It's time to do maintenance on a section of process. You don't want to shut down the entire facility, so you decide to block off and depressurize just the section you're working on. Just upstream is a double block and bleed valve—a trunnion-mounted ball valve with self-relieving seals and a bleed valve to vent the cavity. You close the ball valve and open the bleeder. Now you can de-pressurize the line downstream and open it up to work on it.

No so fast, says Rudy Garza. You may think that valve gives you double isolation, but it doesn't—and that could be dangerous.

Garza stresses that his presentation shouldn't be taken as holy writ, but as how his particular branch of ExxonMobil (i.e. Upstream) looks at the situation in its own industry and the practices it uses. Other companies (including within ExxonMobil) and industries may do things differently and it is up to the users to determine the safety and suitability of a particular practice to their application, he says.

The key message is that a user should look at the design of a particular valve, and find out exactly what the manufacturer means by the term "double positive isolation" or "double block and bleed.", to make sure it's really what's needed in a particular application.

Many users, says Garza, have taken "double block and bleed" as a generic term, and tend to use it when they really mean (and the applicable specification—API 6D, Specification for Pipeline Valves, requires) the use of double isolation and bleed. The key to understanding, Garza says, can be found in API 6D. That specification wasn't always as clear as it could have been in spelling out the difference between DBB and DPI, but the addition in 2008 of several notes has clarified it.

API 6D defines a double-block-and-bleed valve (DBB) 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." The 2008 note points out that this valve does not provide positive double isolation when only one side is under pressure.

By contrast, API 6D defines a double-isolation-and-bleed valve (DIB) 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 note adds that this feature can be provided in one direction or in both directions.

The job of a double isolation and bleed is to stop process fluid from getting into an area where work is being done. Both in-line valves would be closed, then the bleeder would be opened. If any fluid leaked past the first valve the bleeder would drain it off before it pressurized the cavity - the space between the upstream and downstream valves, and at the same time would act as a tell-tale to indicate the leakage. If the bleeder (which is smaller than the in-line valves and may, in fact, be a needle valve) were to be plugged the downstream valve would keep process fluid from getting past it.

So why is the difference between DBB and DIB important? Let's consider 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. Under normal conditions (Figure 1) there is pressure on the upstream seal, which (along with an internal spring) keeps it energized. There's no pressure on the downstream side, so the only thing energizing the seal on that side is a spring. The bleeder valves are open, and the cavity in the ball is at atmospheric pressure.

But it's not uncommon for a valve that's been in service for a while to leak a bit. Figure 2 shows what happens then. The upstream seal is leaking a little, but this should not be a problem because the leakage will be carried away by the bleeder—except when the bleeder is not working, either because one or both of the bleeder valves is closed, or because there's a clog in the bleed line. The pressure in the valve cavity can then possibly reach as high as 200 psi, which overcomes the spring on the downstream seal and forces it off its seat, discharging fluid downstream to where people may be working. This is clearly not a double isolation and bleed valve.