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 ball valves have a hollow ball inside that allow the medium to flow through the valve. Besides the bore, there are two valve seats located upstream (or pipeline side) and downstream of the valve. These valve seats provide the seal or isolation when the valve is closed. When we operate the valve the medium flows inside the cavity. The flow medium trapped in the cavity can get pressurized and heated due to process flow or external source. It could be possible that the pressure in the cavity can exceed the upstream pressure. So one valve seat will be exposed to two different types of pressure or force for sealing. This valve seat can perform two functions of uni-directional or bi-directional determined by the pressure differentials acting on seats.
Uni-directional valve seats or SPE seats (also called as Self Relieving seats) provide sealing if the upstream pressure is greater than the cavity pressure (see Figure 1) but will leak if the cavity pressure is greater than the upstream pressure (see Figure 2). In the closed position of the ball valve the upstream pressure will build due to media pressure, this pressure will try to push the seat and the valve will provide a seal by keeping a tight contact between the ball and the upstream seat ring. However, when the cavity pressure exceeds the upstream pressure, the cavity pressure will push the seat back or away from the ball leaving a gap that will cause a leak or the pressure will self-relieve to the upstream side.
Bi-directional valve seats or DPE seats provide sealing if the upstream pressure is greater than the cavity pressure (see Figure 3) and will also seal if the cavity pressure is greater than the upstream pressure (see Figure 4). So, the DPE seat will provide sealing and will not allow a leak path irrespective of the cavity pressure being higher or the upstream pressure.
It has been a common misunderstanding in the industry that DBB valve provides double isolation barriers, infact it does not. At times designers specify DBB valves where infact they should be refering to DIB valve. To clarify this confusion, API 6D defines the DBB valve as “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”. However, DIB valve is defined as “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.”
DBB ball valves have SPE seats on the upstream and the downstream side. SPE seats will provide a seal in one direction only. Consider the case when the valve is fully closed, the single piston effect at the upstream side actually pushes the valve seat against the ball to provide the seal. In the case of seat wear or seat leakage, the cavity pressure will increase. Excess body cavity pressure will push the downstream seat ring away from the ball creating a gap, and relieving overpressure from the body cavity. It will make the valve seat on the downstream side to leak prior to the upstream seat leakage. The upstream side is pressurized while the downstream side has no or minimum pressure and there would be no force to keep the contact between the ball and the seat at the downstream side. DBB ball valves are not recommended if a true isolation is required for operational safety during shutdown.
DIB ball valves have two different configurations called as DIB-1 and DIB-2. DIB-1 ball valves have both upstream and downstream seats as DPE. However, in case of DIB-2 ball valve, one seat is DPE and the other one is SPE.
DIB-1 ball valves have DPE seats and both upstream and downstream seats will provide a seal in both directions. The cavity pressure cannot release to the upstream or downstream side 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, the cavity pressure will increase in case of upstream seat failure. The cavity pressure will cause a double piston effect on the downstream seat and thereby creating a second seal on the ball. This kind of configuration should have a body cavity relief valve that should relieve the cavity pressure during over-pressurization due to seat leakage.
The main advantage of DBB valve is that it has self-relieving seats. However, if the upstream seat leak the downstream seat will not provide isolation. On the other hand, 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 valve should have body cavity relief valves. If we want to combine positive features of both DIB-1 and DBB 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. It should be noted that the installation of DIB-2 valve is critical and SPE seat should be on the upstream/pressurized side.