Differences Between Fixed Ball Valve DBB, DIB-1, and DIB-2

2026-09-18 09:06:33

There are two common seat sealing configurations for fixed ball valves.

1. DBB (Double Block and Bleed Valve)

A valve with two seat sealing pairs, which in the closed position provides pressure sealing at both ends of the valve by means of venting/draining the body cavity between the two seat sealing surfaces.

2. DIB (Double Isolation and Bleed Valve)

A valve with two seat sealing pairs, each of which in the closed position provides single-source pressure sealing by means of venting/draining the body cavity between the two seat sealing pairs.

Valve Test Methods

1. DBB (Double Block and Bleed) Valve Test

Position the valve in the half-open position and completely fill the valve and body cavity with the test medium. Then close the valve and open the body bleed valve, allowing excess test fluid to overflow from the cavity test connection. Pressure shall be applied simultaneously from both ends of the valve, and seat sealing integrity shall be monitored by observing overflow at the cavity test connection.

2. DIB Valve Test – Divided into DIB-1 and DIB-2

(1) DIB-1 (Double Isolation and Bleed Valve – Bidirectional Double Seats)

Each seat shall be tested in both directions. If a cavity pressure-relief valve is installed, it shall be removed. Position the valve in the half-open position and fill the valve and body cavity with test medium until test fluid overflows from the cavity test connection. To test seat leakage in the cavity direction, close the valve. Test pressure shall be applied sequentially to each end of the valve to test each seat individually for leakage from the upstream side. Leakage shall be monitored by observing overflow at the cavity test connection. Subsequently, each seat shall also be tested as a downstream seat. Open both ends of the valve and fill the cavity with test medium, then pressurize while observing leakage at each seat from both ends of the valve.

(2) DIB-2 (Double Isolation and Bleed Valve – Single-Seat Unidirectional / Unidirectional Double Seats)

Bidirectional seats shall be tested in both directions. If a cavity pressure-relief valve is installed, it shall be removed. Position the valve in the half-open position and fill the valve and body cavity with test medium until test fluid overflows from the cavity test connection. To test seat leakage in the cavity direction, close the valve. Test pressure shall be applied sequentially to each end of the valve to test each seat individually for leakage from the upstream side. Leakage shall be monitored by observing overflow at the cavity test connection. For the bidirectional seat test in the cavity test, pressure shall be applied sequentially to the cavity and the upstream end, and leakage shall be detected at the downstream end of the valve.

Note: The above text is primarily sourced from API 6D-2014. Without repeated reading and careful consideration, it is difficult for non-specialists to fully comprehend the intended meaning

Personal Analysis:

Below, I will use diagrams and textual explanations to illustrate these valve configurations:

1. DBB (Double Block and Bleed) Valve

The figure above shows a typical DBB valve configuration. The ends are labeled "Valve Upstream (Valve Downstream)" and "Valve Downstream (Valve Upstream)" because the end to which pressure is applied is defined as the upstream side, and the other end as the downstream side. During pressure testing, either end of a DBB valve can serve as the upstream or downstream side. The above figure can be simplified as:

The two vertical lines represent the two valve seats, and the arrow directions indicate the sealing directions required. That is, when the valve is in the closed position and the cavity test connection is open, applying pressure simultaneously (or separately) to both ends of the valve allows the cavity test connection to detect whether leakage exists from each end into the cavity.

Note: Theoretically and per standard specifications, a DBB valve does not provide a basis for maintaining pressure at both ends without leakage when pressure is applied from the cavity.

2. DIB-1 (Double Isolation and Bleed Valve – Bidirectional Double Seats)

The figure above shows a typical DIB-1 valve configuration. The ends are labeled "Valve Upstream (Valve Downstream)" and "Valve Downstream (Valve Upstream)" because the end to which pressure is applied is defined as the upstream side, and the other end as the downstream side. During pressure testing, either end of a DIB-1 valve can serve as the upstream or downstream side. The above figure can be simplified as：

The two vertical lines represent the two valve seats, and the arrow directions indicate the sealing directions required. That is, each seat of a DIB-1 valve can withstand pressure from either the valve cavity or the upstream/downstream ends without leakage. When the valve is in the closed position and the cavity test connection is open, applying pressure simultaneously (or separately) to both ends allows the cavity test connection to detect leakage from each end into the cavity. Subsequently, after relieving pressure at both ends, pressure is applied from the cavity to observe whether leakage occurs at either end.

Note: Since the cavity pressure of a DIB-1 valve cannot be automatically relieved to either end, abnormal temperature increases may cause the cavity medium to expand, thereby increasing cavity pressure. When the pressure reaches a certain level, the valve becomes hazardous. Therefore, a pressure-relief valve must be installed on the cavity of a DIB-1 valve.

3. DIB-2 (Double Isolation and Bleed Valve – Single-Seat Unidirectional / Unidirectional Double Seats)

 The figure above shows a typical DIB-2 valve configuration. The ends are labeled "Valve Upstream (Valve Downstream)" and "Valve Downstream (Valve Upstream)" because the end to which pressure is applied is defined as the upstream side, and the other end as the downstream side. During pressure testing, either end of a DIB-2 valve can serve as the upstream or downstream side. The above figure can be simplified as:

The two vertical lines represent the two valve seats, and the arrow directions indicate the sealing directions required. That is, one seat of a DIB-2 valve can withstand pressure from any direction (from the cavity or from either end) without leakage; the other seat can only withstand pressure from the end side without leakage. When the valve is in the closed position and the cavity test connection is open, applying pressure simultaneously (or separately) to both ends allows the cavity test connection to detect leakage from each end into the cavity. Subsequently, when testing the bidirectional seat, pressure should be applied separately to the cavity and the upstream end, and leakage shall be observed at the downstream end. As shown above, pressure is applied to the cavity and the left end, and leakage is observed at the right end.

Note: The advantage of this valve is that when the pipeline requires that, after the valve is closed, the medium must not enter the downstream side, and simultaneously, when cavity pressure rises abnormally, it can be automatically relieved to the upstream side, protecting the valve. Therefore, this type of valve has an installation orientation requirement and special attention must be paid. If installed in the reverse direction, its performance will be no different from that of a DBB valve.