2 seats (one upstream, one downstream). They can have single (SPE) or double piston effect (DPE) seats.
Single Piston Effect (PSE)
Spring loaded seats (uni-directional) are self relieving seats and will seal where the upstream pressure is greater than the cavity pressure. Working automatically to relieve pressure up stream (first seat) and downstream (2nd seat). Seat springs settings are up to 30% over maximum allowable cold working pressure rating of the valve, as per API 6D-2021 Clause 5.5.
Double Piston Effect (DPE)
Seats (bi-directional) will provide sealing regardless of weather the cavity upstream pressure is higher. Configuration for DIB valves can be with one (DIB-2) or two (DIB-1) bi-directional seats. A DIB-1 valve provides double isolation from pressure at both ends of the valve, but requires an external relief system to relieve pressure.
A trunnion ball valve can be specified in three different arrangements:
Two SPE’s seats (DBB)
Two DPE’s seats (DIB-1)
One SPE seat and One DPE seat (DIB-2)
These arrangements are classified by API 6D as follows: Double-block-and-bleed (DBB), Double-isolation-and-bleed type 1 (DIB-1), and Double-isolation-and-bleed type 2 (DIB-2).
Double-Block-and-Bleed (DBB)
The majority of installations referred to supply DBB as the default standard.
According to the API 6D-2021 definition, Section 3.1.15 defines a double-block-and-bleed valve as a: “Valve with two or more sealing surfaces that, in the closed position, provides a seal against pressures from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces.”
The valve is equipped with two SPE self relieving seats. A DBB valve seals from either direction with a bleed between the seats. If the first seal leaks, the second will not seal in the same direction and it does not require an external pressure relief valve. A DBB valve provides sealing in two different directions, each with a separate seal. However, if the first seal leaks, the second seal will not seal in that same direction. The user can request the fitment and testing of an optional cavity drain valve.
For additional safety, tandem twin trunnion ball or twin plug valves are often specified which still have a relatively compact modular design.
Double-Isolation-and-Bleed (DIB)
API 6D-2021 Section 3.1.16 defines a double-isolation-and-bleed valves as a: “Valve with two or more 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 sealing surfaces. This feature can be provided in one direction or in both directions.
The difference is that a double-block-and-bleed valve seals against pressures from both sides of the valve whereas a double-isolation-and-bleed valve provides an additional seal against pressure building in the valve’s cavity.
The DIB-1 configuration has two bi-directional seats one upstream and one downstream. The DIB-2 ball valve design features one bi-directional (DPE) seat and one uni-directional seat (SPE).
DIB-1 valves have DPE seats both upstream and downstream to provide a seal in both directions.
External safety relief valves avoid over pressurisation of the cavity when using the DIB-1 configuration. With the ball in the closed position and upstream side pressure, the cavity pressure will increase in case of upstream seat failure. The cavity pressure will cause a double piston effect on the downstream seat creating a second seal on the ball.
DIB-2 configuration is one seat SPE and the other DPE. An external relief valve is not a requirement because of its bi-direction. The upstream seat will relieve pressure back upstream whilst the uni-directional downstream seat will not allow pressure relief down stream. When compared to DBB, this is the main advantage.
A mark is made on the flange edge, indicating the installation position on the DIB-2.
For more information on double ball & plug/butterfly double block & bleed/double isolation & bleed CLICK HERE
Types of Double Block & Bleed/Double Isolation & Bleed valves including twin ball/plug, slab gate, trunnion & floating ball, plug, expanding plug & expanding gate CLICK HERE
