DIB Ball Valve

 

The term "Double Block and Bleed" (DBB) has been used to describe the basic function of ball valves and gate valves for a long time. Historically, DBB refers to the ability of a valve to isolate pressure at each inlet and vent the cavity between the seats. Venting this cavity allows users to detect leakage past the seats without removing the valve from the piping system.

 

API 6D "Specification for Pipeline Valves" defines a double block-and-bleed (DBB) valve as a "single valve with two seating surfaces that, when closed, provides a seal against pressure from both ends of the valve, with a means of venting/bleeding the cavity between the seating surfaces." It also notes that a DBB valve cannot provide positive double isolation if only one side of the valve is under pressure.

 

In the case of Double Isolation and Bleed (DIB) valves, API defines them as a "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." The standard further explains that a DIB valve can provide positive shutoff when pressure is coming from one direction or both directions.

 

Cavity Pressure Relief Seat

Double Piston Effective (DPE) Design

The medium pressure "Pd" and spring force "Ps" will push the valve seat against the ball, ensuring sealing tightness. When the body cavity pressure increases to its limit, it is released through the body vent bleeder (Safety Valve), so the valve cavity pressure always remains lower than Pd+Ps. This design ensures that the ball valve provides isolation and blocking on both sides, known as Double Isolation and Block (DIB) Valves.

 

Double-Isolation-and-Bleed (DIB) Design

According to API 6D Section 3.1.11, a double-isolation-and-bleed valve is defined as: "A 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 sealing surfaces. This feature can be provided in one direction or in both directions."

 

In API's definitions of DBB and DIB, 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.

 

Two configurations are available for DIB ball valves: DIB-1 and DIB-2. The DIB-1 configuration features bi-directional seats upstream and downstream. The DIB-2 ball valve design features one bidirectional (DPE) seat and one unidirectional seat (SPE).

 

DIB-1 Explained

DIB-1 ball valves have DPE seats both upstream and downstream to provide a seal in both directions. For the DIB-1 configuration, over-pressurization of the cavity is avoided by using an external relief valve. 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, creating a second seal on the ball.

 

DIB-2 Explained

For the DIB-2 configuration, one seat is SPE and the other is DPE. Cavity over-pressurization is internally controlled within the line. If the upstream seat leaks, then there is automatic cavity pressure relief on the upstream side, and the downstream seat will provide isolation due to the double piston effect.

 

Pressure Test of DIB-1 (Two Bi-Directional Sealing Seats)

Each seat shall be tested in both directions, and the installed cavity pressure relief valve shall be removed. The valve shall be half-opened so that the valve and valve chamber can be injected with the test medium until the test liquid spills through the test port of the valve chamber. Close the valve to prevent leakage of the chamber in the direction of the test seat. The test pressure shall be applied successively to each end of the valve to test the leakage of each seat as the upstream seat separately, and then as the downstream seat. Open both ends of the valve to fill the cavity with media and then pressurize while observing leakage at each seat at both ends of the valve.

 

Because the pressure in the cavity of the DIB-1 valve cannot be released automatically, when the temperature of the valve is abnormally raised, the volume of the medium in the valve cavity increases accordingly, thus forcing the pressure in the cavity to increase automatically. When the pressure reaches a certain level, it will be very dangerous; therefore, the cavity of the DIB-1 valve must be installed with a safety valve.

 

Pressure Test of DIB-2 (One Bi-Directional and One Unidirectional Sealing Seat)

One of the seats of the DIB-2 valve can withstand pressure from either the chamber or the end of the valve in any direction without leakage. The other seat can only withstand pressure from the end of the valve. When the valve is closed the valve chamber test interface is open, and both ends of the valve are pressurized (or pressurized separately), the valve chamber test interface can detect whether there is leakage from each end to the valve chamber. A two-way seat test should pressurize the valve chamber and valve upstream to observe whether there is downstream valve leakage.

 

The advantage of the valve is tight protection for the valve. After the valve is closed, the medium will never enter the pipeline downstream, and simultaneously, when the cavity pressure abnormally rises, it can automatically relieve pressure upstream of the valve. Please note that the valve installation direction requirements are the same as for DBB.

 

Size:

½″ - 60″, Reduce and Full bore

 

 

Pressure class:

ANSI 150lbs – 2500lbs, API 3000-15000psi

 

 

End Connections available:

RF/RJ Flanged, compact flanged, SW / NPT / BW

 

 

Standards:

API, ASME, ASTM, BS, ISO, MSS & NACE, Cryogenic service

 

 

Applications:

Oil & Gas, Petrochemical, Pipelines, Offshore/Onshore, Subsea, Chemical, Power Generation, Water Industries

 

When designing ball valves according to API 6D, the most crucial component is the valve seat. Different valve seats enable the ball valve to perform various functions. In API 6D, there are two types of valve seats known as Single Piston Effect (SPE) and Double Piston Effect (DPE). These valve seats facilitate two functionalities in ball valves: Double Block and Bleed (DBB) and Double Isolation and Bleed (DIB).

 

Ball valves feature a hollow ball that allows the medium to flow through the valve. Besides the bore, there are two valve seats located upstream (pipeline side) and downstream of the valve. These valve seats provide sealing or isolation when the valve is closed. When operating the valve, the medium flows inside the cavity. The trapped medium can become pressurized and heated due to process flow or external sources, potentially causing the cavity pressure to exceed the upstream pressure. Thus, one valve seat will be subjected to two different types of pressure or force for sealing. This valve seat can function uni-directionally or bi-directionally, depending on the pressure differentials acting on the seats.

 

Uni-directional valve seats, also known as Single Piston Effect (SPE) seats or Self Relieving seats, function correctly when the upstream pressure is higher than the cavity pressure (see Figure 1), but they will leak if the cavity pressure surpasses the upstream pressure (see Figure 2). When the ball valve is closed, media pressure builds upstream, pushing the seat against the ball to create a tight seal. However, if the cavity pressure exceeds the upstream pressure, it pushes the seat away from the ball, creating a gap that results in a leak or causes the pressure to self-relieve to the upstream side.

 

DIB ball valves come in two configurations: DIB-1 and DIB-2. DIB-1 ball valves have both upstream and downstream seats as DPE. For DIB-2 ball valves, one seat is DPE and the other is SPE.

 

DIB-1 ball valves have DPE seats that provide sealing in both directions. The cavity pressure cannot be released to the upstream or downstream sides, 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, if there is an upstream seat failure, the cavity pressure will increase. This pressure will cause a double piston effect on the downstream seat, creating a second seal on the ball. This configuration should have a body cavity relief valve to relieve cavity pressure during over-pressurization due to seat leakage.

 

The main advantage of DBB valves is their self-relieving seats. However, if the upstream seat leaks, the downstream seat will not provide isolation. Conversely, the primary benefit of DIB valves is that even if the upstream seat leaks, the second seat will maintain the seal, preventing leakage. It's important to note that DIB valves should have body cavity relief valves. To combine the positive features of both DIB-1 and DBB valves, one should use a DIB-2 valve with an SPE seat upstream and a DPE seat downstream. If the upstream seat leaks in a DIB-2 valve, the cavity pressure can be relieved to the upstream side, and the downstream seat will provide isolation due to the double piston effect. It's critical to install DIB-2 valves correctly, with the SPE seat on the upstream/pressurized side.

 

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