DBB or DIB - Background

1. DBB or DIB - Background Ball valves for isolation applications often require a second pressure barrier operating independently of the primary pressure barrier. The need is often due to operational safety requirements, the service (gas service, zero leakage, cleanliness of the produced fluid, etc.). A valve with this independent second barrier is commonly referred to as “DIB” or “Double Isolation and Bleed”. DIB differs significantly from double block and bleed valves, commonly referred to as “DBB”. DBB isolates the pressure from both directions and bleeds the cavity between. It does not provide a second pressure barrier to pressure from either end. A DBB valve only provides a single barrier to pressure from either end. DIB provides two barriers to pressure from the same end of the valve. With DIB the primary seal can be verified by bleeding the cavity between and monitoring that cavity for leakage. The second barrier is in place should the first barrier leak and flood the cavity between.

2. DBB or DIB – EII Valve EII Valve has developed unique uni-directional seats and bi-directional seats for its Type ST3 series of soft-seated, trunnion ball valves that can be used to configure the valve for DIB or DBB capability each achieving class V or VI shutoff. The diagram below shows a trunnion ball valve with one unidirectional seat and one bidirectional seat. DBB is present with zero, one, or two bidirectional seats. DIB in both directions can be achieved by using two bidirectional seats or in one direction by installing only one bidirectional seat downstream. The downstream side is at the left. All EII Valve Type ST3 valves include a body test port fitting to allow testing of the seats whether configured for DIB or DBB only. The valve is closed and each end is pressured, then the body test port fitting is opened to allow pressure to escape from the body. Continued exhaust from the test port indicates a seat leak.

3. DBB or DIB – EII Valve The action of the seats is determined by the pressure differentials that act on the seats. For the unidirectional seat (on the right), upstream pressure urges the seat against the ball and creates a seal between the seat and the ball. The seat is urged in this direction by pressure acting across the differential area between the ball seal contact diameter and the body seal contact diameter. Pressure in the body cavity urges the seat away from the ball and breaks the seal between the ball and seat, thereby relieving body pressure. The bidirectional seat is urged against the ball by pressure regardless of the location of the pressure source (upstream or in the body cavity). This is achieved by moving the effective diameter of the body seal in or out with the reversing ring on the EII bidirectional seat.

4. DBB or DIB – EII Valve An EII Valve Type ST3 with DIB provides a secondary seal and retains the DBB feature. DIB when both seats are bidirectional can cause the valve to trap pressure in the body cavity between the seats. This is typically not a cause for concern in gas service but can be in liquid service. DIB with one bidirectional seat and one unidirectional seat does not have the potential to trap pressure in the body cavity and is often the preferred embodiment for many applications. The limitation is that the DIB function is only effective with the bidirectional seat downstream from pressure.

5. DBB or DIB – Applications Applications for DBB include most ball valve applications. EII Valve Type ST3 ball valves all include the DBB feature. Applications for DIB include metering (particularly gas) and as block valves where line maintenance is expected or common. The DIB design feature provides the second barrier so that while piping is removed downstream (as in a repair or expansion situation) the body cavity can be monitored for upstream seat leakage. The downstream seat provides the second sealing barrier in the event the upstream seat begins leaking during the maintenance or repair. The metering service has a different reason for the DIB. It is driven mainly by the application’s zero leakage requirement. A closed valve that is leaking slightly can create errors in metering. DIB provides a similar result as having two valves in series in either application.

6. DBB or DIB – Applications When two bidirectional seats are used to achieve the DIB feature in both directions, the valve can trap pressure in the valve body cavity between the seats. Liquid trapped in a confined space and heated or allowed to warm could lead to extreme pressures in the body if there is no relief. Using only a single bidirectional seat (as shown in the diagram) eliminates the possibility of building up pressure in the body. For applications where the direction of pressure is known, DIB with one bidirectional seat is preferred.

7. DBB or DIB – Application Assistance Contact EII Valve sales personnel for assistance in determining which seat combination is best suited to meet your process requirements.