In the Oil/Gas industry, although it has become very common, the use of a compact Double Block & Bleed (DBB) valve for pressure energy isolation is still not applied or adopted across the board. I have experienced a few times wherein the use of a DBB valve for process isolation is more of an option rather than a design requirement.
There are safety regulations such as OSHA and engineering codes/practices that gives us guideline on the different kinds of isolation, isolation levels and where to use them. Note that each country/region or even local plants have different standards and policies.
Since there is a variety of actual site requirements during project execution, turnaround, shutdown or maintenance, engineers involved may need to look at all options. The industry today is sensitive on project cost, but it is imperative not to sacrifice safety and reliability, and if possible, time and ease of use.
For this reason, I would like to describe some of the practical advantages of using a dual ball DBB compared to the real-world problems common to other methods of isolation.
Solution with DBB Valves
Compact Modular DBB Valve
The use of a DBB valve can be classified as a high-level isolation, second only to the actual physical separation. A dual ball DBB is a quarter-turn valve which can create a quick barrier by a 90° turn which is really a time-saving option. Even if a gearbox is used, this is the case especially for the larger bore sizes, it is still only a few turns of the hand wheel. It normally has locking facilities to comply any lock-out/tag-out exercise.
Your advantages while satisfying the ease of intervention at the same time:
- Reduce footprint
- Reduce weight
- Reduce Cost
- Reduce risk exposure
Traditional DBB Arrangement
(2 separate block valves)
This is a conventional method where you combine 2 separate ball valves and a spool piece in-between the 2 valves. The bleed valve is often retrofitted in the center spool to form a DBB arrangement. This method takes more space, more weight and more laborious to connect all the individual components. To obtain the true cost, we need to factor in the individual valve testing, individual NDT, individual lifting/installation cost and accessories for each component. This arrangement has several leak paths which could pose hazard to both the workers and the environment.
Other Isolation Methods
It is a common isolation method that comes with a list of disadvantages. To blind the line, a number of workers are needed, equipped with their PPE and tools. For big size and higher rating, lifting equipment will be required (Eg: 6″ CL 2500 blind flange weighs 141 KG). During shutdown and maintenance work, the workers always deal with rusted bolts/nuts and misaligned gaskets which result to very time-consuming, back-breaking, hand-injury causing procedures. Sometimes, old fasteners are sticking that workers need to cut/saw it just to dismantle the bolting. When this is done, flange/counter-flange surfaces need to be cleaned and a new gasket is often required.
This is similar to installing a blind flange except the use of a spectacle blind is more of a permanent installation.
Designed for an easy change-over between a blind or an open spacer, whichever is needed. Just like blinding, this method still requires a physical labor and manual intervention which consumes time and manpower.
Note that for bigger size and higher rating, it could be a back-breaking exercise even by just pivoting/turning the plates. Similarly, the workers will still encounter the same problems like rusted bolting and the challenge of installing the gasket. Since intervention is oftentimes occasional, it is very likely that the blind portion gets corroded and may need to be scraped and cleaned before using it again.
(using a removable spool and installing a blind falnge on the main piping)
This is also one way of getting a positive isolation where it involves an actual and physical separation of the system or equipment. To break achieve a line break, a removable flanged spool piece is detached from the main pipeline, and the main pipeline is blanked with a blind flange. If the main pipeline has a big nominal size, then one will have the same big spool to disconnect, remove, and put in place in a bigger working area.
This is also intensive in terms of equipment and manpower; one probably needs a lifting equipment or forklift. Once operation is resumed, the same spool will be moved and re-position to its place.
In line with innovations in the market, there are proprietary designs of piping equipment where blinding is easier and time-saving. Nevertheless, it is still physical and manual interventions that needs a block or block and bleed isolation in the upstream side.
Since isolation levels depend on the risk associated with the intervention, it is important to evaluate carefully the isolation requirement with respect to safety, cost, time and efficiency.
NOW, imagine if a project requires a huge number of isolations?
Now it becomes clear that a Compact Modular DBB valve is the better solution to reduce weight, cost and risk exposure, while satisfying the ease of intervention at the same time.
(Images source: © xmentoys/Fotolia)