Double Block and Bleed
API 6D defines a double-block-and-bleed valve (DBB) as a “single valve with two seating surfaces that, within the closed place, offers a seal in opposition to stress from each ends of the valve with a way of venting/bleeding the cavity between the seating surfaces.” The 2008 word factors out that this valve doesn’t present optimistic double isolation when just one aspect is underneath stress.
In contrast, API 6D defines a double-isolation-and-bleed valve (DIB) as a “single valve with two seating surfaces, every of which, within the closed place, offers a seal in opposition to stress from a single supply, with a way of venting/bleeding the cavity between the seating surfaces.” The word provides that this function will be supplied in a single path or in each instructions.
The job of a double isolation and bleed is to cease course of fluid from moving into an space the place work is being carried out. Each in-line valves could be closed, then the bleeder could be opened. If any fluid leaked previous the primary valve the bleeder would drain it off earlier than it pressurized the cavity—the house between the upstream and downstream valves, and on the similar time would act as a tell-tale to point the leakage. If the bleeder (which is smaller than the in-line valves and should, the truth is, be a needle valve) have been to be plugged the downstream valve would maintain course of fluid from getting previous it.
So why is the distinction between DBB and DIB essential? Let’s contemplate a typical trunnion-mounted ball valve with self-relieving seats. API 6D defines this as a double block and bleed valve, not a double isolation and bleed valve. Beneath regular situations (Determine 1) there’s stress on the upstream seal, which (together with an inside spring) retains it energized. There isn’t any stress on the downstream aspect, so the one factor energizing the seal on that aspect is a spring. The bleeder valves are open, and the cavity within the ball is at atmospheric stress.
But it surely’s not unusual for a valve that is been in service for some time to leak a bit. Determine 2 exhibits what occurs then. The upstream seal is leaking somewhat, however this shouldn’t be an issue as a result of the leakage can be carried away by the bleeder—besides when the bleeder just isn’t working, both as a result of one or each of the bleeder valves is closed, or as a result of there is a clog within the bleed line. The stress within the valve cavity can then probably attain as excessive as 200 psi, which overcomes the spring on the downstream seal and forces it off its seat, discharging fluid downstream to the place individuals could also be working. That is clearly not a double isolation and bleed valve. Get more about KP-LOK double block and bleed valve
So the place ought to the sort of valve be used? Determine 3 exhibits a scenario which may happen when the valve is utilized in a bypass loop for proving a flowmeter, for instance. The valve is closed and the bleeder is open. This time there’s stress on each the upstream and downstream seals, retaining them absolutely energized. That is the configuration that the API 6D definition of “double block and bleed” supposed when it referred to “two sealing surfaces.” But it surely’s not true double optimistic isolation and bleed, so far as ExxonMobil Upstream is worried, and in sure companies, it should not be used to isolate a piece for upkeep.
To forestall confusion ExxonMobil Upstream kinds valves into 4 classes—A, B, C and D—based on the bodily move blocking capabilities of the valve after which offers steering based mostly on the minimal isolation necessities for a specific software, corresponding to lengthy vs. brief time period, segregation for meter proving, and so on.
- Sort A is a single block valve with a single mechanically energized seal and no physique bleed required.
- Sort Bis a double block and bleed (as outlined in API 6D, however not all the time by trade). It requires stress on upstream and downstream sides concurrently to energise the respective seals. It is the sort proven in Figures 1 by way of 3 (see finish of article).
- Sort Cis a real double isolation and bleed (DIB) per API 6D. The valve is a single physique with twin optimistic seals; it has a single obturator (gate, plug, and so on) and twin positively energized seals (upstream and downstream) with cavity bleed port between them. It requires a cavity overpressure safety machine in expansive fluid companies).
- Sort D is a real double optimistic isolation and bleed valve association, with two impartial obturators (sealing members) in the identical or separate our bodies and two separate actuating mechanisms (i.e. impartial stems). It may be made up of a pair of sure Sort A, B, or C valves, both separate or constructed into one physique. It will need to have a bleeder within the center (between the 2 valves and between every valve’s seals if Sort C valves are used).