When we use API 6D to design ball valves, the most important part is to design the valve seat. Different valve seat will have valve perform different functions. In API 6D we have two different types of valve seats called as Single Piston Effect (SPE) and Double Piston Effect (DPE) seats. Using these two kinds of valve seats, we can come up with two different functions from the ball valves commonly called Double Block and Bleed (DBB) and Double Isolation and Bleed (DIB).
The double block and bleed ball valve (DBB ball valve) is made up of two ordinary ball valves linked by a common inlet end. The double block and bleed ball valve is a product intended to substitute several connections between conventional valve. The distinction between DBB ball valve and the conventional one is that it can effectively overcome leakage that happens during system operation, and it can be closed quickly and discharge easily. This DBB ball valve is a modern form of valve that is commonly used on the market today.
DBB is defined by API 6D as a “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.” When just one side is under pressure, this valve does not have positive double isolation.1. DBB Explained
And OSHA defines DBB as “the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves”.
(1) Spilt Body DBB Ball Valve
Modular or Split Body Type DBB ball valve – The main feature of this design is that it has two balls in one valve to ensure greater safety. A drain needle valve is located between the two balls on the Double Block & Bleed valve. This allows the valve to be drained and the pressure to be reset to zero. Instrumentation and piping all benefit from the modular form, three-piece body construction. Instrumentation valves range in size from 1 inch to 3 inches; piping valves range in size from 1 inch to 24 inches. API 3.000 to API 10.000 in all pressure class scores from 150 to 2500 lbs.
(2) Integral Body DBB Ball Valve
Integral DBB ball valves – These valves are used in the instrumentation industry and come in a variety of designs to meet a variety of needs, including flange to flange, flange to the thread, injection, and sampling, and ball and needle types. A drain needle valve is located between the two balls on the Integral Double Block & Bleed ball valve. This allows the valve to be drained and the pressure to be reset to zero.
3.Benefits of DBB ball valve
Double Block and Bleed Ball valves have been developed to substitute the process of bolting the individual valves together to provide dual insulation.
This new model provides excellent savings in weight, space, and installation time, particularly for insulation in the instrument or instrument cage. These weight savings will amount to up to 60% and tests have shown that a time savings of 70% on installation is also possible. The biggest benefits must be seen, however, in reducing the leakage pathways into the environment, thus reducing the possibility of possible dangers.
DIB Ball Valve
1. DIB Explained
DIB Ball Valve provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the sealing surfaces.
DIB means double isolation and bleed. Single ball valve with two seating surfaces, with two sealing 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.DIB ball valve definition in API 6D:
(1) DIB-1 Ball Valve
Double Piston Seat x Double Piston Seat
For the DIB Double Piston x Double Piston (DIB-1)configuration, over pressurization of cavity is external with a safety valve.
①Upstream and downstream sealing.
③Two seats are bidirectional sealing seats.
Each seat can isolate the fluid from upstream and downstream, if one seat is damaged, the other seat still work to ensure the sealing performance.
For the DIB Self relieving x double piston (DIB-2) configuration, cavity over pressurization is internally controlled within the line.
Upstream and downstream sealing; pressure energized; one seat is bidirectional seat and one seat is unidirectional sealing seat.
For DIB-2 Ball Valve, the fluid can be relieved from one seat to downstream side.
Self-relieving seat, no need to assembly the safety valve for pressure relieving.
Both DIB-1 and DIB-2 are for trunnion mounted ball valve structure.
The double block and bleed valve protects against pressure from either side of the valve while the double isolation and bleed valve offers additional seal against pressure from just one side. It is necessary to use the DIB ball valve instead of the DBB ball valve in applications requiring an external pressure barrier to be sealed separately from the main pressure barrier. This is critical to meet such operational safety standards or depending on the type of the operation, such as low leakage tolerance of fluid cleanliness.
There are normally two unidirectional self-relieving seats with a DBB ball valve. These seats do not depend on an external pressure relief system. On the other hand, the DIB ball valve uses one or two bi-directional seats. The valve allows dual isolation from the pressure at both ends of the valve, but cannot ease the pressure of the body cavity past the seats. DIB ball valves need an external relief mechanism to ease the build-up of pressure.
Both valves could protect in both the upstream and downstream directions, even in high-pressure or high-temperature conditions. Isolation is critical in situations where leakage through the valve can have significant implications. Once the fluid is sealed, the bleed system may drain the area between the two valves or two seating surfaces. This is critical for maintenance and integrity checks in cases where leakage can be tracked.
The main advantage of DBB ball valve is that it has self-relieving seats. However, if the upstream seat leak, the downstream seat will not provide isolation. While 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 ball valve should have body cavity relief valves.
If we combine positive features of both DIB-1 and DBB ball 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.