Double Block and Bleed (DBB) trunnion-mounted ball valves have been traditionally used on pipelines for isolation of pig traps or scraper traps. DBB design ball valves have provision for venting of the body cavity to verify that both upstream and downstream seats provide effective sealing against the line pressure.
What is Single Piston Effect seat in Ball Valve design?
Figure below depicts a configuration of a ball valve is subject to line pressure from one side and the other side is not pressurised. Referring to figure 1a below, if the area A1 of the seat ring is greater than area A2 a net force equal to line pressure x area (A1 - A2) plus the force exerted by the springs acts against the ball creating a seal. Let us assume that the service fluid gets trapped in the body cavity. Referring to figure 1b below, if the area A3 of the seat ring is greater than area A4 of the seat ring a net force equal to body cavity pressure x area (A3 - A4) acts against the spring force. When this net force is sufficient to overcome the spring force, the net force pushes the seat ring away from the ball allowing the service fluid to relieve on the downstream side of the valve.
Thus SPE seats will provide sealing if the upstream or downstream pressure is greater than cavity pressure but will leak if the cavity pressure is greater than upstream or downstream pressure. In a single piston effect design the relief of fluid from the body cavity can occur through both the seats depending on the pressure in the body cavity and the pipeline.
If a valve with two single piston effect (SPE) seats is used for isolation of scraper (pig) trap there is a possibility for the service fluid to be relieved from the valve body cavity into the pig trap. If the process fluid is hazardous, this can lead to a potentially unsafe situation. This limitation can be overcome by using a double piston effect (DBE) seat design.
What is Double Piston Effect seat in Ball Valve Design?
Referring to figure 2b below, if the area A1 of the seat ring is greater than area A2 a net force equal to line pressure x (A1 - A2) plus the force exerted by the springs acts against the ball creating a seal. Let us assume that the service fluid gets trapped in the body cavity. Referring to figure 2b below, if the area A3 of the seat ring is greater than area A4 of the seat ring a net force equal to body cavity pressure x (A3 - A4) plus the spring force acts against the ball. As the net force always acts on the ball creating a seal, there is no possibility of the cavity pressure relieving on side of the valve corresponding to figure 2b.
Thus DPE seats will provide sealing if upstream or downstream pressure is greater than cavity pressure and will also seal if the cavity pressure is greater than upstream or downstream pressure. This type of valve should have a means of relieving the body cavity in case there is overpressurisation of cavity due to seak leakage.
If a valve with single piston effect (SPE) seat on one side and double piston effect (DPE) seat facing the scraper trap side is used for isolation of scraper (pig) trap there is no possibility of the service fluid relieving on the pig trap side. Any relieving of the seat will take place on the SPE seat side of the valve. This combination of seats offers a safe solution for isolation of scraper traps.
What is Double Block and Bleed (DBB) Valve?
As per API 6D / ISO 14313 a double block and bleed valve is defined 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. A DBB valve does not provide positive double isolation when only one side is under pressure. If there is any build-up of pressure in the body cavity, the seat system is designed to automatically vent any excessive pressure to the upstream and downstream side of the pipeline by virtue of a floating seat design. A DBB valve can be consider as a valve with two single piston effect (SPE) seats. This feature as discussed earlier is also called as self-relieving seat design. With this valve feature it is not safe to carry out any maintenance work on the downstream side of the DBB valve.
It is possible to contain the self-relieving seats to only one side of the pipeline by using a double piston effect seat.
A typical DBB valve has the following features:
- It includes two uni-directional seats, one at each end.
- Both seats have conventional upstream sealing which means sealing is only from the upstream side of the seats (from pipeline side to the ball).
- Upstream pressure forces the seat towards the ball and creates the sealing.
- The downstream side of the seat (from body cavity towards the ball) will not seal.
- The body cavity can be relieved through vent or drain connections provided on the body cavity.
The independent upstream and downstream sealing ensures a tight shut-off at the body cavity in the fully open and closed positions. This feature allows for verification of the tightness of the ball valve seals.
What is a Double Isolation and Bleed (DIB) Valve?
As per API 6D / ISO 14313 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 seating surfaces. The DIB feature can be provided in one direction or both directions. As per API 6D, DIB-1 designation shall be used if both seats are required to be bi-directional and DIB-2 designation shall be used if one seat is required to be bi-directional and other seat is required to be uni-directional. Thus, a valve with two double piston effect (DPE) seats has double isolation and bleed (DIB-1) design. A valve with one SPE seat and other DPE seat is a DIB-2 design valve. A DIB-2 valve will have a preferred installation direction.
A typical DIB-1 valve has the following features:
- It inclues two bi-directional seats, one at each end.
- Upstream pressure forces the seat towards the ball and creates the sealing from upstream side.
- Pressure in the body cavity also forces the bi-directional seat towards the ball also called downstream sealing.
- The upstream seat and the downstream seat seal in both directions.
Conclusion
A ball valve with DIB-2 design (SPE seat towards pipeline side and DPE seat towards scraper trap side) is recommended to be used for isolation of pipeline scraper traps.